# Did Mercury clear its neighborhood?

We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

For a body to qualify as a planet according to the IAU definition it must have "cleared its neighborhood". What evidence is there Mercury indeed cleared its neighborhood? Perhaps it migrated there afterwards, when the neighborhood had already been cleared. Does the Grand Tack hypothesis impact our definition of the inner planets as planets?

https://en.m.wikipedia.org/wiki/IAU_definition_of_planet

See Grand Tack Hypothesis.

The present definition of a planet is vulnerable as it seems connected to a model of formation of the solar system. The answer below states that in practice an operational definition used that I believe is adequate.

One calls Mercury a planet because it doesn't share its orbit with any other bodies of comparative size. The fact that it is in its orbit means that that orbit hasn't been cleared by another body (for if it had been cleared by another planet, then Mercury would have been cleared too.)

In practice one doesn't look at "How" an orbit is cleared, only if there is one body that dominates the orbit, or if there are many with no one body dominating. So Mercury is clearly much larger than any other bodies in roughly 88 day orbits. But, (for example) Ceres does not dominate its orbit, as it shares its orbit with Vesta, Pallas and Hygeia and many other asteroids.

## Mercury Retrograde Dates for 2021

Mercury’s next period of retrograde motion in 2021 lasts from May 29 to June 22! According to the age-old practice of astrology, we are all influenced by the effect of Mercury in retrograde. What exactly does it mean for each of us, though? Read on to find out.

Three times a year, the planet Mercury appears to travel backward across the sky. We refer to these periods as times when Mercury is in apparent retrograde motion, or simply ”Mercury retrograde.” To those who practice astrology, these times in particular were traditionally associated with confusion, delay, and frustration. Think undelivered love letters, email blunders, and frazzled travel plans! This is an excellent time to reflect on the past, however, and it’s said that intuition is high during these periods. Coincidences can be extraordinary.

The astronomical explanation for retrograde motion starts with understanding that the planets in our solar system orbit the Sun at different distances and speeds. While we orbit the Sun ourselves, we can observe the other planets moving across our sky, following their own paths.

Sometimes, it can appear (from our perspective) that a planet has abruptly switched directions and has started to move in reverse across the sky. This is, of course, an illusion caused by the position of Earth in relation to that of the planet, since a planet in orbit always travels in one set direction and can’t suddenly reverse course. This is why we call the phenomenon apparent retrograde motion, as it only looks like the planet is moving backward (“retrograde motion”)!

### When Is Mercury in Retrograde in 2021?

In 2021, Mercury will be in apparent retrograde motion during the folowing ranges of dates:

• January 30 to February 20
• May 29 to June 22
• September 27 to October 17

Please note that dates reflect Eastern Time U.S., not Universal Time.

### What You Should Do When Mercury is Retrograde

The planet Mercury rules communication in all forms—listening, writing, reading, speaking, and so on—as well as activities closely related to communication, like negotiations and contracts. It also rules travel, automobiles, shipping, and mail.

So, when Mercury is retrograde, try to remain flexible, patient, and understanding, allow extra time for travel, and avoid signing onto any new contracts that you’re unsure of. Double check your email responses and check in with reservations before you take that trip.

Review projects and plans at these times, but wait until Mercury is direct again to make any final decisions. You can’t stop your life, but plan ahead, have back-up plans, and be prepared for people’s shorter fuses and miscommunication.

Take a Moment to Reflect

Some blame Mercury retrograde for all chaos that happens in their lives. However, this is a good time to sit back and review what you put your energy toward.

For example, if family and faith are important to you, are you putting enough of your energy into them or have you become overextended in other areas such as your career or a hobby? Losing the balance between different parts of your life can cause all of them to suffer, so being aware of the connections between them—especially during Mercury retrograde—can help you to maintain or at least better understand how that balance works.

Take a moment to reflect. Mercury retrograde can be an excellent time to take a step back and reanalyze who you are and what you are doing—but do refrain from making any drastic changes until after retrograde has ended.

### Mercury and the Zodiac

The type of influence you feel also depends on which of the 12 zodiac signs Earth is in when Mercury goes retrograde.

## Contents

The Bible explicitly dates the universe as being the same age as the Earth and just over 6000 years old. In fact, according to the Bible, all other celestial bodies are slightly younger than the Earth. At the moment the Earth was created there were no other planets, stars, comets, or other such bodies in the universe. None of these came into existence until the fourth day of the Creation Week (See: Genesis Chapter 1). There is indeed much evidence to support the contention that our solar system, galaxies and even that the entirety of the universe is very young.

In contrast, secular scientists date the universe as being approximately 13.7 billion years using standard cosmologies. [2] The universe is believed to have begun with a cosmic inflation known as the Big Bang, which is then followed by the formation of stars, planets, and galaxies. Based on this chronology the Earth is believed to have formed after our Sun and is dated to be near 4.6 billion years old.

Vector site provides a nice summary of what we know about the planets. That will be the source for my answer.

Some planets were fairly well known to the ancients, but they could only use their eyes until the birth and proliferation of the telescope (starting in the 1600s) and then modern telescopes and space probes (1900s).

The planets known to the ancients were Mercury, Venus, Mars, Jupiter and Saturn. Of these, Venus and Mars could be more closely observed by the eyes (with Mars appearing red), but Jupiter and Saturn appear as brighter distant stars. However, they had a clear orbit and so they fell into the "planet" category. Even the closer planets are confusing to the eye - Mercury and Venus are low in the atmosphere and/or coincide with the sun, making it hard to see any detail with the eye.

Telescopes from the 1600s into the early 1900s could only tell so much. Using math and orbital mechanics, combined with telescope observations, astronomers could determine relative density, mass, orbital parameters and so on. Prominent moons could be seen too: Galileo spotted four moons with his first observation of Jupiter in 1610.

The more distant planets required stronger telescopes. Uranus wasn't spotted until the 1780s and its rings weren't spotted until 1977.

Understanding of Uranus' orbital mechanics developed in the 1800s to suggest the presence of another planet, and so astronomers started looking. With the latest understanding of Uranus' strange orbit and some math, Le Verrier plotted the position of what had to be the source of the issue - and thus Neptune was found in 1846. Galileo had actually spotted Neptune in 1613 but chalked it up to a star. Neptune's first moon Triton was spotted shortly afterward, but appeared so small and faint as to be hard to calculate its properties.

Pluto wasn't spotted until 1930. I'll avoid the planet debate here, but information about Pluto was slow to come and difficult to interpret. Charon, which defines some of Pluto's behavior and being unusual as a binary system, wasn't found until 1978.

Telescopes could also be misleading. The most famous case is Mars' canals. The presence of canals, which appeared to link oceans or oases, appeared in observations in the 1800s. Giovanni Schiaparelli produced a map of Mars in 1877 that showed them prominently. The canals and their possible origins captured the imagination of astronomers and the sci-fi community for years, although other astronomers claimed they couldn't see them. This craze included a book, MARS AND ITS CANALS, by Percival Lowell in 1906. When Mariner 4 observed Mars in 1965, there were no canals or oases - just a barren planet.

The limits of classic telescopes started to be reached in the late 1800s. For example, the fifth known moon of Jupiter would be observed in 1892 and would be the last until photography and imaging technology (i.e., modern telescopes) became useful. Radar astronomy would provide additional data and would correct some previous observations, such as showing Venus to be very hot in the 1950s (previously Venus was thought to be swampy).

Most of these earth-based observations were still relatively basic by the 1960s. Space probes had to clarify and confirm many of these observations, starting in the early 1960s with simple probes. For the gas giants, Pioneers 10 and 11 were the first visitors but were fairly basic, but the soon-to-follow Voyagers provided fantastic data that added much to what is now known - although there are still plenty of mysteries out there. Earth-based observatories still play a prominent role as well.

To summarize the answer, only a few planets had color and appearance that could be estimated and guessed until relatively recently. The telescope certainly helped starting in the early 1600s, but there were still errors and limitations (the Mars canal controversy being an example). Earth-based observations have expanded our knowledge but are concerned more with physical properties (mass, density, spin rates, atmospheric parameters, etc). Space probes have been the best at determining color and appearance.

## 3. The IAU definition unfortunately mixes up being with doing

Philosophers wonder whether it is more important to be or to do. According to the graffiti on the wall:

Saying a planet must be round by its own gravity is a part of its being. Saying it must have cleared its orbital neighborhood is a part of its doing. There’s also an implicit requirement that a planet must not be so big that it starts nuclear fusion, because then it would be a star. That’s part of its being. And furthermore, the IAU says a planet must orbit a star instead of going rogue or orbiting another planet. That’s more of its doing. This mixed-up set of being and doing requirements reflects the chaos of human history, because we didn’t understand what planets were in the earliest days of astronomy, and we never said, “let’s just start over and create a completely new system that makes sense.”

Mixing these would be crazy in any other branch of science. Suppose we said that unless a species eats meat, then it’s not a real mammal it’s just a dwarf mammal. Or consider the tiny chameleon pictured above. It could never clear the alligators out of its neighborhood, so it wouldn’t qualify as a true species of reptile it’s just a dwarf reptile, and dwarf reptiles are not reptiles. Historical note: at one time the biologists argued whether whales should be classified as mammals or fish. They eventually realized that being a mammal is more fundamental than doing like a fish.

That’s not to say that both being and doing classifications don’t still exist in biology, because they do. Some mammals and some reptiles are classified as carnivores while others are classified as herbivores. Biologists just don’t intermix the doing and being categories into the same classification.

## Sketchy neighborhood and this hobby

• This topic is locked

### #26 wxcloud

Yeah been wanting to try and move for some time now, find quieter area that's more conducive to this and other hobbies. Ran things through the ol noodle a few times but there is a lot of stuff that needs to happen, like housing costs to drop.

Then again here in the state seems few nights are decent for the hobby as it is. So, kind of ties into the other thread about throwing in the towel.

Can't jump ship just yet, but also don't want to completely toss the towel in either. Until I can possibly uproot, perhaps going ahead and trying to automate what I got and try to secure the area a bit better. Maybe do visual away from the area.

### #27 rhetfield

It's the bullets that I'm most concerned about. Wasn't the best neighborhood when we moved in but now it's getting a bit scary to be outside any more

If random bullets are the concern, it is time to move. Too many people die in their homes because the walls won't stop them. Wife grew up having to play under the kitchen table because it was in the center of the house and thought to be the safest place. Nobody should have to live that way.

### #28 mg07

My neighborhood isn't terribly sketchy but it's urban. There's plenty of property crime and I've had bikes stolen and such. One thing about astronomy equipment (well, depending on what you have) is that it is super bulky. That deters at least one type of criminal. That said I never leave my gear outside, I assume the people of ill intent can and will see what I'm doing in my yard (though I don't think my 8 inch dob or my Starblast puts much of a target on my back). In some ways being in my yard at night is actually a great deterrent--people interested in breaking in and theft are not keen on there being others present.

That said I assume when I leave anything in my garage, that I might have to simply accept that it could disappear at some point. That is in part why my bicycle is a cheap one. If I buy a more expensive scope I will likely leave the money end (mirror box) at least inside.

Edited by mg07, 04 May 2021 - 04:08 PM.

### #29 DSOGabe

I'm a member of a model railroad club. Clubhouse is in a rough area of town. A member has a family member who is a cop, so he invited the guy over a couple of times. Cop thought it was cool, so he started to pass by regularly when he was patrolling.

Time to make friends with a couple of cops who patrol that area!

### #30 t_image

The other strategy haven't seen yet posted,

is to (shocker) actually learn who your neighbors are.

Anything you can help with (cut grass/shovel snow alongside,rake leaves/help jump start car or fix a flat tire, hire them to cut grass,etc) or even the offer can help 'open the door of friendliness.'

Partly it's about practicing human social skills that'll always be useful, so don't look at it as just means to an end.

It might be me but I've found it's possible to befriend even the rough types. Sometimes they'll be the best protectors---'hey, don't bother them, they're cool!'

Maybe even find the grandma that everyone listens to, obeys.

I get the fear of 'putting yourself on their map' and everyone loves privacy,

but sounds like your setting needs the neighbors to be involved watching out for each other.

maybe you can be a force of change for good!

### #31 JOEinCO

. Then again here in [Colorado] seems few nights are decent for the hobby as it is. So, kind of ties into the other thread about throwing in the towel.

I don't quite understand this comment. I have probably averaged 200+ nights a year for the last 30+ years in Colorado. And I'm only about 12 miles south of you.

Even this morning is clear and it was raining at midnite (four hours ago). The scope is acclimating, as I type, for some Saturn time.

### #32 epee

Get out of there ASAP. Never mind astronomy, if stray bullets are an issue.

If housing prices are high, now is a good time to sell. Maybe look into a mobile home on some semi-rural land.

### #34 csa/montana

Den Mama & Gold Star Award Winner

Others may still drop by and offer more solutions.

### #35 doc_cj

Broncos just need to win more. that's the root cause of this issue

Wouldn't mind adding the Rockies to that list. At least we still have the AVS!

Back to the topic, I'm still new to Colorado but bought up near Fort Collins. So far, we are avoiding some of the crazy that you're getting in Denver. I fear, my friend, that things are going to get worse, so it might be time to look for better digs. Of course, that's easy to say if you've got the money to move up, and I'm guessing you might be there yet.

Maybe an option is to find folks in outlying areas who would host your equipment and viewing times on their property. Right now I'm using some land east of me near the Pawnee Grasslands that is much better spot for viewing or AP. Actually been thinking of renting a tiny house in the area to store my equipment and such. I figure get 6 - 10 friends together, split the rent and basic utilities for $11o to$185 each per month and you've got a place to get away from it all. Might be cheaper than moving your whole household to a more expensive house.

### #36 Cpk133

Get yourself a tactical vest and a couple good plates. Wear it cutting the grass, observing or whatever. You can use the ammo pouches to hold eyepieces. Outside of moving, it's your only option. I was also thinking you could stack some sandbags and make a little bunker in the backyard. The sandbags will block the wind, stray light, and bullets. Sand bags are cheap. Make a plan and commit to moving as others recommended.

Edited by Cpk133, 09 May 2021 - 12:57 PM.

### #37 KI5CAW

I once lived in a bad area of Albuquerque, one of the most dangerous cities in the USA. All my neighbors had six foot concrete block walls around the property most were old Hispanic families who inherited their houses and were not about to surrender to the gangs. I observed from the back yard with only one incident. A car full of malcontents pulled into the alley behind us, and four inebriated teenagers saw me observing, and hopped the six foot fence to see what I was doing. I was scared out of my wits, but when they saw Saturn at 150X in my Newtonian, they thought it was completely cool. they hopped back over the fence and went on their way.

Now I live way out of town, and the worst crime I have to worry about is wayward SUVs careening about while their drivers are texting.

### #38 Stacyjo1962

Having lived in Oakland, CA for a number of years, I found that despite the random gun fire, knowing your neighbors was key. on our little cul de sac like street, we all kept a keen eye out for each other. and anyone who caused any problems, well, one of our neighbors (former military) made sure that the hooligans had second thoughts of ever venturing into our neck of the woods again. (For real, two cars carreening at top speeds turn into our street, obviously not knowing there was no way out except the way they came in. neighbor was ready for them with a very swift bat to the windshield of the car, a Glock and a camera. 2nd car crashed into a light pole and the youngsters got out running. quite entertaining. and the car that mind melded with the light pole? Well, it took OPD 3 weeks to come, the car, get prints and have it towed away. (Military neighbor was born in Niger, immigrated as a 4 year old, joined Marines, became Special Forces, had a wall around his house and raised the US Colours every morning. never felt any type of apprehension when he was around).

### #39 Angeles

not sure if the poster is still here or not as he said to cancel the thread but ill add some info here if its not too late.

Iam kinda there , i just moved this this new area about a yr ago.

Although his area (posters) seems more rougher then my new area by the sounds of it Iam not in a great area myself, its not really bad but not good either so/so i would say

You can kind of tell by how much you pay for a property. I paid 300k for my town house which is cheapish, although iam very close to the edge of the city so iam about 1 hr and 10 min from the downtown area by transit. So being this far on the edge prices are alot cheaper BUT still again i know area not great as 4 min drive other town homes go for 100k to 200k and more. so those area are abit better areas.

I have lots gear i dont list all my gear on my sig but i have about 11 telescopes, 1 costs as much as a brand new car, 2 others costs as much as a good quality used cars etc etc.

Anyway 1st thing dont show-off your items where people can see you have expensive things. PS i also have an expensive car, probably the best one in the complex, its an underground parking & i keep a car cover on it every time its not in use so no-one sees what it is. I would not have bought this place if my car had to be outside even in a private parking spot as then anyone from the street could see the car, at least underground its only the people who in here can see it.

Same with my scope they are inside 100% of the time and only go out when i observe only, they come back inside when im done. My curtains are thick and always closed.

The poster did mention about costs of housing coming down etc so i don't think he own the property but rents and city pays for some of the rent etc. So him moving may or may not work or as some suggested buy a place else where. Some of those waiting list where i live can be 5 to 10 year wait list so you cant just go anytime you want.

Some housing may or may not have your own private backyard, sometimes you get a shared mutual courtyard, maybe if he comes back he can explain abit more if he has a backyard or mutual shared court yard. If its a court yard thing then thats even harder as then you have no choice but to view with your gear out for everyone to see. Altho then as another person wrote up above he then needs to get to know all the neighbor's and make friends.

Another suggestion forget imaging just do visual & my thinking is imaging takes alot more to set-up sometimes alot more gear (guilders , laptops power sources) & can take hours to do. If you have no choice to view from a public court yard do your viewing then go inside (even 1 hr is enough).

Something i did i have a smallish backyard only12x wide x 20 ft long. my fence is a common thing meaning corp own it and repairs it etc. It semi privacy meaning if you look diagonally you can see through it. Iam sure most you understand. What i did last year i bought more fence boards and nails them on my inside then making it soild fence so no-one can see though it anymore.

2nd thing i did is make my fence just a tad taller. It was 5ft 6 inch tall i bough 2x6 PT boards and screwed it on top of the fence making it 6ft taller. so only tall people can see over it and even if they do they may only see a small view of my head etc instead of the original fence where anyone 5.6" could see over. This also worked as i do see a few top of peoples top hair sometimes but for the vast majority i dont see anyone & they cant see in although they could still hear me or i can hear them. I still fear the corp one day may say take off the top 2x6 plank wood but if that happens i just unscrew it etc

My last suggestions is if you have those shared courtyard then it has to have lights and probably alot since they have to have safety all over their property & it cant be too dark. In this case you dont need a large or expensive scope it may only be for the moon and planets and some double stars. a 80mm to 4" refractor is all u need for this kind of of viewing that fast east to carry back and forth.

I hope some these are good suggestions for u

Edited by Angeles, 13 May 2021 - 07:31 AM.

### #40 edwincjones

do not observe along-get a group of observing buddies.

Join a local club and go to their star parties.

Edited by edwincjones, 13 May 2021 - 07:54 AM.

### #41 alphatripleplus

I certainly hope the OP can stay safe until he has a chance to move. If he occasionally has the chance to travel somewhere safe, e.g. visiting friends overnight or vacation, it might make sense to have a portable astro set-up to at least do some astronomy under safe conditions.

### #42 wxcloud

Edited by wxcloud, 13 May 2021 - 12:23 PM.

### #43 grif 678

We live in dangerous times these days. The neighbor hood I live in has changed so much in the last 25 years. I knew all my neighbors when we first got here, all were very nice. Did not even think about something happening when I observed back then. But in the last 25 years, most have moved out, new ones have moved in. No one cares about property any more, houses are getting run down. All kinds of unknown cars driving in sub division now. 800 new houses being built right across the road from me now. But on a fixed income, can not afford to move again. There is really no where to go, that in a few years, the same thing will be happening there also.

I never thought years ago that I would need to carry a couple small weapons outside to look at the sky, but now a days it is necessary. I do not leave anything outside, but you do not have to leave anything outside, some people will break in and get what they want, even is you are home.

I have said this before, getting old is depressing at times, but I am so glad that I came up when I did. This could be another whole subject, so I will stop here.

### #44 gwd

"Now I live way out of town, and the worst crime I have to worry about is wayward SUVs careening about while their drivers are texting."

The serious crime map is one reason we choose to live in the East mountains rather than in town when we moved to the ABQ area from DC. A real estate agent tried to push us into a house in town in a neighborhood where most of the houses looked like jails with bars on the doors and all the windows. When I remarked that it tells me the neighborhood has a lot of crime the agent disagreed. She said it's a southwest style element that "adds value to the property."

In the east mountains, the neighbors said to watch for bears when the apples ripen. No city jail "style elements" in the neighborhood but some neighbors had observatory domes.

### #46 ian408

The other strategy haven't seen yet posted,

is to (shocker) actually learn who your neighbors are.

Anything you can help with (cut grass/shovel snow alongside,rake leaves/help jump start car or fix a flat tire, hire them to cut grass,etc) or even the offer can help 'open the door of friendliness.'

Partly it's about practicing human social skills that'll always be useful, so don't look at it as just means to an end.

It might be me but I've found it's possible to befriend even the rough types. Sometimes they'll be the best protectors---'hey, don't bother them, they're cool!'

Maybe even find the grandma that everyone listens to, obeys.

I get the fear of 'putting yourself on their map' and everyone loves privacy,

but sounds like your setting needs the neighbors to be involved watching out for each other.

maybe you can be a force of change for good!

When I'm outside blowing the leaves, I often do the neighbors on both sides. I rarely see either neighbor though I do often see others. It definitely pays to be at least friendly to your neighbors, you don't have to chat them up, just wave and say hello.

### #47 BigC

Another suggestion is limit the amount and value of astro gear you take out.NEVER brag or complain about how expensive your astro gear was better ,if true, mention how little it cost you.Bragging about expensive things gets the wrong kind of attention too often.

If visual observer there are plenty of inexpensive sets in the grab-n-go category that can give good views.

If contemplating a multi-kilobuck setup you might want to reconsider your priorities.

Have a vest with plenty of pockets for eyepieces and filters.

Ideally the gear should require only one trip in or out.

Install a screw anchor in the ground at your observing spot .Use a bicycle cable/ lock or similar to secure the scope . This allows you to abandon the scope ,if necessary , at the first sign of potential trouble, whilst making it much less likely to be taken.

### #48 Cpk133

The other strategy haven't seen yet posted,

is to (shocker) actually learn who your neighbors are.

Anything you can help with (cut grass/shovel snow alongside,rake leaves/help jump start car or fix a flat tire, hire them to cut grass,etc) or even the offer can help 'open the door of friendliness.'

Partly it's about practicing human social skills that'll always be useful, so don't look at it as just means to an end.

It might be me but I've found it's possible to befriend even the rough types. Sometimes they'll be the best protectors---'hey, don't bother them, they're cool!'

Maybe even find the grandma that everyone listens to, obeys.

I get the fear of 'putting yourself on their map' and everyone loves privacy,

but sounds like your setting needs the neighbors to be involved watching out for each other.

maybe you can be a force of change for good!

When I'm outside blowing the leaves, I often do the neighbors on both sides. I rarely see either neighbor though I do often see others. It definitely pays to be at least friendly to your neighbors, you don't have to chat them up, just wave and say hello.

I love when people talk about being friendly as if it's a panacea. Sometimes being friendly is just an invitation for more trouble. Go blow leaves for the drug addict whos buddies show up full of kneck tats dropping and picking up packages and exchanging money. Heck, you might have to cut the grass before you blow the leaves. One time i lived with some guys in off campus housing at an urban university. One of my roomates thought hed be pals with some slime balls across the street. Next thing you know theres a knock on the door "you guys have any catsup?" haha, no big deal, then it litterally turned into "you have any meat?" no joke. Shortly thereafter, we were broken into and litterally all of our stuff stolen. A couple weeks later, i see one of the scumbags wearing one of my shirts. I even had to go buy my textbooks back from the local bookstore, my physics book with my notes written in the margins. I transferred the next semester. Dont gaslight people that have to deal with scumbags. Scumbags view kindness as weakness. That doesnt mean walk around with a scowl or a chip on your shoulder, but dont think for a second that youre going to win everyone over with warmth and smiles. I think the ops last post is right on the money. I used to supervise some rough characters. One time this guy told me, "you look like Oppie Taylor, you need to chew some garlic, smoke some cigarettes, and come in here with some dirty hair so people will be scared of you" I'll never forget that, loved that guy, funny and serious at the same time, he was a good worker. Its called street smarts, its no joke. Another guy in that same crew, always giving me trouble, I drove him out of my crew. A few years later he was found full of holes trying to buy prescription drugs off some kids on facebook at 3am.

## Planetary history

The term "planet" originally comes from the Greek word for "wanderer." Many ancient cultures observed these "moving stars," but it wasn't until the advent of the telescope in the 1600s that astronomers were able to look at them in more detail. Small telescopes revealed moons circling Jupiter — a big surprise to Galileo Galilei (the likely discoverer) and his opponents at the Catholic Church — as well as rings around Saturn and an ice cap on Mars.

Telescopes also revealed the existence of objects not known to the ancients, because they are too far away and small to be spotted with the naked eye. Uranus was found on March 13, 1781, by the prolific astronomer William Herschel. Ceres was discovered between Mars and Jupiter in 1801. It was originally classified as a planet, but it was later realized that Ceres was the first of a class of objects eventually called asteroids. Neptune was discovered in 1846. [Related: Solar System Planets: Order of the 8 (or 9) Planets]

Astronomers continued scouring the solar system's outer reaches in search of a large "Planet X" that was believed to be disturbing the orbits of Uranus and Neptune. While these irregularities were later discounted by further observations, Clyde Tombaugh did spot a smaller object in 1930 beyond the orbit of Neptune. Called Pluto, the object (then called a planet) was relatively small and had a highly eccentric orbit that sometimes even brought it closer to the sun than Neptune is.

## Fighting for Pluto's Planet Title: Q & A With Planetary Scientist Alan Stern

Alan Stern has beenfighting for Pluto's planethood ever since the icy body was demotedto "dwarf planet" in 2006.

That year, theInternational Astronomical Union (IAU) came up with a new definitionof "planet": A body that circles the sun without beinganother object's moon, is large enough to be rounded by its owngravity (but not so big that it undergoes nuclear fusion, like astar) and has "cleared its neighborhood" of most otherorbiting bodies.

Since Pluto sharesorbital space with many other objects in the Kuiper Belt &mdash the ringof icy bodies beyond Neptune &mdash it was relegatedto the newly created categoryof dwarf planet.

Stern, a planetaryscientist at the Southwest Research Institute in Boulder, Colo., andleader of NASA's New Horizons mission to Pluto, strongly disagreeswith Pluto's demotion.

In an e-mailinterview with SPACE.com, Stern lays out the casefor Pluto's planethood and explains whyit matters what we call Pluto and other objects in the solar system.

What are yourmajor concerns with the current IAU definition of "planet?"

One problem I havewith the IAU definition is that it's sloppy when it says asufficiently large object has to "clear its orbital zone"to qualify, because none of our solar system's planets orbit in afully cleared zone. All have some form of asteroids, comets or KuiperBelt objects passing through their orbital zone.

So the IAUdefinition leaves its proponents having to argue why they didn'treally mean what they wrote, that instead they really meant thatzone-clearing is the "ability" to clear a zone in aperfect-world computer simulation. [POLL:Should Pluto's planet status be revisited?]

But a larger problemis that so many of its proponents said back in 2006 that the IAUdefinition was specifically designed to limit the number of planetsto a small number. That, in my opinion, is not scientific. It'ssomewhere between personal preference and political.

And mostimportantly, I and many other planetary scientists &mdash like thealmost 400 that signed a petition against the IAU in 2006 &mdash have aproblem with the IAU definition because the implications of it arejust nonsensical.

Here's why. TheIAU's "zone-clearing" criteria, when worked outmathematically, means that to qualify as a planet at larger andlarger distances from the sun, a body has to have more and more massthan it would in a closer orbit. This is in part because the zonesget larger (like distance cubed, or volume) as you go outward it'salso in part because orbital speeds are slower further out, sozone-clearing takes longer.

The end result isthat many objects that can clear Mercury's zone can't clear thezone at Earth's orbit (including Mercury, as I recall), and anobject that can clear the zone at Earth might or might not be able toclear the zone at Pluto. In fact, Earth &mdash the one object I thinkeveryone agrees is a planet &mdash is too small to clear Pluto's zonein the age of the solar system, and would not be a planet by theIAU's way of thinking.

So when people sayPluto can't clear its orbital zone, they should be fair and alsopoint out that planet Earth also couldn't clear a zone this farout, so the IAU definition would exclude an Earth &mdash and a Mercury,a Venus or a Mars &mdash at Pluto's distance.

Thisdistance-dependent planethood criterion produces situations whereidentical objects don't classify identically at differentlocations, and results in a ridiculous and chaotic classificationscheme that isn't good for anyone &mdash not for school kids, not forthe public or even for researchers.

You've said thatthe decision to strip Pluto of its planethood was motivated partly byan unscientific desire to keep planets "special." What didyou mean by that?

Last week, youinterviewedMike Brown about planet definitions.Mike once said to me during an NPR interview we did together in 2006,"We can't have 50 planets. My little daughter won't be ableto remember all their names."

Well, I have twodaughters of my own, and really appreciate a father's love, butthis is hardly a scientific rationale. Did we limit the number ofstars for memorization convenience when Galileo turned his telescopeto the sky and found there weren't any more a countable number ofthem? Did we limit the number of galaxies when a similar discoverywas made about them? Do Earth scientists limit the numbers ofmountains or rivers to be named? Do biologists limit the number ofspecies?

In all these casesthe answer is no, because science is about accepting new facts,independent of their convenience or inconvenience. And that should bethe case with planets, too.

So I responded toMike on NPR that evening by saying, "Well, if we can't have 50planets but only eight, so kids can remember their names, then Iguess we ought to be going back to eight states, too."

But to answer yourquestion more fully, many proponents of the IAU's planet definitionwant to limit the number of planets and say so all the time. When Ihear that, I think, why would anyone care to keep the number ofplanets small, except to protect their status as special?

But you know, datais data. In planetary science, we've learned over the past 15 or soyears we were wrong &mdash our solar system does not have nine or 10planets as we long thought it had, but more like 900. Moreover, we'velearned that most of the planets in our solar system are verydifferent from the first handful we knew about because they are muchsmaller, though they share virtually all other attributes in common.

It's as if we'dgrown up on a desert island with only NBA-player-sized people andthen, on learning that the other people of the world are mostlysmaller, saying those people aren't really what we want to callhuman, because that would make us on the home island feel differentand less special.

So when it comes tosmall planets outnumbering big ones (just as small stars outnumberbig ones), I say, "Cool, that was unexpected, but bring it on!"After all, reacting to new data and changing one's paradigm tobetter fit the totality of the facts is what science is all about.

What definitionof "planet" do you prefer, and why?

I like to tellaudiences that we'd be better off with the "Star Trek"test for planethood than what the IAU adopted.

Whenever a starshipon "Star Trek" pulls up tosomething in space and turns on the viewfinder, the audience and theship's crew know, within about a second, whether it's a planet,or a star, or another spaceship, or a comet, or a nebula or anasteroid. Whatever. They don't need to know what else is nearby,they don't need to conduct a survey of the solar system andintegrate orbits to determine what objects have cleared their zone,they don't even need a Ph.D astronomer to advise them.

They just know bylooking. They know, because it's not all that hard to tell a planetwhen you see one, and they know that it doesn't matter if it'salone or in a flock: if it's big and roundish, "not on firewith fusion," and not a spaceship, it's a planet.

But more rigorously,since a 1991 article where I coined the term "dwarf planet"in analogy to dwarf stars and dwarf galaxies, I've promoted what wenow call the Geophysical Planet Definition, or GPD: A planet is anyobject in space that is massive enough for its self-gravity to createa state of hydrostatic equilibrium, but not so massive that its corecan at some point ignite in sustained nuclear fusion.

This simpledefinition nicely traps planets as objects that are too big to actlike rocks (which control their shape by chemical bonds rather thangravity), and [too small to act like] stars (which are big enough tobe in hydrostatic equilibrium but which have done or currently donuclear fusion in their interiors).

And in GPD, nothingelse matters &mdash doesn't matter where we find it, in what kind oforbit or what it's close to or far from. In fact, that's how weclassify most other object types in astronomy &mdash based on what itis, not where it is or what orbit it's in.

What do you sayto those who argue that Pluto is just a big Kuiper Belt object?

I say they'reright! But that doesn't really relate to whether Pluto is a planetor not. Just because Pluto orbits with many other dwarf planetsdoesn't change what it is, just as whether an object is a mountainor not doesn't depend on whether it's in a group or in isolation.

What we see in theKuiper Belt is a third class of planets, the dwarf planets, or DPs.Most, like Pluto and Eris, have primarily rocky compositions (likeEarth), moons, and polar caps, atmospheres, seasons and otherattributes like the larger planets. They're just somewhat smaller.

Back before theKuiper Belt was discovered, Pluto did look like a misfit that didn'tbelong with either the terrestrials or the giant planets. Turns outthat was exactly right, but now we know why: Pluto looked like amisfit because our technology back then couldn't see that it wasjust the brightest and easiest to detect of a large new class ofplanets.

In fact, that'swhy it's clear Ceres [the largestobject in the asteroid belt] wasa planet all along, but was misclassified for a time because wedidn't have enough similar examples to recognize dwarf planets astheir own category. Today, however, it's clear the DPs outnumberboth of the other two planet classes we know of in our solar system &mdashthe giants and the terrestrials. Which types look to be the misfitnow, versus the norm?

Some people seem tobe uncomfortable with that fact. I see it as just another step in theCopernican revolution that began by displacing the Earth from thecenter of the universe.

Do you thinkastronomers will ever come to a consensus?

I do. I expect thatbefore long, consensus will be that our solar system was good atmaking planets in very large numbers, and that most of them arefaraway dwarfs, rather than the closer big guys that we knew about inchildhood.

Then schools willteach that, like the rivers of Earth, there is a huge number ofplanetsin our solarsystem, and you only need to remember the namesof the ones that are nearby or particularly famous.

Does it matterwhether we call dwarf planets full-fledged planets or not? Is thisjust a semantic issue, or does it hit on something more important?

I think it matters.One thing scientists do is to find order among a large number offacts, and one way to do that across fields as diverse as biology,geology, physics and astronomy, is through classification.

Yes, in planetaryscience we have found our old classification scheme wanting becausewe've recently discovered a large number of small planets far outin the solar system, as well as hot Jupiters and super-Earths aroundother stars, and planets with strange new orbits, and planetsorbiting pulsars.

I don't think ourjob as planetary scientists and astronomers is to react to this newdata that planets are much more diverse in size, orbit, and locationthan we knew by saying there's no longer any reason to classifythem, and that the word planet has lost all meaning.

I think our job isfind a better classification scheme that makes sense in light of thenew data. As I said about the "Star Trek" test, it reallyisn't so hard to do.

And after all, thefield is called "planetary science." To say that what aplanet is doesn't matter would be to imply that a planetary scientistcouldn't explain to someone what the field is about. How would thatbe a good thing?

## Planet Pluto

So the IAU says Pluto is no longer a planet. A true planet "has 'cleared the neighborhood' around its orbit". But Pluto crosses Neptunes orbit. If that's the case, Neptune has not cleared the neighborhood around its orbit either. So by definition, Neptune is not a planet!

### #4 BrooksObs

So the IAU says Pluto is no longer a planet. A true planet "has 'cleared the neighborhood' around its orbit". But Pluto crosses Neptunes orbit. If that's the case, Neptune has not cleared the neighborhood around its orbit either. So by definition, Neptune is not a planet!

A highly naive take on the true meaning of the phrase, I'm afraid. "Clearing Its Orbit" means that the larger body in question has removed all the material traveling within the region of the path that it follows. This is not meant to include orbit-crossing objects. Following your concept of the meaning would make none of the larger solar system bodies qualify as planets, because none of them have removed all the orbit-crossing objects in their sectors either.

Pluto along with the other Kuiper Belt Objects are nothing more than accumulated bits of debris left over from the formation of the major planets in the solar system. They do not and should not qualify as planets. When any of these objects have their orbit shifted so as to come into the inner solar system they present themselves as nothing more than simply comets. Jupiter, Saturn and Uranus family of comets all appears to be escaped KBO. In fact, asteroids, KBO, comets, and Oort Cloud Objects are one and the same, simply the solar system's leftover rubble. Only their location within the solar system differentiates them for us and as such Pluto should ever remain just a minor object.

Edited by BrooksObs, 19 September 2017 - 11:27 PM.

So the IAU says Pluto is no longer a planet. A true planet "has 'cleared the neighborhood' around its orbit". But Pluto crosses Neptunes orbit. If that's the case, Neptune has not cleared the neighborhood around its orbit either. So by definition, Neptune is not a planet!

A highly naive take on the true meaning of the phrase, I'm afraid. "Clearing Its Orbit" means that the larger body in question has removed all the material traveling within the region of the path that it follows. This is not meant to include orbit-crossing objects. Following your concept of the meaning would make none of the larger solar system bodies qualify as planets, because none of them have removed all the orbit-crossing objects in their sectors either.

Pluto along with the other Kuiper Belt Objects are nothing more than accumulated bits of debris left over from the formation of the major planets in the solar system. They do not and should not qualify as planets. When any of these objects have their orbit shifted so as to come into the inner solar system they present themselves as nothing more than simply comets. Jupiter, Saturn and Uranus family of comets all appears to be escaped KBO. In fact, asteroids, KBO, comets, and Oort Cloud Objects are one and the same, simply the solar system's leftover rubble. Only their location within the solar system differentiates them for us and as such Pluto should ever remain just a minor object.

Nice opinion spoken as fact.

### #6 MikeTahtib

". Only their location within the solar system differentiates them for us and as such Pluto should ever remain just a minor object."

Which is why Pluto should remain a planet, along with its location in history. The story I read is that the decision to demote Pluto was taken by a small group within the IAU, with little input or debate.

Even more irritating to me is the IAU's redefinition of the word constellation. It was society's word, adn had a clear deinition - similar or identical ot the definition now assigned to "asterism". They really should have chosen a new term for "region of sky".

And while we're at it, how about the name Bootes? It's just an awkward name to pronounce, for such an attractive asterism and constellation.

### #7 NiteGuy

Perfect argument. I say we have a march to reinstate Pluto as a planet. Finally, something worth marching for.

Sadly, ever since the hippies faded into the shadows, no one seems to think anything on Earth is much worth marching for.

### #8 SeaBee1

Me, I'm kinda like that kid when the teacher told him to sit down. I sat down, but in my mind I was defiantly standing up, and watching her apoplectic face get all scrunched up and red.

SS5 Plus describes Pluto as a "Dwarf Planet". but as I recall, in human terms, "Dwarfs are people too". in my mind, and in practice, Pluto will ALWAYS be the 9th.

Another opinion nicely stated as fact.

### #9 rowdy388

Pluto is now king of the Kuiper Belt objects and Mercury is now the runt of the planets.

Edited by rowdy388, 20 September 2017 - 08:59 AM.

### #10 *skyguy*

If a dwarf star is still a star . then a dwarf planet must still be a planet and Pluto is still a planet. The IAU needs to get its act together if it wants to regain any credibility with the general public.

### #11 grif 678

I do not understand why the IAU would waste time and money degrading a planet that has always been considered a part of out solar system. Whether it is a planet or not will not affect anything in our lives, it will not change anything on this earth, so why did not they just leave it alone. Percevell and Clyde spent their lives finding this planet, so some Johnny come lately, who has nothing better to do, decides all that hard work is useless, and just wants to start a controversy, so he can be somebody.

### #12 epee

I do not understand why the IAU would waste time and money degrading a planet that has always been considered a part of out solar system. Whether it is a planet or not will not affect anything in our lives, it will not change anything on this earth, so why did not they just leave it alone. Percevell and Clyde spent their lives finding this planet, so some Johnny come lately, who has nothing better to do, decides all that hard work is useless, and just wants to start a controversy, so he can be somebody.

Because science is about taxonomy. When Pluto was discovered, as when Ceres was discovered, it was singular to science. However, improving instruments and technique uncovered more and more similar objects KBOs in Pluto's case and Asteroids in Ceres'. This left science with the prospect of opening the door for millions, if not trillions, of planets or else deciding what makes Earth, Saturn, & Jupiter "special" among "stuff orbiting The Sun".

It's not personal, it's not about denigrating someone's work, it's not about getting noticed, and it's certainly not going to change a darn thing about the object we call Pluto it is about classification. Speak to a biologist about the importance of classification.

Edited by epee, 20 September 2017 - 09:37 AM.

### #13 BillP

So the IAU says Pluto is no longer a planet.

So, the IAU says a lot of things Pluto is a planet of our solar system. Period!

### #14 BillP

A true planet "has 'cleared the neighborhood' around its orbit".

Hmmmm. I guess this is a problem for Earth as well since we have that pesky little bugger 2016 HO3.

### #16 RussL

Perfect argument. I say we have a march to reinstate Pluto as a planet. Finally, something worth marching for.

### #17 Jim Nelson

Pluto wasn't "demoted" or "degraded". These aren't prizes or levels of command or ranks of royalty that are being given out.These are attempts at making scientific sense of the variety and complexity among the vast number of objects that constitute our solar system.

Pluto has always seemed like a weird "planet", but as more evidence comes in it bears a stronger family resemblance to the class of objects we've come to call "Kuiper Belt Objects" than the class of objects that have been traditionally called "Planets" (and I can't imagine that's a controversial summary of the current state of knowledge).

There may well be good arguments for formalizing a class of objects we call "planets" that does include Pluto, but the bizarre emotionality that this topic induces has always been mystifying to me.

Edited by Jim Nelson, 20 September 2017 - 11:24 AM.

### #19 rowdy388

An awful lot of the science I learned in school is now known to be wrong. I don't worry

about it. The nature of science is that what we think we know changes as our knowledge

increases. Move on or get left behind.

### #20 ninelives

There may well be good arguments for formalizing a class of objects we call "planets" that does include Pluto, but the bizarre emotionality that this topic induces has always been mystifying to me.

Same here. I was at university when the classification changed and was surprised that so many people were upset. With new knowledge comes re-classification, right? I didn't (and still don't) understand what was so controversial about that.

### #21 BrooksObs

I do not understand why the IAU would waste time and money degrading a planet that has always been considered a part of out solar system. Whether it is a planet or not will not affect anything in our lives, it will not change anything on this earth, so why did not they just leave it alone. Percevell and Clyde spent their lives finding this planet, so some Johnny come lately, who has nothing better to do, decides all that hard work is useless, and just wants to start a controversy, so he can be somebody.

Time to face reality boys ans girls. Astronomy is an eternally evolving science and over time concepts within it often change radically. Even in my own time amazing changes have occurred, including the very yardstick for measuring the scale of the universe doubling in size overnight! Mars had always been believed to be a world probably harboring at least very primitive life, maybe Venus too. and then we saw the images from the probes and realized that these surely nothing but totally dead worlds. You should have seen the controversies surrounding QSO and blackholes! The list goes on and on.

Turning to Pluto, if one reads the period literature surrounding Pluto's discovery immediately following the event, not the semi-whitewashed versions one mostly sees today, it casts a very different light on the thoughts of its being a true planet right from the start. It was immediately realized that there was something extremely peculiar about the body. Many were uncertain about assigning it planetary status right from the start! Reading the original reports and calculations concerning the object's mass raised all manner of outrageous ideas of its density to allow it to fit it into viable planetary status. In the 1930's it was well known that Pluto could not be larger than our moon, possibly even much smaller and that it was simply a huge asteroidal object, or even a runaway moon of Neptune's because of its orbit. Reading the profession and semi-professional journals of the day cast a very different light on the problems regarding Pluto than what is mostly written today.

Its time to realized that we are just dabblers in the field of astronomy, a very changeable scientific pursuit, and unless you wish to look foolish and naive you need to accept it when alterations come along, not baselessly challenge them. The matter of Pluto being a true planet is closed and my guess is that in time all the small bodies in the solar system will be consolidated into a single group or class, just as I noted in my earlier post. because that is where they all belong.

## Did Mercury clear its neighborhood? - Astronomy

Patterned mainly after the Sun, had 13 months that were linked to Sirius and the Nile.

Patterned after movements of the Sun and moon, alternated between 12 and 13 month years.

Our calendar today has many characteristics of the early calendars. We have mainly patterned ours after the Roman calendar which had 365 days in a year and seven days in a week. Julius Caesar realized that there had to be a little time left at the end of the year (in other words, a year was longer than just 365 days). His calculations revealed that there were actually close to 365.25 days in a year. He is credited for incorporating a leap year every four years to keep the seasons from slipping into months in which they don’t belong. Pope Gregory the 8 th took it even further. He reduced the number of leap years to make the calendar even more accurate. This one should last us for the next 3 thousand years or so.

A topic that fits well with calendars is seasons. So, why does Earth have them? The simple answer is because Earth is tilted on its axis and obits the sun. The northern hemisphere is pointed toward the Sun during part of our orbit and away from it during another part. A couple of terms you should know are Solstice and Equinox. Many people think the seasons are caused by our moving closer to or farther away from the Sun. In actuality, the Earth is closer the Sun during winter than summer by about three million miles.

There are many people and cultures that played huge roles in helping us get to the understanding of the universe. You need to learn about just a few of them. Please do the “Early Observers and Who’s Who of Early Astronomy” assignment. (Click Here for pages from the book.) The overview of the answers to this assignment can be found by clicking HERE.

Early observers located and named familiar stars and linked them together. Many people refer to these star patterns as constellations. We still use many of the same names, but the understanding is a little different. Constellations are more than just stick figures in the sky that link stars together. They are entire sections of the sky that happen to contain recognizable patterns of stars. We have divided the sky into 88 different constellations which make locating stars, galaxies, etc much easier.

We also use other coordinates for describing the locations of stars. Learn the following vocabulary:

Also, understand and know how to describe what circumpolar stars are.

The Solstice, Equinox and the Seasons

When one is asked to describe the solstice, the usual answer includes something about the longest or shortest day of the year. If you consider the day or night to be the amount of time spent in darkness or light, then this works out pretty good. However, we all know that a day is really the amount of time required for the Earth to rotate once on its axis (about 24 hours), and this doesn't change with much of a measurable amount of time. We can really think of the solstice as the two times during the Earth's orbit that its axis is pointing as close to or as far away from the sun as it can get. Therefore, we know that the solstices occur during the summer and the winter. The reason that these two locations provide different amounts of daylight is because of where the shadow line is found on the Earth compared to where the Earth's geographical north and south poles are located (i.e. where Earth's axis is located).

Directly between these two solstice points of Earth's orbit is an orientation of the Earth that we call equinox. It might be easier to remember what the equinox is by remembering the word equal because during these two times of Earth's orbit, the shadow line on the Earth runs directly through the north and south geographical poles of Earth's axis. Therefore, no matter where you are on the Earth, you will always experience about 12 hours of daylight and 12 hours of darkness. Equal daylight and darkness. Equinox.

Galileo Galilei was the first to be given credit for using the telescope to view and collect astronomical data with an optical telescope. He did not invent the telescope, but he certainly made some major improvements to it. He increased the magnification power by at least 10x that of other telescopes of his time. He did this by teaching himself to grind lenses and other necessary skills with the lenses that he used. They have come a long way since he made his first observations of the moon.

The Electromagnetic Magnetic Spectrum

Energy can be transferred in many different forms. One really important form of energy that we will be discussing in multiple systems of the Earth is the electromagnetic spectrum. This is really just the range of energy types that travel on waves of electricity and magnetism. The range of energies varies from high energy, high frequency short waves such as gamma and x-rays to low energy, low frequency long waves such as radio and microwaves. One of the important things to remember about electromagnetic radiation is that there is a big connection between frequency, wavelength and energy. The shorter the wavelength, the higher the frequency and energy of the wave. If the wavelength is longer, the frequency and energy are lower.

Optical Astronomy--telescopes that collect wavelengths of visible light we see it with our eyes (optical).

Reflecting: telescopes that focus visible light with a mirror. These typically have an advantage over refracting telescopes because it is easier to make a large diameter mirror than lens. This allows the telescope to collect more light. However, they tend to be a little more expensive.

Refracting:telescopes that focus light using a lens. Because of the weight of the glass and the high probability of impurities, it is difficult to fabricate a large diameter lens to collect light. However, there are many refracting telescopes that are plenty large enough to make a ton of awesome discoveries out there in space. They are also a little cheaper on average than a reflecting telescope.

Non-optical Astronomy--These are telescopes that collect wavelengths of the EMS other than visible. Perhaps the most common type is the radio telescope. These look like giant dish antennas. Non-optical telescopes are super important because there are so many objects out there in space that don't give off visible light (or at least give off a ton of other types of radiation).

There are many different types of telescopes all over the world. However, if a telescope is on the surface of Earth, the light (in whatever form, optical or non-optical) must pass through the atmosphere before being collected by the instrument. What a royal bummer! That light has been traveling for many light years. Then, in the last, tiny bit of its journey, it gets all jacked up and distorted by the gases in our atmosphere.

To solve this problem, scientists began putting telescopes in space (like the Hubble Space Telescope). These instruments can collect the EM radiation before being distorted by the Earth's atmosphere.

Nebular Theory of Solar System Formation

Most of us created our first model of the solar system in about the third grade, so we know quite a bit about it as we have been able to gather tons of information from many scientists over the last few hundred years. We are really at a great advantage if you consider all of the awesome nuggets of information and insight that we take for granted about what is out there in space, close by and far away.

So, we all know that the most dominant object in our solar system is the sun. However, I'm shocked each year at how many students have the misconception that stars are smaller than planets and other objects within the solar system. It is almost as if those students don't consider the sun as a star. Anyway, if the sun is the most dominant object in the solar system, what would be a good definition for the solar system? I think that something like a star and all of its satellites is a great way to describe the solar system.

Early views of the solar system consisted of the Ptolemaic theory. The Greeks understood the Earth to be at the center of everything. They recognized five planets--Mercury, Venus, Mars,. Jupiter, and Saturn. In about 1500 a.d., Copernicus proposed that the sun was at the center, and Galileo's telescopic discoveries lead to an acceptance of the Copernican model.

Today, we understand our solar system to contain one star, the sun, eight planets, at least five dwarf planets, over 165 moons, and numerous asteroids, comets and smaller bodies. Of all of these objects, planets have been at the forefront of our interest for a long time. Not too long ago, there was a ninth planet in our solar system, Pluto. However, in 2006 Pluto was reclassified as a dwarf planet because it was clear that Pluto was not quite as dominant as it appeared in 1930 when it was first discovered (Pluto and its moon were indistinguishable from one another, so they looked like one body). In 2006, astronomers set the following criteria for a planet: 1. must orbit a star, 2. its gravity must make it a sphere, 3. it cannot be a star, and 4. it must gravitationally clear its orbit of smaller objects (clearing its neighborhood)--it needs to be the most gravitationally dominant object in its orbit. This last point did not apply to Pluto, so it was downgraded to a dwarf.

If we really think about it, we realize that space is really much more than emptiness. There are lots of things out there in space. The kicker is that most of the stuff is not spread out evenly. Most of it is clumped together in large globs like stars. There really isn't very much stuff between these large gobs of stuff. On average, there is about 1 particle of matter in space for every cubic centimeter. That really isn't very much, way less dense than the air in our atmosphere.

One big bunch of stuff that we can find in space is called a nebula. These are large clouds of gas and dust. Most of the gas stuff is made of hydrogen and helium. Most of the dust stuff is made of carbon and iron. These clouds are huge, many times spanning multiple light years across. Most of these clouds are dark. There is very little movement of the particles within nebulae, so the temperatures are usually very, very cold. As far as we can guess, our solar system began like we believe most other star systems began, as a nebula cloud that began to condense in on itself to form our star, planets and other smaller bodies that we see in our solar system today.

Because the particles within a nebula are made of matter, they also have gravity. Pressure is generated between particles in a nebula as they bump into each other. These two forces counter balance each other in a nebula to yield a net movement that is very slight. The particles within the nebula are mostly at an equilibrium--not moving toward or away from each other. In order for the cloud to begin to condense, this balance between forces must be disrupted.