Astronomy

How rare are earth-like solar eclipses?

How rare are earth-like solar eclipses?


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Solar eclipses on the earth are so spectacular because the moon has roughly the same size as the sun when viewed from the earth's surface. This is an incredible coincidence, and my guess is that it is a very uncommon phenomenon in the universe.

Is it possible to say anything sensible about the likelihood of a planet having a moon and a sun with same apparent size? Is it possible to make an educated guess about how many places in the universe you would be able to see an earth-like solar eclipse?


We can certainly speculate.

There are dozens of moons in our own Solar System. I've done some preliminary calculations of their apparent size as seen from the planet vs. the apparent size of the Sun at that distance. The large moons (Jupiter's 4 Galilean satellites, Saturn's Titan, Neptune's Triton) are all substantially larger in the sky than the Sun is. I think there are some satellites that are fairly close to the apparent size of the Sun, but I haven't done all the calculations.

The apparent size of our Moon has changed over time, as the Moon has gradually moved farther away. We are coincidentally in a period of history in which it happens to be very nearly the same apparent size as the Sun.

The criterion for an exoplanet to have solar eclipses like the spectacular ones we have here on Earth is a moon that happens to have an angular size large enough to cover the photosphere, but not so large that it also hides the corona. For a larger moon, you'd have eclipses in which the corona is visible, but not all the way around the moon. For a smaller moon, you'd only have annular eclipses. (It could also get interesting if you consider eclipses as seen from other moons.)

So given the distribution of sizes and distances of moons in our Solar System, we can guess that situations where the apparent size of a moon and sun closely match is fairly rare, but since there are moons whose apparent size is smaller than the Sun and others whose apparent size is larger than the Sun, it probably happens sometimes.

The one piece of the puzzle that we're still missing, I think, is the typical sizes and distances of satellites of Earth-like planets. Moons the size of ours orbiting habitable planets might be common or very rare. If they're rare (say, if Mars-like moon systems are far more common), then Earth/Moon style eclipses might be very rare for habitable planets.


Is Earth extremely rare to have a perfect Solar Eclipse and Lunar Eclipse?

What are the chances that the size of a planet is just perfect enough to eclipse it's moon just right, and the size of the moon (relative to that planet) is just right to eclipse it's sun resulting in perfect eclipses? I am not advocating for creationism or "intelligent design", but am actually genuinely curious. Thank you!

Lunar eclipses aren't anything special. The Earth is much bigger than the minimum size required for lunar eclipses and the Moon is much closer that the maximum distance required.

Solar eclipses are a different matter. The apparent size of the Moon and the Sun in the sky are very similar. During a solar eclipse, the Moon is just big enough to fully block the Sun. Because the Moon is only just big enough, solar eclipses don't last very long (totality usually lasts several minutes at most) and total eclipses are only visible in a narrow strip of land.

However, the distance between the Earth and the Moon is increasing (at the breakneck speed of several centimeters per year) and thus the apparent size of the Moon is decreasing. In several hundred million years, the Moon will be too far away to fully obscure the Sun and instead of total eclipses we'll only be able to see annular eclipses, where the outer ring of the Sun is still visible.

Since the orbit of the Moon is not a perfect circle (and the distance between Earth and Moon varies during the orbit), annular eclipses are already possible nowadays, but as time passes annular solar eclipses will become more frequent while total solar eclipses become less frequent.

It's hard to determine how long ago the first "perfect" solar eclipse was. But there's enough overlap between the sun/moon min and max sizes, and the rate at which the moon's apparent size is shrinking is slow enough that it's entirely possible that every critter that possessed eyes lived in an era when perfect solar eclipses were a thing. And they will continue for at least half a billion years more.

Lunar eclipses? Meh. All the moons of Jupiter are eclipsed for a while on every orbit, for example. Lunar eclipses are really dependant on the size of the planet, the size of the moon, and the orbital plane of the moon in relation to the planet/sun orbital plane -- if the planes are similar then a lunar eclipse will happen more often. The more dissimilar the less often they will occur. Our Moon orbits at few degrees off the Earth/Sun plane, so that's why we don't get a lunar eclipse every times there is a full moon. (It's also why we don't get a solar eclipse every time there is a new moon).

The rare one is the solar eclipse. The Sun is 400 time bigger than the Moon, and the Sun is also 400 times further away than the Moon. So both appear to be the same size in the sky, allowing for the perfect total solar eclipses we see. By "perfect" I mean that the Moon covers the full disc of the sun and only that, allowing the corona to still be seen. If the Moon were much closer to us (or much bigger, or the sun much smaller) then it would cover the sun and the surrounding area of the sun. And if the Moon was further away (or smaller, or the sun bigger) then there would still be a ring of sun visible around the Moon (an annular eclipse, which we do get at time when the Moon is at apogee, or the furthest part of it's orbit from the Earth*).

So, yeah, it's a cosmic coincidence that this occurs so perfectly. But it's really a timing thing too -- the Moon's orbit is increasing, so the Moon is gradually moving away from us. In a few million years total eclipses will never occur again, only annular eclipses.

* circular orbits are rare, if indeed any natural circular orbits exist at all. Orbits are elliptical. So the Moon will come closer to the Earth (and will travel faster), and the move further from the Earth (and also slow down). When the Moon is at it's closest point we say it is at perigee. When at it's further point, apogee. These are just names for the nearest and furthest points of an orbit. A full moon at perigee is sometimes referred to as a "supermoon". Hence the "super blood moon" you would have seen for the lunar eclipse this week. I much prefer the actual phrase to descibe this -- "perigee syzygy". The word syzygy means that 3 bodies are lined up in a straight(ish?) line. And it's fun to say.


Earth-like biospheres on other planets may be rare

Credit: Pixabay/CC0 Public Domain

A new analysis of known exoplanets has revealed that Earth-like conditions on potentially habitable planets may be much rarer than previously thought. The work focuses on the conditions required for oxygen-based photosynthesis to develop on a planet, which would enable complex biospheres of the type found on Earth. The study is published today in Monthly Notices of the Royal Astronomical Society.

The number of confirmed planets in our own Milky Way galaxy now numbers into the thousands. However planets that are both Earth-like and in the habitable zone—the region around a star where the temperature is just right for liquid water to exist on the surface—are much less common.

At the moment, only a handful of such rocky and potentially habitable exoplanets are known. However the new research indicates that none of these has the theoretical conditions to sustain an Earth-like biosphere by means of 'oxygenic' photosynthesis—the mechanism plants on Earth use to convert light and carbon dioxide into oxygen and nutrients.

Only one of those planets comes close to receiving the stellar radiation necessary to sustain a large biosphere: Kepler−442b, a rocky planet about twice the mass of the Earth, orbiting a moderately hot star around 1,200 light years away.

The study looked in detail at how much energy is received by a planet from its host star, and whether living organisms would be able to efficiently produce nutrients and molecular oxygen, both essential elements for complex life as we know it, via normal oxygenic photosynthesis.

By calculating the amount of photosynthetically active radiation (PAR) that a planet receives from its star, the team discovered that stars around half the temperature of our Sun cannot sustain Earth-like biospheres because they do not provide enough energy in the correct wavelength range. Oxygenic photosynthesis would still be possible, but such planets could not sustain a rich biosphere.

Planets around even cooler stars known as red dwarfs, which smolder at roughly a third of our Sun's temperature, could not receive enough energy to even activate photosynthesis. Stars that are hotter than our Sun are much brighter, and emit up to ten times more radiation in the necessary range for effective photosynthesis than red dwarfs, however generally do not live long enough for complex life to evolve.

"Since red dwarfs are by far the most common type of star in our galaxy, this result indicates that Earth-like conditions on other planets may be much less common than we might hope," comments Prof. Giovanni Covone of the University of Naples, lead author of the study.

He adds: "This study puts strong constraints on the parameter space for complex life, so unfortunately it appears that the "sweet spot" for hosting a rich Earth-like biosphere is not so wide."

Future missions such as the James Webb Space Telescope (JWST), due for launch later this year, will have the sensitivity to look to distant worlds around other stars and shed new light on what it really takes for a planet to host life as we know it.


A crescent sunrise

In much of northeastern North America, the eclipse will already be in progress at sunrise. If the skies are clear, a spectacular crescent-shaped sun will appear above the horizon for people in major metropolitan areas, including Boston, New York, Philadelphia, and Washington, D.C.

Because the eclipse occurs around sunrise, it’s important to scout out a good viewing spot beforehand with easy views of the eastern horizon. The biggest challenge to catching the Devil’s Horns effect for city dwellers will be finding a spot that has a totally unobstructed view of the sunrise.

“The essential things are to be on time, make sure you have a good line of sight to the sun, and be serious about eye safety,” says Graham Jones, an astrophysicist who works with the website timeanddate.com. “For a sunrise eclipse, you need a position with a clear view to the horizon, like a hilltop or tall building.”

When the eclipsed sun rises, optical effects may make it appear redder and larger than it actually is. The sun’s distinct coloration at dawn and dusk is due to the fact that sunlight must pass through more of the atmosphere to reach our eyes than when we see it at high noon. When light has to travel longer distances through the gases in our atmosphere, much of the shorter, bluer wavelengths get absorbed and scattered, while longer, redder wavelengths reach our eyes.

Particles in Earth’s atmosphere can also affect the sun’s colors. How orange or red the sun will appear while near the horizon depends on how much dust or pollen is in the air at the time.

In addition, the rising sun may appear larger to our eyes than it actually is. Since there are objects closer to us in the foreground, our brains compare them to the rising sun, creating the illusion that it is larger near the horizon.


Where Umbra Meets Antumbra

The Moon's umbra and antumbra, surrounded by the penumbra.

Hybrid solar eclipses happen when the Earth travels through the area where the Moon's umbra meets its antumbra.

Because the Moon is smaller than the Sun, both its umbra and its antumbra are V-shaped. The diameter of the umbra decreases with growing distance from the Moon. Where the umbra ends, at the tip of the V, the antumbra starts. This part of the shadow increases in diameter as we move away from the Moon. Together, both shadows look a bit like an hourglass, if viewed from the side (see illustration).

The location where the umbra transitions into the antumbra marks the spot where, if you look at the Sun and the Moon, the apparent sizes of both celestial bodies are exactly equal. If you move towards the Moon from that spot, you enter the umbra and see a total solar eclipse if you move away from the Moon, you enter antumbral territory, so the Moon begins to appear smaller than the Sun, creating an annular eclipse.


Royal Astronomical Society of Canada: Toronto Centre

The Earth’s orbit carries it around the Sun once a year – in 365.24 days, like clockwork. In the meantime, every 27 days, 7 hours, and 43 minutes, the Moon completes a single orbit around the Earth. This is where we get the term “month” from, and the two reliable and regular orbits are the bases for our calendars.

At the moment in the Moon’s orbit when it crosses the straight line between the Earth and the Sun, a New Moon occurs. Within a day or so on either side of the moment of New Moon, we can’t see the Moon within the Sun’s glare and, in any case, the half of the Moon pointing towards the Sun is fully lit, while the half facing us is dark. Roughly two weeks later, the Moon has travelled half of its orbit to place the Earth in the middle of the trio. This time, the Earth crosses the straight line between Sun and Moon, creating a Full Moon. At that moment, the entire side of the Moon facing Earth is illuminated by Sunlight, and the Full Moon rises just as the Sun sets at our backs.

If the Sun, Earth, and Moon were all travelling in the same plane (imagine a model of them on a tabletop), every New Moon would cross over the Sun and cast a circular shadow on the Earth, producing a Total Solar Eclipse visible by people inside the shadowed area. At every Full Moon the Earth would hide the Sun from the Moon as it passed through our shadow for a few hours, producing a Total Lunar Eclipse. So why do these events seem to be so rare?

The main reason eclipses occurs rarely, and are visible from different locations on Earth, is because the Moon’s orbit is tilted (or Inclined) by about 5° with respect to the Earth’s orbit around the Sun. While the bodies in the Solar System orbit the Sun in roughly the same plane, each body’s orbit is tilted by a small angle that carries it sometimes above and sometimes below the Solar System’s “tabletop”, like the horses on a revolving carousel.

On infrequent, but predictable occasions, the moments of New Moon or Full Moon occur when the Moon is situated in the place where its orbit and the Earth’s orbit intersect, also known as a node. This is the geometry that creates eclipses. The Earth has a much larger diameter than the Moon, so an eclipse can still occur when the Moon is below or above the node. That is why eclipses are visible from different parts of the Earth and why the tracks of totality change in both latitude and longitude.

Solar Eclipse
By cosmic coincidence, the size of the Sun’s disk and the Moon’s disk observed from Earth are almost exactly the same, even though the Sun is about 400 times larger! If this were not so, we would not be treated to the splendor of a Total Solar Eclipse.

During a Total Solar Eclipse, the Moon’s round shadow sweeps across the Earth, completely blocking the Sun for observers along the track of the shadow for only a few minutes. The best eclipses have 250 km wide tracks and yield many minutes of totality. In zones alongside the track, observers see only part of the Sun obscured – the farther from the path of totality, the less of a “bite” is taken out of the Sun. This is called a Partial Solar Eclipse.

It is NEVER safe to directly look at a Solar Eclipse, with one exception. Observers experiencing the few minutes of totality may turn their gaze upon the Moon-obscured Sun and see the glorious corona. Everywhere outside of totality, some of the Sun’s disk is exposed, and any amount of unprotected viewing is harmful to your eyes. The next nearby Total Solar Eclipse will cross the continental USA on August 21, 2017.

Lunar Eclipse
In a Lunar Eclipse, totality lasts several hours because the Earth is larger than the Moon and casts a shadow that is much wider than the Moon’s. The Moon in totality tends to turn a ruddy red, orange, or pinkish colour because all the sunlight reaching it has travelled over the Earth’s horizon, scattered by our atmosphere. This is the same light that colours the landscape (and us) during sunrises and sunsets. It also means that some sunlight will always get to the Moon and prevent it from going completely dark.

Due to the Earth’s atmosphere, the shadow cast by our planet has a dark circular core called the Umbra surrounded by a ring of partial darkness called the Penumbra. As the Moon traverses these zones, we get eclipse phases known as Penumbral and Umbral. In a Total Lunar Eclipse, the Moon can cross the penumbra twice.

Since Full Moons are already 100% safe to look at with unaided eyes or a telescope, a Full Moon during Lunar Eclipse is just as safe to view.


How does this rare event occur?

The annular solar eclipse is a rare sight. A total solar eclipse occurs when the Moon passes between Earth and the sun and blocks the Sunlight resulting to create a ring-like figure around itself. At this time, Moon is also near its farthest point from the Earth. It is this phenomenon that creates a &lsquoRing of Fire&rsquo in the sky. As per NASA, annular solar eclipses occur every 18 months somewhere on Earth and they are visible only a few minutes, unlike lunar eclipses. Also Read - Surya Grahan 2021 Impact on Zodiac Signs: Taurus, Libra, Capricorn, Gemini to be Worst Affected | Remedies And What Not to do


A Rare Earth-like Planet Has Been Discovered in Our Galaxy

Scientists are calling it a once-in-a-lifetime discovery.

Scientists in New Zealand have spotted a rare glimpse at a planet that is comparable to the size and orbit of Earth within our galaxy, USA Today reported. Although this is certainly a once-in-a-lifetime discovery, you may not want to be packing your bags for life on a new planet just yet.

According to The Astronomical Journal, the planet was discovered using a “microlensing” technique, a method that can detect planet- or star-sized objects regardless of how much light they emit.

“The combined gravity of the planet and its host star caused the light from a more distant background star to be magnified in a particular way,” said study lead author Antonia Herrera-Martin, from New Zealand’s University of Canterbury, to USA Today. “We used telescopes distributed around the world to measure the light-bending effect.”

The planet has a mass somewhere in the range between Earth’s mass and that of Neptune, according to USA Today, and its orbit makes for a year that lasts around 617 days.

While this may seem promising, there is one drawback. The planet’s host star (which would give warmth and light to the planet), is only about 10 percent of our sun’s mass while also being the same approximate distance from its planet as our sun is to Earth.

"Although it’s not too much bigger than Earth, and orbiting its star at a similar distance, this planet would be very cold because its star is smaller than the sun and emits much less light," said study co-author Michael Abrow, of the University of Canterbury, to USA Today.

Abrow added that water is unlikely to exist on the planet in the form of liquid and life is not expected to exist because of the harsh conditions.

While this discovery is certainly exciting and interesting, it’s also unlikely that we will be able to see the planet again any time soon, since finding objects with the microlensing technique is extremely rare, according to USA Today.


How rare are earth-like solar eclipses? - Astronomy

The Sun's face is hidden by the moon's dark disk as the corona glows softly around it. Stars and planets are visible that are usually washed out by the Sun's bright light. The air has cooled. Some flowers have closed their petals and night-blooming ones are opening. Bees are back in their hives and birds have stopped singing. It's a solar eclipse.

Solar eclipses have been viewed as ill omens, which is not surprising given our dependence on the Sun. Some ancient civilizations thought a dragon, a demon or some other creature was eating the sun. The Chinese would frighten it away with drums and shouting and other noise.

As seen from Earth, the Moon and Sun appear to be about the same size. Otherwise total solar eclipses couldn't occur. Although the Sun is 400 times bigger than the Moon, the Moon is 400 times closer to us.

The Moon's orbit is elliptical (shaped like a squashed circle), so the Moon's distance from us varies. If it's at its most distant at the time of an eclipse, its disk won't cover the Sun, so there is a ring of sunlight surrounding it. This is an annular eclipse.

A solar eclipse occurs only during a new moon, and then only if the Earth, Moon and Sun are aligned. This doesn't happen every month. Earth orbits the Sun in what is called the ecliptic plane – this is also the plane of the Sun's apparent motion through the sky. Although the Moon orbits the Earth close to the ecliptic, it's slightly tilted to it. The two points where its orbit crosses the ecliptic are called nodes. If a new moon occurs when the Moon is near a node, then everything is lined up for a solar eclipse.

Since it's small compared to Earth the Moon's shadow is narrow. Any place where the darker part of the shadow (the umbra) falls would have a total eclipse, but within the outer part of the shadow (the penumbra) the eclipse is only partial.

The irregular landscape of the Moon means that its edge isn't perfectly smooth as it passes in front of the sun. It looks as if there are beads of bright sunlight coming through valleys and between mountains. They are called Baily's beads in honor of British astronomer Francis Baily who first explained them. It also causes the lovely diamond ring effect.

A total or annular eclipse occurs somewhere in the world about every nine months. But at any particular location they are rare. For example, there was a total eclipse in Cornwall, England in 1999, but the next one isn't until 2090.

Predicting eclipses before the days of computers was tricky. It's based around the Saros cycle of just over eighteen years and was probably known to the ancient Babylonians. However working out from where on Earth the eclipse will be visible is quite complex.

Solar eclipses used to be the only time astronomers could study the solar corona. This is the region surrounding the Sun and it’s about a million times fainter than the disk of the Sun we normally see. Coronagraphs, especially on space telescopes, can now block out the bright center. However this doesn't give a complete picture, so solar eclipses continue to add to our understanding of the Sun.

Famously, Arthur Eddington made Einstein an international celebrity by testing his General Theory of Relativity during a solar eclipse. Yet it isn't just astronomers that are interested in eclipses. Biologists also study the effects of solar eclipses on animal behavior. Birds and insects seem to be particularly sensitive to the changes in light levels.

Modern eclipses aren't accompanied by demon-scaring clamor. The sounds would be the clicks of cameras, gasps of wonderment and perhaps delighted applause of onlookers.

Reference:
Tony Phillips, "There Goes the Sun," NASA Science News, 05.08.99
ROG learning team, "The Solar System: Eclipses," National Maritime Museum [accessed 05.07.10]

Credit header image: NASA/SDO and the AIA, EVE, and HMI science teams
Credit Bailey's beads image: Catalin Beldea (Descopera Magazine)
Credit diamond ring image: Rick Feinberg

Content copyright © 2021 by Mona Evans. All rights reserved.
This content was written by Mona Evans. If you wish to use this content in any manner, you need written permission. Contact Mona Evans for details.


See Rare Sunrise Spectacular As Solar Eclipse Hits Parts of U.S. and Canada This Week

It’s time to dig out the solar glasses from the back of the cupboard, because on the morning of June 10—that’s this Thursday—the Sun, Moon, and Earth will bring us a highlight of the summer stargazing season as a solar eclipse hits the Northern Hemisphere.

If you’re in the Lower 48 you’ll want to be in the northeast, or in parts of the Midwest and Mid-Atlantic, to glimpse a surreal sunrise where the sun has ‘solar horns’. Maine will see 78% coverage of the sun in Washington, D.C. it will be 55% covered—creating a fascinating crescent shape.

If you’re in Ontario, Nunavut, or Quebec in Canada, if the skies are clear you’ll see an impressive ‘Ring of Fire’ solar eclipse a little after sunrise—but only if you’re north of Lake Superior (sorry, Torontonians.)

Drew Rae

If you’re further in the west of Canada or the U.S? You may as well have a lie-in, as the eclipse will begin and end its show before sunrise hits.

The Weather Network has great maps containing more info on what you’ll see, and when, in your location in North America.

So how do solar eclipses work? According to NASA, “A solar eclipse happens when the Moon moves between the Sun and Earth, casting a shadow on Earth, fully or partially blocking the Sun’s light in some areas.

“During an annular eclipse, the Moon is far enough away from Earth that the Moon appears smaller than the Sun in the sky. Since the Moon does not block the entire view of the Sun, it will look like a dark disk on top of a larger, bright disk. This creates what looks like a ring of fire around the Moon.”

P.S. It’s not just up in the direction of the sun you’ll want to look at during Thursday’s spectacular sunrise. Look on the forest floor, according to The Washington Post, and you could see crescent-shaped patches of light tucked among the trees’ shadows as the sun’s image gets projected on the ground.

(WATCH NASA’s visualization of Thursday’s eclipse in the video below.)

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