THE SOUTHWORTH PLANETARIUM
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70 Falmouth Street Portland, Maine 04103
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Founded January 1970
Julian date: 2457769.16
"Just living is not enough... one must have sunshine, freedom, and a little flower."
-Hans Christian Andersen
THE DAILY ASTRONOMER
Tuesday, January 17, 2017
Extra Terrestrial Solar Eclipses
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STAR DOME ASTRONOMY
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Six Tuesday evenings 7:00 - 9:00 p.m.
An introductory astronomy class that combines the lecture
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The glorious uncertainty of it all.
We reach into Pandora's depths and withdraw a mysterious parchment. We call it mysterious because we don't know what will be written on it until we see it. Sometimes the question will have a simple answer.. At other times, the answer will be much more involved, Mind you, we don't discriminate. All questions are welcome!
On today's parchment we found a question that required a rather in-depth answer, so we will address only one Pandora question today. However, we will still reach our 100 total by August!.
PANDORA PARCHMENT # 13: Do other planets experience solar eclipses like Earth does?"
-I. Morse, Portland
Greetings!
Other planets do experience solar eclipses, but we have some qualifications.
First, our own moon and Sun have almost precisely the same apparent angular diameter in our sky. (Approximately half a degree, with some variation throughout the year.) Astronomers know of no other moon in the solar system whose angular diameter is the same as the angular diameter of the Sun. So, total solar eclipses, in which a moon just covers the Sun and allows the outer corona to become visible, only occur on Earth. (At least in OUR solar system.)
Total solar eclipse. Earth is the only planet in the solar system where one can observe a total solar eclipse, here defined as an eclipse caused by a moon that appears precisely as large as the Sun. Other moons in the solar system do "eclipse" the Sun, but these eclipses are either occultations or transits. The former occurs when the much larger moon (in apparent size) blocks the Sun. The latter occurs when the much smaller moon only partially blocks the Sun.
Some moons in orbit around other planets can cause solar eclipses. We'll methodically proceed through the solar system to find those moons that will occasionally block the Sun.
MERCURY.
No moon.
No eclipses.
We proceed.
VENUS
No moon
No eclipses
Let's move on
EARTH
One moon.
The only place in the solar system in which total (precise) solar eclipses are possible.
MARS
Two moons: Phobos and Deimos
Both of these satellites can move directly in front of the Sun. However, they are so small that neither will completely block the Sun. We call these events transits. Phobos, being both larger than Deimos and closer, would appear larger to a Martian observer during these transits. These transits occur very quickly. A Phobos transit lasts about thirty seconds. A Deimos transit lasts about two minutes.
Transit of Phobos. The larger of the two Martian satellites can "transit" the Sun. As this planet completes an orbit once in about seven and a half hours, these eclipses of Phobos last less than a minute. The Curiosity Rover captured these images of the transiting Phobos on August 20, 2013. Image: NASA
JUPITER
Known moons: 67
Only five of these moons, -Io, Callisto, Ganymede, Europa and Amalthea- appear large enough to completely block the Sun. The first four, known collectively as the "Galilean moons," are the four largest moons and all appear much larger than the Sun. (Callisto appears six times larger than the Sun!)
When these bodies pass directly over the Sun, an eclipse does occur. The moon would cast a shadow onto Jupiter's upper layers. Any observer within this dark region would observe the Sun suddenly blocked out by a moon.
The other moons, being either farther away and/or smaller, will only appear to transit the Sun from Jupiter.
AN IONIAN ECLIPSE: We see a Hubble image of the moon Io casting a shadow onto Jupiter's upper layers. Five Jovian moons appear large enough to completely block out the Sun on occasion. These moons are Io, Europa, Ganymede, Callisto and Amalthea. Image: NASA
SATURN
Known moons: 62
Seven of Saturn's 62 moons are large enough and/or close enough to completely block out the Sun. I won't bother to name these moons now, as I don't want to bore you to tears and despair. (Might be too late.) As is true with Jupiter, these moons will all appear larger than the Sun. At Saturn's distance, the Sun is only about 3 arc-minutes across: quite small, but still blindingly bright. As long as a Saturnian moon appears larger, it can completely occult, or eclipse, the Sun/
URANUS
Known moons: 27
Twelve Uranian moons appear larger from the perspective of Uranus' upper layers than the Sun. The Sun from Uranus subtends an angle of about 2 arc-minutes! These moons can completely occult the Sun.
ARIEL PASSAGE: This Hubble image shows the Uranian moon Ariel casting a shadow on the gas giant's upper cloud layers. Image: NASA
NEPTUNE
Known moons: 14
The Sun subtends an angular diameter of only 1.25 arc-minutes. Fifty percent of Neptune's moons appear larger than the Sun and can completely block it.
Before proceeding to the last planet, we should mention that some of these eclipses are very rare. Eclipses require precise alignments, especially when the Sun appears to be so small in the sky.
PLUTO
Known moons: 5
Three of Pluto's moons appear large enough to occult the Sun, which is slightly smaller than 1 arc-minute at this distance. Interestingly, Pluto is tidally locked with Charon and so both bodies experience eclipses. Charon occults the Sun on Pluto; Pluto occults the Sun on Charon. Due to their tidal lock, these eclipses can only occur at certain regions on either body.
Solar eclipses are common in the solar system.
Precise total solar eclipses are unique to Earth.