Monday, 10 July 2017

Eclipses on other planets?

Is the Earth the ONLY planet to enjoy a solar eclipse?

Virtually any astronomer can assure you that eclipses are not an uncommon thing. An eclipse occurs when one object casts a shadow on another such as when the Moon's shadow hits the Earth. That is when a solar eclipse takes place. When the Earth's shadow hits the Moon, a lunar eclipse takes place. Using a modest backyard telescope one can see the shadow of the Jovian moons hitting the upper cloud decks of Jupiter causing a solar eclipse there. And if the angle is right one can also see the moons of Jupiter emerge or get engulfed by the shadow of the giant planet.

Thus the simple answer is that Earth is not the only planet to enjoy eclipses.

But do they see really good eclipses on other planets? A good eclipse is where the apparent size of the eclipsing moon is the same as the size of the Sun in the sky. Here on Earth the Sun and Moon appear to be about the same size in the sky. The reason is one of perspective. The Sun is massive. 400 times bigger than the Moon. Yet it is 400 times farther away making it look the same size.

You can see this effect by using two coins, a big one and a small one. Put the larger coin on your desk and hold on to the smaller one. Now block the view of the big coin with the small one by holding the smaller one closer. (It helps to close one eye.) Move the smaller coin back and forth until it matches the size of the larger, more distant coin. Now measure the distance to the larger and smaller coins. Also measure the size of the two coins. If you compare the ratio of the larger value over the smaller value you will find they match up. (Example, two coins, one is three times larger will match in size if the larger coin is three times further away. Try it!)

Now back to the question about the other planets. Do any of them have a moon that appears to be the same size as the Sun? There are over 180 known moons in the solar system and they vary in size greatly. There are moons that are very big and there are moons that are very small (kilometer or so).

To figure this out the first step is to determine how big the Sun appears from the other planets with moons. All of these planets are further away meaning that the Sun will look smaller to someone observing from that location.

The next step is to find out about the various moons of the planets and determine how big they appear in the sky.

Now we can compare the sizes to see if any match up or come close. Given the diversity of the solar system moons there will be some that appear to be many times the size of the apparent Sun as well as some that will be mere dots in the sky. And as we reach farther and farther away, the Sun itself starts to appear as a dot. This means we would need a powerful telescope to see the eclipse.

The results of one such analysis I did can be viewed at:

I created a table of just over 100 various moons with various sizes and found, as expected, that an exact matching ratio is rare. Just not as rare as I anticipated.

It was expected to find that only the Earth and Moon had such an arrangement of matching angular sizes. But what I found were two other moons, Pandora and Epimetheus both in orbit about Saturn, also has a near perfect fit just like the Earth and Moon! Check out the study to see which moon was actually the best.

The study revealed at least three more moons that create total eclipses where one could see the corona all the way around the eclipsing moon. I say at least because that is a subject open to debate. I set the test up to look for any circumstance where the moon was two times the size of the Sun or less. That means the corona must be visible out two solar radii, not an uncommon situation (the corona can be seen out to six solar radii with dark adapted eyes from inside the Earth's atmosphere, so one could increase this cut off level and see even more matching situations).

It turns out that such coincidences are not as rare as I had believed!

There is quite a range in sizes. The moon Triton  appears 26 times larger than the Sun from Neptune! Now this does not mean it fills the sky. Neptune is pretty far away from the Sun. Without a telescope, the Sun would appear as a bright dot in the sky. Triton appears to be about half a degree, the same as our Moon appears to us over Earth. On the other end of the scale some moons appear to be a dot against the solar disk. The worst case was the Sun appearing over 16,000 times larger than the eclipsing moon!

BUT - this is important!

The eclipses of the Sun as seen from the Earth are the best. Why? Earth is the closest planet to the Sun with a natural moon in orbit. And thus it has the closest view. From Saturn, we would need a 10x telescope to see the same thing we see with the naked eye from the comfort of Earth.

cartoon by Bill Kramer

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