Earthrise Is Impossible (Or Is It?)

The prologue:

This post is inspired by Greg's comment on this resharing of Blogstronomy's How Big Would Jupiter Look From Europa post. That post was inspired by a conversation between Tanya and myself about the potential awesomeness of a Jupiter-rise in the skies of Europa, and this, of course, prompted Greg's comment "Unfortunately, Europa is tidally locked, so you'll never actually see a Jupiter rise from its surface." Now, I'm not one to readily admit defeat, and 'never' is a pretty definitive word, and I'm always tempted to do battle against those. So here's my attempt to cobble something together which shows that I was right all along and didn't just forget about Europa being tidally locked to Jupiter*.

Tidal Locking

In order to attempt to redeem myself, I'm going to take you back to Earth and ask you to look up into a clear night sky. The Moon, hanging there in our sky, always has the same face pointing towards us. Forgetting momentarily about the phases, you can always see the same details, and there are some details you will never see from the surface of the Earth because they are on the far side** of the Moon and that never comes into view. The reason for this is that the Moon is tidally locked to the Earth- it, for reasons that I'll go into in another post if you ask me nicely, has an orbital period that exactly matches up with its rotation period.

Now lets go to the Moon. We're standing on the surface, and we're looking up at the Earth. What do we see? We see the Earth hanging there in the sky, never changing position. The only thing that really changes is that over the course of a month we see the same kind of phases changes as we do on the Moon as viewed from back home on Earth.

Because the Moon and Earth are tidally locked, the Earth just sits there.



This is nothing to do with books or star signs. When I said that the Moon always has the same face pointing at the Earth, I lied a bit. It was only a little white lie, and is mostly true, but not completely. During the course of a month the Moon, as viewed from Earth, actually wobbles backwards and forwards a little bit, almost like it's shaking its head at us disapprovingly, but not so much that it actually wants us to notice. This effect is called 'libration', and has the effect that, with patience and a decent telescope, little slithers of the Moon's 'far side' can be observed to come into view and disappear again.

Back to the Moon: standing on its surface, this means that or view of the Earth in the sky seems to wobble backwards and forwards*** as a month passes. This wobbling moves the Earth about in an imaginary box in the sky that is a bit more than 15 degrees wide and a little more than 13 degrees high****. Given that the Earth (from the Moon) has an angular size of about 2 degrees, if we're standing in one of the little slithers of far-side that appear and disappear, we'll see the Earth similarly appear and disappear (or rise and set) following the idea of 'if they can't see us, we can't see them'.

Hang on, we were talking about Europa...

Yes, yes, I'm getting to that. The issue is that, like the Moon and Earth, Europa is tidally locked to Jupiter. This means that Europa turns with the same period that it orbits Jupiter, and always has the same face looking at it.

Europa also librates, though I'm not sure how much. Given that Jupiter takes up a whopping 12 degrees of Europa's sky, Europa would have to librate like a mad thing to generate full Jupiter-rises and -sets, but if we were to stand in the right place we'd certainly see some sort of semi-rising and -setting over the course of a Europan orbit.
By Apollo 8 crewmember Bill Anders, via Wikimedia Commons

But wait, there's more!

On December 24th 1968 an American man took a groundbreaking photo. It was this one (right). He was on Apollo 8 at the time, which had just passed around the far side of the Moon. As it came back around to the near-side, the Earth appeared to rise above the Moon's horizon, which has given this famous and influential photograph its colloquial name: Earthrise.

So if you want to experience a planet-rise from a tidally-locked moon, get yourself into orbit.

Just for the snap-happy:

For those infected with the shutterbug, James Lovell***** was recorded stating the settings: 1/250th of a second, at f/11. Just in case you find yourself in the same place and don't want to miss that perfect Earthrise shot.

* Honest.
** Sometimes, and erroneously, referred to as the 'dark side' of the Moon.
*** Actually, it appears to move in a figure-of-eight.
**** If you're not sure what that means, you might like to have a read through this post.
***** Yes, he of Apollo 13 fame. He gets around a bit, doesn't he?


  1. "On December 24th 1948 an American man took a groundbreaking photo"

    That should be 1968.

  2. Of course it should, and now it is! Thanks!


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