Do they really think the moon follows a polar orbit?

It may well be this new thing called "perspective", Ramses. ;-)

*laugh*

A good defence, sir, a good defence.

It's possible they think it's a polar orbit, but I think this is more an issue of broken (or severely bent) perspective and they're trying to show the moon beyond earth not over the pole.

And yes, Herschel and planck will be orbiting L2. This isn't an orbit in the strictest sense since L2 is an unstable point, but the expendables needed to keep them in this orbit are minimal compared to any other similarly favoured point in the Earth-Sun-Moon system.

It *was* you I heard on Material World broadcast last Friday, wasn't it?

Yep! It was this Cthulhoid One. Waddyathink?

Thursday - yes.

Of course! (I blame being on an intensive training course for making me lose track of which way was up, never mind what day of the week it was <G>)

Most enjoyable broadcast - which reminds me, must pick up the podcast.

I think the diagram is mean to be a perspective view from above the plane of the Earth's orbit, so we are seeing the moon on the far side of the Earth, rather than 'above' it.

L2 'orbits': the term 'orbit' is slightly misleading, as we tend to understand it as meaning orbiting about a large mass such as the sun or a planet. Here's my arm-waving explanation of L2.

- If a probe is put in an orbit around the sun slightly outside Earth's orbit, then it will take longer than Earth to orbit the sun, and so would normally tend to lag behind Earth.

- But Earth's gravity will add to the sun's gravity, and in effect strengthen it. As such the probe has to orbit a bit faster to counteract the extra gravitational pull.

- Get the distance right, and the extra speed around the orbit exactly makes up for the orbit being bigger (and thus slower) than Earth's, so the probe still takes exactly one year to go around the sun, hence keeping pace with Earth.

- This still works if the probe is not exactly on the sun-Earth line. But if it is off a little, then what it ends up doing is following a path that from the perspective of Earth is a circle around the 'L2' point. It's not orbiting that point - it's orbiting the sun - but from Earth it is going in a circle around it, so 'orbit' is the term that tends to get used.

The three-body problem, and the special restricted solutions to it, is a very challenging and non-intuitive aspect of orbital mechanics. L1, L2 are reasonably easy to understand when you look at it in the way I've described. (For L1, the point inwards of Earth, just run the argument the other way, with the Earth's pull reducing the sun's apparent gravity). L3, on the far side of the sun, is rather weirder. L4 and L5, 60 degrees ahead of and behind the Earth but in the same orbit, are just bizarre, but fall out of the maths - and turn out to be the most stable points of all.

Actually, I have least problem with L4/L5 - I just see them as like a three-weight bolas.

I think my problem with the L2 orbit is that although I know that it should be an attractor, and that therefore something slightly to one side will slowly oscillate through it like a pendulum swing, I don't automatically consider oscillations in orthogonal directions, giving an elliptical path.

(And in orbital terms, I tend not to remember the theoretical 'orbit' of a mass dropping from the North Pole through the centre of the earth to the South Pole and back again. That is not elliptical at all, but (theoretically, excluding Earth's solar orbit, etc. etc.) just a straight line. Which is what I unthinkingly expect for something adjacent to the centre of the L2 region.)

Edited at 2009-05-11 10:40 am (UTC)

L2 isn't a simple attractor: it's a saddle, so a spacecraft needs active assistance to keep it on station.