Pia Asa asked:
My daughter recently turned 10 and we started talking about planets rotating around the sun (as you do).
We were wondering whether you end up in exactly the same spot in the universe on your birthday ie since the earth goes around the sun in 365 days, does that mean that each year on your birthday you would be in exactly the same spot in the universe with respect to the sun? (all the other planets would be in different places wouldn't they?)
Or is it constantly shifting, and if so, then how?
Chris Smith put this to Andrew Norton, astronomer from the Open University...
Andrew - Well, thatís a really good question. You're absolutely right. In one year, the Earth will return to the same spot with respect to the sun. but the sun is moving through space and the sun is moving, dragging all the rest of the planets in the Solar System with it. the Sun is moving with respect to the nearby stars. All those nearby star,s including the Sun, are in turn rotating around the galaxy Ė our galaxy, the Milky Way. Our galaxy, the Milky Way is also moving through space with respect to the other galaxies in our local group of galaxies. That local group of galaxies including the Andromeda galaxy, is moving through space with respect to the Virgo Supercluster of galaxies, of which weíre a part, and that Virgo Supercluster of galaxies is itself moving through space with respect to the cosmic microwave background, the background glow of the Big Bang, which we can imagine as a sort of static reference against which everything else moves. So, everything is moving.
Kat - Also, thatís assuming that itís exactly 365 days to go around the Sun, but itís not, is it?
Andrew - Itís not. Itís about 365 and a quarter days, which is why we get that extra day every leap year, every four years. So yeah, there's a little bit of adjustment due to that as well. But with everything else going on. Itís a lot of movement.
Kat - Who cares a quarter day here and there, February 29.
Chris - So relative to the sun, you are in the same spot on your birthday. But relative to everywhere else on the universe, you're not.
Andrew - After 365 and a quarter days, you're relative to the same spot with the Sun. but yeah, itís complicated.
The shape of Earth's orbit is not constant due to the gravitational pull of other planets, see Ö
Yep, its always beneath your feet, unless you're standing on your hands, that is.
Isn't the same position in the universe a rather meaningless concept ?
So, with the orbit around the sun, you would be within about Ī1į of the same place every year with the variability due to the leap years.
I think you should ask space.com forum this question. Ask them if the plough will always be in the same position, altitude and compass bearing; in the night sky at 9pm GMT on your daughters birthday. Then check it out!! Make sure they have got it right!!