that always used to puzzle me. how come the sun is closer in the
winter than in the summer ?
i know it is to do with the tilt of the earth on its axis.
can you explain it more clearly ?
There are two different peculiarities you are mixing up. One is that the
orbit is an ellipse. As I mentioned, the sun is at one of the foci of an
ellipse, and the ellipse is *almost* a perfect circle.
The other issue is the earth's axis of rotation is tilted with respect to
the orbit. As you may recall from your school days, this tilt is
responsible for the seasons. The tilt is something like 23.5 degrees. The
axis stays pointing in the same direction in space as the earth orbits
around the sun (oh, allright, there is a ~26 000 year 'nutation', but lets
ignore that).
Seasons are caused by the tilted axis remaining oriented the same way in
space (relative to distant stars) as we orbit the sun. When the earth is on
one side of the sun, the north end of the axis is 'leaning' towards the sun.
Summertime in the northern hemisphere, later the orbit carries earth to
opposite side of orbit and the north pole is further away from sun than the
south pole (winter in northern hemisphere).
Now, the orientation of the axis in space, and the orientation of the
ellipse traced out by the earth's orbit, *ARE UNRELATED*. It is purely a
coincidence that in these modern times, the earth's axis is oriented for
summer in the northern hemisphere when at the closest point of approach in
orbit.
Indeed, as I mentioned earlier, the axis does 'wobble' (called 'nutation')
with a period of ~26 000 years. So in about 13 000 years (1/2 cycle from
now), the earth will be closest to the sun in winter (for the northern
hemisphere). The axis will have shifted to point to another distant star.
The 'north star' (Polaris) won't be very close to the north pole anymore.
But I won't have to learn any new celestial navigation because I won't be
here in 13 000 years ;-)
IIRC, the ellipse that is the earth's orbit also moves. But I believe that
movement is slower by several orders of magnitude from the planet's axis
nutation. But now we've reached my limit on celestial mechanics, I'd have
to go look it up to go further ;-)
There, does that help??
daestrom