"jojo" wrote in message
.. .
so, the planets revolve around the sun in an elliptical orbit?
all the models I have seen show the orbits as circular.

That's because many popular books have over-simplified diagrams.

This explains why we have summer and winter, and why mars is closer to us
now?

No to the first, yes to the second. The difference in the distance of the
earth from the sun in winter as compared to summer is negligible. The
seasons are actually caused by the tilt of the earth's axis. When it is
summer in the northern hemisphere, that hemisphere is tilted toward the sun.
When it is winter in the northern hemisphere, it is tilted away from the
sun. (Just the opposite is true for the southern hemisphere).

RM