Answer 1:
Thanks for your great question. Just a few
weeks ago, the planet Mars was closer to the Earth
than it has been in 60,000 years  maybe you saw
it as the brightest thing in the sky besides the
moon.
Naturally, you might wonder how close the other
planets get to E arth, as well, and why the
planets get closer and farther away at all. To
find the closest distance between Saturn and
Earth, we have to understand the orbit of each
planet around the sun. Although we usually think
of planets traveling in circles around the sun,
they actually travel in an ellipse where the
sun isn't exactly in the center. You can draw
a perfect ellipse yourself. Place a loop of string
around two pins stuck through a piece of paper
into a piece of cardboard. Use a pencil to keep
the loop taut. If you move the pencil carefully,
it should trace out an ellipse. The pushpins are
located at the 'foci' of the ellipse, which are
two very special points. If the two 'foci' are at
the same point, then you draw a circle. (So
circles are a special kind of ellipse, just as
squares are a special type of rectangle.) The
sun is at the foci of planet orbits, which are
always ellipses. The fact that all repeating
orbits are ellipses is a very cool feature of the
force of gravity and was discovered by a physicist
named Johannes Kepler in 1609!
Now to figure out how close Saturn gets to
earth, we can figure out how far each of their
orbits is away from the sun, and subtract the two.
We can give a very rough approximation by assuming
the orbits ARE circles and subtracting the average
distances from the sun: Earth is 1 Astronomical
Unit from the sun, which is about 150,000,000
kilometers, while Saturn is about 9.5 AU, or
1,425,000,000 km away, which makes Saturn
approximately 1,300,000,000 km from Earth.
But if you pay attention to the fact that the
orbits are elliptical, then sometimes Saturn will
be farther away from the sun while Earth is
closer, and vice versa, so the distance between
Saturn and Earth changes, or oscillates, over
time.
So to find the closest approach, and when that
happened, we have to do some hefty calculations
using the actual orbital paths of the planets. The
answer is that on December 17th of last year
(2002), Saturn got to within 1,200,000,000 km
(750,000,000 miles) of Earth. When Saturn is
that close, it is brighter than all other stars
except for Sirius and Canopus, so you can see it
really well with a telescope. (The maximum
distance between the two planets is nearly 1.7
billion km.) At Saturn's closest position to
Earth, Saturn and the sun were on opposite sides
of the Earth. Known as opposition, the situation
takes place about every 13 months. But the last
one was the closet in three decades because Saturn
happened to be making its closest approach to the
sun in its lopsided orbit.
>If you are interested in seeing the orbits of
the planets and how they change, you can visit
NASA's solar system website:
NASA
solar system. It's pretty cool.
