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What was the closest distance between Saturn and Earth?
Question Date: 2003-09-14
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 earth, 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 pice 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 Saturns 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:
http://liftoff.msfc.nasa.gov/academy/space/solarsystem/solarsystemjava.html
It's pretty cool.

Answer 2:

I don't know off-hand; that's something that you can look up. The distance from the Earth to the Sun is eight light minutes. I don't remember exactly what the distance between the Sun and Saturn is, but I think that it's about two light hours. So, if Saturn and the Earth are on the same side of the Sun, then the distance between them is the distance between the Sun and Saturn minus the distance between the Sun and Earth.1 light second = 300,000 kilometers or 186,000 miles.


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