|MadSci Network: Astronomy|
Pluto is the smallest planet in the solar system, even smaller than a few of the moons in the solar system, including our own Moon. There's even debate as to whether Pluto should be classified as a "planet" at all.
We know Mercury's size fairly well because it's close to us, so we can see it well, and we've sent spacecraft by it to take close up pictures. Pluto is so far away that it's very hard to measure how big it is - we've never sent a spacecraft there, and it just looks like a very tiny dot in the telescope rather than a round disk (though this is changing with the Hubble Space Telescope). The way we've measured the size and mass of Pluto is by looking at the orbit and eclipses of its moon, Charon.
Imagine that you're standing on one end of a field and on the other end there's a big building and a road running behind the building. A car is driving down the road at 25 miles per hour (37 feet per second) goes behind the building, and then appears again on the other side 5 seconds later. We could calculate from this that the building is roughly 185 feet long (37 ft/sec times 5 sec). In the same way, astronomers have calculated how fast Charon is going, and then measured how long it takes for Charon to go behind Pluto.
Try the links in the MadSci Library for more information on Astronomy.
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