The Earth is one of nine planets. Like the other eight, it is controlled by sun and revolves about it. All the planets move in the same direction, but each at a different distance from the sun. And although they are said to belong to the same family and act alike in many ways, they are quite different in others, just as brothers or sisters often are.
Some planets, since they are so closer to the sun than others, are very hot while those far away are very cold. Some are smaller than the earth and some are much larger. As the length of a day on any planet is the time that it takes to rotate once on it's axis and the length of a year is the time that it takes to revolve once in it's orbit around the sun, all have days and years of different lengths.
The force of gravity is also quite different on the various planets. A 100-pound boy would weight only 37 pounds on mercury and 38 pounds on mars, while on jupiter he would weight 264 pounds. Some planets have no air around them and very little moisture. How many problems we would have if the dream of some people should come true- the one day we might step into a spacecraft and visit another planet!
The smallest planet is mercury, just a little longer than our moon. It is also the planet nearest to the sun. Being the nearest, mercury has the shortest journey around the sun, also its speed is the greatest. And, of course, it is the hottest, at least it has the hottest day. As compared with our year of 365 days, it takes mercury only 88 days to make one complete trip around the sun. But mercury turns only once on its axis every 59 days, meaning that its day is two-thirds as long as its year. Photographs of mercury taken from unmanned spacecraft show that the planet is covered with many craters, much like the earth's moon. When mercury can be seen from the earth, which is not often because it is not so near the sun,it appears in the western sky right after sunset or in the eastern sky just before sunrise. It then looks as bright as a star.
Venus is next and is about twice as far from the sun as mercury. It is almost as large as the earth, and its orbit is closer to the earth than any other planet. For this reason, venus was the first planet to which the unmanned spacecraft were sent. Several of these craft actually landed on the venus, where they recorded temperatures as high as 900 degree fahrenheit. Scientists believe that life as we know it could not exist on Venus.
Venus turns even more slowly on its axis than does mercury. One full day is about 243 days or about 18 earth days longer than Venus's year!
After Venus comes the earth, then, mars, which is often called the ''RED PLANET'' because of its appearance in the night sky. The orbit of mars is beyond that of the earth and mars take takes twice as long to go around the sun as the earth takes. If there is life on any planet in the solar system other than the earth, it is probably on mars. The red planet is not as warm as the earth, and the nights are extremely cold. Moreover, there is very little oxygen in the atmosphere. The day on mars is about the same length as ours. Mars has two tiny moons, about 10 to 155 miles in diameter, which travel around the planet much as our moon circles the earth.
The next planet, jupiter, is more than three times as far from the sun as mars is. Jupiter is truly the giant of the planets, containing more matter than the eight planets put together. Jupiter spins very rapidly on its axis, making a complete rotation in less than 11 hours. Seen through a telescope, jupiter shows a surface marked with many colored bands, which are actually the tops of dense clouds. The giant planet has 13 known moons, of which four are about as large as our moon.
Saturn is nearly as large as jupiter and almost twice as far from the sun. Through a telescope, saturn can be seen to have a series of bright rings around it. These rings are probably made up of rocks of various sizes. Saturn has 10 moons, of which one-Titan-is about the size of the planet mercury and has an atmosphere.
Next comes Uranus, the farthest planet that can be seen without a telescope. It has five moons and is nearly twice as far from the sun as saturn. Still farther away is neptune. The temperature there must be very low, for neptune 30 times as far away from the sun as the earth.
Pluto is so far away that no one even suspected that it existed until the early 1900's and it was not sighted until 1930. Not much is known about it yet except that it takes 250 years to make one trip around the sun, and that its diameter is probably less half that of the earth.
The word planet means "wanderer" and it might seem at first, as it did to the ancient Greeks who gave them the name, that they wander about in space. In particular, the planets whose orbits are beyond that of the earth seem to wander across the night sky. Then they seem to stop and begin moving to the west. After a time, the entire sequence is repeated.
We can see only that part of any planet that is in a position to reflect sunlight towards us, since the planets, like the moon, have no light or their own. Also, they appear brighter as they get closer to the earth and dimmer as they get farther away in their wide sweeps around the sun. No wonder the ancient Greeks thought they were wanderers.
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