Though a satellite of Earth, the Moon is bigger than Pluto. Some scientists think of it as a planet (four other moons in our solar system are even bigger). There are various theories about how the Moon was created, but recent evidence indicates it formed when a huge collision tore a chunk of the Earth away.
How the Moon's phases change
Because it takes 27.3 days both to rotate on its axis and toorbit Earth, the Moon always shows us the same face. We see the Moon because of reflected sunlight. How much of it we see depends on its position in relation to Earth and the Sun.
The 27.3-day number is what scientists call a sidereal month, and it is how long it takes the Moon to orbit the Earth in relation to a fixed star. Another measurement, called a synodic month, is measured between in relation to the Sun and equals 29.5 days. Full moons and new moon are measured by the synodic month.
Earth's gravity keeps the Moon in orbit, while the Moon's gravity creates tides on our oceans
On the moon
Like the four inner planets, the Moon is rocky. It's pockmarked with craters formed by asteroid impacts millions of years ago. Because there is no weather, the craters have not eroded.
The Moon has almost no atmosphere, so a layer of dust -- or a footprint -- can sit undisturbed for centuries. And without an atmosphere, heat is not held near the planet, so temperatures vary wildly. Daytime temperatures on the sunny side of the Moon reach 273 degrees F; on the dark side it gets as cold as -243.
In June of 1999, reserchers discovered by accident that a huge cloud of sodium gas trails behind the Moon.
The Lunar Prospector in 1998 provided evidence of ice near the Moon's poles, perhaps as much as 6 billion tons of it.
The Moon travels around the Earth at a little more than half a mile per second; its speed is slowing and the satellite is gradually moving away from Earth.
The fourth planet from the sun has always captivated our imagination, and while scientists haven't proven there's any life, not even the microscopic variety, the dusty red planet still commands our attention (and a lot of space missions).
On the planet
The surface of Mars is more interesting than most planets. Like Mercury, Venus and Earth, Mars is mostly rock and metal. Mountains and craters scar the rugged terrain. The dust, an iron oxide, gives the planet its reddish cast. A thin atmosphere and an elliptical orbit combine to create temperature fluctuations ranging from minus 207 degrees Fahrenheit to a comfortable 80 degrees Fahrenheit on summer days (if you are at the equator). Researchers have recently monitored huge storms swirling on Mars. The storms are very similar to hurricanes on Earth.
Mars has two moons, Phobos and Deimos.
Is there water?
Mars was most likely warm and wet about 3.7 billion years ago. But as the planet cooled, the water froze. Remnants exist as ice caps at the poles (as shown here). A recent image of Mars taken by the Hubble Space Telescope shows evidence of water-bearing minerals in large amounts, and scientists say the deposits may provide clues to the planet's water-rich background.
Is there life on Mars?
It has not yet been proven that there is life on Mars. A NASA announcement in 1996 about microscopic life found in a meteorite has failed to convince skeptics, and the search continues.
The apparent odd motion of Mars as seen from Earth stumped scientists for centuries, finally leading in the early 1600's to the notion that planets orbited the sun in an elliptical pattern. Percival Lowell, an amateur astronomer who studied Mars into the early 1900s, thought he saw canals that must have been dug by inhabitants. Upon closer examination with modern telescopes and planetary probes, they turned out to be optical illusions.
In 1938, Orson Welles broadcast an Americanized version of a 40-year-old British novel by H.G. Wells -- The War of the Worlds. The radio drama was perceived by many as a real newscast about a Martian invasion near Princeton, New Jersey.
The fifth planet from the sun is a huge ball of gas so massive it could hold all the other planets put together. What we can see of the planet are bands of the highest clouds in a thick atmosphere of hydrogen and helium. Traces of other gases produce the bright bands of color.
The Red Spot
Jupiter's most familiar feature is swirling mass of clouds that are higher and cooler than surrounding ones. Called the Great Red Spot, it has been likened to a great hurricane and is caused by tremendous winds that develop above the rapidly spinning planet. Winds blow counterclockwise around this disturbance at about 250 miles per hour. Hurricanes on Earth rarely generate winds over 180 miles an hour.
The Red Spot is twice the size of Earth and has been raging for at least 300 years. It is one of several storms on Jupiter.
At Jupiter's center is a core of rock many times the mass of Earth. But the bulk of the planet is a thick gaseous murk that appears smeared through a telescope because the planet moves so rapidly beneath. Jupiter's rapid rotation causes it to bulge, making the diameter 7 percent greater at the equator than at the poles.
Jupiter has thin, barely perceptible rings and at least 16 satellites. The four largest-- Io, Europa, Ganymede and Callisto -- are called the Galilean moons. They orbit in the same plane and are all visible in a telescope.
JUPITER: RULER OF THE ROMAN GODS, ALSO JOVE
Jupiter was believed by Mesopotamians to be a wandering star placed in the heavens by a god to watch over the night sky. In 1610, Galileo Galilei used a 20x telescope to observe three "stars" around Jupiter. Over several nights he observed these "stars," but each night they were in different positions, leading to his conclusion that they were bodies orbiting the giant planet.
In 1994, astronomers around the world watched as the fragments of comet Shoemaker-Levy 9 struck Jupiter -- an event that had been forecast. This image shows a bright cloud more than 8,600 miles in diameter caused by