Solar system

MERCURIUS: ROMAN WINGED MESSENGER OF THE GODS

Historical notes

Mercury has been known since ancient times. Its elusiveness generated the name Hermes, given by the Greeks, later translated to Mercurius by the Romans.

Venus.

The second planet from the sun bakes under twice as much solar radiation as Earth and reaches temperatures of 895 degrees Fahrenheit (480 degrees Celsius). Pressure from the dense atmosphere of sulfuric acid gas is about 95 times greater than Earth's and would crush a human.

The thick cloud cover around Venus rotates much faster than the planet itself -- once every four days. After the moon, Venus is the brightest object in the sky.The surface of Venus is mostly a rocky desert (this computer-generated view shows lava flows around Sif Mons). Like Mercury, Earth and Mars, Venus is composed of mostly rock and metal.

VENUS: ROMAN GODDESS OF LOVE AND BEAUTY

Historical notes

The Greeks believed Venus was two separate objects -- one in the morning sky and another in the evening. Because it is often brighter than any other object in the sky -- except for the sun and moon -- Venus has generated many UFO reports.

Final facts

While all of the planets orbit in an ellipse, Venus' orbit is the closest to a perfect circle. It is the only planet in the solar system whose day (241 Earth days) is longer than its year (225 Earth days).

Earth

The third planet from the sun is, in scientific terms, quite similar to the first two. In fact, the four planets of the inner solar system (Mercury, Venus, Earth and Mars) all share rock and metal as their primary ingredients. Each of these so-called terrestrial planets has a solid surface, unlike the gaseous planets of the outer solar system.

Perhaps Earth's most distinguishing factor, at least from our point of view, is the presence of water, which contributed to the formation of life some 3,000 million years ago. Most of us ought also to be fond of Earth's unique atmosphere, rich in life-sustaining nitrogen and oxygen.

Final fact

The Earth's surface is rotates about its axis at 1,532 feet per second -- slightly over 1,000 miles per hour -- at the equator, and the planet zips around the sun at more than 18 miles per second.

The Moon

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 to orbit 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