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
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.
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
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
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
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.
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