Earth - third rock from the sun

Lesson 9: Continuing Our Visit Close To Home

We're not used to thinking of Earth as a planet, though we all know that it is. This "third rock from the sun" is also the fifth in size. The diameter of the earth at the equator is about 7926 miles, but that's not the whole story. Because the earth is not a perfect sphere but is slightly flattened at the poles, the diameter of the earth measured around the North Pole and the South Pole is about 7899 miles.

Seventy-one percent of the Earth's surface is covered with water, the only planet where it exists in its liquid form on the surface. This may account for the fact that the Earth is the only planet known to contain life.

While it's fairly easy to see portions of the Earth with the unaided eye, our close proximity prevents us from seeing it in its entirety.

One needs to travel into space for that view, and I hear it's spectacular.

The Earth has a satellite, moon, called Luna. It's 238,000 miles from the Earth and has a diameter of 2155 miles. Because of its size and rocky composition, the moon has also been called a terrestrial planet along with Mercury, Venus, Earth, and Mars. It has no atmosphere, but there is water ice in some deep craters. The moon is the only extra-planetary body that a human has visited.

Besides the sun, the moon is the brightest object in the sky and very easily seen with the naked eye. Using a telescope, you can easily map out many of the Lunar features.

The Red Planet, Mars, the seventh largest, is the fourth planet from the sun at an average distance of 141 million miles.

Mars has a very thin atmosphere made up mostly of a tiny amount of carbon dioxide (95.3%) plus nitrogen (2.7%), argon (1.6%) and traces of oxygen (0.15%) and water (0.03%).Named for the Roman god of war, Mars is about half the diameter of Earth (about 4212 miles), with one tenth Earth's mass. It's gravity is about one third that of Earth. Although Mars's surface is closest to that of Earth of any of the planets, it would still be a very harsh place to live, with temperature extremes between -225 and +60 degrees Fahrenheit, with an average of -67 degrees.

When it is in the night sky, Mars is easily visible with the naked eye. A good telescope will allow the viewer to make out details, such as the famous canals.

Mars has two moons, Phobos and Deimos. Compared to our moon, these satellites are quite small. It is believed they may have been asteroids, captured into Mars orbit. Neither satellite is visible to the unaided eye, but can be viewed with a fairly decent telescope.

Finally, this week, we'll talk a little about asteroids. The majority of asteroids fill a space between Mars and Jupiter known as the Asteroid Belt. The largest of these asteroids is 1 Ceres, at a whopping 578 miles across. The smallest are mere pebbles. The total mass of all the asteroids is less than that of the moon.

Although they are not visible to the naked eye, many asteroids can be viewed with binoculars or telescopes.

Next week, we'll finish our tour of the Solar System and our course. The end of the tunnel is in sight.