Mercury, Venus, Mars, Jupiter, and Saturn

Lesson 9: Continuing Our Visit Close To Home

Before the late 1700s, people were aware of only five other planets besides the Earth; Mercury, Venus, Mars, Jupiter, and Saturn. In 1781, Sir William Herschel, a German-born British musician and astronomer, discovered Uranus using a telescope. Citing wobbles in the orbit of Uranus, two astronomers John Couch Adams of Great Britain and Urbain Jean Joseph Leverrier of France, each independently calculated the existence and position of a new planet in 1845 and 1846, respectively.

Mercury, the second smallest planet, is the closest planet to the Sun. Its average distance is approximately 36 million miles. Mercury's diameter is 3,032 miles, and its volume and mass are about one-eighteenth that of Earth. Mercury is approximately as dense as Earth and denser than of any of the other planets.Using Leverrier's calculations, Johann Gottfried Galle of Germany first observed Neptune in 1846. The final planet, Pluto was discovered by a massive telescopic search started in 1905 by American astronomer Percival Lowell. He theorized the existence of a distant planet beyond Neptune because of slight anomalies in the orbit of Uranus. The Lowell Observatory staff, continued the search started by the man it was named for until the search ended successfully in 1930. An American astronomer, Clyde William Tombaugh, found Pluto near the position Lowell had predicted.

This lesson, we'll concentrate on the Inner Solar System, the so-called Terrestrial Planets; Mercury, Venus, Earth, and Mars as well as the asteroid belt. Next lesson, we'll look at the other planets of our solar system.

Mercury's orbit takes it around the Sun approximately every 88 Earth days. One Mercury day, the time it takes to revolve around its axis, is equal to just under 59 Earth days. Mercury can be viewed with binoculars or even the naked eye, but it is always close to the Sun and hard to see in the twilight sky.Its gravity on the surface is about one-third of the Earth's and about twice that of the Moon.

Venus, the sixth largest planet, is the second in distance from the sun. It's average distance from the sun is around 67 million miles. It has a diameter of around 7,500 miles. Conditions on the surface of Venus are fairly stable, but would be very unpleasant for humans. The temperature is about 864° F and the surface pressure is 96 bars (Compare that to 1 bar for Earth). Venus's atmosphere is nearly all carbon dioxide (CO2). It has a cloud base at about 31 miles, made mostly of sulfuric acid.

Besides the sun and the moon, Venus is the brightest object in the sky. It is known as the morning star when it appears in the east at sunrise, and the evening star when it is in the west at sunset. It is easily visible with the unaided eye, and when viewed through a telescope, exhibits phases like the moon.

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.

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

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.