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, 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. Its gravity on the surface is about one-third of the Earth's and about twice that of the Moon.
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.
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.