The moon is somewhat more than one-quarter the diameter of the earth. This makes it the largest substantial satellite in the solar system in comparison to its parent planet. The moon orbits the earth every 27 1/3 days and the phases repeat every 29 ½ days. The fact that the moon has large flat areas called maria and craters was discovered by Galileo when he first turned his telescope to look at the moon in 1609. Meria means "seas" though there is no water in these lunar seas. These "seas" have, instead, been made flat by volcanic material —lava— that flowed from beneath the surface more than 3 billion years ago. When observing the moon with a telescope or binoculars it is best done when the there is a line separating the light and dark (day and night on the moon).
The planet names are derived from Roman and Greek mythology, except for the name Earth which maybe Old English in origin. The five planets easily visible with the unaided eye (Mercury, Venus, Mars, Jupiter, and Saturn) have been observed for all human history and they were called different things by different cultures. The Romans named these planets according to their movements and appearance. For example, Venus, the planet that appears the brightest, was named after the Roman goddess of beauty, while the reddish Mars was named after the god of war. These Roman names were adopted by European languages and culture and became standard in science. Uranus, Neptune and Pluto where discovered much later in time and have been given mythological names by astronomers to reflect the other planet Roman—Greek names. The inner planets, Mercury, Venus, Earth and Mars, are called the terrestrial or earth like planets. They are rocky planets with metal cores which have solid surfaces. The outer planets, Jupiter, Saturn, Uranus and Neptune, are Jovian or Jupiter like planets. They have no solid surfaces, and are made mostly of hydrogen, helium, or methane which is why they are also known as the gas giants.
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