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Aardvark Communications:

A Practical Guide to Astronomy


Our Moon


Defining the Moon

The Earth's only known natural satellite, shining by the sun's reflected light, revolving around the Earth from west to east in about 29.5 days with reference to the sun or about 27.3 days with reference to the stars. It has a diameter of 3475 km (2160 miles) and a mean distance from the Earth of about 384,000 km (238,700 miles).


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The moon is an airless spherical rock about 1/4 the size of the Earth. The moon has had a violent and turbulent history. Most of the craters on its surface were caused by meteoroids crashing into it about 3-4 billion years ago. The darker shades of the moon are called 'maria' (seas). These were caused by underground lava that flowed to the moon's surface.

Gravity on the Moon

The Moon is much smaller than the Earth and so it has a weaker gravitational pull. When the USA sent astronauts to the moon, they had to be weighed down to help them walk. It is gravity that causes ocean tides on the Earth to rise and fall. The gravity of the Sun and Moon pulls the Earth's oceans into bulges. The Earth rotates beneath these bulges causing two high tides per day.

Phases of the Moon

The moon keeps the same side facing the Earth at all times. As the Moon travels around the Earth, we see a varying degree of this side each day. These stages are called phases of the Moon

The Moon's surface is covered with craters, ridges, mountains and valleys. The dark maria are the smoothest parts because of the lava that created them flowed over and covered old craters. The biggest Moon crater that can be seen from Earth is called Bailly. It's about 290 km across.

Where did the Moon come from?

The Moon & Earth are roughly about the same age. Several theories have been discussed:

Little Known Facts

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