In antiquity, its phases gave us our week. In more recent years, it has served as a focal point and target for the space programs of many countries. In the not-too-distant future, it will serve as a stepping stone and mission base for our continued manned exploration of our Solar System.
Our nearest celestial neighbor, a natural satellite of Earth, the Moon has fascinated Man throughout the ages. The fifth largest planetary satellite in the solar system, and when full it is the second-brightest object in our sky after the Sun.
As the Moon spins on its axis in the same time it takes to orbit the Earth, it only shows one face to us; this is known as synchronous rotation, though eccentricities in the Moon’s orbit called librations allow a few areas of the far side to occasionally be seen. All in all, we can therefore see up to 59% of the Moon’s surface over time.
Because the Earth is moving around the Sun, the Moon takes 29.53 Earth days to return to the same position relative to the Sun as seen from Earth, completing its cycle of phases. This makes a lunar day the same length: 29.53 Earth days between successive sunrises on the Moon.
In ancient times one of the main tasks of astronomers was to regulate the calendar. The main point of reference was the cycle of the Sun giving us day and night. Then came the progress of the Moon through its cycle of phases, sometimes not being seen, then ‘waxing’ from a thin crescent to a half disc, then through a gibbous phase to a full disc and then ‘waning’ in reverse to the opposite crescent and disappearing again. This gave rise to the concept of a ‘month’, so early calendars were therefore based on the Moon.
Early astronomers also realized the Moon is illuminated by the Sun, is a sphere, and by analogy they realized that the Earth must also be a sphere, something confirmed by the shape of the Earth’s shadow on the Moon during a lunar eclipse.
Here are some facts and figures about the Moon:
Average distance from Earth – 384,400 km (238,900 miles)
Diameter – 3,476 km (2,160 miles)
Angular diameter as seen from Earth – 29.3 to 34.1 arc minutes (about half a degree)
Mass (Earth = 1) 0.012
Gravity at equator (Earth = 1) 0.165
Volume (Earth = 1) 0.02
Rotational period – 27.32 days
Orbital period – 27.32 days
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