Shape, Size & Distance

2.1 - Know the shape of the Moon

2.2 - Be able to use information about the mean diameter of the Moon (3500 km)

The Moon's diameter is 3476 km (approximately 3500 km). It's about 25% of the size of the Earth (diameter of 12,756 km). This is considered to be large for a satellite. Some astronomers regard the Earth and Moon as a 'double planet' similar to Pluto and Charon.


While 3476 km diameter at the equator, it is 3472km at the poles. It is not a perfect sphere and so technically an oblate spheroid. However, it is often referred to as being 'egg shaped'. The moon has a larger bulge towards the Earth. Imagine the large part of the egg pointing that way. This is thought to have been caused by tidal forces.


The average distance of the Moon from Earth is 384,000 km. That works out at about thirty Earth diameters away. You may also think of it as ten Earth circumferences away.


The moon takes 27.32 days to orbit once around the Earth. However, since the Earth is orbiting the Sun at the same time, the interval between two new moons is 29.53 days.



Mean distance of the Moon from Earth
384,400 km

Greatest distance of the Moon from Earth (Apogee)
406,740 km

Shortest distance of the Moon from Earth (Perigee)
356,410 km


Why does the Sun appear to be the same size as the Moon (when viewed from Earth), when it is many times larger?


NASA 'Shooting the Moon' with Satellite Laser Ranging