New Moon occurs this Thursday evening May 9, 2013 at 6:31:40 p.m. CDT. The phrase new moon means the lunar phase that occurs when the Moon, in its monthly orbital motion around Earth, lies between Earth and the Sun, and is therefore in conjunction with the Sun as seen from Earth. More precisely, it is the instant when the Moon and the Sun have the same ecliptical longitude.

The Moon takes 27.3 days to orbit Earth, but the lunar phase cycle (from new Moon to new Moon) is 29.5 days. The Moon spends the extra 2.2 days “catching up” because Earth travels about 45 million miles around the Sun during the time the Moon completes one orbit around Earth.

Why do we always see the same side of the Moon? Only one side of the Moon is visible from Earth because the Moon rotates about its axis at the same rate that the Moon orbits the Earth. This is known as synchronous rotation or tidal locking.

The Moon’s diameter is 2,159 miles. This is about 3.7 times smaller than the Earth, making it the Solar System’s fifth largest moon, both by diameter and mass, ranking behind Ganymede, Titan, Callisto, and Io.