HMXEarthScience

The Moon revolves around the Earth once every 27.3 days. It also rotates on its axis once every 27.3 days (both of these numbers can be found on the Solar System Data Table on page 15 of the Earth Science Reference Tables. Because these two periods are the same, from Earth, we are only ever able to see one side of the Moon. Also, a result of these motions, along with the motions of the Earth, the Moon appears to rise 50 minutes later each night. This diagram shows how the Moon and Earth move.
 

Lunar Phases

At any given moment, only the side of the Moon facing the Sun is illuminated...the other side is dark. When the Moon revolves around the Earth, we see varying amounts of this illuminated side...this is why we see the phases of the Moon. During the waxing phases, the illuminated portion is getting bigger each night and the lit-up portion of the Moon's face is on the right. During the waning phases, the illuminated portion is getting smaller each night and the lit-up portion of the Moon's face is on the left. View an interactive moon phase diagram. Because the Earth is revolving around the Sun while the Moon is revolving around the Earth, a complete cycle of lunar phases takes approximately 29.5 days.
 

Eclipses

A lunar eclipse occurs when the Earth passes between the Sun and the Moon causing the Moon to be blocked out by the shadow of the Earth. Lunar eclipses only occur during the Full Moon phase.

A solar eclipse occurs when the Moon passes directly between the Sun and Earth causing the Sun to be blocked out by the Moon. Solar eclipses only occur during the New Moon phase.

Eclipses don’t occur every month because the Moon’s orbit is slightly tilted relative to the Earth’s path around the Sun. This causes the Moon to be either above or below the Earth most months.
 

Tides

Tides occur because of the gravitational pull of the Moon and Sun on the Earth's oceans. There are two high and two low tides each day. Tides are cyclic and predictable. The amount of time from one high tide to the next high tide (or from a low tide to the next low tide) is approximately 12 hours and 24 minutes.

Spring Tides occur when the Earth, Sun, and Moon are lined up causing higher than normal tides. Spring tides occur only in the New and Full Moon phases.

Neap Tides occur when the Moon acts on its own causing less severe high tides. Neap tides occur only in the First and Last Quarter phases.