Moon Phases

Moon Phases The Moon is a sphere and it reflects the Sun’s light. No matter where the Moon is positioned in its path around the Sun, only half the Moon is illuminated at one time- the half turned toward the Sun. The Moon appears to change shape from Earth because… The Moon rotates on its axis meaning the side of the Moon that receives the Sun’s light always changes. Moon orbits Earth at the same time that Earth orbits the Sun, the area of the illuminated Moon that is visible from Earth also changes each day.

Moon Phases The apparent shape of the Moon in our sky depends on how much of the Moon’s illuminated side is visible from Earth based on the Moon’s rotation on its axis and revolution around Earth. Phases are seen, because only part of the illuminated side of the Moon is visible from Earth, and the part of the Moon that is in its own shadow cannot be seen. As Earth orbits the Sun, the Moon orbits the Earth. As a result, the Moon has to travel a little more than one complete orbit around Earth to realign itself with the Sun and Earth to get to a particular phase. The sidereal (orbital) month is the time it takes the Moon to orbit Earth one complete revolution- 27.3 days long. The synodic (lunar) month is the time its takes between successive repetitions of the same lunar phase. The lunar phase cycle repeats on average every 29.5 days.

New Moon and Full Moon When the Moon passes between the Sun and Earth, none of the illuminated side of the Moon is visible and only the non-illuminated side faces Earth. This is called a “new moon”. When the Earth is between the Moon and the Sun, the entire illuminated side is visible from Earth. It is called a “full moon”.

Waxing and Waning During the waxing phases, the illuminated portion of the Moon is continually growing (waxing). Right side of the Moon is getting bigger. During the waning phases, the illuminated portion of the Moon is continually shrinking (waning-winding down). Right side of the Moon is getting smaller.

What else? The moon is visible during the day because it reflects the Sun’s light. Earth’s rotation causes the Moon to rise and set like the Sun. The Moon doesn’t appear at the same time and in the same place in the sky every day. The Moon rises slightly less than one hour later each following day. Each phase of the Moon last a little over half a week (3-4 days). During a 24-hour period, everyone on Earth will see essentially the same phase of the Moon. The Moon moves only a short distance in its orbit relative to Earth as Earth makes one complete rotation on its axis.

First and Third (Last) Quarter First quarter is when the Moon is one quarter (¼) through its orbit around Earth. Third (last) quarter is when the Moon is three quarters (¾) through its orbit around Earth. These moon phases are also called “half moon” because half of the Moon appears to be illuminated from Earth.