A full moon occurs when the sun, Earth, and moon are aligned and Earth is between the sun and moon in its orbit.
Fifth phase of the lunar cycle
A full moon is the fifth phase of the lunar cycle, as it comes after a new moon, a waxing crescent moon, a first quarter moon and a waxing gibbous moon.
The meaning of a full moon
The moon is called “full” because it is the time when the Earth-facing side of the moon is fully illuminated, or 100% illuminated.
Rise and set time of a full moon
A full moon rises around sunset, reaches its highest point in the sky (overhead point) around midnight, and sets around sunrise.
A full moon rises around sunset and sets around sunrise because it is located on the opposite side of the sun relative to Earth.
Second quarter moon
A full moon is a second quarter moon because it is the moment when the moon passes two quarters of the way in its orbit around Earth.
The duration of a full moon
A full moon has no duration as it occurs at a specific moment in time. It is the moment when the sun and moon are on opposite sides of Earth.
Occurs once every 29.531 days
The moon reaches the opposite side of the sun relative to Earth in its orbit once every 29.531 days. This is called a synodic month.
So a full moon occurs once every synodic month or 29.531 days.
Age of a full moon
A full moon is about 14 days old because it occurs about two weeks after the new moon.
A Blue Moon is a full moon
Since a full moon occurs once every 29.531 days, it is possible to fit two full moons into a calendar month.
When two full moons occur in a calendar month, then the second full moon of that month is called a Blue Moon.
For that to happen, the first full moon must be on the 1st or 2nd of a calendar month so that the second full moon is on the 30th or 31st.
Supermoon
When a full moon occurs near perigee, the closest point to Earth in its orbit, it’s called a supermoon.
A supermoon is about 7% larger and 16% brighter than a regular full moon.
No lunar eclipse at every full moon
A lunar eclipse occurs when Earth is in a straight line between the sun and moon, and Earth’s shadow falls on the moon.
A lunar eclipse doesn’t occur at every full moon because the lunar orbit around Earth is tilted 5° with respect to Earth’s orbit around the sun. As a result, Earth’s shadow usually passes above or below the moon.
Read about all eight phases of the moon:
New moon
Waxing crescent moon
First quarter moon
Waxing gibbous moon
Full moon
Waning gibbous moon
Third quarter moon
Waning crescent moon
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