The revolution of the Moon around the Earth makes the Moon appear as if it is changing shape in the sky. This is caused by the different angles from which we see the bright part of the Moon's surface.
These are called "phases" of the Moon. Of course, the Moon doesn't generate any light itself; it just reflects the light of the Sun. The Moon passes through four major shapes during a cycle that repeats itself every 29.5 days. The phases always follow one another in the same order.
Today's Moon Phase
by the U.S. Naval Observatory
The moon phases were understood as early as 500 BC by the Greeks. The key clue is that the phase of the Moon correlates with its angular distance from the Sun. For instance, the Moon is in its crescent phase when it is near the Sun in the sky but full when it is opposite the Sun. The Greeks realized this implies the Moon is a solid sphere, in orbit about the Earth, half of which is always illuminated by the Sun.
In the figure above you are looking down on the Earth's North Pole. The Earth spins counterclockwise (in 24 hours), and the Moon orbits counterclockwise (in 27 days). The fraction of the Moon's sunlit hemisphere which we can see from Earth determines the lunar phase at any time. We see a "full," "crescent," or dark ("new") Moon depending on the angle between the Sun and Moon as viewed from Earth.
New Moon
This is the first of the moon phases
Waxing Crescent Moon
"Waxing" means growing and refers to
the size of the illuminated part of the moon that is increasing.
Full Moon
The full moon occurs when the Moon lies on the opposite side of Earth from the Sun. The moon as seen from the surface of the earth is fully illuminated by the sun at this time, presenting a "full" round disc to viewers on earth. As always, only half the total surface of the moon is illuminated.
The full moon reaches its highest elevation at midnight
A full moon is the only time when a lunar eclipse is possible; at that time the moon may move through the shadow cast by the earth. However, because of the tilt of the moon's orbit around the earth relative to the earth's orbit around the sun, the moon may pass above or below the shadow, so a lunar eclipse does not occur at every full moon.
Full moons are generally a poor time to conduct astronomical observations, since the bright reflected sunlight from the moon overwhelms the dimmer light from stars.
Waning Gibbous Moon
"Waning" means shrinking.
When the the Moon is said to be waning, we see a little less of the Moon each day until it completely disappears when the Moon is New.
In the northern hemisphere, if the right side of the Moon is dark, the light part is shrinking: the Moon is waning (moving towards a new Moon). If the left side is dark, the Moon is waxing (moving towards a full Moon). The acronym mnemonic "DOC" represents this ("D" is the waxing Moon; "O" the full moon; and "C" the waning moon). In the Southern hemisphere, this is reversed, and the mnemonic is "COD".
Blue Moons
There are also months with two full moons. The second full moon in a month is known as a blue Moon. Because this happens fairly infrequently, it has resulted in the expression "once in a blue moon."
Months without a full moon
There are months in which no full moons occur. For example, there was a full moon at 18:25 universal time on Feb. 28, 1991. This means there was no February full moon in east Asia and the Pacific, where it was already March. Years in which February lacked a full moon are 1809, 1847, 1866, 1885, 1915, 1934, 1961, 1999, 2018, 2037, 2067, 2094 (Meeus 1995, Odenwald)
Moon rise times
The New Moon always rises at sunrise.
The first quarter Moon rises at noon.
The Full Moon rises at sunset.
The last quarter Moon rises at midnight.
Moonrise takes place about 50 minutes later each day than the day before.
The percent of the Moon's surface illuminated is a more refined, quantitative description of the Moon's appearance than is the phase. Considering the Moon as a circular disk, the ratio of the area illuminated by direct sunlight to its total area is the fraction of the Moon's surface illuminated; multiplied by 100, it is the percent illuminated. At New Moon the percent illuminated is 0; at First and Last Quarters it is 50%; and at Full Moon it is 100%. During the crescent phases the percent illuminated is between 0 and 50% and during gibbous phases it is between 50% and 100%.
For practical purposes, phases of the Moon and the percent of the Moon illuminated are independent of the location on the Earth from where the Moon is observed. That is, all the phases occur at the same time regardless of the observer's position.
Because of time zone differences, the full Moons may occur on different dates in your region.
The time is in Greenwich Mean Time (GMT).
If your local time zone is currently on Daylight Saving time, please add one hour to the standard times listed below.
Legend of Eclipse Types:
T - Total Solar Eclipse
A - Annular Solar Eclipse
H - Hybrid (Annular/Total)
P - Partial Solar Eclipse
t - Total Lunar Eclipse (Umbral)
p - Partial Lunar Eclipse (Umbral)
n - Penumbral Lunar Eclipse
