Lunar phase

In common usage, the four major phases are the new moon, the first quarter, the full moon and the last quarter; the four minor phases are waxing crescent, waxing gibbous, waning gibbous, and waning crescent.

A lunar month is the time between successive recurrences of the same phase: due to the eccentricity of the Moon's orbit, this duration is not perfectly constant but averages about 29.5 days.

On average, the intermediate phases last one-quarter of a synodic month, or 7.38 days.

Due to lunar motion relative to the meridian and the ecliptic, in Earth's northern hemisphere: Non-Western cultures may use a different number of lunar phases; for example, traditional Hawaiian culture has a total of 30 phases (one per day).

Lunar libration causes part of the back side of the Moon to be visible to a terrestrial observer some of the time.

The "old moon" is a waning sliver (which eventually becomes undetectable to the naked eye) until the moment it aligns with the Sun and begins to wax, at which point it becomes new again.

When an illuminated hemisphere is viewed from a certain angle, the portion of the illuminated area that is visible will have a two-dimensional shape as defined by the intersection of an ellipse and circle (in which the ellipse's major axis coincides with the circle's diameter).

Assuming that the viewer is in the Northern Hemisphere, the right side of the Moon is the part that is always waxing.

In the Southern Hemisphere, the Moon is observed from a perspective inverted, or rotated 180°, to that of the Northern and to all of the images in this article, so that the opposite sides appear to wax or wane.

Closer to the Equator, the lunar terminator will appear horizontal during the morning and evening.

Since the above descriptions of the lunar phases only apply at middle or high latitudes, observers moving towards the tropics from northern or southern latitudes will see the Moon rotated anti-clockwise or clockwise with respect to the images in this article.

Archaeologists have reconstructed methods of timekeeping that go back to prehistoric times, at least as old as the Neolithic.

The natural units for timekeeping used by most historical societies are the day, the solar year and the lunation.

The ancient Roman calendar was broadly a lunisolar one; on the decree of Julius Caesar in the first century BCE, Rome changed to a solar calendar of twelve months, each of a fixed number of days except in a leap year.

[7] The approximate age of the Moon, and hence the approximate phase, can be calculated for any date by calculating the number of days since a known new moon (such as 1 January 1900 or 11 August 1999) and reducing this modulo 29.53059 days (the mean length of a synodic month).

However, this calculation assumes a perfectly circular orbit and makes no allowance for the time of day at which the new moon occurred and therefore may be incorrect by several hours.

The lunar phases and librations in 2025 as viewed from the Northern Hemisphere at hourly intervals, with titles and supplemental graphics
The lunar phases and librations in 2025 as viewed from the Southern Hemisphere at hourly intervals, with titles and supplemental graphics
A full moon sets behind San Gorgonio Mountain in California on a midsummer's morning.
The phases of the Moon as viewed looking southward from the Northern Hemisphere . Each phase would be rotated 180° if seen looking northward from the Southern Hemisphere . The upper part of the diagram is not to scale, as the Moon, the Earth, and the Moon's orbit are all much smaller relative to the Earth's orbit than shown here.
Animation showing progression of the Moon's phases.
This video provides an illustration of how the Moon passes through its phases – a product of its orbit, which allows different parts of its surface to be illuminated by the Sun over the course of a month. The camera is locked to the Moon as Earth rapidly rotates in the foreground.
Diagram of the Moon's phases: The Earth is at the center of the diagram and the Moon is shown orbiting.
The observed orientation of the Moon at different phases from different latitudes on Earth (the different orientation displayed between the phases at each latitude show merely the extremes of orientation due to libration )
An overexposed photograph of a crescent Moon reveals earthshine and stars.
A crescent Moon over Kingman, Arizona
As the Earth revolves around the Sun, approximate axial parallelism of the Moon's orbital plane ( tilted five degrees to the Earth's orbital plane ) results in the revolution of the lunar nodes relative to the Earth. This causes an eclipse season approximately every six months, in which a solar eclipse can occur at the new moon phase and a lunar eclipse can occur at the full moon phase.
The lunar phase depends on the Moon's position in orbit around the Earth and the Earth's position in orbit around the Sun. This animation ( not to scale ) looks down on Earth from the north pole of the ecliptic.