Lunar phase

The lunar phases and librations in 2019 as viewed from the Southern Hemisphere at hourly intervals, with music, titles, and supplemental graphics
A full Moon sets behind San Gorgonio Mountain (in California) on a midsummer's morning.

The lunar phase or phase of the Moon is the shape of the directly sunlit portion of the Moon as viewed from Earth. The lunar phases gradually change over the period of a synodic month (about 29.53 days), as the orbital positions of the Moon around Earth and of Earth around the Sun shift.

The Moon's rotation is tidally locked by Earth's gravity; therefore, most of the same lunar side always faces Earth. This near side is variously sunlit, depending on the position of the Moon in its orbit. Thus, the sunlit portion of this face can vary from 0% (at new moon) to 100% (at full moon). The lunar terminator is the boundary between the illuminated and darkened hemispheres.

Each of the four "intermediate" lunar phases (see below) is around 7.4 days, but this varies slightly due to the elliptical shape of the Moon's orbit. Aside from some craters near the lunar poles, such as Shoemaker, all parts of the Moon see around 14.77 days of daylight, followed by 14.77 days of "night". (The side of the Moon facing away from Earth is sometimes called the "dark side of the Moon", although that is a misnomer.)

Phases of the Moon

In western culture, the four principal phases of the Moon are new moon, first quarter, full moon, and third quarter (also known as last quarter). These are the instances when the Moon's ecliptic longitude and the Sun's ecliptic longitude differ by 0°, 90°, 180°, and 270°, respectively.[a] Each of these phases occur at slightly different times when viewed from different points on Earth. During the intervals between principal phases, the Moon's apparent shape is either crescent or gibbous. These shapes, and the periods when the Moon shows them, are called the intermediate phases and last one-quarter of a synodic month, or 7.38 days, on average. However, their durations vary slightly because the Moon's orbit is rather elliptical, so the satellite's orbital speed is not constant. The descriptor waxing is used for an intermediate phase when the Moon's apparent shape is thickening, from new to full moon, and waning when the shape is thinning.

The eight principal and intermediate phases are given the following names, in sequential order:

Principal and intermediate phases of the Moon
Moon Phase Northern Hemisphere Southern Hemisphere Visibility Mid-phase
standard time
Average
moonrise time
Average
moonset time
Northern Hemisphere Southern Hemisphere Photograph
(view from
Northern Hemisphere)
New Moon Disc completely in Sun's shadow
(lit by earthshine only)
Invisible (too close to Sun) Noon 6 am 6 pm
Moon phase 0.svg
Moon phase 0.svg
Not visible
Waxing crescent Right side, 0.1%–49.9% lit disc Left side, 0.1–49.9% lit disc Late morning to post-dusk 3 pm 9 am 9 pm
Moon phase 1.svg
Moon phase 7.svg
Waxing crescent moon 20131108.jpg
First Quarter Right side, 50% lit disc Left side, 50% lit disc Afternoon and early evening 6 pm Noon Midnight
Moon phase 2.svg
Moon phase 6.svg
Daniel Hershman - march moon (by).jpg
Waxing gibbous Right side, 50.1%–99.9% lit disc Left side, 50.1%–99.9% lit disc Late afternoon and most of night 9 pm 3 pm 3 am
Moon phase 3.svg
Moon phase 5.svg
Lune-Nikon-600-F4 Luc Viatour.jpg
Full Moon 100% illuminated disc Sunset to sunrise (all night) Midnight 6 pm 6 am
Moon phase 4.svg
Moon phase 4.svg
20110319 Supermoon.jpg
Waning gibbous Left side, 99.9%–50.1% lit disc Right side, 99.9%–50.1% lit disc Most of night and early morning 3 am 9 pm 9 am
Moon phase 5.svg
Moon phase 3.svg
2013-01-02 00-00-55-Waning-gibbous-moon.jpg
Last Quarter Left side, 50% lit disc Right side, 50% lit disc Late night and morning 6 am Midnight Noon
Moon phase 6.svg
Moon phase 2.svg
Waning gibbous moon near last quarter - 23 Sept. 2016.png
Waning crescent Left side, 49.9%–0.1% lit disc Right side, 49.9%–0.1% lit disc Pre-dawn to early afternoon 9 am 3 am 3 pm
Moon phase 7.svg
Moon phase 1.svg
2011-11-19-Waning crescent moon.jpg
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 is much farther from Earth than shown here.
A crescent Moon above Earth's horizon is featured in this 2010 photograph by an Expedition 24 crew member.
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.

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).[1]

Waxing and waning

When the Sun and Moon are aligned on the same side of the Earth, the Moon is "new", and the side of the Moon facing Earth is not illuminated by the Sun. As the Moon waxes (the amount of illuminated surface as seen from Earth is increasing), the lunar phases progress through new moon, crescent moon, first-quarter moon, gibbous moon, and full moon. The Moon is then said to wane as it passes through the gibbous moon, third-quarter moon, crescent moon, and back to new moon. The terms old moon and new moon are not interchangeable. 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.[2] Half moon is often used to mean the first- and third-quarter moons, while the term quarter refers to the extent of the Moon's cycle around the Earth, not its shape.

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). If the half-ellipse is convex with respect to the half-circle, then the shape will be gibbous (bulging outwards),[3] whereas if the half-ellipse is concave with respect to the half-circle, then the shape will be a crescent. When a crescent moon occurs, the phenomenon of earthshine may be apparent, where the night side of the Moon dimly reflects indirect sunlight reflected from Earth.[4]

Orientation by latitude

In the Northern Hemisphere, if the left (east) side of the Moon is dark, then the bright part is thickening, and the Moon is described as waxing (shifting toward full moon). If the right (west) side of the Moon is dark, then the bright part is thinning, and the Moon is described as waning (past full and shifting toward new moon). Assuming that the viewer is in the Northern Hemisphere, the right side of the Moon is the part that is always waxing. (That is, if the right side is dark, the Moon is becoming darker; if the right side is lit, the Moon is getting brighter.)

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.

The lunar crescent can open upward or downward, with the "horns" of the crescent pointing up or down, respectively. When the Sun appears above the Moon in the sky, the crescent opens downward; when the Moon is above the Sun, the crescent opens upward. The crescent Moon is most clearly and brightly visible when the Sun is below the horizon, which implies that the Moon must be above the Sun, and the crescent must open upward. This is therefore the orientation in which the crescent Moon is most often seen from the tropics. The waxing and waning crescents look very similar. The waxing crescent appears in the western sky in the evening, and the waning crescent in the eastern sky in the morning.

Earthshine

An overexposed photograph of a crescent Moon reveals earthshine and stars.

When the Moon as seen from Earth is a thin crescent, Earth as viewed from the Moon is almost fully lit by the Sun. Often, the dark side of the Moon is dimly illuminated by indirect sunlight reflected from Earth, but is bright enough to be easily visible from Earth. This phenomenon is called earthshine and sometimes picturesquely described as "the old moon in the new moon's arms" or "the new moon in the old moon's arms".