Moon Phase and Libration, 2020 South Up
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The animation archived on this page shows the geocentric phase, libration, position angle of the axis, and apparent diameter of the Moon throughout the year 2020, at hourly intervals. Until the end of 2020, the initial Dial-A-Moon image will be the frame from this animation for the current hour.
More in this series:
Moon Phase and Libration Gallery
Lunar Reconnaissance Orbiter (LRO) has been in orbit around the Moon since the summer of 2009. Its laser altimeter (LOLA) and camera (LROC) are recording the rugged, airless lunar terrain in exceptional detail, making it possible to visualize the Moon with unprecedented fidelity. This is especially evident in the long shadows cast near the terminator, or day-night line. The pummeled, craggy landscape thrown into high relief at the terminator would be impossible to recreate in the computer without global terrain maps like those from LRO.
The Moon always keeps the same face to us, but not exactly the same face. Because of the tilt and shape of its orbit, we see the Moon from slightly different angles over the course of a month. When a month is compressed into 24 seconds, as it is in this animation, our changing view of the Moon makes it look like it's wobbling. This wobble is called libration.
The word comes from the Latin for "balance scale" (as does the name of the zodiac constellation Libra) and refers to the way such a scale tips up and down on alternating sides. The sub-Earth point gives the amount of libration in longitude and latitude. The sub-Earth point is also the apparent center of the Moon's disk and the location on the Moon where the Earth is directly overhead.
The Moon is subject to other motions as well. It appears to roll back and forth around the sub-Earth point. The roll angle is given by the position angle of the axis, which is the angle of the Moon's north pole relative to celestial north. The Moon also approaches and recedes from us, appearing to grow and shrink. The two extremes, called perigee (near) and apogee (far), differ by more than 10%.
The most noticed monthly variation in the Moon's appearance is the cycle of phases, caused by the changing angle of the Sun as the Moon orbits the Earth. The cycle begins with the waxing (growing) crescent Moon visible in the west just after sunset. By first quarter, the Moon is high in the sky at sunset and sets around midnight. The full Moon rises at sunset and is high in the sky at midnight. The third quarter Moon is often surprisingly conspicuous in the daylit western sky long after sunrise.
Celestial south is up in these images, corresponding to the view from the southern hemisphere. The descriptions of the print resolution stills also assume a southern hemisphere orientation. (There is also a north-up version of this page.)
The phase and libration of the Moon for 2020, at hourly intervals. Includes supplemental graphics that display the Moon's orbit, subsolar and sub-Earth points, and the Moon's distance from Earth at true scale. Craters near the terminator are labeled, as are Apollo landing sites and maria and other albedo features in sunlight.
The phase and libration of the Moon for 2020, at hourly intervals. Includes music, supplemental graphics that display the Moon's orbit, subsolar and sub-Earth points, and the Moon's distance from Earth at true scale. Craters near the terminator are labeled, as are Apollo landing sites and maria and other albedo features in sunlight.
Music Provided By: Universal Production Music -“Weightless” Composers: Erica Driscoll [BMI], Wally Gagel [ASCAP], Xandy Barry [ASCAP] Publishers: Killer Tracks [BMI], Open Note [ASCAP]
Watch this video on the NASA Goddard YouTube channel.
The First Point of Aries is at the 3 o'clock position in the image. The Sun is in this direction at the March equinox. You can check this by freezing the animation at around the 1:03 mark, or by freezing the full animation with the time stamp near March 20. This direction serves as the zero point for both ecliptic longitude and right ascension.
The south pole of the Earth is tilted 23.5 degrees toward the 12 o'clock position at the top of the image. The tilt of the Earth is important for understanding why the north pole of the Moon seems to swing back and forth. In the full animation, watch both the orbit and the "gyroscope" Moon in the lower left. The widest swings happen when the Moon is at the 3 o'clock and 9 o'clock positions. When the Moon is at the 3 o'clock position, the ground we're standing on is tilted to the left when we look at the Moon. At the 9 o'clock position, it's tilted to the right. The tilt itself doesn't change. We're just turned around, looking in the opposite direction.
In the animation, the blue dot is the sub-Earth point, and the yellow dot is the subsolar point. The lunar latitude and longitude of the sub-Earth point is a measure of the Moon's libration. For example, when the blue dot moves to the left of the meridian (the line at 0 degrees longitude), an extra bit of the Moon's eastern limb is rotating into view, and when it moves above the equator, a bit of the far side beyond the south pole becomes visible.
At any given time, half of the Moon is in sunlight, and the subsolar point is in the center of the lit half. Full Moon occurs when the subsolar point is near the center of the Moon's disk. When the subsolar point is somewhere on the far side of the Moon, observers on Earth see a crescent phase.
The animation follows the imaginary line connecting the Earth and the Moon as it sweeps around the Moon's orbit. From this vantage point, it's easy to see the variation in the Moon's distance. Both the distance and the sizes of the Earth and Moon are to scale in this view. In the HD-resolution frames, the Earth is 50 pixels wide, the Moon is 14 pixels wide, and the distance between them is about 1500 pixels, on average.
Note too that the Earth appears to go through phases just like the Moon does. For someone standing on the surface of the Moon, the Sun and the stars rise and set, but the Earth doesn't move in the sky. It goes through a monthly sequence of phases as the Sun angle changes. The phases are the opposite of the Moon's. During New Moon here, the Earth is full as viewed from the Moon.
Feature labels. Crater labels appear when the center of the crater is within 20 degrees of the terminator (the day-night line). They are on the western edge of the crater during waxing phases (before Full Moon) and to the east during waning phases. Mare, sinus, and lacus features are labeled when in sunlight. Apollo landing site labels are always visible. The frames include an alpha channel.
The Named PhasesThe following is a gallery containing examples of each of the Moon phases that have names. New, full, and quarter phases occur on specific days, while crescent and gibbous phases are the transitions between these points and span multiple days. The quarters are so named because they occur when the Moon is one fourth or three fourths of the way through its cycle of phases. Many people find this confusing, though, since visually they are half moons. It might be helpful to remember that the visible half of the Moon's disk is really only one quarter of its spherical surface.
New Moon. By the modern definition, New Moon occurs when the Moon and Sun are at the same geocentric ecliptic longitude. The part of the Moon facing us is completely in shadow then. Pictured here is the traditional New Moon, the earliest visible waxing crescent, which signals the start of a new month in many lunar and lunisolar calendars.
Please give credit for this item to:
NASA's Scientific Visualization StudioMusic Provided By: Universal Production Music - "Weightless" Composers: Erica Driscoll [BMI], Wally Gagel [ASCAP], Xandy Barry [ASCAP] Publishers: Killer Tracks [BMI], Open Note [ASCAP]
MissionsThis visualization is related to the following missions:
Datasets used in this visualization
LRO DEM (A.K.A. Digital Elevation Map) (Collected with the LOLA sensor)
DE421 (A.K.A. JPL DE421)
Lunar Reconnaissance Orbiter LROC WAC Color Mosaic (A.K.A. Natural Color Hapke Normalized WAC Mosaic) (Collected with the LRO Camera sensor)
Note: While we identify the data sets used in these visualizations, we do not store any further details, nor the data sets themselves on our site.