What causes the phases of the moon?

Despite we always see the same side of the Moon, it still looks a little different every night. Sometimes we see only a thin crescent, sometimes a half-moon, sometimes a full moon, and other times in-between. Sometimes even the Moon seems to disappear entirely. These “shapes” called lunar phases or phases of the Moon. A lunar phase is the shape of the directly sunlit portion of the Moon as viewed from Earth.

It is a common misconception that the Earth’s shadow causes the phases of the Moon – but that’s totally wrong. Just like the Earth, the moon is only half-lit by the Sun at any time. We see it from different angles, that’s how moon phases occur. The sunlit portion of the lunar side which always faces Earth can vary from 0% (at new moon) to 100% (at full moon). Yes, occasionally, the Earth’s shadow falls on the moon’s face – that’s called a lunar eclipse. This can only happen during a full moon – when the Earth can get in the way between the Sun and Moon.

In the videos published by NASA below, you can clearly see why – and how do phases of the moon occur.

Moon Phases 2018 – Northern Hemisphere – 4K

Moon Phases 2018 – Southern Hemisphere – 4K

These 4K visualizations above show the Moon’s phase and libration at hourly intervals throughout 2018, as viewed from the Northern Hemisphere (1st video) and from the Southern Hemisphere (2nd video). Each frame represents one hour. In addition, this visualization shows the moon’s orbit position, sub-Earth and subsolar points, distance from the Earth at true scale, and labels of craters near the terminator. Credit: NASA’s Goddard Space Flight Center/David Ladd (USRA)

Ernie Wright (USRA): Lead Visualizer
John Keller (NASA/GSFC): Scientist
Noah Petro (NASA/GSFC): Scientist

The images were taken by the Lunar Reconnaissance Orbiter (LRO), which is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit. It 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 (the boundary between the illuminated and darkened hemispheres), 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.

Phases of the Moon

Phases of the Moon
From this birdseye view, it’s somewhat easier to see that the phases of the Moon are an effect of the changing angles of the sun, Moon and Earth. The chart above, created by NASA, shows why the phases of the Moon happens. The center ring shows the Moon as it revolves around the Earth, as seen from above the north pole. Sunlight illuminates half the Earth and half the Moon at all times. But as the Moon orbits around the Earth, at some points in its orbit the sunlit part of the Moon can be seen from the Earth, and at other points, we can only see the parts of the Moon that are in shadow. The outer ring shows what we see on the Earth during each corresponding part of the Moon’s orbit.


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