The phases of the Moon are determined by the changing angles of the Sun, Moon, and Earth, not by the Earth’s shadow, which only causes lunar eclipses. Like the Earth, the moon is only half-lit by the Sun at any time. We see it from different angles at different times, that’s how moon phases occur.
Although we always see the same side of the Moon, it still looks a little different every night. Sometimes we see only a thin crescent or a half-moon, sometimes a full moon, and other times in-between. Sometimes even the Moon seems to disappear entirely. These “shapes” are 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 wrong. The Moon is always half-lit by the Sun, similar to Earth. As we observe the Moon from different angles during its orbit, its visible sunlit portion changes, creating the lunar phases. These range from 0% visibility at the new moon to 100% at the full moon. Lunar eclipses, often mistaken for lunar phases, occur separately when Earth’s shadow falls on the Moon during a full moon.
These NASA animations show how and why the moon’s phases occur [Moon Phases 2024]
In the videos published by NASA below, you can see why – and how phases of the moon occur.
These 4K visualizations above show the Moon’s phase and libration at hourly intervals, 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.
The images in the animations above were taken by the Lunar Reconnaissance Orbiter (LRO), which is a NASA robotic spacecraft currently orbiting the Moon in an eccentric polar mapping orbit.
This is especially evident in the long shadows cast near the terminator (the boundary between the illuminated and darkened hemispheres), or the day-night line. The pummeled, craggy landscape thrown into high relief at the terminator would be impossible to recreate on the computer without global terrain maps like those from LRO.
Diagram: How Phases of the Moon occur
Looking at it from above, it’s clearer to understand the Moon’s phases. They happen because of how the Sun, Moon, and Earth move and align with each other. NASA’s chart helps explain this. It shows the Moon going around the Earth, viewed from above the North Pole. The Sun always lights up half of the Earth and half of the Moon. As the Moon moves around the Earth, we see different parts of its sunlit side. Sometimes we see mostly the bright side of the Moon and other times we see mostly the dark side. The chart also shows what the Moon looks like from Earth at each point in its orbit.
Sources
- What Causes the Phases of the Moon? on the NASA website
- What are the Phases of the Moon? on Sky and Telescope
- “Lunar Phase” on Wikipedia
- “Moon Phase and Libration” on the NASA website
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