As we watch the Moon from our vantage point on Earth, how much of its face we see illuminated by sunlight depends on the angle the Sun makes with the Moon. Here is a simple experiment to show you what we mean: stand about 6 feet in front of a bright electric light in a completely dark room or outdoors at night and hold in your hand a small round object such as a tennis ball or an orange. Your head can then represent Earth, the light represents the Sun, and the ball the Moon. You will see phases just like those of the Moon on the ball.
Another good way to get acquainted with the phases and motions of the Moon is to follow our satellite in the sky for a month or two, recording its shape, its direction from the Sun, and when it rises and sets. The trick to this figure is that you must imagine yourself standing on Earth, facing the Moon in each of its phases.
Note that in every position on Figure 1, the Moon is half illuminated and half dark as a ball in sunlight should be.
The difference at each position has to do with what part of the Moon faces Earth. Figure 1: Phases of the Moon.
The appearance of the Moon changes over the course of a complete monthly cycle. The pictures of the Moon on the white circle show the perspective from space, with the Sun off to the right in a fixed position. The outer images show how the Moon appears to you in the sky from each point in the orbit. Imagine yourself standing on Earth, facing the Moon at each stage. Note that the distance of the Moon from Earth is not to scale in this diagram: the Moon is roughly 30 Earth-diameters away from us.
The Moon is said to be new when it is in the same general direction in the sky as the Sun position A. Here, its illuminated bright side is turned away from us and its dark side is turned toward us. In this phase the Moon is invisible to us; its dark, rocky surface does not give off any light of its own. See Subscription Options. Go Paperless with Digital. Alan P. Boss of the Carnegie Institution of Washington provides an answer to this question: "The moon keeps the same face pointing towards the Earth because its rate of spin is tidally locked so that it is synchronized with its rate of revolution the time needed to complete one orbit.
Get smart. Many other large moons in the solar system are tidally locked with their partner. Of the larger moons, only Saturn's moon Hyperion , which tumbles chaotically and interacts with other moons, is not tidally synchronized. The lunar rotation determined whether the infamous Man in the Moon, a face-like pattern of dark maria on the Earth-facing side, wound up pointing toward our planet.
Gravity created an Earth-side bulge in the moon, slowing down its rotation in the past to create the synchronous rotation and keeping the longer lunar axis toward our world. Recent research suggested that the side of the moon facing Earth was determined by how quickly the lunar rotation slowed. Because the moon lost speed slowly, there was about a two-to-one chance that the Man in the Moon would wind up facing Earth rather than keeping a space-bound view. Instead, the real coincidence is that the moon's slowdown was just enough to load the coin.
The situation is not limited to large planets. The dwarf planet Pluto is tidally locked to its moon Charon , which is almost as large as the former planet. Earth and other planets do not escape completely unscathed. Just as the Earth exerts friction on the spin of the moon, the moon also exerts friction on the rotation of the Earth.
As such, the length of day increases a few milliseconds every century. Earth rotates every day! This means that observers in many different parts of the world have their turn looking at the Moon throughout the day, just like we all see the same Sun over the course of 24 hours. This is an optical illusion. Prove it for yourself here! The Moon does not make its own light. All moonlight is reflected sunlight. The bright part of the Moon is experiencing daytime.
The Moon is a natural satellite , or a space object that orbits around something else. In general, a moon is a natural satellite of a planet, and a planet is a special kind of natural satellite that orbits a star and also meets other conditions.
Learn more: What makes a planet a planet? The leading theory is that a Mars-sized object collided with Earth billions of years ago, and debris from this collision eventually formed the Moon. Here is an animation that shows what this might have looked like:. Animation showing the Moon's early history.
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