Why moon phases

Saturn has at least 31 satellites, Uranus has 27, and Neptune has 13 — and more are being discovered all the time! Pluto has one moon — Charon — the largest moon with respect to the size of the planet it revolves around. Only Mercury and Venus do not have any known satellites. Satellites are not restricted to planets; tiny Dactyl was discovered orbiting the asteroid Ida in ! LPI Education. Why does our Moon's shape change?

Nearside view of Earth's Moon as seen by the Galileo Spacecraft. Farside view of Earth's Moon as seen. False color view of Saturn's moon Epimetheus. Jupiter's moon Europa. Jupiter's moon Io. Mars' moon Phobos.

Get the solar system in your inbox. The "quarter" used to name these phases refers to the respective fraction of an orbit that the Moon has completed since new Moon. The illuminated part of the Moon gradually transitions between these phases.

To remember the in-between phases you'll need to understand these terms: crescent, gibbous, waxing, and waning. Crescent refers to phases where the Moon is less than half-illuminated, while gibbous means more than half is illuminated. After new Moon, a slice of reflected sunlight becomes visible as a waxing crescent.

The lunar crescent grows until first-quarter Moon. As the sunlit portion of the Moon continues to increase to more than half of the Moon's face, the Moon turns waxing gibbous. Then, after the full Moon the sunlit fraction begins to decrease again though it still takes up more than half the face of the Moon to make a waning gibbous and then a third-quarter Moon.

The slice of sunlight continues to decrease until the moon is a waning crescent and then a new Moon. The whole cycle from new Moon to new Moon takes about The Moon's phases are actually related to orbital motion, and there's a simple and fun observation that shows how they're connected.

All you'll need is a Ping-Pong ball to simulate the Moon—actually, any small, white sphere would work. Then head outside about an hour before sunset, or around the time of a first-quarter Moon.

Find the Moon in the southern part of the sky, then hold the ball up at arm's length right beside it. You'll see that the ball shows exactly the same phase as the Moon. A first quarter moon rises around noon and sets around midnight. A full moon rises around sunset and sets around sunrise.

The Moon will appear full for a couple of days before it moves into…. The Moon rises later and later each night. A last quarter moon, also known as a third quarter moon, rises around midnight and sets around noon. The Moon is nearly back to the point in its orbit where its dayside directly faces the Sun, and all that we see from our perspective is a thin curve. When we think of the way the Moon seems to change over the course of a month, we think of phases.

But frequent Moon observers know that the Moon also appears to twist, nod, and roll slightly during its journey across the sky, allowing us to peek around the Moon's shoulder and catch glimpses of the farside.

This phenomenon is called libration. Because the Moon's orbit is not perfectly circular, its distance from Earth and its speed in orbit both change slightly throughout the month.

When the Moon is farthest from Earth and orbiting at its slowest, its rotation gets a little ahead, and we see a bit more of its western side.

Learn more about lunar eclipses here! This phase occurs between the full and last quarter and describes the Moon when it is more than half lit, but not fully. At the beginning of this stage in the Northern Hemisphere, we see a disk almost fully lit except for a tiny sliver on the right side that is in darkness. In the Southern Hemisphere, the same happens, only the light shrinks from left to right.

This stage is sometimes also called Third Quarter. In the Northern Hemisphere, the left side is illuminated; in the Southern Hemisphere, it is the right side. During a last quarter phase, the Moon is said to be at west quadrature, meaning that it is 90 degrees west of the Sun when viewed from Earth. This phase occurs between the last quarter and new Moon phases.

Eventually, the entire disk will be in darkness, at which point it will be the new Moon phase and another lunar cycle will have begun. In the Southern Hemisphere, the same thing happens, only the lit area would have started on the right side and shrunk from left to right, until a thin crescent remained on the right.

Once the Sun rises, it is not easy to see this slim phase; the best time is before the glare of sunrise. The term " Moon's age " is not a reference to how long the Moon has existed about 4. As mentioned above, the span of time between one new Moon and the next is called a lunar cycle , lunation , lunar month , or synodic month and on average lasts for This translates to 29 days, 12 hours, 44 minutes, and 3 seconds.