At first quarter in the Northern Hemisphere, the right side of the Moon is illuminated in the Southern Hemisphere, it’s the left side. In fact, both the first and last quarter phases are sometimes called a Half Moon.

It’s a confusing label, though, because at this time from our perspective, 1/2 of the Moon’s surface is lit. This phase got its name because at this point the Moon has traveled 1/4 of the way through its orbit. Some lunar and lunisolar calendars, such as the Islamic (or Hijri) calendar, define the start of a month as when the Moon first becomes visible, which is usually a day or so after the new Moon, during its waxing crescent stage. (In the Southern Hemisphere, the same thing happens, only on the left side.)

The lit area slowly widens each day, covering more and more of the right side of the Moon’s surface until the first quarter phase, when the Moon’s entire right side is illuminated. At the beginning of this stage, we see a thin, crescent-shape Moon, which, in the Northern Hemisphere, appears on the right side. This phase occurs between the new Moon and first quarter phases. The months of some calendars, such as the Chinese lunisolar calendar, begin at the time of the new (or dark) Moon. These events are only visible from a small portion of Earth and require special eye protection to be viewed safely. Occasionally, if the new Moon’s position lines up correctly between the Sun and Earth, from our viewpoint it will cover part or all of the Sun’s disk, causing a solar eclipse. But if we were to travel to the other side of the Moon, the part that faces the Sun, it would be totally illuminated. From our perspective, the Moon appears totally dark: We can not usually see it because we are facing the Moon’s shadowed side, which does not receive any direct sunlight. At this time, the Sun and Moon are in conjunction, meaning that they are closest together in the sky, on the same side of Earth (Sun→Moon→Earth). This phase is named as such because it starts a new lunar cycle. (Depending on where you live, you may or may not be able to see the exact moment of a phase, in part because the Moon may not have risen yet in your area.) The secondary phases, however, represent a span of time rather than a specific moment. The primary phases occur at a specific moment, no matter where you are on Earth, which is then converted to local time. There are also four secondary phases: Waxing Crescent, Waxing Gibbous, Waning Gibbous, and Waning Crescent. (See “What’s the Moon’s Age?” below for more information.)Īstronomers have broken down this cycle into four primary Moon phases: New Moon, First Quarter, Full Moon, and Last Quarter. The length of the cycle can vary slightly, but on average, it is 29.53059 days. We see the Moon’s disk change from all dark to all light to all dark again: This span of time is called a lunar cycle, lunation, lunar month, or synodic month. (The Moon itself produces no light of its own.) As a result, the amount of sunlight that reflects off the Moon and travels to our eyes changes every day. About the Moon's Phases What Are Moon Phases?Īs the Moon orbits around Earth and Earth orbits around the Sun, the angle between the Sun, Moon, and Earth changes.

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