How much do you know about Moon phases? Do you know which phase is invisible to us? Or what the difference is betweent waxing and waning? Here’s a crash course on understanding Moon phases.
Understanding Moon Phases and the Lunar Cycle
First, the Moon is on a 30-day cycle when it makes its orbit around the Earth. Actually, the entire trip takes 29.53 days, to be exact, and is measured from one “New Moon” to the next.
The Phases of the Moon
Here, we’ll break down each of the 8 phases of the Moon as it revolves around the Earth. Refer to the corresponding numbered phases in the diagram above in this article to follow along.
- New Moon. When we are in the “New” Moon phase, the Moon is not visible from our perspective because it is positioned between the Sun and Earth. The portion of the Moon that is actually getting sunlight is the back side of the Moon, the half that we cannot see. It’s called the New Moon because it’s the beginning of the lunar cycle. Push the “reset button” and let the 30-day orbit begin!
- Waxing Crescent – After the New Moon, the Moon continues its journey around the Earth, becoming visible as it moves on its path toward becoming a Full Moon. The sunlit portion is increasing. A crescent Moon is easy to identify as it looks like a sliver in the sky. Waxing = growing in illumination.
- First Quarter Moon. This one confuses a lot of people. In this case, the term is used because the Moon is in the first quarter of the 30-day cycle, but it appears half full. The First Quarter and Last Quarter Moons (both called “half moons”) happen when the Moon is at a 90 degree angle with respect to the Earth and Sun. So we are seeing exactly the half of the moon that gets hit by the Sun’s light.
- Waxing Gibbous – Still growing as we head toward full. More than half of the Moon is illuminated by the Sun. Gibbous = more than half. Waxing = growing in illumination.
- Full Moon. The full Moon comes about 15 days (14.8 to be exact) after the New Moon, the mid-point of the cycle (half of 30 = 15). The Moon is now in alignment with the Earth and Sun again, just as in the New Moon phase, but this time, the Moon is on the opposite side of Earth, so the entire portion of the Moon that is lit by the Sun is facing us. After this phase, which lasts only an instant, visibility starts decreasing.
- Waning Gibbous – Waning = decreasing in illumination as we head toward the darkness of the next New Moon. Gibbous = more than half.
- Last Quarter Moon (sometimes called the Third Quarter Moon). This is when the Moon completes the third quadrant of its phase cycle, about 22.1 days after the New Moon phase. And, as in #3, it looks like a half Moon to us again, except this time, it’s heading toward the New Moon phase (disappearing) instead of growing toward the full Moon phase.
- Waning Crescent – The Moon is a little sliver of a crescent, just as in #2, but the illuminated part is decreasing. Waning = decreasing in illumination. Now, the illuminated crescent is facing the opposite direction as when it was a waxing crescent (see #2).
Two terms to memorize:
Waxing = Growing in illumination
Waning = Shrinking in illumination
How Can You Tell if The Moon Is Waxing Or Waning?
If you look in the sky and see a crescent or gibbous Moon, here’s how to tell if it is in the waxing or waning phase by the direction it’s curving.
So there you have it. We’re happy to answer any questions to help you understand this fascinating and complex changes in Earth’s satellite. Just leave them in the comments below.
Want to see the whole process in animation using real images of the Moon? Watch this amazing video courtesy of NASA: