What are Moon Phases?

Next time you’re outside checking out the sky and the Moon happens to be out, notice what shape it is. Does it look round and full? Or, more like a banana or even a lopsided ball? Throughout each month, the Moon appears to change shape while it appears in the sky at different times. These changes are something you can observe as they happen, and they may even surprise you. For example, many people think the Moon is only up at night. But, as you will find out, it can also be up during the day.

It’s Just a Phase It’s Going Through
What you are witnessing are the phases of the Moon. A lunar phase is simply the shape of the sunlit part of the Moon as seen from Earth. That shape changes for the following reasons:

the Moon orbits Earth;
both Earth and the Moon orbit the Sun;
the Moon’s orbit is the same length as the length of time it spins on its axis (about 28 Earth days), which means that we see the same part of the lunar surface all month;
the Sun illuminates both Earth and the Moon.

When you put these all together, it means that the changing positions of the Moon and Sun relative to Earth show us different phases of the Moon (that is, different sunlit parts of the lunar surface). This happens each month in the same cycle of phases.

Get to Know Your Lunar Phases

There are eight phases of the Moon that you can track each month.

New Moon: during New Moon, the side of the Moon facing us is not illuminated by the Sun. At this time, the Moon is not up at night, but it is up during the day. We just can’t see it.

Waxing Crescent: as the Moon waxes (grows) into its crescent phase, it begins to show up low in the sky right after sunset. Look for a silvery-looking crescent. The side facing the sunset direction will be lit up.

First Quarter: seven days after New Moon, the Moon is in first quarter. Only half of it is visible 1/2 of the moon is visible for the first half of the evening, and then it sets.

Waxing Gibbous: after First Quarter, the Moon appears to grow into a gibbous shape. You can see most of it, except for a shrinking sliver over the next seven nights. Look for the moon at this time during the afternoon, too.

Full Moon: During Full Moon the Sun lights up the entire surface of the Moon that faces Earth. It rises just as the Sun sets and disappears beneath the western horizon when the Sun rises the next morning. This is the brightest phase of the Moon and it washes out the nearby part of the sky, making it difficult to see stars and faint objects such as nebulae.

Waning Gibbous: after the glorious appearance of Full Moon, the lunar shape starts to wane — meaning “get smaller”. It’s visible later at night and into the early morning, and we see a steadily shrinking shape of the lunar surface that’s being lit up. When you do spot the Moon, the side that is lit up is facing toward the Sun, in this case, the sunrise direction. During this phase, look for the Moon during the day — it should be in the sky in the morning.

Last Quarter: at Last Quarter we see exactly half the sunlit surface of the Moon and it can be in the early morning and daytime sky.

Waning Crescent: the last phase of the moon before returning to New Moon is called Waning Crescent, and it is exactly what it says: a steadily-shrinking crescent phase. We can see only a small sliver from Earth. It’s visible in the early morning and by the end of the 28-day lunar cycle, it has vanished almost entirely. That brings us back to New Moon to start the new cycle.

Make Your Own Lunar Phases

You can demonstrate this to yourself by setting up a light in the center of the room. Take a white ball in your hand and stand a few feet away from the light. Next, turn yourself in a circle, as if you are the Moon spinning on its axis. Watch how the ball is illuminated as you turn.

Observing the Moon throughout a month is a great school project, as well as something you can do simply on your own (or with family and friends). Check it out this month!

Source: About.com

Carolyn Collins Petersen, Space/Astronomy Expert