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Phases of the Moon

by Ron Kurtus (revised 25 February 2016)

Probably the most interesting thing about our Moon is how its appearance varies from night to night. These are called the phases of the Moon.

We can see a portion of the Moon that is illuminated by the Sun and another portion that appears darkened. The bright part is where it is daytime on the Moon and the dark part is where it is nighttime. At different times of the month, the Moon may appear as a crescent, half circle, or full disk.

These phases of the Moon are determined according to the angle that the Sun shines on the Moon. You can check the sky to see today's phase.

Questions you may have include:

This lesson will answer those questions.



Cycle of phases

A phase of the Moon is a classification of how it appears to observers on Earth. This concerns the relationship between the part that is bright and the part that is dark. A detailed list of the phases of the Moon is:

Most people just use an abbreviated list of phases:

Lunar month

The Moon goes through a cycle of these phases during the 29.53 days of what is called the Lunar Month.

The Moon goes through a cycle of phases

The Moon goes through a cycle of phases

New Moon

The cycle of phases start with a New Moon, where the sunlight is completely behind the Moon. The illumination you see is light reflected off the Earth to the Moon.

First Quarter

In the nights that follow, the relative angle of the Sun shining on the Moon changes such that the part you can see slowly increases to the First Quarter. It is really misnamed, since it looks like half the Moon is illuminated.

Full Moon

The relative angle of the Sun shining on the Moon continues to increase until you see a Full Moon. This is where you see the surface facing you illuminated 100%. It means that the Sun is directly behind the Earth and fully illuminating the Moon.

Blue Moon

Since the Lunar Month is less than a month on Earth, every 2.7 years there is a month that has two Full Moons. The second Full Moon is called a "Blue Moon" for some reason or another. It isn't really blue but is responsible for the expression "Once in a Blue Moon."

Cycle completed

The cycle continues with the view of the Moon getting smaller until you see the Waning Crescent sliver of the Moon.

Relationship with Sun and Earth

To explain how the view of the illuminated area of the Moon changes according to the relationship between the Moon, Earth and Sun, see the illustration below.

Views of the Moon from a spot on the Earth at night and during the day

Views of the Moon from a spot on the Earth at night and during the day

You can see that from a given position on the Earth, and observer will see different views of the Moon, depending on the day. The three views from 9:30 am and from 2:00 am cover a span of about 10 days. In other words, looking at the Moon at 2:00 am each night, you will see a different view and phase of the Moon.

But also notice that the Moon may be visible during the day.

Don't notice Moon in day

You mainly see the Moon at night, although you may see it once in a while during the daytime. In reality, the Moon is in view as many times in the day as it is in the night. The problem is that you just don't notice the Moon as well in the day as in the night.

When no Moon is seen at night

There are times when the Moon is not visible at night. What happens is that on the day after the Full Moon, the Moon will rise one half-hour after sunset. The Moon then rises about one hour later each night, resulting in increasing time where there is no Moon visible in the sky. The night is usually quite dark during these periods.

For example, if sunset is at 7:00 pm, the Moon might not rise over the horizon until around 12-midnight, five days after the Full Moon. To a person looking in the sky at 9:00 pm, there is no Moon visible in the sky.

Today's Phase

The graphic below shows the phase of the Moon today. Look in the sky to verify the Moon is in that phase.

Moon

moon phase

Summary

The Moon's appearance varies from night to night. We can see a portion of the Moon that is illuminated by the Sun and another portion that appears darkened. The different views are called the phases of the Moon. At different times of the month, the Moon may appear as a crescent, half circle, or full disk. These phases are determined according to the angle that the Sun shines on the Moon. You can check the sky to see today's phase.


Shoot for the Moon with your goals and plans


Resources and references

Ron Kurtus' Credentials

Websites

Facts About the Moon - From NASA

The Moon - Good details from Nine Planets site

U.S. Navy Sun and Moon times - - Times of sunrise, sunset, moonrise, moonset, transits of the Sun and Moon

Astronomy Resources

Books

From Blue Moons To Black Holes: A Basic Guide To Astronomy, Outer Space, And Space Exploration by Melanie Melton Knocke; Prometheus Books (2005) $19.00

Observing the Moon by Peter T. Wlasuk; Springer (2000) $39.95 - Reference book for anyone seriously interested in the Moon and its geology

Welcome to the Moon: Twelve Lunar Expeditions for Small Telescopes by Robert Bruce Kelsey; Naturegraph Publishers (1997) $11.95 - Well written "how to" for novice astronomers

Top-rated books on the Moon


Questions and comments

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.


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