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Phases of the Moon
by Ron Kurtus (updated 18 January 2022)
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:
- What is the cycle of phases of the Moon?
- What is the motion of the Moon relative to Earth?
- What is today's phase?
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:
- New Moon
- Waxing Crescent
- First Quarter
- Waxing Gibbous
- Full Moon
- Waning Gibbous
- Last Quarter
- Waning Crescent
Most people just use an abbreviated list of phases:
- New Moon
- First Quarter
- Full Moon
- Last Quarter
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
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
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.
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
Websites
Facts About the Moon - From NASA
The Moon - Good details from Nine Planets site
The Phases of the Moon - From TimeandDate.com
Moon Phases Visualized - From TimeandDate.com
Why Is The Moon So Bright? - From NightSkyPix.com
Books
(Notice: The School for Champions may earn commissions from book purchases)
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
Students and researchers
The Web address of this page is:
www.school-for-champions.com/astronomy/
moon_phases.htm
Please include it as a link on your website or as a reference in your report, document, or thesis.
Where are you now?
Phases of the Moon