Understanding Moon Phase

Posted in: Weather 101

Understanding Moon Phase

What is Moon Phase, you ask? The phases of the moon are determined by the amount of direct sunlight that shines on the moon’s surface at any given point in time. The moon is always illuminated by sunlight, to some degree. The amount of light reflected off the moon changes as the moon moves through its orbit. This change gives the moon a distinct appearance when observed from Earth, and that appearance changes and also repeats itself over a period of time.

Phases of the Moon

The location of the moon, relative to the sun and earth, determines the different moon phases that can be observed. The first quarter moon and last quarter moon occur when both the moon and the Earth are the same distance away from the sun, illuminating half of the moon. These moon phases are referred to as “quarter” moons because when the moon is in this position, it has either completed one quarter of its orbit, or has one quarter of the orbit left in its path to be completed. A full moon appears when the moon is the farthest away from the sun, exposing the fully illuminated surface of the moon to people on earth. The new moon phase occurs when the moon’s orbit takes it closest to the sun. At this point, the sun is illuminating the “far” side of the moon, leaving the dark or unilluminated side facing the earth.1

The Cycle

A complete moon cycle takes approximately 29-and-a-half days to progress through the eight distinct moon phases. Since this cycle is shorter than the average calendar month, a single moon phase may occur twice in one month. You will be able to see the same phase at the very beginning and very end of the calendar month.

Many to help you track the current phase of the lunar cycle. The following image is one example of the moon phase icons included on AcuRite products:

acurite weather station thermometer moon phase icon

new moon phase

New Moon: During the New Moon phase, the sun is illuminating the "far" side of the moon, so the part of the moon that is facing the Earth is not illuminated or visible (except during a solar eclipse).

waxing crescent moon phase>

Waxing Crescent: Less than one-half of the moon is visible and the illuminated area is increasing.

first quarter moon phase

First Quarter: About one-half of the moon is visible and the illuminated area is increasing.

waxing gibbous moon phase

Waxing Gibbous: More than half, but less than all of the moon is visible and the illuminated area is increasing.

full moon

Full Moon: During a full moon, the moon is completely visible. The moon is “full” of illumination.

waning gibbous moon phase

Waning Gibbous: More than half, but less than all of the moon is visible and the illuminated area is decreasing.

last quarter moon phase

Last Quarter: About one half of the moon is visible and the illuminated area is decreasing.

waxing crescent moon phase

Waning Crescent: In this phase, less than one half of the moon is visible and the illuminated area is decreasing. As the illuminated area of the moon disappears, the lunation will repeat and begin again with the New Moon phase.

Blue Moon

You may have heard the expression “once in a blue moon” before. Although a full moon may appear to have some subtle blue hues, the expression does not describe the moon’s color. The “blue moon” expression originates from observation of the phases of the moon. Native Americans used moon phase and changes in weather to mark the start of a season. Later adopted by European settlers, this method of prediction based on natural observations became extremely important for farming and agriculture.2

The Maine Farmers’ Almanac in 1937 noted that each year contained twelve (12) full moons, but occasionally contained thirteen (13). This means that each season would contain three complete moon cycles (one per month), with some seasons containing four (4). Unique names were assigned to the moons to help keep track of the changing seasons. Since these names are based on the 12-moon year, when four moons appear in a season, the third is referred to as a “blue” moon. This ensures that moons signifying naturally reoccurring events, such as animal migration or weather conditions, remain constant from year to year.3

Amateur astronomer and author James Pruett brought the term “blue moon” into popular use in his 1956 article in Sky & Telescope, “Blue’ Moons in May. Based off of the Maine Farmers’ Almanac, Pruett determined that a year with 13 moons would see one full moon per month, with the remaining month containing two full moons. Pruett reported, “The second in a month, so I interpret, was called Blue Moon.” 4

Due to the difference in time between a moon cycle and a calendar month, a blue moon occurs approximately once every 33 months. Since Pruett’s article, the expression “once in a blue moon” has been widely accepted as a meaning “long period of time.” 3

Related Stories

This year 2018 started off with two exciting moon events in January. Read about the Full Wolf Moon and Supermoon and the Super Blue Blood Moon. While we won't see any full moons in February, we will have another Blue Moon on March 31st!

Further Reading



2Full Moon Names and Their Meanings- Farmers’ Almanac. (n.d.). Retrieved September 16th, 2014 from http://farmersalmanac.com/full-moon-names/

3What is a Blue Moon? (2012, May 3). Retrieved September 16, 2014 from http://www.space.com/15455-blue-moon.html

4Olson, D., Fienberg, R., & Sinnot, R. (2006, July 27). What is a Blue Moon? Retrieved September 16, 2014. From http://www.skyandtelescope.com/observing/what-is-a-blue-moon/

May 2, 2018
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