Goodnight Crescent Moon

Marianne Dyson, November 2019

Despite the art you may find in some popular children’s books, the crescent Moon seen out the window when tucking a child into bed will never be shaped like a letter C (unless you live in the southern hemisphere). The waxing crescent, first quarter, and a full Moon are the only phases visible between sunset and 9 pm (standard time).

The waxing crescent Moon rises about 9 am (in the east), is highest around 3 pm (due south), and sets about 9 pm (in the west). This is the only crescent Moon that may appear outside the window in the early evening hours. Note the large round Sea of Crises near the equator. Observers in the southern hemisphere see this image “upside down,” in a C shape. [NASA 2015 Photo]

The Earth and Moon are spherical, so one side is always sunlit and the other side is in shadow. The side facing the Sun changes as each world rotates. The Moon rotates once for each orbit around Earth, making dayspan and nightspan for any location (except the poles) each about two weeks long.

The length of Earth’s day and night at any location depends on the season and latitude. But at the equator (and elsewhere at the equinoxes), day and night are 12 hours long. The Earth’s rotation carries observers counterclockwise (as seen from above the north pole) which is west to east. Observers at the location crossing the terminator from night to day experience sunrise at around 6 am. They continue rotating east so that the Sun seems to rise up from the east and reach its highest point (south) at noon. As the rotation continues, the Sun slides down to set in the west around 6 pm.

When the Moon is new, it is between the Earth and Sun. As the observers rotate through the day, the Moon moves with the Sun: it rises in the east with the Sun, is high at noon, and sets with the Sun in the west at 6 pm.

Each day the Moon moves east in its orbit. The day after the new Moon, the Sun rises again at 6 am as the terminator is crossed. But the Moon is “ahead” of the Sun, so the observer must rotate farther around to the day side before it appears to rise in the east. The waxing crescent (a backwards C) rises about 50 minutes after sunrise. The Sun again sets at 6 pm, and the Moon sets at about 6:50 pm.

The next day, the Moon has moved east some more and rises another 50 minutes later: at about 7:40 am. It sets at about 7:40 pm.

About a week after new Moon, the crescent has grown to the first quarter, showing the eastern half of the near side. The first quarter Moon rises around 9 am, is overhead at 6 pm, and sets about 9 pm.

The Moon continues moving in its orbit, rising 50 minutes later each day, and shifting the times it rises, is overhead, and sets by 50 minutes. After two weeks, the Moon is full and halfway through its orbit. So instead of rising at sunrise with the Sun, the full Moon rises at sunset, is overhead at midnight, and sets at 6 am.

Three weeks after new Moon, the Moon is in last quarter phase, with only the western side of the near side showing. It rises at midnight, is overhead at 6 am, and sets at noon. As it moves ever eastward, it turns into the waning crescent “C-shaped” Moon that rises around 3 am, is overhead at 9 am, and sets around 3 pm. This is not the Moon seen out the window when putting a child to bed!

The waning crescent Moon is a C shape (viewed from the northern hemisphere of Earth). It rises about 3 am (in the east), is highest around 9 am (due south), and sets about 3 pm (in the west). This is the only crescent Moon that may appear outside the window in the early morning hours. Note the small dark Crater Grimaldi near the equator. Observers in the southern hemisphere see this image “upside down,” in a backwards C shape. [NASA 2015 Photo]

Next time you read a children’s book with a Moon in the sky, check that the phase and direction of the Moon are correct for the time of day (or night) in the story. If not, let the child know, and then take them outside to see the real Moon for themselves!

Share Space for the Holidays

Give a gift that encourages a child to read and possibly sparks a life-long interest! Many studies show that children’s success in school correlates highly with the number of books in the home. For the younger children (kindergarten to grade 4) on your list, I recommend the pop-up book, To the Moon and Back: my Apollo 11 Adventure that I coauthored with Buzz Aldrin to celebrate the 50th anniversary of the first Moon landing this year. For middle school (and gifted upper elementary) students, I recommend Welcome to Mars which won the Best STEM book award and its new companion, Welcome to the Moon which is available in print or eBook formats.

For high school and college students considering a space career and adults interested in biographies of women and insiders’ stories of the space program, I suggest my memoir, A Passion for Space: the Adventures of a Pioneering Female NASA Flight Controller. To order copies of my books, visit my website Book Orders page which has PayPal/credit card info for autographed books and links to Amazon for discounted copies and eBooks. And Thank You!

Teachers, librarians, and event organizers, please consider me for Author Visits in 2020!