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!

Luna 13 a Solid Success

Author Marianne Dyson’s
May 2019 Science Snacks Newsletter

Hello, and a special welcome to new subscribers.

As we celebrate the 50th anniversary of Apollo, I thought I’d devote this Science Snack to some little-known lunar science performed by Soviet scientists in preparation for their own planned human landings on the Moon. The following is mostly an excerpt from my new book, Welcome to the Moon, which will be released June 17 from the Aldrin Family Foundation. (See book ordering information below.)


Buzz Aldrin tested soil hardness with this boot print. The length of the shadows reveal this print is less than an inch deep. (NASA/B. Aldrin)

One of the first questions that scientists had to answer is How Hard is it? “It” being the lunar surface! How strong do the legs of a lunar lander need to be? Will the foot pads sink deep into fluffy powder, break rocks into glasslike shards, or smack into solid stone? To find out, Russian engineers devised an experiment for Luna 13 to test the hardness of the lunar surface.

Like its predecessors, at an altitude of 46 miles, Luna 13 inflated airbags and fired its landing rockets. When it was 16 feet above the surface, the engines shut down as a sensor contacted the ground (a method still employed by Soyuz capsules). The landing capsule, in its airbag cocoon, was ejected and bounced several times before coming to rest in the Ocean of Storms.

After bouncing to a stop, Luna 13’s airbag deflated, and two booms sprang out from the body of the spacecraft. One boom contained a small solid rocket, pointed down. The rocket shot a titanium cone penetrator (with a diameter of 1.4”) into the ground. A pin slid along a groove in the side of the casing to measure how deep it went. The engineers had tested this penetrator on 14 different surfaces on Earth, including dust and concrete, and in a vacuum chamber ahead of time. Depending on the surface material, the penetrator could drill down two inches.

On Christmas Eve of 1966, the engineers got the gift of data from the Moon. The penetrator dove in 1.7 inches. The team concluded that the surface was volcanic rock (basalt) covered by a layer of powder.

Spacecraft cameras revealed rocks scattered on the surface. Of 181 counted, most were pebbles. Only three were larger than four inches and all less than eight inches in diameter. The experiment gave the engineers confidence they could safely land a cosmonaut on this surface.

To learn more about the historical and current science of the Moon in “layman’s” terms (written for gifted middle-school students), order your copy of Welcome to the Moon
via my Book Orders page. (And Thank You!)

Writing about Space

To share what the first journey to the Moon was like, Buzz and I teamed up with pop-up artist Bruce Foster to create To the Moon and Back: my Apollo 11 Adventure. We hope you’ll share this historical American story with the whole family.

I’m pleased to announce that my fact article, In Defense of the Planet, won the AnLab Readers’ poll! It is available FREE on the Analog website.

Speaking about Space

I offer programs appropriate for school-aged children up through senior citizens, as well as science workshops for students and teachers. See my list of programs on the Author Visits tab of my website.

Friday May 31, WriteFest Weekend Festival, Anderson-Clarke Center (6100 S. Main St. Houston, 77005), Rice University. The weekend festival includes panels, presentations, agent pitch sessions, and a book fair. Look for me on panels at 2:45 and 4 pm.

See my website’s contact page for a complete appearance schedule.

Miles and Miles? Judging Distance in Space

Marianne Dyson, April 2019

On Apollo 14, Alan Shepard famously hit the first golf ball on the Moon. Because of the stiff space suit, he had to hit one-handed. After several tries, he sent it off camera and claimed it went “Miles and miles and miles.” But did it really go that far?

Judging distance in space is tricky. Studies have shown that even on Earth, people routinely underestimate horizontal distances by ten percent. On the other hand, heights are usually overestimated by as much as 30 percent, especially when looking down or from a distance. (A pyramid appears steeper from a distance than it does up close.)

NASA AS14-67-9367.
Credit: NASA AS14-67-9367.

Does the Apollo 14 lander seem closer than 650 feet (two football fields) or taller than 10’7”? The near horizon, sharp shadows, and a tendency to underestimate distance and overestimate height of objects makes judging distances  difficult on the Moon. 

A study conducted on space station astronauts shows these effects are exaggerated in space. Astronauts underestimated distances by as much as 35 percent, even for objects at close range. Astronauts with long arms perceived targets within reach that were out of range. They also perceived objects to be taller than on Earth. This effect may be in part because people use the height of their eyes above the ground to provide scale—and there is no floor when floating in space. [Reference: Distance and Size Perception in Astronauts during Long-Duration Spaceflight]

On the surface of the Moon, many of the cues used to judge distance, such as trees and trucks, are missing. The lack of air also makes objects appear sharper and thus closer—adding to the tendency to underestimate distance and size of objects. Finally, the Moon is a smaller world than Earth, so the horizon is much closer. From a height of about six feet, the horizon is about 1.5 miles away (compared to about 2.8 miles on Earth). Combining all these effects means that what first appears to be a small nearby rock is actually a distant boulder.

Astronauts also have difficulty predicting the motions of objects in space. During a space shuttle mission, catching balls moving at constant speeds was difficult. People are used to balls accelerating as they fall on Earth. So astronauts think they are moving faster than actual and reach for the balls too soon (and miss the catch!). [Reference: Does the brain model Newton’s laws?]

So did Shepard’s ball go miles and miles and miles? [Watch video.] The record for a golf drive (Mike Austin, 1974) on Earth is 515 yards/0.3 miles. Some people have speculated that because of the Moon’s low gravity and lack of air, a golf ball hit that hard might sail more than two miles. Considering Shepard was likely underestimating the distance by up to a third, I wouldn’t be surprised if the ball went a mile—but not more than 1.5 miles since it didn’t disappear over the horizon.

On a more serious note, distortions in perceived distance, height, and motions could have grave consequences during space missions. A poor sense of closing speed has been cited as a contributing factor in a collision with a docking port on the Mir space station in 1997. [Reference: Shuttle-Mir’s lessons for the ISS]

More studies on how people judge distances and react in space will help us better understand our ingrained biases when it comes to judging distances in space.  Laser range finders and future AI lunar golf advisers may even help us figure out how much of a handicap to give an astronaut in a stiff space suit!

Writing about Space

I’m thrilled to announce my newest space book! Watch my website Book Orders page for Welcome to the Moon ordering information.

A Summer 2019 Release!

My fact article about a practice drill for what to do if an asteroid threatens Earth, In Defense of the Planet, is a finalist in the AnLab Readers’ poll. It is available FREE on the Analog website until the winners are announced at the Nebula Awards in May.

Speaking about Space

I offer programs appropriate for school-aged children up through senior citizens, as well as science workshops for students and teachers. See my list of programs on the Author Visits tab of my website.

Saturday, May 11, Comicpalooza, GRB convention center, Houston. I’m on two panels in the Literary Track (upstairs rooms).  From 3-4 PM, Worldbuilding Tips and Tricks: How to Create Believable Worlds, and from 6-7 PM, Writing Historical Fantasy—Getting the Details Right!

Thursday, May 23, Bay Area Writers League, Clear Lake Park, Houston, 7 pm. “Beyond Self-Publishing: Becoming a Publisher. What are the financial, legal, personnel, quality, quantity, and time considerations of creating books for companies or individuals?

Friday May 31, WriteFest Weekend Festival, Anderson-Clarke Center (6100 S. Main St. Houston, 77005), Rice University. The weekend festival includes panels, presentations, agent pitch sessions, and a book fair. Look for me on panels and at the book fair. Register early for the best price ($95 to $185 one day only & $180-$375 F-Sun).

See my website’s contact page for a complete appearance schedule.