Curiosity Corner: The Phases of the Moon


The Phases of the Moon                                                                                                                                                                   

Curiosity Corner


Dr. Jerry D. Wilson59271dff495ed.image

Emeritus Professor of Physics

Lander University


A couple of questions – one technical, and the other more well-rounded.


QUESTION: The Moon goes through phases – new, first quarter, full and third (or last) quarter. Yet, when we see a first- or third-quarter moon, the face of the Moon we see is one-half illuminated. Is there something wrong – shouldn’t we have half-moons? (Submitted by a lunar-observant column reader.)


REPLY: Well, the thing to keep in mind is that about half of the Moon’s surface is always illuminated – the half of the spherical surface that is toward the Sun. On Earth, we see phases (or different portions) of the Moon illuminated because of the relative positions of the Sun, Earth and Moon. For example, we see first- and third-quarters when the Sun is 90o east and 90o west of the Moon, respectively. On Earth then we see half of the Moon’s face illuminated. (Think of someone shining a flashlight on a basketball and you are 90o from the person. Depending on whether they are shining the light from left or right, you’ll see only half of the ball – left or right – illuminated.)


Got it? OK, the phase count starts with the new moon, when the Sun and the Moon are overhead at 12 noon. Astronomers refer to phases in reference to how far around the Moon is in its orbit. One-quarter of the way around, the Moon is in first-quarter phase, but we see it half-illuminated, as described above. Halfway around, it is full moon (12 midnight; the Moon and Sun are on opposite sides of the Earth). Third-quarter moon (6 a.m.), the Moon is three-quarters of the way around in its orbit, but again, only half of the Moon is illuminated for us poor Earthlings.


QUESTION: I bought a hat the other day, and I wear a size 7-3/8. Does the hat size number mean anything?

REPLY: I checked into it, and, oddly enough, the size seems to be based on the roundness of your head (or hat). Most American manufacturers use the length or circumference (c) of the band inside the hat and divide by pi (π = 3.14…). This gives the diameter (d) of the hat if you were round-headed, d = c/π.

For example, if the hatband had a length of 23 inches, then 23/π = 7-3/8 (to the nearest 1/8). The head size number gives us an indication of fit. If you have a fitting problem, you can always try a baseball cap with the adjustable plastic strap in the back. 

C.P.S. (Curious Postscript): The best thing about the future is that it comes only one day at a time. —Abraham Lincoln

Curious about something? Send your questions to Dr. Jerry D. Wilson, College of Science and Mathematics, Lander University, Greenwood, SC 29649, or email Selected questions will appear in the Curiosity Corner. For Curiosity Corner background, go to

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