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Werewolves, Relax

A full moon will never be seen during the middle of the day. Learn more on this Moment of Science.

Moon with cloud during the day

Photo: LeonidasGR (flickr)

A full moon will never be seen during the day due to its phases and their relationship to the sun.

If you are a werewolf, you may have had the following concern.

Every now and again you see the moon during the daytime. What if the full moon were suddenly to rise? Imagine your embarrassment as you start growing fangs at three in the afternoon.

Well, luckily a full moon will never be seen during the middle of the day. You may see one close to sunrise or sunset, or you may see an almost-full moon, but not a completely full one.

To understand why not, let’s think about what causes the phases of the moon. From our perspective, the illuminated part of the moon seems to wax and wane. In fact, except for eclipses, one half of the moon is always illuminated. It’s just that the bright part moves around as the moon changes position with respect to the sun.

Think of a line with the sun at one end, then the moon in the middle, then the earth. In this configuration, the sun’s light will fall on the side of the moon we can’t see. From our vantage point, the moon seems dark. This is called a new moon.

For us to see the illuminated half completely, it would have to be on the other side of the earth. Put the moon on that side in your mind. Now you can see it shining from earth, but do you notice anything else?

Yep, in order to be looking at that full moon, you now have to be standing on the dark side of the planet. That’s the catch. You can’t see a completely full moon unless it’s also nighttime where you are standing.

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