WRAL WeatherCenter Blog

Moon Misconceptions

Posted September 10, 2007

Morehead Planetarium and Science Center’s monthly skywatching session is Saturday, Sept. 15. It will run from 8 p.m. until 10 p.m. at Jordan Lake’s Ebenezer Church Recreation Area. Although there is no special astronomical event that night, we should have great views of a crescent moon very low in the western sky (setting shortly after the session starts), along with Jupiter in the southwest.

One of the most frequently asked questions that we get at Morehead Planetarium and Science Center is “Why does the moon change shape?” Both children and adults are curious about the moon and often have misconceptions about it. Our planetarium show, “Earth, Moon and Sun” does a great job of clarifying lunar phenomenon with images and animations. To explain it in writing is more challenging, but I’ll give it a shot.

First, remember two basic characteristics of the moon: a.) It orbits the Earth, making one complete revolution about every 28 days and b.) The moon does not make its own light – it reflects light from the sun.

Here are the most common misconceptions about the Moon that I’ve heard from our visitors:

MISCONCEPTION #1: The moon’s phases are caused by Earth’s shadow.

The earth’s shadow has nothing to do with lunar phases. It does cause a different astronomical phenomenon: the lunar eclipse. When the noon passes through Earth’s shadow and its surface is darkened (partially or totally), we call that a lunar eclipse. Lunar eclipses happen zero to three times per year, depending on the positions of the earth, moon and sun.

The true reason behind the phases is a combination of things: The motion of the moon around Earth, and the lighting of the moon by the sun. One half of the moon is lit at any given moment. As it orbits Earth, we see different amounts of the lit side and the dark side. Once per revolution, the moon is positioned on the opposite side of Earth from the sun, and we see the entire lit side. We call this phase the full moon. When the moon is positioned in between the earth and the sun, the entire opposite side of the moon is lit, and the dark side faces Earth. We call this phase the new moon. In between the new and full phases, we see the moon partially lit. It can range from a thin crescent to nearly full.

MISCONCEPTION #2: The moon is only visible at night.

The moon does not have a preference for the night sky. It is in the daytime sky just as often as the night sky. However, it is much easier to see the moon at night because it is so much brighter than the rest of the sky. When the moon is up in the daytime sky, it doesn’t jump out at you as much, especially when it is near the sun in the sky.

The easiest time to see the moon in the daytime sky is about 7 to 10 days after a new moon, when you’ll see a waxing gibbous moon low in the eastern sky in the few hours before sunset, or about 18 to 21 days after a new moon, when you’ll see a waning gibbous moon low in the western sky in the few hours after sunrise.

MISCONCEPTION #3: There is a side of the moon that is always dark, the “dark side of the moon.”

It is true that at any given moment, one half of the moon is dark. However, it is not always the same half. As the moon rotates (once per each revolution around Earth) the half that is lit shifts so that no area on the moon is dark or light more than another area.

The “dark side of the moon” makes as much sense as the “dark side of the earth” – at any given moment, one half of the rarth is dark, but 12 hours later, the dark side and the light side will have flipped because of  Earth’s rotation. The moon’s rotation is much longer – 28 days – so if you were to visit the moon, you’d have 14 days of light followed by 14 days of darkness.

I hope this clears up some misconceptions about the moon! If you have any questions, please submit them via the "Ask Morehead Planetarium and Science Center” blog.

6 Comments

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  • harinootsak Sep 11, 2007

    There another misconception about tides in what I added about the earth/moon rotation. I was always fascinated of Richard Feynman's treatment of this rotation in his "Lectures on Physics." But after researching tidal forces further, the opposite side tidal bulge is NOT inertial in any way as incorrectly glossed over by Dr. Feynman and many physics textbooks to follow. Rather it is the slight variations of the vector directionals of the gravitational field from the moon, and the complex interaction of land masses and bodies of water that cause the twice daily tides. A good overview of this misconception can be found here: http://www.lhup.edu/~DSIMANEK/scenario/tides.htm

  • jrichuso Sep 11, 2007

    harinootsak, I like #4 - that's another good one, a bit more advanced. Please list more lunar misconceptions if you can think of any!

    Here's #5: We can see the flag (or something left behind by the astronauts) on the Moon with a telescope. That would be cool, but unfortunately the stuff we left behind (the biggest being the descent stage of the Lunar Module) is just too small and too far away for us to see it, even with the best telescopes on Earth.

  • m0nky Sep 11, 2007

    I wish i could find the link, but i read an article not too long ago that some rediculous percentage of adults in the united states think that the sun revolves around the earth.

    i mean...come on...

  • SailbadTheSinner Sep 11, 2007

    Steve,

    We, as a nation, are rapidly falling behind the rest of the world - especially China and India – in technical expertise.

    Our students are much more interested in marketing, law, business administration, etc., subjects that are much easier to learn than engineering and, unfortunately, more profitable, at least in the short run.

    Once long ago I actually heard someone say that they thought that daylight savings time was a bad idea. They thought that the extra hour of daylight would kill the grass ....

    Who knows. Maybe they were right and that’s what is really causing the drought ....

    STS

  • harinootsak Sep 11, 2007

    MISCONCEPTION #4: The moon solely rotates around earth

    The earth and moon rotate around each other, dancing around the center of their combined masses, about 1/3 of the way into our planet. This is the reason there are 2 tidal bulges one on the moonside and one opposite. The moonside bulge is the moon's gravity countering the earth's gravity holding down the water. The opposite side from the moon is caused by the centripetal force of the water as the earth dances in a circle with the moon.

  • Steve Crisp Sep 11, 2007

    It never ceases to amaze me how people can get through high school and not understand those things. When I used to teach observational astronomy, there would be at least one student each semester who would complain that the lab was too late at night and why we could not hold it earlier. Sometimes you gotta wonder about what goes through some skulls.