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.