Seven Moon Myths

Seven Moon Myths

We’ve all seen the moon. Thanks to the Apollo missions, we know that the moon is not made of green cheese; it is made of rocks that are similar to igneous rocks found in Earth’s crust. The biggest difference between Earth rocks and moon rocks is that moon rocks lack water trapped in their crystals. Students often have misconceptions about our moon. I would like to discuss the following seven.

Myth 1. The moon revolves around Earth in one day. It takes the moon about a month to revolve around Earth. Because Earth rotates on its axis, we see the moon rise about once a day.

Myth 2. The far side of the moon is always dark. The far side of the moon receives as much light as the side that faces Earth. When we have a new moon, the far side of the moon is illuminated by the sun. During the first and last quarter phases, half of the side facing Earth and half of the side facing away from Earth are lit by the sun.

In the illustration above, the large moons represent what we would see from Earth’s surface. The small moons show how the moon is illuminated. I cut this figure from Amsco’s Reviewing Earth Science: The Physical Setting by Thomas McGuire.

Myth 3. Only one side of the moon faces Earth because the moon does not rotate on its axis. It is true that only one side of the moon faces Earth. However, the moon does rotate on its axis. Its period of rotation on its axis is equal to its period of revolution around Earth. That is, the moon rotates on its axis one time during each revolution around Earth.

Myth 4. The moon is never visible during the day. During certain phases of the moon it is easily visible during the day you just have to know where to look. The phases you can see during the day are: new crescent, first quarter, new gibbous, old gibbous, last quarter, and old crescent. For you to be able to see the moon during the day, the moon must be up in the sky at the same time as the sun, but not so close to the sun in the sky that you can't see it.

Myth 5. The phases of the moon are caused by Earth’s shadow falling on the moon. As you can see from the diagram above, the phases of the moon are the result of the amount of the moon’s illuminated surfaces that faces Earth. Earth’s shadow falls on the moon during a lunar eclipse.

Myth 6. The phases of the moon are caused by the moon moving into the sun’s shadow. The sun is a sphere that illuminates in all direction, and therefore it does not produce a shadow. See myth #5 and the diagram.

Myth 7. Different countries see different phases of the moon on the same day. I fear that my illustration may lead to this misconception. If I could animate the diagram you would see that Earth would rotate on its axis about 30 times as the moon revolved once around Earth. All parts of Earth see the same phase of the moon on the same day. However, people in different countries see the moon in a slightly different way. In the Southern Hemisphere, a person from the Northern Hemisphere would say that the moon is “upside down.” The sunlit side is the opposite from that seen in the Northern Hemisphere.

Moon phases as seen from the Northern Hemisphere.

Moon phases as seen from the Southern Hemisphere.