Astronomy

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Video

Materials

• 1 spun cotton ball
• 1 pencil
• 1 flashlight

Short explanation

In this demonstration, you are Earth, the spun cotton ball is the Moon, and the flashlight is the Sun. The Moon constantly revolves around Earth, and this gives rise to the phases of the Moon. Half of the Moon is always illuminated by the Sun. This half is called the dayside. The side of the Moon that doesn't receive any sunlight is called the nightside. When you, from Earth, see all of Moon's dayside, it's called the full moon. When you see half of Moon's dayside, it's called the first or last quarter of the Moon. And when you only the Moon's nightside, it's called the new moon.

Long explanation

The Moon makes one orbit around Earth in 29.5 days. During this time, the appearance of the Moon varies as seen from Earth - from new moon, to first quarter moon, to full moon, to last quarter moon, and then back to new moon. A new moon is when the Moon looks completely dark from Earth. A first quarter moon - and a last quarter moon - is when half the Moon looks illuminated from Earth. A full moon is when the whole Moon looks illuminated from Earth. But half the Moon is in reality always illuminated by the Sun. It's only at full moon that we see this whole side, because the Sun is then "behind" Earth.

A more true error in this model is that there is always a solar eclipse during a new moon. That is, the Moon blocks the Sun as seen from Earth. In fact, a solar eclipse would occur during every new moon if the Moon rotated around Earth in the same plane as Earth's orbit around the Sun. But the Moon's orbit is inclined 5 degrees relative to Earth's orbit around the Sun. Therefore, Earth, the Moon and the Sun only end up in line with each other between two and five times a year. Most of these times there is also no total eclipse, as only part of the Moon passes in front of the Sun. In addition, the Moon's orbit is elliptical ("oval"), which means that it's usually too far away from Earth at these times to cover the entire Sun.

When Earth ends up in between the Moon and the Sun, lunar eclipses can occur. Earth - your head in this model - will block the sunlight so that it doesn't reach the Moon. Lunar eclipses will probably occur during every full moon in your model, while they in reality occur less frequently due to the inclination of the lunar orbit. However, they are much more common than solar eclipses - between two and five occur each year. This is because the shadow that Earth casts on the Moon is much larger than the shadow Moon can cast on Earth. Therefore, it's more difficult for the Moon to "escape" it although it has tilted orbit.

One difference between a lunar eclipse and a solar eclipse is that a lunar eclipse can be seen from the entire part of Earth facing the Moon, while a solar eclipse can only be seen from a small part of Earth.

Earth's orbit around the Sun is not included in this model. Earth makes one revolution around the Sun in 1 year. But to study the phases of the moon, and the solar and lunar eclipses, we can ignore the motion of Earth through space and only study the Moon, because the lunar phases are solely caused by the Moon's orbit around Earth.

Experiment

You can turn this model and demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What would happen to the lunar phases if the Moon's orbit were closer to Earth?
• What would happen to the lunar phases if the Moon's orbit were further away from Earth?
• What would happen to how you see the lunar phases if Earth would stop spinning around itself?
• What would happen to how you see the lunar phases if Earth would spin twice as fast around itself?
• What would happen to the lunar phases if there were two suns?
• What would happen to the lunar phases if the Moon was flat?
• What would happen to the lunar phases if the Moon had a very elliptical (oval) orbit around Earth, like that of a comet orbiting around the Sun?

Variation

You can create a more permanent installation, where you have the Moon inside a box and then look at it from different directions through doors in the box. You can find this experiment here: Moon in a box.

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