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Moon phases

Author:Rafael Losada Liste
This activity belongs to the GeoGebra book Earth and Sun. In this construction you can observe the phases of the Moon. Keep particular attention to the small white semicircle in the left graphic view: it represents the illuminated part of the Moon (in the construction, the sunlight comes from the left). Now imagine you are looking at the Moon from Earth. The illuminated part of the Moon that you see from Earth (the part of the white semicircle that is inside the lunar orbit) is the one shown in the right graphic view. There are 8 phases: 4 main (new Moon, first quarter, full Moon and last quarter) and another 4 intermediate between these states. The small arrow indicates, when turning, the direction of rotation of the Earth. This does not affect the moon phase itself, but it is useful to know, approximately, in what interval of hours we can see each moon phase (imagine that you are looking from Earth as indicated by that arrow). If you want to see it in detail, stop the animation and check the Visible sky box. You can stop the animation by setting the speed to zero or simply by moving the time slider. If you want to move it slowly, stop time slider and press the - and + buttons. To return to animation, just change the speed. You can also move the latitude from where we observe the Moon. This latitude influences how the visible side of the Moon looks. Note that, from the northern hemisphere, the Moon "waxes and wanes to the left", while from the southern hemisphere it does so to the right. Notes:
  • In the previous paragraph, the area between the tropics is not considered, since an observer located in that area varies its position with respect to the Earth's orbital plane (ecliptic) depending on the time of year, which causes the look of the Moon also changes.
  • The representation of the Earth, the Moon and the Sun, as well as their distances, are not on a real scale, they only make up a diagram that helps to understand the phenomenon of the phases.
  • The orbit of the Moon is not in the same plane as the orbit of the Earth, but rather inclined (about 5º) with respect to it. If it were, every time there was a new Moon there would be a solar eclipse, and every time there was a full Moon there would be a lunar eclipse.
  • The number of phases depends on the traditional culture. The natives of the Hawaiian Islands can distinguish 30 phases (one per day).

1. How are the Earth, the Moon and the Sun located when it is a new Moon?

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2. How are the Earth, the Moon, and the Sun located when it is a full Moon?

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3. How are the Earth, the Moon and the Sun located when it is a first quarter or last quarter?

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4. Why do we see the full Moon rise when the Sun sets?

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5. What is the only phase of the Moon in which a solar eclipse can occur (every long time)?

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6. What is the only phase of the Moon in which a lunar eclipse can occur (every long time)?

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7. In what phase can the Moon be perfectly seen in the sky throughout the afternoon? (Look at the little arrow and the visible sky.)

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8. In what phase can the Moon be perfectly seen in the sky throughout the morning? (Look at the little arrow and the visible sky.)

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9. Why is it difficult to see the waxing or waning Moon during the day?

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10. Why are lunar eclipses much more frequent than solar eclipses?

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Author of the construction of GeoGebra: Rafael Losada

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