Astronomy in questions and answers
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The Moon [Questions 9-14]

9. If the Moon rotates, why do we always see the same side of it?

We can see only one side of the Moon because it rotates in the same length of time that it takes to revolve about the Earth. That combination of motions means it always keeps the same side of itself toward the Earth.

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10. Who first saw the Moon through a telescope?

Galileo Galilei looked at it through a telescope in 1609.

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11. Is the dark part of the Moon the shadow of the Earth?

No, it isn't. It's the Moon's own shadow. It is the part of the Moon that is turned away from the Sun, so that the Sun's light cannot reach it. It is the Moon's night. The shadow of the Earth causes an eclipse of the Moon.

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12. Which way does the Moon move in the sky?

The Moon revolves around the Earth from west to east, in the same direction in which the Earth rotates on its axis and in the same direction in which the Earth revolves about the Sun. The Moon changes its position by 12.2 every day, moving to the east.

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13. How much of sunlight that strikes it does the Moon reflect?

The Moon reflects only 7% of the light of the Sun which reaches its surface. The Earth reflects 39%.

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14. What is the cause of the phases of the Moon?

One half of the Moon is always fully lighted by the Sun, but we are not always able to see all the lighted side. At the time of the astronomical new Moon, we cannot see the Moon at all, because the Moon is then between the Earth and the Sun and its dark side, away from the Sun, is turned toward the Earth. After a day or two the Moon has moved to the east, out of line between the Earth and the Sun. So that we can see the edge of the lighted half of the new Moon. In a little more than a week we can see half of its lighted side. The other half turned toward us is dark. Since it is a quarter of the Moon's surface, we call it the first quarter. After another interval of a little than a week, the Moon has moved on around the Earth until it is opposite the Sun. The Moon rises as the Sun sets. We then see all of the lighted side. This is the full Moon. Then the Moon continues to move around the Earth, and we again see half of its lighted side. When the Moon is like this, we call it the third-quarter Moon. This time we see the other half of the Moon. The next interval of a little more than a week returns the Moon to its original position, almost in line between the Earth and the Sun, and produces another new Moon.

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phases
See also
• The Sun
• The Earth
• Mercury, Venus and Mars
• Jupiter, Saturn, Uranus, Neptune and Pluto
• Meteors and Comets
• Stars
• The Constellations
• Galaxies
• Scientists and Astronomers
 
 
 

 
 
Related Internet Links
• Atlas of the Moon
• The Moon Museum
   
  
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