The Motions of the Earth and Moon

The Reason for Seasons: Earth Motions

• Q. Why is there Day and Night? The Earth rotates on its axis once every 24 hours.
  

• Q. Why are there Seasons?

  The Earth's spin axis is tilted with respect to its orbital plane around the Sun.

  
  

This tilt has several ramifications.

• For the Northern hemisphere, in the winter we look to the south to see the Sun. In the summer it goes more nearly overhead.

• Winter daylight hours are fewer and the intensity of sunlight is less.
  Thus it is colder in the Northern Hemisphere winter even though we are actually a little closer to the sun.
  

• Seasons are reversed in the Northern and Southern Hemispheres.

A little more detail:

	The tilt of earth's axis increases the # of summer daylight hours and decreases the # of winter daylight hours. In the image, it is summer in the northern hemisphere and the regions above the equator will remain in the sunlight for a longer time. The north pole is receiving 24 hours of daylight.
	The tilt of earth's axis alters the intensity of sunlight received during different seasons - it is increased during summer and decreased during winter. This occurs because the beam of sunlight, which carries the same amount of energy, is concentrated over a smaller area in the region tilted towards the sun (where it is summer), and is spread over a larger area in the opposite hemisphere (where it is winter).

• Note:

  We see different stars during different seasons as a consequence of the Earth's orbital motion around the sun.
  

It's Just A Phase: Moon Motions

As the moon orbits around the earth, we see different moon phases depending on the positions of the earth, moon, and sun. In the diagram below, you are looking down on the north pole of the earth. You can see that the lit half of the moon always faces the sun, but as the moon orbits around the earth, our viewing angle of the lit half changes. The phase that we see when the moon is in a particlar position is shown in the box beside the moon's orbit.



• Q. What time does the Full Moon rise?
  (a) Midnight, (b) Noon, (c) Sunset, (d) Any old time and this is a trick question
  

  
  

  
  
  

• Q. It takes the moon 27 days to make a trip around the Earth. Then why is it more than 29 days between full moons? It takes 27 days for the moon to orbit the Earth with respect to the stars. However, the Earth orbits around the sun as the moon orbits around the Earth, so the moon must travel through an extra distance to make one orbit with respect to the Sun. Thus it takes 29 days for the moon to reach the same position with respect to the Sun.