Download Earth Motions

The Motions of the Earth and Moon
The Reason for Seasons: Earth Motions
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Q. Why is there Day and Night? The Earth rotates on its axis once every
24 hours.
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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 particular 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.