Problem Set 1 Solution
Review and Discussion:
6, 7, 10, 12, 17
Why does the Sun rise in the east and set in the west each day? Does the Moon also rise in the
east and set in the west? Why? Do stars do the same? Why?
Although the Sun appears to rise in the east and set in the west, in fact the Sun is essentially stationary
relative to the Earth. The Earth rotates from west to east, giving rise to the apparent motion of the Sun.
The Moon, stars, and all other astronomical objects appear to do the same because we view them from the
surface of the rotating Earth.
How and why does a day measured by the Sun differ from a day measured with respect to the
The true rotation of the Earth is measured with respect to the stars, the sidereal day. But during that
interval of time, the Earth has moved in its orbit around the Sun (by about 1 degree). In order for the Sun
to appear in the same location as it did on the previous day, the Earth must rotate slightly more. The solar
day is therefore slightly longer than the sidereal day. Refer also to Figure 1.10.
Why are there seasons on Earth?
The seasons of the Earth are caused by the tilt of the Earth's equator relative to its orbit around the Sun.
This results in the Sun appearing higher in the sky during spring and summer months and causes higher
rates of heating. In the fall and winter months the Sun appears much lower in the sky, its light falling
more at an angle to the Earth's surface, and heats it less.
What is precession, and what causes it?
Precession is a slow shift in the direction of the tilt of the Earth's axis of rotation. Over a period of 26,000
years the axis moves through a circle, always keeping an angle of about 23.5 degrees. It is caused by the
gravitational pull of the Moon and Sun.
What is parallax? Give an everyday example.
Parallax is the apparent change in position of a foreground object, relative to distant background objects,
due to the change in the position of the observer. Example: look at your finger with one eye and then the
other. Notice how your finger seems to move relative to distant objects in the background.
3, 6, 9, 10, 14
How, and by roughly how much, would the length of the solar day change if Earth’s rotation were
suddenly to reverse direction?
Due to Earth's orbital motion, it must now rotate for an additional 4 minutes for the Sun to come back into
its same position as on the previous day. If the Earth's rotation were reversed, it would have to rotate 4
minutes less in order to put the Sun in the same position. The net effect would be a day 8 minutes shorter
than the current one.