Download Title

Revolution and Rotation
Most objects in the solar system have a regular and predictable motion.
These motions explain such phenomena as a day, a year, phases of the Moon,
eclipses and tides. The apparent motions of the Sun, Moon, planets and stars
across the sky can be explained by the Earth’s rotation and revolution.
Although often confused, there is a distinct and important difference in the
concepts of revolution and rotation. Earth has what is called an axis. It is an
imaginary line that goes through the center of the Earth from the North Pole to the
South Pole. Earth rotates, or spins, on its axis at an angle of 23 ½ °. Rotation
means to spin on an axis. When you rotate, you do not move from place to place.
You only spin around.
Earth’s rotation on its axis causes the length of one full day to be
approximately 24 hours. It is also the reason we have a cycle of light days and
dark nights. As we spin, different portions of our planet face the sun and it is day.
Other portions face away from the sun, and it is night. Our rotation also causes the
apparent movement of the Moon, stars, and the Sun across the sky from east to
west. These objects are not actually moving that way, it only seems like they do
because we spin on our axis. This is why it seems to us here in NY State that the
sun rises in the East and sets in the West.
Revolution is different from rotation. As stated, rotation is spinning on an
axis, but the object doesn’t move from its location. Revolution is when an object
moves around another object. Earth actually rotates on its axis while it revolves
around the Sun.
Earth’s revolution around the sun causes the length of a year to be 365 ¼
days. Every four years that extra quarter of a day is added up and we get a leap
year with 366 days, the extra day being at the end of February. Our orbit, the path
we travel in around the Sun, is in the shape of an ellipse (oval shaped). Sometimes
our orbit brings us closer to the sun than at other times.
Rotation and revolution, simple ideas on their own, combine on our planet
and give us the complex cycles that we know of as a day, a night and a year.
1.
2.
3.
4.
5.
What is an axis? What is the angle of Earth’s axis?
Define rotation. Explain how it causes day and night.
What effect does our rotation have on the objects we see in the sky?
Define revolution. Around what object do we revolve?
What is an orbit? What shape is Earth’s orbit?