Earth, Moon, and the Sun
- Bre Bell
How does the Earth Travel in Space?
The Earth moves through space in two ways...
Number one: Rotation. Earth's axis is the imaginary line that passes through Earth's center and the North and South poles. The spinning of Earth on it's axis is called rotation.
Day and night is caused by the rotation of Earth. The sun appears to move westward across the sky as the Earth rotates eastward. It takes about 24 ours to rotate once, each 24 our cycle of day and night is called a day.
Number two: Revolution. Revolution is the movement of one object around another. A complete revolution of Earth around the sun is called a year. Earth follows an orbit (path) as it revolves around the sun. The orbital movement of Earth isn't very circular. It is a slightly elongated circle, or ellipse.
How Earth Moves
What Causes the Cycle of Seasons on Earth?
A calender is a system of organizing time that defines the beginning, length, and divisions of a year. One of the first calenders was was made by ancient Egyptians. Egyptian astronomers counted the number of days between each first appearance of the Sirius star in the morning. Using that they found out that there are about 365 days in a year. The time from one full moon to the next is about 29 1/2 days. A year of 12 of these months adds up to only 364 days. Crazy right!? Well the ancient Egyptian calender had 12 months of 30 days each, with 5 extra days at the end. Earth actually orbits around the sun in about 365 1/4 days. The Romans had adjusted they ancient Egyptian calender by adding one day every four years. The fourth year is known as a leap year. During a leap year February has 29 days instead of 28.
Gravity and Motion
What determines the strength of gravity between two objects?