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# Period Definition

A period is the time for a cycle to complete once, like Earth's rotation or orbit. For example, Earth's rotation defines day and night and takes place over a 24 hour period.

Grades 3-5 VideoEarth’s Orbit & Rotation
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## Earth’s Orbit & Rotation

### Fun Facts

• Earth rotates on its axis every 24 hours.
• Earth orbits around the sun every 365.25 days.
• Since it takes Earth a period of 365.25 days to orbit the sun, we add an extra day to our calendar every 4 years.

### Why Do We Need To Know About Period

Learning how Earth spins and moves around the sun over certain periods of time helps us know why we have day and night and why we see different stars throughout the year. This is really important for people who need to find their way when traveling. It also affects our daily lives, when we work, and some of our traditions.

Knowing about the period of Earth’s spin also helps us keep time all over the world.  Twenty four time zones help us talk and do business with people in other places. Leap years and using pendulums to show how the Earth spins are ways we make sure our timekeeping is right. This shows how important understanding time and basic science is for technology, teaching, and making calendars.

Why do we have a leap year in our calendar every four years?
Since it takes Earth 365.25 days to orbit the sun, but our normal calendar is 365 days, we add an extra day to our calendar every four years. (4 years x 0.25 day = 1 day added every 4 years.)
How are our 24 time zones related to Earth’s rotation?
Each day the sun appears to rise, move across the sky and set. However, this doesn’t happen at the same time in every location on Earth. To adjust our clocks so that everyone has daytime during the same hours, it is necessary to use time zones. This way, anywhere you are on earth, the sun will be as high in the sky as it can be at noon.
Why can we only see certain planets at certain times?
Earth and all the other planets in our solar system are both rotating and orbiting the sun. Sometimes planets are on the same side of the sun and sometimes on opposite sides. The view of planets from earth changes all the time.
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