What are the Effects of Earth's Rotation?
Rotation and apparent motions
The rotation of the Earth causes several illusions:
- The Sun appears to move across the sky each day from East to West
- The Moon appears to move through the night's sky
- The stars appear to spin around Polaris in the night's sky (as shown in the image to the right)
ll of these are examples of apparent motion - meaning they aren't really occurring. The motion that is occuring in reality is the Earth's rotation.
Since all of these motions are caused Earth's rotation, they all appear to move at the same speed as the Earth.
- The sun moves 15 degrees through the sky per hour
- The moon moves 15 degrees through the sky per hour
- The stars spin around Polaris 15 degrees per hour
Rotation and Time
The Earth's rotation is the bases for our whole system of measuring time.
As we learned in the previous lesson, Earth rotates at a rate of 15 degrees per hour. To complete one rotation the Earth has to spin around 360 degrees.
At a rate of 15 degrees per hour it takes 24 HOURS TO COMPLETE 1 ROTATION.
In the past, people would use the Sun's position in the sky to tell time. This is called SOLAR TIME.
- When the Sun first rises, the solar time is approximately 6AM
- When the Sun is directly overhead, the solar time is 12PM noon.
- When the Sun sets, the solar time is approximately 6PM.
The problem is, the Sun rises, and sets at different times in different places. For example, when we see the sunrise in NYC, it is still dark in Los Angeles. When the sun is directly overhead in NYC, it is still the morning in Denver.
To standardize time, so that we could all agree on the time no matter where we were, we developed different TIME ZONES.
To make time zones, we divided the Earth up into 24 different sections, with each one 15 degrees apart. This makes it so that every time zone is a one hour difference.
- As you travel East each time zone decreases by one hour
- As you travel West each time zone increases by one hour.