Why stars move in circles at night
1 min 50 s. The words are spoken and shown in the picture; the player can also show them as subtitles.
For the Earth and Space Sciences Regents. Topic: Periods of revolution and rotation, with the past questions on it.
What Dr. Mars says (the words of the video)
Why do stars seem to move in circles at night?
At night in New York, Dr. Mars aims his camera north.
He points it at Polaris, the North Star.
Earth rotates once in about twenty-four hours.
So Earth rotates fifteen degrees each hour: three hundred sixty divided by twenty-four.
His camera keeps recording: each star leaves a curved trail.
Earth turns fifteen degrees each hour, so every trail grows by fifteen degrees.
After four hours, each trail is a sixty degree arc.
The stars seem to turn counterclockwise around Polaris.
Earth rotates, so the sky seems to turn.
Polaris barely moves.
Earth's axis points almost straight at it.
Dr. Mars measures Polaris with his protractor.
Polaris is about forty-one degrees above the northern horizon.
This angle is called its altitude.
The altitude of Polaris equals your latitude.
New York's latitude is about forty-one degrees north of the Equator.
Check yourself! Each trail is a forty-five degree arc. How long was the camera open? A, one hour. B, three hours. C, six hours.
Earth rotates fifteen degrees each hour.
Answer: B! Forty-five divided by fifteen equals three hours.
Dr. Mars's star trails grew fifteen degrees each hour.
Dr. Mars. Free practice for the New York Earth and Space Sciences Regents.