# Motions of the Earth, Moon and Sun

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Motions of the Earth, Moon and Sun
Topic 4: Motions of the Earth, Moon and Sun

What is an apparent motion?
When an object appears to move.

What is the daily motion of the stars?
The daily motion of the stars appear to move across the sky, from east to west. Stars near Polaris (N.S) appear to move around Polaris in a circle.

What is the rate at which celestial objects move through the sky?
All motion occurs at a constant rate of 15 degrees per hour 360 degrees/ 24 hrs

What is the apparent motion of the planets?
The apparent motion is similar to stars, however over time the planets movement compared to stars is not uniform. It appears complex – (retrograde).

What are the apparent motions of the Moon?
The moon follows a daily east to west motion, but appears to rise 50 minutes later each day and shifts eastward.

What are the apparent motions of the Sun?
From sunrise to sunset the apparent path is an arc. It changes both the position and length of the path with the seasons.

How does the path of the Sun vary with the seasons?
In June, our summer, the path is the longest. This gives the N. hemisphere more hours of daylight. In both March & Sept., our spring & Fall, the path is in between therefore our daylight hours = Night time hours In December, our winter, the path is the shortest. This gives the N. hemisphere less hours of daylight. 15 night hrs 9 day hrs

How does the altitude (angle of the sun above the horizon) of the Sun vary with the seasons? (Refer to the same picture) As the season go from winter to summer the sun’s angle increases, getting closer to 90 degrees. Therefore the suns rays are more direct in the summer. Direct rays make a 90 degree angle with Earth’s surface.

What is the geocentric model?
An Earth-Centered Model, in which Earth is stationary and most celestial objects revolve around it.

Why is the geocentric model incorrect?
When observing the planets in the sky, they exhibit a motion called retrograde. This means that the planet appears to move backwards. This occurs because all planets are moving at different orbital speeds. When one planet passes the other, it will appear to be moving backwards (just because it is going slower) The model does not explain this apparent motion of planets. (if we were stationary, it would look like a smooth cycle, not retrograde) Does not explain the behavior of a freely swinging pendulum. Over time, the pendulum will appear to rotate, because Earth rotates.

What is the Heliocentric model?
A sun centered model, in which Earth rotates on its axis and revolves around the sun in orbit.

What evidence supports the heliocentric model?
The model explains: The daily motion of stars (Sun rising and setting) The complex motion of planets (retrograde) The motion of a pendulum