Download 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