1. Motions of the Night Sky
Stars, Sun, Moon, Planets

2. Motions of the Stars
Objects rise in the east move across the sky, and set in the west.
Motion is known as daily or diurnal motion.

3. Motions of the Stars (2)
Stars appear to rotate around the North Celestial Pole, earth’s axis of rotation projected out into space.

4. The points of sunrise and sunset change with the seasons
During the summer, the sun rises far to the north of east, sets far to the north of west, and spends up to 15 hours above the horizon.

5. Compare these 3 sunrise points for the summer solstice, fall equinox, and winter solstice.
Photos by Rick Pirko

6. This change in the sun’s apparent behavior is due to the 23
This change in the sun’s apparent behavior is due to the 23.5o tilt of the earth’s axis, and is also the cause of our seasonal weather changes. The hyperlinks below illustrates further.

7. Motions of the Moon
You can make 4 observations if you watch the moon move through the sky over several months:
First, the apparent size of the full moon varies from month to month.

8. http://terpsichore. stsci. edu/~summers/viz/perigee_full_moon/index

9. Motions of the Moon
From the different sizes of the full moon, we can conclude that the moon’s orbit is quite elliptical.
In fact it varies by 13.3% of the average distance.
Perigee or closest approach is 356,000 km. Apogee or farthest approach is 407,000 km, with an average of 384,000 km.

10. Motions of the Moon 2nd Observation
The moon doesn’t orbit the earth on the ecliptic, but rather spends half its time above and half its time below the ecliptic.
The moon’s orbit is tilted or inclined
(ecliptic - the plane of the earth’s orbit and the apparent path of the sun against the background of stars)

12. Motions of the Moon 3rd Observation
The moon moves eastward against the background of stars and the moon lags behind the stars, rising 53 minutes later every evening.
The orbital motion results in the changing lunar phases.

13. http://www. sumanasinc. com/webchttp://www. sumanasinc
The hyperlink above illustrates the changing lunar phases as the moon orbits the earth.

14. Motions of the Moon The 4th observation
The same side of the moon always faces the earth. This means that the moon rotates on its axis at exactly the same rate at which it orbits the earth. We call this “locked” rotation synchronous rotation.

15.
Synchronous Rotation of the Moon

16. You’re going the wrong way!
Retrograde Motion
You’re going the wrong way!

17. Motions of the Planets
The planets rise on the eastern horizon and set in the west, due to the earth’s rotation. However, like the moon, the planets usually move eastward against the background of stars.
How fast they move depends on their distance from the earth and their orbital distance from the sun.

18. Motions of the Planets
Once or more each year the planets do something strange. They pause in their normal eastward (or prograde) motion, and begin moving westward (or retrograde) for a few weeks, pause again, then resume their normal eastward prograde motion.

19. History of Retrograde Motion
Ancient Greeks noticed that certain celestial objects changed their locations from time to time
They called them “wanderers”; ‘planet’ comes from that Greek word
Greeks charted stars and planets, noticed that planets seemed to stop and change direction sometimes

20. Retrograde Motion
On Earth, planets appear to move from east to west due to our rotation (prograde motion)
Once in a while, planets appear to stop and move west to east (retrograde motion)

21. Time lapse of Mars

22. Retrograde Motion For planets further from the Sun than Earth
Earth orbit is faster than these planets
Earth catches up to a planet in its orbit
Planet appears to stop and move backwards (West to East)

23. YouTube “Retrograde Motion and the Opposition of Mars”

24. Motions of the Planets
More than any other motion of objects in space, this retrograde motion of the planets confused the ancient peoples and seriously affected the development of astronomy as a science.