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

2. Motions of the Stars
Like all objects in the sky, the stars rise in the east with the horizon, move across the sky, and set in the west
This ordinary motion that we see every clear night is known as daily or diurnal motion.

3. Other motions of the sun
Besides the normal rise/set motion of the sun, the points of sunrise and sunset change with the seasons, as does the altitude or elevation of the noon-time sun.
During the summer, the sun rises far to the north of east, sets far to the north of west, has a noon-time altitude of 73.5o, and spends up to 15 hours above the horizon.

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

5. Other motions of the sun (2)
On the equinoxes, the sun rises due east, spends 12 hours above the horizon, and sets due west.
In the winter, the sun rises far to the south of east, spends as few as 9 hours above the horizon, and sets far to the south of west.

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 hyperlink 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 (2)
From the different sizes of the full moon, we can conclude that the moon’s orbit is elliptical.
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 (3)
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.
From this, we can conclude that the moon’s orbit is tilted or inclined

12. Motions of the Moon (4)
The tilt is evidence that the moon was NOT formed at the same time and in the same region of space as the earth was formed, but that another process caused the origin of the moon.

13. Motions of the Moon (5)
The orbital motion results in the changing lunar phases.

14. Motions of the Moon (6)
The 4th observation is that 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. Motions of the Planets
Like all objects, 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.

17. Motions of the Planets (2)
Once or more each year, however, 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.

18. East or Prograde West or Retrograde
Photographs of Mars, several days apart, over about a
6 month period.

19. Motions of the Planets (3)
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.

20. Motions of the Planets (4)
The inferior planets, Mercury & Venus, are never seen far from the sun. Mercury is never farther than 27o and Venus never farther than 47o from the sun.
These planets appear to move back and forth from one side of the sun to the other, alternately appearing at sunrise, then at sunset.

21. Motions of the Planets (5)
Not only are the 2 inferior planets always found near the sun, but they also exhibit phases.
Today, we interpret these motions to indicate that the planets are orbiting the sun. Ancient astronomers had some less straight-forward explanations.

22. http://lpc1. clpccd. cc. ca

23. Motions of the Planets (6)
The superior planets, Mars, Jupiter, and Saturn (Uranus, Neptune, and Pluto aren’t visible to the naked eye) also exhibit retrograde motion, but somewhat differently.
These outer planets form loops or S-curves in the sky.

24.
The hyperlink above will take you to a retrograde motion animation.

25. Motions of the Planets (7)
Today, we realize that the retrograde motion of a superior planet results from the earth “lapping” or passing the planet. The line of sight from the earth to the planet angles backward against the very distant stars, causing the planet to appear to be moving backward.

26. Motion of the Planets (8)
Consider an analogy – 2 race cars moving around two concentric circular tracks at the same speed. The car that is on the inner track will periodically lap the car on the outer track. Both cars are constantly moving forward, but from the point of view of the inner car, the outer car appears to move backward.

27. The car on the inner track laps the outer cars
The car on the inner track laps the outer cars. To the inner driver, the outer cars appear to move backward, while actually moving forward.

28. The Next Step
In the next lessons, we’ll see how the motions of the objects through the sky, especially those of the planets, affected the concept of how the solar system functions, and the development of astronomy as a science.