1. The Sun-Earth-Moon System
(Power Point 05)
Image Credit: wordpress.com

2. Student Learning Objectives
Construct a model of the Solar System
Associate cycles in the sky with time
Describe why Earth has seasons
Determine the phase of the moon
Compare and contrast all types of eclipses

3. What are the motions of the Earth, Moon, and Sun?
All of the apparent motion in the sky is due to the motions of the Earth.
Rotation (spin)
Revolution (orbit)
Counterclockwise
Image Credit: Ashish Meshram

4. Revolution/Rotation Resonance
The synchronous rotation of the moon results in always seeing the same face of the Moon.
Image Credit: NASA

5. Precession
Precession is caused by differential gravitational tugs from the Sun and Moon on Earth’s equatorial bulge.
In ~13,000 years
Vega
is North Star
In ~26,000 years
Polaris
is North Star again
Image Credit: eearth.com

6. Video Credit: Robert Simmon, NASA GSFC
Video Credit: Robert Simmon, NASA GSFC

7. Practice
Does precession have any effect on the celestial poles or celestial equator?
How many rotations does Earth complete in one revolution?
Do you think the Sun rotates and/or revolves?

8. What are the cycles that determine time keeping?
Many early calendars were based on the cycles of Moon phases.
We currently use the Gregorian calendar (1582).
Leap years must be divisible by 4
Leap years exclude years divisible by 100
The first indications of time keeping are notched bones which may have been used to record moon phases (Africa and Europe 20,500 BC)

9. Video Credit: astronomy.ohio-state.edu
Practice
What is the path called that is being shown in the video?
Which time keeping cycle does it show?
Video Credit: astronomy.ohio-state.edu

10. Synodic vs Sidereal
Synodic motion relates to an Earth rotation/revolution which results in the same view of the Sun or Moon
Sidereal motion relates to the process of returning to the same position with respect to the background stars

11. Slideplayer.com

12. One sidereal year = (1 + 1/26,000) tropical years
Sidereal vs Solar Year
A sidereal year is the time for our Sun to appear to return to a particular place in the sky
Pisces Constellation
A solar year is the time for one complete revolution of Earth around our Sun
One sidereal year = (1 + 1/26,000) tropical years

13. Practice
Why is there a difference between the sidereal year and the solar year? What causes the difference?
Which happens first, sidereal or synodic time block?

14. The month is based on the motion of our Moon.
One synodic month = days (complete cycle moon phases)
One sidereal month = days (one moon orbit)
Image Credit: astronomy.ohio-state.edu

15. A Day The day is based on the rotation of Earth.
A sidereal day = 23 h, 56m, 4.09 s
A solar day = 24 hours
1 → 2
Earth Observer re-points to distant star (sidereal)
Earth rotates 360°
1 → 3
Earth Observer re-points to sun (synodic)
Image Credit: astronomy.ohio-state.edu

16. Practice In general, what does sidereal time measure?
Why do we have leap year?
What is the change in position of the moon (number of degrees) in the sky, in 24 hours?

17. What causes Earth to have Seasons?
The changing distance between the Earth and the Sun is not the cause of the seasons.
Our distance from the Sun only varies by 2-3%
Image Credit: scienceblogs.com

19. Solstices & Equinoxes
The more direct sunlight a hemisphere receives, the more it is heated.
Summer = most direct sunlight
Winter = least direct sunlight
Season
Date
Location of Sun
Spring
Mar 22
Vernal Equinox
Summer
Jun 21
Summer Solstice
Fall
Sep 21
Autumnal Equinox
Winter
Dec 21
Winter Solstice

20. Only on the date of an equinox does the Sun rise directly east and set directly west.
The altitude of the Sun changes with the seasons.
Image Credit: brightblueball.net

21. Practice
Locate the position of the Sun on your Celestial Sphere handout, for each season.
Will the seasons on Earth ever change? Explain.
At what latitude is the Sun directly overhead at noon, on the first day of our “summer”? What is this latitude called?
At what latitude is the Sun directly overhead at noon, on the first day of our “winter”? What is this latitude called?

22. A Full Moon
Mount Victoria Lookout in Wellington, New Zealand by Mark Gee

23. Why do we see different shapes of the Moon?
As the amount of reflected sunlight that we can see changes, we see the phases of the Moon.
New Moon Phase – No illumination
Full Moon Phase – All illumination
1st and 3rd Quarter – Half
Gibbous – More than half
Crescent – Less than half
Waxing –Increasing
Waning – Decreasing
Image Credit: NASA

24. Video by songsofhigherlearning.scom
Video by songsofhigherlearning.scom

25. Practice
There is always half of the Moon’s sphere reflecting sunlight. (True or False)
Is the moon ever visible during the day?
3) If the moon in the image has been increasing in size each night, what is the phase of the moon?
Image Credit: NASA

26. What are the characteristics of solar & lunar eclipses?
An eclipse is a shadow.
Umbra: complete shadow
Penumbra: partial shadow
The Eclipse 2017 Umbra Viewed from Space (NASA)

27. Eclipse Seasons
Eclipses only occur when the Earth, Moon, and Sun all lie in the same plane (the ecliptic) and are in the same location of space at the same time.
Image Credit: Wikipedia.org

28. Practice How often is it possible for an eclipse to occur?
Which type of eclipse can be seen by more people, solar or lunar? Explain.

29. Total Solar Eclipse
The umbra of the Moon's shadow touches Earth’s surface. Full disk of Sun is blocked. (Sun-Moon-Earth)
Image Credit: NASA

30. Annular Solar Eclipse
The umbra does not reach Earth, anti-umbra touches Earth.
Sketch not to scale
Image Credit: NASA

31. Partial Solar Eclipse
The penumbra touches Earth. Part of Sun disk is blocked by Moon.

33. Practice What is the phase of our Moon during a total solar eclipse?
Our Moon is much smaller than our Sun. Explain how it is possible for a solar eclipse to occur.
Image Credit: pinterest

34. Lunar Eclipses
Total Lunar Eclipse: umbra of our Earth's shadow covers our Moon
Partial Lunar eclipse: penumbra of Earth's shadow covers our Moon
Image Credit: socratic.org

35. Refraction The fully eclipsed Moon appears red.

36. Practice What is the phase of the Moon during a total lunar eclipse?
Why is the shadow dark and then red, during a total lunar eclipse?