1. Our Moon

2. Goals of this Lecture Understand the Lunar Phases
Introduce tides and tidal forces
Understand Lunar Eclipses
Understand Solar Eclipses

3. Assigned Reading and Announcements
Chapter 3, all of it
Homework #1 will be posted today
Due in class on Thursday 18

4. Lunar Phases

5. Earth & Moon Like planets, the moon does not emit its own right.
The Moon is ~1/4 of the Earth in diameter and ~1 percent in mass.
The rotation of the Moon and its revolution around the Earth are synchronized and take 27 days,7 hours, and 43 minutes.
The length of the Lunar orbit around Earth (Sidereal Period) is days.
The length of the Lunar Phase (Synodic Period) is days.
We only see one side of the moon.

6. The Phases of the Moon (1)
The Moon orbits Earth in a sidereal period of days.
Earth
27.32 days
Moon
Fixed direction in space

7. The Phases of the Moon (2)
Fixed direction in space
29.53 days
Moon
Earth
Earth orbits around Sun => Direction toward Sun changes!
The Moon’s synodic period (to reach the same position relative to the sun, i.e., the same lunar phase) is days (~ 1 month).

8. One rotation
in 24 hours
One orbit in days

9. Lunar Phases
sunlight
As seen from Amherst

10. Survey Question Lunar Phases
sunlight
As seen from Amherst

11. A third quarter moon is rising. What time is it? 1) sunset 2) sunrise
Survey Question
A third quarter moon is rising. What time is it?
1) sunset
2) sunrise
3) midnight
4) noon

12. Earth Rising on the Moon
Apollo 11

13. Survey Question
In this picture of the Earth rising, what is the predominant direction of the sun in relation to the observer?
1) above the observer
2) below the observer
3) behind the observer
4) behind the Earth

14. How long does it take for the moon to rotate once?
One day
About 27 days
Half a year
One year

15. How long does it take for the moon to rotate once?
One day
About 27 days
Half a year
One year

16. Lunar and Solar Eclipses
Lunar, from “Luna”
Solar, from “Sol”

17. Lunar Eclipse
Whose shadow is this?
Why is the shadow curved?

18. Reminder: angular (apparent) diameter
Angular diam / 206,265” = linear diam./distance
NOTE: linear diam. and distance MUST be in the same units
It so happens that both the Moon and the Sun have angular diameter of about 0.5 degree, the exact value at a given time depending on the exact distance.

19. Shadows

22. A Total Lunar Eclipse (1)
Motion of the moon against the background of stars
Motion of the moon against the horizon

23. A Total Lunar Eclipse (2)
A total lunar eclipse can last up to 1 hour and 40 min.
During a total eclipse, the moon has a faint, red glow, reflecting sun light scattered in Earth’s atmosphere.

25. Partial lunar eclipse

26. During a lunar eclipse, what would you see from the moon?

32. Geometry of Solar Eclipses
Umbra (shadow): sun is completely blocked out.
Penumbra (almost shadow): sun is partially blocked out.

33. Earth and Moon’s Orbits Are Slightly Elliptical
Apogee = position furthest away from Earth
Earth
Perihelion = position closest to the sun
Moon
Perigee = position closest to Earth
Sun
Aphelion = position furthest away from the sun
(Eccentricities greatly exaggerated!)

34. Annular Solar Eclipses
The angular sizes of the moon and the sun vary, depending on their distance from Earth.
When Earth is near perihelion and the moon is near apogee, we see an annular solar eclipse.
Perigee
Apogee
Perihelion
Aphelion

35. Annular eclipse

37. Credit: Mir Space Station Crew
Looking Back on an Eclipsed Earth; 1999 August 11
Credit: Mir Space Station Crew

38. Total Eclipse of June 21st 2001
The umbra is never more than 270 km in diameter and sweeps across Earth's surface at 1600 km/hr (1000 mph)!

39. Survey Question
The phase of the moon at a solar eclipse is?
a) new
b) 1st quarter
c) full
d) it depends on the year

40. How come there isn't an eclipse every full moon and new moon?
The main reason is that the moon’s orbit is tilted from the earth’s orbit.
A total eclipse from a given point on the surface of the Earth is not a common occurrence. For example, it will be two decades before the next total solar eclipse visible in North America occurs.

41. Draw a diagram showing the related objects (Earth, the moon, or Sun) during
a) a partial solar eclipse,
b) an annular eclipse,
c) a total lunar eclipse,
d) a partial lunar eclipse.

42. Effects of Moon on Earth: Tides
To sun

43. Spring Tides
Occur at every new and full moon

44. Neap tides
Occur at every first- and third-quarter moon

45. Earth's rotation slows down by 0.0023 s/100 years.
Friction drags the tidal bulges eastward
out of the direct earth-moon line
Earth's rotation slows down by s/100 years.
Only 900 million years ago, Earth' day was 18 hrs long.
The moon's orbit is growing larger by about 4 cm/yr.

46. We see only one side of the moon, because the tidal friction has slowed the rotation of the moon so that its rotation and revolution are synchronized

47. The Tidally-Locked Orbit of the Moon
The Earth exerts tidal forces on the moon’s rocky interior.
 It is rotating with the same period around its axis as it is orbiting Earth (tidally locked).
 We always see the same side of the moon facing Earth.

48. Solar day and Sidereal day
The apparent solar day = the time from high noon to high noon.
The average length of the apparent solar day = 24 hours
Sidereal day = the time from a star to the same position.
Because the sun moves in respect to stars, the sidereal day is different from the solar day.

49. The Length of the Solar Day
Earth revolves about 360o/365 ~ 1o around Sun
Earth rotates at a rate of 360o/(24x60)=0.25o/minute
So need an extra 1o/0.25o =4 minutes to complete a solar day