1. Arny, 3rd Edition, Chapter 6
The Moon
Arny, 3rd Edition, Chapter 6

2. Introduction Introducing the Moon Earth’s nearest neighbor is space
Once the frontier of direct human exploration
Born in a cataclysmic event into an original molten state, the Moon is now a dead world – no plate tectonic or volcanic activity and no air
Suffered early impact barrage
Plays major role in eclipses and tides
The Moon

3. Description of the Moon
General Features
Moon is ¼ the Earth’s diameter
A place of “magnificent desolation” – shapes of gray without color
Surface Features
Surface divided into two major regions
Highlands – Bright rugged areas composed mainly of anorthosite (a rock rich in calcium and aluminum silicates) and pitted with craters
Maria – Large, smooth, dark areas generally surrounded by highlands and composed primarily of basalt (a congealed lava rich in iron, magnesium, and titanium) which is more dense than anorthosite
Figure 6.1, Figure 6.2, Figure 6.3
The Moon

4. Description of the Moon
Surface Features (continued)
Other surface features
Craters - circular features with a raised rim and range in size from less than a centimeter to a few hundred kilometers – some of the larger crater have mountain peaks at their center
Rays – Long, light streaks of pulverized rock radiating away from many craters and best seen during full Moon
Rilles – Lunar canyons carved either by ancient lava flows or crustal cracking
Figure 6.4, Figure 6.5
The Moon

5. Description of the Moon
Origin of Lunar Surface Features
Nearly all lunar features (craters, maria, rays) are the result of impacts by solid bodies early in the Moon’s history
A circular crater forms when a high-velocity projectile disintegrates upon impact in a cloud of vaporized rock and fragments which blast a hole in the surface
The highlands are the result of the very intense bombardment by solar system bodies soon after the Moon formed and created a solid surface
A mare forms when early in the Moon’s history, a few large bodies (over 100 kms) strike the Moon, blasting a huge crater, pushing up mountain chains, and cracking the Moon’s crust to the point that still-molten interior material floods the newly-formed lunar depression and eventually congeals
The Moon

6. Structure of the Moon Introduction Crust and Interior
The Moon lacks the folded mountain ranges and variety of volcanic peaks seen on Earth
Lack of activity due to Moon cooling off much faster than Earth
Moon’s higher surface-to-volume ratio (relative to Earth) allows heat to escape from it faster
Being much less massive than the Earth, the Moon also has a smaller source of radioactive material to supply heat
Crust and Interior
Interior (including crust) studied by seismic detectors set up on Moon by astronauts – essentially found to be inactive and has simpler structure than Earth’s
The Moon

7. Structure of the Moon Crust and Interior (continued)
Features of Moon’s surface layer
Surface layer is shattered rock chunks and powder (from repeated impacts) forming a regolith tens of meters thick
Regolith is basaltic in maria and anorthostic in highlands
Regolith may extend to several hundred meters in some places
Below regolith is Moon’s crust
Average thickness of 100 km, although crust is thinner on side that faces Earth – reason for asymmetry is not clear, but may be related to the difference in the Earth’s gravitational force across the Moon
Very few maria exist on side of Moon away from Earth
Crust is composed of silicate rocks rich in aluminum and poor in iron
Figure 6.9
The Moon

8. Structure of the Moon Crust and Interior (continued)
Beneath the crust is the mantle
Relatively thick extending 1000 km down
Probably rich in olivine
Appears too cold and rigid to be stirred by the Moon’s feeble heat
The Moon’s core
The Moon’s low average density (3.3 g/cm3) tells us interior contains little iron
Some molten material may be below mantle, but core is smaller and contains less iron and nickel than Earth’s
The relatively cold Moon interior, low iron/nickel content, and slow rotation imply no lunar magnetic field – found to be the case by the Apollo astronauts
The Moon

9. Structure of the Moon The Absence of a Lunar Atmosphere
Moon’s surface is never hidden by lunar clouds or haze, nor does reflected spectrum show any signs of gas and hence no winds
Lack of an atmosphere means extreme changes in lunar surface temperature from night to day
No atmosphere for two reasons
Lack of volcanic activity to supply source of gas
Moon’s gravitational force not strong enough to retain gases even if there was a source
Lack of atmosphere and plate tectonics implies that the Moon has been relatively unchanged for billions of years and will continue to be so into the foreseeable future
The Moon

10. Orbit and Motions of the Moon
Introduction
The Moon’s orbit around the Earth is elliptical with an average distance of 380,000 km and a period of 27.3 days relative to the stars
Determining the Moon’s distance can be done with high precision by bouncing a radar pulse or laser beam off the Moon
The Moon’s Rotation
The Moon keeps the same face toward the Earth as it orbits
The fact that the Moon rotates at the same rate as it orbits the Earth is called Synchronous Rotation
Figure 6.10, Figure 6.11
Animation: The rotation of the Moon
The Moon

11. Orbit and Motions of the Moon
Oddities of the Moon’s Orbit
The Moon’s orbit is tilted about 5° with respect to the ecliptic plane
It is also tilted with respect to the Earth’s equator – very unlike most of the moons in the solar system which lie almost exactly in their respective central planet’s equatorial plane
The Moon is also very large relative to its central planet – again unlike most of the other moons in the solar system
These oddities indicate that the Moon formed differently from the other solar system moons
Figure 6.12
The Moon

12. Origin and History of the Moon
Before Apollo missions, three hypotheses of the Moon’s origin:
Moon originally a small planet orbiting the Sun and was subsequently captured by Earth’s gravity during a close approach (capture theory)
Earth and Moon were twins, forming side by side from a common cloud of gas and dust (twin formation theory)
The Moon spun out of a very fast rotating Earth in the early day of the Solar System (fission theory)
The Moon

13. Origin and History of the Moon
Each of these hypotheses gave different predictions about Moon’s composition:
In capture theory, the Moon and Earth would be very different in composition, while twin theory would require they have the same composition
In fission theory, the Moon’s composition should be close to the Earth’s crust
Moon rock samples proved surprising
For some elements, the composition was the same, but for others, it was very different
None of the three hypotheses could explain these observations
The Moon

14. Origin and History of the Moon
The new Moon formation hypothesis:
Moon formed from debris blasted out of the Earth by the impact of a Mars-sized body
Age of lunar rocks and lack of impact site on Earth suggests collision occurred at least 4.5 billion years ago as the Earth was forming
This “large impact” idea explains:
The impact would vaporize low-melting-point materials (e.g., water) and disperse them explaining their lack in the Moon
Only surface rock blasted out of Earth leaving Earth’s core intact and little iron in the Moon
Easily explains composition difference with Earth
Figure 6.13, Figure 6.14
Animation: The birth of the Moon
The Moon

15. Origin and History of the Moon
This “large impact” idea explains (continued):
The splashed-out rocks that would make the Moon would more naturally lie near the ecliptic than the Earth’s equatorial plane
Earth’s tilted rotation axis is explained
As Moon’s surface solidified, stray fragments from original collision created craters that blanket highlands
A few of the larger fragments created the large basins for the maria to form
By the time the Maria filled with molten material and solidified, little material was left for further lunar bombardment – thus the smooth nature of the maria
Figure 6.13, Figure 6.14
Animation: The birth of the Moon
The Moon

16. Eclipses Introduction Rarity of Eclipses
An eclipse occurs when one astronomical body casts its shadow on another
For observers on Earth, two types of eclipse:
Lunar eclipse – Earth’s shadow falls on Moon
Solar eclipse – Moon’s shadow falls on Earth
Some past and upcoming solar and lunar eclipses
Rarity of Eclipses
Because of the Moon’s tilt relative to the ecliptic, eclipses will not necessarily occur at new and full Moon – the shadows will fall either above or below their target
Figure 6.15, Figure 6.16
The Moon

17. Eclipses Rarity of Eclipses (continued)
Since the Moon’s orbit tilts nearly in the same direction through the year, twice a year the Moon’s orbit will pass through the Sun giving the possibility of an eclipse – these times are called eclipse seasons
When a solar occurs at new Moon, conditions are right for a lunar eclipse to occur at the full Moon either before or after the solar eclipse
Since the Moon’s orbit actually precesses once every 18.6 years, the time between eclipse seasons is less than 6 months
Lunar eclipses can be seen from anywhere on Earth as long as the Moon is above the horizon, while an observer must be in the path of the Moon’s small shadow to see a solar eclipse
Figure 6.17
Animation: Eclipses and the Moon’s orbital inclination
The Moon

18. Eclipses Appearance of Eclipses Lunar eclipse Solar Eclipse
In a total lunar eclipse, the Earth’s shadow takes about an hour to cover the Moon
At totality, the Moon generally appears a deep ruddy color
The color of the eclipsed Moon is caused by Earth’s atmosphere scattering out most of the blue in sunlight and bending the remaining reddish light at the Moon
Solar Eclipse
Hardly noticeable at first, at totality, a solar eclipse will give the appearance of nightfall
Solar corona is also evident at totality
Figure 6.18, Figure 6.19
The Moon

19. Tides Introduction Cause of Tides
The regular change in height of the ocean surface is called the tides
Tides are mainly caused by the Moon
Cause of Tides
The Moon exerts a gravitational force on the Earth that is stronger on the side closest to the Moon and weakest on the far side
This difference in force from one side of an object to the other is called a differential gravitational force
This differential force draws water in the ocean into a tidal bulge on the sides facing and opposite the Moon
Earth’s rotation leads to 2 high/low tides per day
Figure 6.20, Figure 6.21, Figure 6.22
Animation: Tidal Forces
The Moon

20. Earth Tides
Most internet explanations of the Earth Tides are in fact wrong. They usually explain that since one side of the Earth is closer to the moon than the other far side, we get a difference in forces that pulls the water off the Earth on the near side, and pulls the Earth from the water on the far side (the toffee effect). Thus providing two tides per day. Cool, but WRONG!
If that was the case, why don’t we get tides in lakes and ponds or even in your bathtub?

21. Earth Tides
Now, that model is correct in the sense that there is a stronger force on the Earth’s side closer to the Moon, that the far side of the Earth. The problem is that the difference between these two forces is along the Earth-Moon line is:
So, you can’t pull something up with a force that is about 1 ten millionth of is weight.

22. Earth Tides Then what gives?
We have to look at the tidal accelerations that are NOT on the Earth – Moon line.
At A, it feels a pull in the direction of the Moon. But the whole Earth feels a pull towards the Moon. Adding these vectors we get a Net Force downward (Radially inward).

23. Earth Tides
So, if we map out the radial accelerations at all points around the Earth then we obtain the following animation:

24. Earth Tides
Now, we must keep in mind that the Earth’s own gravity is 10,000,000 stronger that these forces. But, there is a lot of water in the oceans. These tiny force vectors add up over the whole ocean given us the two Tidal Bulges

25. EartH Tides
So rather that the Oceans being pulled toward the Moon, they are in effect getting squeezed from the top and bottom and have only one direction to go. Outwards.
This is the same effect that happens if you squeeze a pimple. It bulges outward.

26. Earth Tides
So why doesn’t the lakes get tides? Simply even though it does experience the same squeezing effect. It just isn’t big enough to bulge out. Calculation show that very large lakes such as in the great lakes we are only talking about a bulge of a couple of centimetres. Way too small to measure as compared to the normal sloshing of water caused by boats on the lakes.

27. Tides Solar Tides Tidal Braking
The Sun creates tides as well, although not as large in variation
When the Sun and Moon create identically oriented tidal bulges (new and full Moon), the abnormally large “spring” tides occur
With the Moon at first or third quarter, the so-called neap tides occur with tides not as extreme as normal tides
Tidal Braking
Tides create forces that slow the Earth’s rotation and move the Moon farther away - tidal braking
Tidal braking caused the Moon’s synchronous rotation
Figure 6.23, Figure 6.24
The Moon

28. Moon Lore
Folklore filled with stories concerning the powers of the Moon over humans
Claims that the Moon triggers social behavior – hence the word “lunatic”
Claims the full Moon responsible for accidents, murders, etc.
No scientific backing for these claims
Some “Moonisms” have a touch of truth
“Once in a blue Moon”, meaning a rare event, may be related to an unusual atmospheric effect in which the Moon appears blue
“Harvest Moon”, the full Moon nearest in time to the autumn equinox, rises in the east at sunset giving farmers additional light for tending to crops
The Moon

29. The Moon. (Courtesy Stephen E. Strom.)
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30. Photograph showing the different appearance of the lunar highlands and maria. The highlands are heavily cratered and rough. The maria are smooth and have few craters. (© UCO/ Lick Obs image.)
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31. (A) Overlapping craters in the Moon's highlands
(A) Overlapping craters in the Moon's highlands. (B) Isolated craters in the smooth Mare. (©UC Regents; UCO/Lick Obs image.)
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32. The long, narrow, white streaks radiating away from the crater at the bottom are lunar rays. (©UC Regents; UCO/ Lick Obs image.)
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33. Photograph of a lunar rille. (Courtesy NASA.)
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34. An artist's impression of the Moon's interior
An artist's impression of the Moon's interior. Notice the thinner near-side crust and the displacement (exaggerated for clarity) of the core toward the Earth.
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35. Finding the distance from the Earth to Moon by triangulation and radar and laser ranging.
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36. The Moon rotates once each time it orbits the Earth, as can be seen from the changing position of the exaggerated lunar mountain. Notice that at (A) the lunar peak is to the right, while at (B) it is to the left. Thus, from the Earth, we always see the same side of the Moon even though it turns on its axis.
To help see that the Moon rotates even though it keeps the same face toward the Earth, put a coin on the figure of the Moon and move it around the Earth so that the same edge of the coin always faces the Earth.
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37. The Moon's orbit is tipped 5° with respect to the Earth's
The Moon's orbit is tipped 5° with respect to the Earth's. The angle is exaggerated for clarity.
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38. The birth of the Moon.
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39. A rain of debris creates craters on the young Moon
A rain of debris creates craters on the young Moon. Large impacts late in the process form the maria basins. Lava floods the basins to make the maria.
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40. (A) Sketch of how the Moon's shadow travels across the Earth
(A) Sketch of how the Moon's shadow travels across the Earth. (B) Location of some recent and upcoming total solar eclipses. (From Kuhn, Karl F. Astronomy,p. 204, St. Paul, Minn., 1989 West Publishing.)
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41. The Moon's orbit keeps approximately the same orientation as the Earth orbits the Sun. Because of its orbital tilt, the Moon generally is either above or below the Earth's orbit. Thus the Moon's shadow rarely hits the Earth, and the Earth's shadow rarely hits the Moon, as you can see in (A) and (C). Eclipse seasons occur when the Moon's orbital plane “points” at the Sun. A solar eclipse will then occur at new moon and a lunar eclipse at full moon, as you can see in (B) and (D).
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42. (A) Precession of the Moon's orbit
(A) Precession of the Moon's orbit. Notice its similarity to twisting a tilted book that has one edge resting on a table. (B) Precession of the Moon's orbit causes eclipses to come at different dates in successive years.
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43. (A) Photograph of a total lunar eclipse
(A) Photograph of a total lunar eclipse. (Photo courtesy Dennis di Cicco.) (B) As sunlight falls on the Earth, some passes through the Earth's atmosphere and is slightly bent so that it ends up in the Earth's shadow. In its passages through our atmosphere, most of the blue light is removed, leaving only the red. That red light then falls on the Moon, giving it its ruddy color at totality.
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44. (A) Photograph of a total solar eclipse
(A) Photograph of a total solar eclipse. The bright halo of light is the Sun's corona, its outer atmosphere. (Photo courtesy Dennis di Cicco.) (B) The landscape is eerily lit during a total solar eclipse. The dark color of the sky is the Moon's shadow. (Courtesy Richard Wainscoat.)
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45. Tides are caused by the Moon's gravity creating tidal bulges.
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46. (A) Arrows schematically show Moon's gravitation force at different points on the Earth. (B) Tidal forces from the point of view of an observer on the Earth. These arrows represent the difference between the Moon's gravitational force at a given point and its force at the Earth's center.
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47. As the Earth rotates, it carries points along the coast through the tidal bulges. Because there are two bulges where the water is high and two regions where the water is low, we get two high tides and two low tides each day.
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48. The Sun's gravity creates tides too, though its effect is only about one-third that of the Moon. (A) The Sun and Moon each create tidal bulges on the Earth. When the Sun and Moon are in line, their tidal forces add together to make larger than normal tides. (B) When the Sun and Moon are at 90° as seen from Earth, their tidal bulges are at right angles and partially nullify each other, creating smaller than normal tidal changes.
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49. Tidal braking slows the Earth's rotation and speeds up the Moon's motion in its orbit.
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