The Illusion of changing Lunar Size

Sequence of photos over Seattle, with the final one a longer exposure

Synodic vs Sidereal for the Moon

View of the Moon’s Orbit

Attributes of the Earth-Moon System

Making

Eclipses

Anatomy of Solar Eclipses

Eclipse

Tracks

Bailey’s Beads

Lunar Eclipses

A Lunar Eclipse

Share Question

In order for a solar eclipse to occur, the Moon must be:

a) high in the sky b) near first or last quarter c) near new moon d) near full moon e) over another country

Sequence of Lunar Phases

Orbital View of Lunar Phases

Geometry of Lunar Phases: Earth Perspective

Snapshots of Lunar Phases: What doesn’t

change?

Lunar Rotation

Share Question

As seen from the Moon, how often does the Sun rise? a) Never. b) About every 24 hours. c) About once per week. d) About once per month. e) About once per year.

As seen from the Moon, how often does the Earth set? a) Never. b) About every 24 hours. c) About once per week. d) About once per month. e) About once per year.

Prominent Lunar Missions

• Luna 3: 1959, Soviet1st pictures of Moon’s far side

• Apollo 11: July 20, 1969Astronauts Armstrong, Aldrin, and

Collins first on Moon• Apollos 11-17: 1969-1972

Returned ~400 kg of Lunar samples• Moon exploration continues to search for

water and to study its structure

Soviet Commemorative Stamps

Dark Side of the Moon

Apollo 11

Buzz Aldrin

Flag on the Moon

Lunar Footprint Mare Basalt

Highland Breccia Highland Anorthosite

Water Ice on the Moon

Lunar probes Clementine and Lunar Prospector have provided suggestive evidence for the existence of water ice in permanent shadows near the lunar poles.

Water Found on the Moon• Four spacecraft recently reported

small amounts of H2O and/or OH at the Moon:• India’s Chandrayaan mission•NASA’s Cassini mission•NASA’s EPOXI mission•NASA’s LCROSS mission

The first three measured the top few mm of the lunar surface. LCROSS measured plumes of lunar gas and soil ejected when a part of the spacecraft was crashed into a crater.

• How much water? Approximately 1 ton of lunar regolith will yield 1 liter of water

This false-color map created from data taken by NASA’s Moon Mineralogy Mapper (M3) on Chandrayaan is shaded blue where trace amounts of water (H2O) and hydroxyl (OH) lie in the top few mm of the surface.

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How was Water Detected?

• Lunar soil emits infrared thermal radiation. The amount of emitted light at each wavelength varies smoothly according to the Moon’s temperature.

• H2O or OH molecules in the soil absorb some of the radiation, but only at specific wavelengths

• All four infrared spectrographs measure absorption by water.

An infrared spectrum measured by LCROSS (black data points) compared to models (red line)

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Wavelengths where water absorbs light

model with thermal radiation only

model with thermal radiation and absorption by molecules

The Big Picture• Lunar water may come from ‘solar

wind’ hydrogen striking the surface, combining with oxygen in the soil. It may also arrive via meteorite and comet impacts. Both processes are likely.

• Lunar water may be ‘bounced’ by small impacts to polar regions, forming ice in permanently shadowed craters

• Similar processes may occur on other airless bodies (e.g., Mercury, asteroids)

• Water-laden lunar regolith may be a valuable resource, possibly supporting future lunar exploration activities

Discovery of water on the moon may support future activities on the lunar surface and beyond. Artwork from NASA / Pat Rawlings.

Tidal ForcesTidal forces are a consequence of how gravity from

one body acting on a second body varies across that second body.

Gravity isa) A vectorb) Changes with distance

Both Moon and Sun contribute to tides at the Earth (Sun’s tidal force about half of Moon’s)

Spring tide - when they add up (Sun, Earth, and Moon aligned at New and Full phases)

Neap tide - when tides are at odds (1st qtr. and 3rd qtr. Lunar phases)

The Tidal Force

Lunar Deformation

This is a false-color plot of the Moon’s deviation from spherical shape. Blue is “squashedness” (near the poles) and red is “stretchedness” (mostly at front and rear faces). Based on data from Clementine.

Roche LimitGravity scales like, FG ~ 1/r2

Tidal forces scale like, FT ~ 1/r3

Different dependence on distance suggests that tidal forces (although weak) could overcome surface gravity at some close distance, which is called the “Roche Limit”

One kind of Roche Limit is to ask how close a moon must come to a planet before the tidal force of the planet lifts a rock off the surface of the moon.

The Roche Limit

Tidal Evolution of Lunar Orbit

Lunar Surface

• Regolith – layer of rock and dust debris built up from meteoritic impacts

• Mare – few craters• Terrae – many craters• Galileo discovered Lunar craters in 1609• Craters:

– Reveals properties of sub-surface– Amount of cratering related to surface

age

Stretch of Lunar Terrain

Lunar “Climate”• The escape speed from the Moon is 2.3

km/sec, and so it has essentially no permanent atmosphere

• Moon does keep a transient atmosphere from capture of solar wind and radioactive decay in rocks (composition mainly He, Ne, Ar, and H)

• Without an atm., the sky is always “black”, and there are large day-night temperature swings, from 400 K (260 F) to 100 K (-280 F)– Earth has a 20 K (36 F) temperature swing on

average

Crater Formation

FEATURES:• Impact produces a crater• Sprays ejecta• Often leaves a rim• Sometimes with associated

bright rays (radial “spokes”, possibly a consequence of color contrast)

Crater Formation

Share Question

Why are some large crater walls sharp and steep, while others are more rounded?

a) different volcanos make different craters b) age differences c) size differences among the impact bodies d) composition differences among the impact

bodies e) seismic activity on the Moon

The Copernicus Crater

Rays

Crater Dating• Count number of craters, divide by

cratering rate, get an age!• CAUTIONS:

a) Erosionb) Cratering rate can vary over timec) Crater saturation – overcrowding (craters

upon craters)

• NOTE – Erosion can also bias relative numbers of different sized craters, since smaller craters tend to get “erased” faster.

Moon’s Interior

Origin of the Moon

1) [The Moon is old.]2) The chemical composition of the Moon

and Earth is similar but not identical.3) Near absence of the iron core in Moon.4) The Moon’s orbit is inclined to the Earth’s

equator, somewhat inclined to the ecliptic, and is prograde.

Models

• Fission Theory: Moon “spun off” from rapidly rotating Earth after iron core formed

• Binary Accretion Theory: formed as a pair

• Capture Theory: having formed elsewhere, the Moon was “captured” in a close encounter

• Giant Impact Theory: collision between Mars-sized body and Earth – debris collects to form our Moon

Model Schematics

Challenges

• Fission:– Moon’s orbit

inclined to Earth equator

– Needed rotation at once per 4 hrs – why was Earth so fast?

• Binary: – why no iron in

Moon?

• Capture:– Capture difficult

(how to slow down?)

– Are Earth and Moon compositions too similar?

• Giant Impact:– favored

Artistic Impression of Giant Impact

Giant Impact Theory

a. Similar composition to Earth because debris contains mantle material

b. Different composition owing to the impacting body

c. Moon’s orbit being inclined is not surprising

d. Body was iron-poor and Earth’s iron core was already formed