and Shadows © Bob Field 2006 How bright is the ?

Why does its brightness vary so much with its phase?

What is the ash gray light of the moon?

What is the ?

Why do we see only one side of the moon?

 Bob Field 1998 Location of Moon During Each Phase waxing

Q1

G C

Full New

G C Q3

waning drbobfield©2006 waxing Chronological Phases of the Moon 7

10 4

0 15 age in days 1998 only age  + 26 20 1999 add 11 23 2000 add 22 if > 30 waning subtract 30 When & Where does the Moon Wax & Wane?

Waxing Phases Waning Phases

S full new new n moon moon n moon Dusk Dawn

not to scale S What can you see at 12 o’clock?

Dim Phases Bright Phases

waxing n waning waning n waxing Noon Midnight

not to scale S Waxing Gibbous and Phases The waning gibbous Moon’s face has turned 120 in about eight hours

6 hours before sunrise 2 hours after sunrise also note that the blue sky is brighter than the Moon’s dark ‘seas’ 12º

7 6 5 4 3

2

1

0 Daily Growth of the Waxing Moon Days 1-7

7 6 5 4 3 2 1

The crescent and nearly full moon phases linger because the appears to move slowly near the moon’s limbs

3 am Photo 2 Days After Waning Last Quarter Moon

Predawn Photo 4½ Days Before Waning Crescent Becomes

Predawn Photo 1½ Days Before Waning Crescent Becomes New Moon Dusk Photo 3 Days Before Waxing Gibbous Becomes Full Moon

Unlike the , the moon can hide from view easily 1 - below the 2 - behind a hill or tree 3 - above the clouds or vapor 4 - among the clouds 5 - within 30 of the glare of the sun 6 - near horizon where light scatter is highest 7 - in plain view overhead 8 - in your blind spot or off your fovea the visible moon changes shape, brightness, height in sky, location, rise & set time The new moon occasionally reveals its presence dramatically with a solar

S S S

partial total annular

duration is less than 8 minutes because moon lags sun by one diameter per hour totality is highly localized on Earth not to scale Angular diameter of Moon & Sun are each ½.

The sun is 400 times larger  and 400 times more distant 8000 than the moon.

Is this a coincidence? 800,000 Why are solar rare?

penumbra umbra sun

moon If you were on the Moon, you wouldn’t see much change Earth A more accurate animation would show the Moon and its shadows moving

not to scale How long is a Month? sidereal M one revolution M 27.3 days Earth M

Earth synodal 1 1/12 revolutions 29.5 days

Moon lags sun by 24 hours / 29.5 days or ~50 minutes / day or ~12 / day S not to scale Same hemisphere always faces earth How do we know?

Is this a coincidence?

Exceptions are called : motions of the earth and moon allow us to peek at 18% of back side How Bright is the Full Moon? Moon reflects about 7% of incident sunlight over a large area of space. The moon is darker than black velvet. The sun appears to be half a million times brighter than the full moon. What color is the sky at night? The stars are visible at night. The sky is blue because the moon (above the scene) is full. The stars produce trails during a long time exposure. The sky is blue because the moon (not within the scene) is full. How Bright is the Moon?

reflectance of surface features varies from 5% to 15%

What color is the Moon? If the Moon Were Made of Green Cheese

color digitally altered Full Moon around Midnight Flattened Full Moon Setting in West Before Sunrise How Bright is the Moon? one stellar magnitude is a change 150M sun from earth -27 of 2.5 in apparent brightness 10M skylight -24 5 magnitudes = 100x change 25K earth from moon -18 300 full moon -13 200 twilight -15 to -9 30 half full moon -10.5 1 crescent -8 to -5? 1 clear -7

0.1 Venus -4.3 0.03 -2.8 0.02 -2.5

0.01 Sirius -1.46 0.001 Betelgeuse +0.7 0.0001 average star +3 0.0001 +2 to +4 The Ash Gray Light of the Moon

 Bob Field 1999

The Earth T T T as seen from the moon T T

T Night and Day:  T T The Earth goes through phases like the Moon, but opposite.

The Earth is only visible from the .

Its location in the lunar sky varies greatly with lunar vantage point, but barely changes over time. This is what an impending lunar eclipse looks like from the near side of the Moon

Sun moves Earth is one nearly diameter S stationary per hour  and dark

Our lunar eclipse is a for the Moon. Lunar surface temperatures drop 150 C in one hour.

not to scale A partial lunar eclipse can last four hours

penumbra

full moon

Earth umbra sunlight The Earth’s shadow is moving faster than the Moon

The Moon may appear reddish due to refracted twilight passing through the Earth’s to the Moon not to scale S S S S S S S S S S S S S S The Sun S S S S S S as seen from the moon S S S S S S S S S S S S S S S S The lunar day is S S S S S ~30 days long S S S S S because the Moon S S S rotates once in ~30 S S S S S S S S days about its axis S S S S S S S S S S S S

Sun moves ~ one diameter per hour not to scale

Moonlight

phases: sunlight intensity varies with incident angle brightness depends on observer’s location

features: composition: lighter rays & darker maria structure: craters & mountains origin: impact theory 4.5 bya unusually large relative to its planet 1990’s impact theory - glancing blow 4.5 bya ejection by planetesimal approximately size of Mars from based on size, density, lunar explorations earth

capture Science Year 2000 Year Science

co-formation

planetesimal impact

Earth Story Lunar Features

Selenography

craters maria rays albedo

true light regions have lower density materials like the calcium and aluminum silicates in the earth’s low gravity due to size and density

cannot hold atmosphere or surface water

minimal erosion forces

no biological agents

craters persist unlike on earth

 observer

The view from the crater floor may be like a flat plain

The moon is small and some craters are large. The rim of the crater is beyond the horizon.

Microstructure & Composition of the Moon

most fine grains and dust are bright because of light scattering

Moon dust has dark and titanium coatings because meteorites deposit chemicals and solar wind sputters lighter atoms

“gardening” effect on the moon: meteorites have tilled the surface to about 20 cm (at the rate of 1 cm per 10 million years) craters may be meteoric or plutonic (volcanic) darker regions like maria have heavier basaltic materials such as iron and magnesium silicates which come from flows and have smoother appearance opposition effect the brightness of the full moon reveals the extreme roughness of the the moon

The Moon is NOT a mirror !

The surface is not smooth, so sunlight scatters rather than specularly reflect. The surface is not an ideal spherical shape, so shadows are cast by irregularities like craters.

How Bright is the Earth (as seen on the moon)? Albedo of Earth averages 35% mostly due to clouds, vs. 7% for Moon. T T T T T Area of the Earth is T T 16 times the Moon.  T Full earth shines on moon 5x16=80 times as bright as the full moon on earth, or 6000 times less bright than the sun. M longitudinal librations slow Moon’s orbit is

not quite circular. M

M It speeds up as it nears Earth. But Moon rotates at constant rate on Earth its axis.

fast M not to scale diurnal librations earth’s radius is 1/60th orbital radius

dusk observation n

M Earth n

dawn observation

not to scale latitudinal librations

6.5 tilt of lunar axis n

n EarthH new full moon moon

what about tilted plane of moon’s orbit?

not to scale Why does the Moon appear Larger near the horizon? The moon is actually slightly farther away (2%) from the observer when it is near the horizon, so it should appear smaller. n

Also refraction by the atmosphere flattens the moon by up to 20%. How high in the sky is the Moon?

summer full full M M M M M S moon M moon (winter) Q3 winter Q1 (summer) (spring) (spring) not to scale

New moon and are near the sun all year. Full moon appears to be in the opposite . Quarter are off by ± a quarter cycle. The Full Moon Rises at & Sets at Sunrise

same as sun solstices and equinoxes

30 N Winter Spring Summer Fall New 7 am SE 6 am E 5 am NE 6 am E Moon 5 pm SW 6 pm W 7 pm NW 6 pm W Full 5 pm NE 6 pm E 7 pm SE 6 pm E Moon 7 am NW 6 am W 5 am SW 6 am W opposite of sun approximate sun time (add 20 - 30 minutes to moonset due to moon’s orbital speed) When Q1 reaches its daily apex, the Earth has advanced a quarter rotation into the next “season”. Full Moon is two “” ahead and Q3 is three “seasons” ahead. sun & thin crescents also solstices and equinoxes 30 N Winter Spring Summer Fall

New 7 am SE 6 am E 5 am NE 6 am E approximate sun time Moon 5 pm SW 6 pm W 7 pm NW 6 pm W Q1 noon E 11 am NE noon E 1 pm SE wax midnight W 1 am NW midnight W 11 pm SW Full 5 pm NE 6 pm E 7 pm SE 6 pm E Moon 7 am NW 6 am W 5 am SW 6 am W Q3 midnight E 1 am SE midnight E 11 pm NE wane noon W 11 am SW noon W 1 pm NW full G G 2 Q3 3 1 Q1 4 0 C 50 million miles C Q3 Q1 in one month new new C C

speed (mph) Earth Moon in orbit 70,000 2200 rotation on axis 1000 9 a commercial jet can’t keep up with the sunset on earth, but a marathoner can track the terminator on the Moon! Sunlight varies across the near face

60 65% 42% 22% 45 5%

30 

0

F G Q C

Does brightness vary across full moon? Why does the moon’s face rotate during the day?

the face of the moon appears to sweep through an arc as the earth rotates throughout the day

the leading edge of the sunlit moon is a constant point on the limb during the day The Moon is upside down when viewed from down under n

 n  My list of interesting things about the Moon’s appearance and location 1 - goes through phases: crescent, half, full, etc. 2 - surface is not quite black after lunar sunset 3 - moves slower than sun / frequently unseen 4 - appears to have same diameter as sun 5 - seems larger when approaching horizon 6 - has many craters and dark seas 7 - same hemisphere always faces earth 8 - solar and lunar eclipses are rare 9 - location does not correlate exactly with Geology & Natural History of the Moon

Origin Theories? Composition and Structure? Comparisons to Earth? Volcanic Craters and Meteor Craters Lack of Water and Wind Erosion Evidence of Buried Water near Poles? Human & Robotic Lunar Exploration How the Far Side Differs from the Near Side The terminator moves at constant speed: view from above Moon’s north pole W E All About Tides

What causes tides? Why are there two high tides per day? Why is one high higher than the other? What are spring tides and neap tides? Why isn’t the Moon overhead during high tides? Why is the Moon more important than the sun for tides? Why are tide variations so low in the Riviera and Venice and so high in Glacier Bay and the Bay of Fundy? Where does tidal energy come from and go? Why does the Moon face the earth, but not vice versa?

If the Moon were a spherical mirror, we would only see a small intense spot

Q G C

F The Full Moon is in the opposite season from the Sun.

Reminder: Summer is hotter than Winter because Sun’s rays are more direct because Earth’s axis is tilted 23.45

full full moon moon “winter” summer “summer”

M noon S M midnight winter

not to scale Moonlight and Shadows

How bright is the moon?

Why does its brightness vary so much with its phase?

What is the ash gray light of the moon?

What is the moon illusion?

Why do we see only one side of the moon?

 Bob Field 1998 sunrise terminator - waxing phases The terminator moves at constant speed full G QN C new when viewed from above the Moon’s north pole

W E

n new C Q G full sunset terminator - waning phases sunrise terminator - waxing phases The terminator moves at constant speed full G QN C new when viewed from above the Moon’s north pole

W E

n new C Q G full sunset terminator - waning phases The Full Moon Rises at Sunset & Sets at Sunrise

same as sun solstices

30 N Winter Spring Summer Fall New 7 am SE 5 am NE Moon 5 pm SW 7 pm NW Full 5 pm NE 7 pm SE Moon 7 am NW 5 am SW opposite of sun approximate sun time (add 20 - 30 minutes to moonset due to moon’s orbital speed) The Full Moon Rises at Sunset & Sets at Sunrise

same as sun solstices

30 N Winter Summer New 7 am SE 5 am NE Moon 5 pm SW 7 pm NW Full 5 pm NE 7 pm SE Moon 7 am NW 5 am SW opposite of sun approximate sun time (add 20 - 30 minutes to moonset due to moon’s orbital speed) When Q1 reaches its daily apex, the Earth has advanced a quarter rotation into the next “season”. Full Moon is two “seasons” ahead and Q3 is three “seasons” ahead.

sun & thin crescents also solstice 30 N Summer

New 5 am NE approximate sun time Moon 7 pm NW Q1 noon E wax midnight W Full 7 pm SE Moon 5 am SW Q3 midnight E wane noon W