Moonlight and Shadows © Bob Field 2006 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 Location of Moon During Each Phase waxing
Q1
G C Sunlight
Full Earth New
G C Q3
waning drbobfield©2006 waxing Chronological Phases of the Moon 7
10 4
0 15 age in days 1998 only age month + day 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 full moon 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 Crescent 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 terminator 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 New Moon
Predawn Photo 1½ Days Before Waning Crescent Becomes New Moon Dusk Photo 3 Days Before Waxing Gibbous Becomes Full Moon
Unlike the sun, the moon can hide from view easily 1 - below the horizon 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 eclipse
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 eclipses 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 librations: 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 night sky -7
0.1 Venus -4.3 0.03 Mars -2.8 0.02 Jupiter -2.5
0.01 Sirius -1.46 0.001 Betelgeuse +0.7 0.0001 average star +3 0.0001 lunar eclipse +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 near side of the Moon.
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 solar eclipse 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 atmosphere 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: planetesimal impact theory 4.5 bya Origin of the Moon 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 crust 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 iron 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 lava 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 crescents are near the sun all year. Full moon appears to be in the opposite season. Quarter moons are off by ± a quarter cycle. The Full Moon Rises at Sunset & 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 “seasons” 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 tides 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 tide 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