revolution and rotation of the moon diurnal (daily) motion of the moon is due to the rotation of the...

Revolution and Rotation of the Moon• Diurnal (daily) motion of the Moon is due to the

rotation of the Earth around its axis• Monthly motion of the Moon responsible for its phases

– Sidereal month (27.32 days) = length of time for Moon to complete one orbit about the Earth = length of time for Moon to return to same position in sky relative to the stars

– Synodic month (29.53 days) = length of time for Moon to return to same position in sky relative to the Sun = time for Moon to go through its cycle of phases

– The difference in time (about 2 days) arises because Earth is moving in its own orbit about the Sun

• It takes an extra 2 days for the Moon to “catch up” with the Sun in the sky

• If Earth were much farther from the Sun, sidereal month ≈ synodic month

Revolution and Rotation of the Moon• The Moon’s orbit about the Earth is elliptical

– Average orbital distance = 384,400 km– Orbital distance ranges from 356,400 (perigee) – 406,700

km (apogee)

• The Moon’s orbit lies nearly along the ecliptic, but is tipped by about 5°– This orbit is closer to the Earth’s orbital plane than it is to its

equatorial plane (unusual for a planetary satellite)– Affects rise and set times of the phases throughout the year

• The two points where the orbit of the Moon passes through the ecliptic plane are called nodes– Ascending node = point where Moon moves northward

across the ecliptic– Descending node = point where Moon moves southward

across the ecliptic

Sun’s Path (Ecliptic) vs. Moon’s Path in the Sky

M.A. Seeds, The Solar System, 5th Ed., Thomson/Brooks-Cole, 2007

Revolution and Rotation of the Moon• The Moon rotates and revolves in the same amount

of time (the sidereal month)– “Synchronous rotation”– Thus we only ever see one face of the Moon– If the Moon didn’t rotate, we could actually see both faces

of the Moon during a month– Both faces of the Moon receive sunlight during the month

There really is no such thing as the “dark side” of the Moon

• Better to use the terms “near” and “far” sides of the Moon

• Appearance of Earth from the Moon– Earth would go through same cycle of phases (although

they would be “opposite” to those of Moon at a given time)– Earth would hang nearly motionless in the sky

Phases of the Moon• When sunlight falls on the Moon, it illuminates only

half of the Moon’s surface– Portion of illuminated half that reflects back to Earth

makes illuminated part look bright

• Other half of Moon is turned away from Sun and receives no sunlight– Absence of reflected sunlight makes this part of Moon

appear dark

• Phase of Moon depends on how much of the side turned toward us is illuminated by the Sun– It is not due to the Earth’s shadow! – Earth’s shadow is responsible for lunar eclipses

Phases of Moon• Phases of Moon determined by the position of

Moon relative to the Earth and Sun

Phases of the Moon• Moon travels from west to east in its orbit around

Earth• When Moon is new, angle between Moon and Sun

essentially 0° (only strictly true during a total solar eclipse)

• At waxing crescent phase, Moon lies east of Sun in sky by angle less than 90°– “Waxing” means bright part is growing larger

• At first quarter phase, Moon is (approximately) 90° east of Sun in the sky

• At waxing gibbous phase, Moon lies east of Sun by an angle between 90° and 180°

• When Moon is full, it is 180° away from Sun on the opposite side of the sky

Phases of Moon• At waning gibbous phase, Moon is west of Sun in

sky by an angle between 180° and 90°– Since Moon lies more than 180° east of Sun, better to use

smaller angle west of Sun– “Waning” means bright part is getting smaller

• At last quarter, Moon is about 90° west of Sun• At waning crescent phase, Moon is less than 90°

west of Sun• Since Earth rotates from west to east, sky appears

to turn from east to west around us– Sun, Moon, stars, and planets thus rise in east and set in

west

• When Moon lies east of Sun (waxing phases), its motion lags behind that of Sun

Phases of Moon• When Moon lies west of Sun (waning phases), its

motion is ahead of that of the Sun• Implications of relative motion between Moon and

Sun:– Waxing phases are visible at sunset, but not visible at

sunrise– Waning phases are visible at sunrise, but not visible at

sunset

• Motion of Moon during visible hours near Delaware:– Rises in eastern part of sky, sets in western part– Climbs into southern sky after rising, just like Sun– After rising, it moves to SE sky, then crosses meridian to

the south, then moves to SW sky, then sets in west

Phases of the Moon• For great interactive animations about the

phases of the Moon, check out the following website:

http://astro.unl.edu/classaction/

(Contains the same animations that you saw

in class, and more!)

Moon Phase Wheel• Assume that Sun rises in east at 6 AM, crosses

meridian to south at 12 noon, and sets in west at 6 PM (at midnight, Sun will be below horizon to our north)– Simplification that ignores complications of daylight savings

time, changing length of day for different seasons, and fact that we are not in center of our time zone

• We can represent this schematically by a circle called the Moon Phase Wheel (MPW): noon

S

N midnight

W 6 PM6 AM E

Moon Phase Wheel• MPW is like a 24–hour clock with Sun serving as

the hour hand– Location of Sun set by what time it is

• Location of Moon relative to Sun set by its phase– Waxing phases are counterclockwise around the wheel

from Sun– Waning phases are clockwise around the wheel– Counterclockwise (clockwise) corresponds to going east

(west) on the sky

• Relationship between angle from Sun to Moon and phase: Angle Phase

< 90° crescent

90° quarter

between 90° and 180° gibbous

180° full

Moon Phase Wheel Example #1It is sunrise, and the phase of the Moon is waning

gibbous. In what direction must you face to see the

Moon? noon S

N midnight

W 6 PM6 AM E

Southwest

Moon Phase Wheel Example #2It is sunset, and the Moon is in the southeast sky.

What is the phase of the Moon?

noon S

N midnight

W 6 PM6 AM E

Waxing gibbous

Moon Phase Wheel Example #3At what time is the last quarter moon on the meridian

in the south?

noon S

N midnight

W 6 PM6 AM E

6 AM

Moon Phase Wheel Example #4It’s 3 AM, and you see the Moon rising. What is

the phase of the Moon?

noon S

N midnight

W 6 PM6 AM E

Waning crescent

Eclipses• A lunar eclipse occurs when the Moon passes

through Earth’s shadow during the full moon phase close to the ecliptic plane

– Visible from entire night side of the Earth– Someone on the Moon would see a solar eclipse

Umbral vs. Penumbral Shadows


Eclipses• A total lunar eclipse occurs when the Moon passes

through the umbral shadow of the Earth• The only sunlight that enters the umbra is light

refracted (bent) by the Earth’s atmosphere– More red sunlight than blue sunlight is refracted, so the

Moon appears a dim red during a total lunar eclipse

Earth

Moonsunlight

(Progression of total lunar eclipse during 10/27/2004)

Eclipses• A solar eclipse occurs when part of the Earth enters

the Moon’s shadow

– Only occurs during new moon phase close to ecliptic plane– Only parts of the Earth experience the eclipse

Eclipses• A total solar eclipse occurs when the Moon’s umbral

shadow sweeps across the Earth

– The umbral shadow actually needs to reach the Earth’s surface (sometimes it doesn’t because of the Moon’s elliptical orbit)

Eclipses• If the Moon is on the far side of its elliptical orbit

when the solar eclipse occurs, the result is an annular eclipse in the areas that would otherwise have had a total eclipse

Eclipses – View From Space


Eclipses• There are at least 2 eclipses each year (solar), and

a maximum of 7 (lunar and solar)• How can you predict when the next eclipse will

occur (say a solar eclipse) in your neighborhood?– Moon must pass through the plane of the ecliptic during a

new moon phase– It takes 346.6 days to move from one such alignment to

the next identical one (eclipse year)– Calculate how many whole synodic months equal some

whole number of eclipse years– An accurate calculation shows that this interval of time,

called the saros, is 18 years 11⅓ days• Eclipses separated by the saros interval form an eclipse series

– Because of the extra ⅓ day in the saros interval, you would actually have to wait 3 full saros cycles (54 years, 34 days) before eclipse came back to your neighborhood

Upcoming Lunar Eclipses


Upcoming Solar Eclipses


revolution and rotation of the moon diurnal (daily) motion of the moon is due to the rotation of the...

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