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Motion of the Moon
Line of Nodes
East to west in arc across sky
Eastward w.r.t. stars - half degree
per hour
Sidereal Period - 27.32166 days
Moon rises ~ 50 minutes later each day
Phases of the
Moon
Why does the Moon Shine?
Reflected sunlight
How much of the Moon is lit
up by Sun?
Half of the Moon is
always lit up
Fraction of lit surface we
see from Earth - phase
Synodic Period - 29.530588
days
Sidereal & Synodic Periods
Phases of the
Moon
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Sidereal Period:
27.3 days
The changes in the Moon’s phase
are due to changes in the angle
between the Sun, Moon and the
Earth
Synodic Period:
29.5 days
– 0 degrees : New Moon
– 90 degrees: Quarter Moon
(First or Third)
– 180 degrees: Full
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Question
• If the Sun sets at 6pm, when
does a first quarter Moon rise?
Eclipses
A First Quarter Moon rises at noon
The Moon Moves Over the Face of the
Sun
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• A solar eclipse occurs when the Moon moves
between the Earth and the Sun
• A lunar eclipse occurs when the Earth moves
between the Sun and the Moon and the Moon moves
through the Earth’s shadow
• In ancient cultures eclipses were bad omens
Nearly Covered
The Sun is Nearly Covered
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The Diamond Ring
Angular Size of Sun and Moon
• The Sun and Moon have very different
physical sizes
– Radius of Sun is 7x105 km
– Radius of Moon is 1.7x103 km
– So the Sun is 400 times bigger than the
Moon!!
• How can they appear to be nearly the
same size during an eclipse?
Angular Size
An Annular Solar Eclipse
• Answer: the Moon and Sun,
coincidentally, have nearly the same
angular size
• Angular size of an object depends on
two things
– The physical size of the object
– The distance to the object
Angular size (radians)
Size
= Physical
Distance
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Shadows & Eclipses
Question
• Why does an annular eclipse look
different?
• The Moon’s
shadow moves
over the Earth
during a solar
eclipse
Next solar eclipse visible from USA
Aug. 21st 2017
•
A total solar eclipse can seen from only a small region on
the Earth
– entire disk of the Sun covered
– In umbra or inner shadow the Moon
•
Partial solar eclipses are seen over a larger area
– only part of the Sun is covered by the Moon
– in the penumbraor Moon’s outer shadow
Paths of Solar Eclipses
When and How Often Are Eclipses
Solar eclipses occur
at new moon
New moon crosses
ecliptic
partial, total or
annular eclipse
- 3 to 5 times
each year
total eclipse 0 to 3 times a
year
http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html
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The Lunar Eclipse
A Lunar Eclipse
• total lunar eclipse - the Moon moves completely through
the Earth’s umbra
• partial lunar eclipse - the Moon moves partially through
the Earth’s umbra
• Penumbral eclipse – the Moon passes through the
penumbra but does not come in contact with umbra
• Lunar eclipses are visible anywhere on the night-time
side of the Earth
When & How Often?
Question
•
• Why is the Moon’s surface still visible
during a total lunar eclipse?
•
Lunar eclipses occur at FULL
Moon PHASE
Lunar eclipses do not occur
every month because the
Moon’s orbital plane is tilted
with respect to the ecliptic
– a total lunar eclipse occurs
when the Moon crosses the
ecliptic at full Moon
– since the Earth’s shadow is
much bigger than the Moon,
total lunar eclipses occur
more often than solar
eclipses
Lunar eclipses occur 2 to 5 times per year
Motion of the Planets
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Motion of Planets
The planets are the brightest
objects in the night sky (with the
exception of the Moon)
Rise in the east and set in the west
Planet means “Wanderer”
– The planets move slowly among
the stars staying near the ecliptic
– Different planets move at
different speeds relative to the
stars (of the visible planets,
Mercury is the fastest, Saturn is
the slowest)
– They move in complex patterns
changing their direction of
motion
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Motion of the Planets
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Inferior Planets
Planets generally move eastward relative to the stars
Planets undergo retrograde motion
– motion relative to stars:
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slows
reverses
moves westward relative to stars
slows again
reverses again
resumes its general eastward motion
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This motion is unique among all astronomical objects
This confused & perplexed people for centuries & led people of
many cultures to attribute superior powers to the planets
Superior Planets
• The visible superior
planets are Mars,
Jupiter and Saturn
• Can appear far from
the Sun but remain
near the ecliptic
Mars & Venus
Fresco from Pompeii
(ca. 1 A.D.)
Venus and Mercury
Stay near the Sun on
the sky
Are visible only near
sunrise and sunset
(“morning & evening
stars”)
Move away from the
Sun and then move
toward it
Mercury - (ca. 15th
century)
Mercury is holding a
bag for commerce and
twin snakes for healing
powers.
Reigns over Virgo &
Gemini
Venus depicted in the Aztec
Codex: Aztec god, Xolotl
(evening star) at the
crossroads of fate. Later
became twin of Quetzalcoatl
(morning star & supreme god)
Can you ever see Venus at midnight?
Why do inferior planets always appear near the
Sun
(i.e. in the evening or morning)?
Mars is the war-god
& Venus, the god of
love.
Saturn (ca. 18th century) - Arabic
illustration showing agricultural
activities under direction of Saturn
Planets, Gods & Days of
Week
Planets show Phases
15th century
engraving of
days of the
week and
their
astrological
counterparts
• English names for most
of the days of the week
come from Norse gods
• Tuesday : Tiwes - god
of war
• Wednesday: Woden god of day & night
• Thursday: Thor - god of
thunder - head god
• Friday: Frega - goddess
of spring
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Ancient Astronomy
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Since pre-historic times, ancient
people have observed and
recorded the regular & cyclic
patterns in the sky
To explain these motions, a dual
development in human thought
began:
– search for natural &
unchanging laws
– creation of mythology
The Earliest Calendars
• During the stone age (50,000 years ago) people first
began to leave a record in carvings and paintings.
These included:
– Pictures of constellations
– The first calendars
14 Phases of the moon seen in cave paintings
in Lascaux, France (18,000 BC)
25,000 year old Ishango bone (Congo, Africa)
with possible lunar phase calendar
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