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Motion of the MoonEast to west in arc across sky

Eastward w.r.t. stars - half degreeper hour

Sidereal Period - 27.32166 days

Line of Nodes

Moon rises ~ 50 minutes later each day Phases of theMoonWhy does the Moon Shine?

Reflected sunlight

Half of the Moon isalways lit up

Fraction of lit surface wesee from Earth - phase

How much of the Moon is litup by Sun?

Synodic Period - 29.530588days

Sidereal & Synodic Periods

Sidereal Period: 27.3 days

Synodic Period: 29.5 days

Phases of theMoon

• The changes in the Moon’s phaseare due to changes in the anglebetween the Sun, Moon and theEarth

– 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, whendoes a first quarter Moon rise?

A First Quarter Moon rises at noon

Eclipses

• A solar eclipse occurs when the Moon movesbetween the Earth and the Sun

• A lunar eclipse occurs when the Earth movesbetween the Sun and the Moon and the Moon movesthrough the Earth’s shadow

• In ancient cultures eclipses were bad omens

The Moon Moves Over the Face of theSun

•

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 differentphysical 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• Answer: the Moon and Sun,

coincidentally, have nearly the sameangular size

• Angular size of an object depends ontwo things– The physical size of the object– The distance to the object

Angular size (radians)= Physical Size

Distance

An Annular Solar Eclipse

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Question

• Why does an annular eclipse lookdifferent?

Shadows & Eclipses

• A total solar eclipse can seen from only a small region onthe 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

• The Moon’sshadow movesover the Earthduring a solareclipse

Paths of Solar Eclipses

Next solar eclipse visible from USAAug. 21st 2017

http://sunearth.gsfc.nasa.gov/eclipse/eclipse.html

When and How Often Are Eclipses

partial, total orannular eclipse- 3 to 5 timeseach year

total eclipse -0 to 3 times ayear

Solar eclipses occurat new moon

New moon crossesecliptic

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The Lunar Eclipse

• total lunar eclipse - the Moon moves completely throughthe Earth’s umbra

• partial lunar eclipse - the Moon moves partially throughthe Earth’s umbra

• Penumbral eclipse – the Moon passes through thepenumbra but does not come in contact with umbra

• Lunar eclipses are visible anywhere on the night-timeside of the Earth

A Lunar Eclipse

Question

• Why is the Moon’s surface still visibleduring a total lunar eclipse?

When & How Often?• Lunar eclipses occur at FULL

Moon PHASE• Lunar eclipses do not occur

every month because theMoon’s orbital plane is tiltedwith respect to the ecliptic– a total lunar eclipse occurs

when the Moon crosses theecliptic at full Moon

– since the Earth’s shadow ismuch bigger than the Moon,total lunar eclipses occurmore often than solareclipses

Lunar eclipses occur 2 to 5 times per year

Motion of the Planets• The planets are the brightest

objects in the night sky (with theexception of the Moon)

• Rise in the east and set in the west• Planet means “Wanderer”

– The planets move slowly amongthe stars staying near the ecliptic

– Different planets move at different speeds relative to thestars (of the visible planets,Mercury is the fastest, Saturn isthe slowest)

– They move in complex patternschanging their direction ofmotion

Motion of Planets

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Motion of the Planets• Planets generally move eastward relative to the stars• Planets undergo retrograde motion

– motion relative to stars:• slows• reverses• moves westward relative to stars• slows again• reverses again• resumes its general eastward motion

• 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

Inferior Planets• Venus and Mercury• Stay near the Sun on

the sky• Are visible only near

sunrise and sunset(“morning & eveningstars”)

• Move away from theSun and then movetoward it

Mercury - (ca. 15th

century)

Mercury is holding abag for commerce andtwin snakes for healingpowers.

Reigns over Virgo &Gemini

Venus depicted in the AztecCodex: Aztec god, Xolotl(evening star) at thecrossroads of fate. Laterbecame twin of Quetzalcoatl(morning star & supreme god)

Superior Planets• The visible superior

planets are Mars,Jupiter and Saturn

• Can appear far fromthe Sun but remainnear the ecliptic

Mars & VenusFresco from Pompeii(ca. 1 A.D.)

Mars is the war-god& Venus, the god oflove.

Saturn (ca. 18th century) - Arabicillustration showing agriculturalactivities under direction of Saturn

Can you ever see Venus at midnight?Why do inferior planets always appear near the

Sun(i.e. in the evening or morning)?

Planets show PhasesPlanets, Gods & Days of

Week• English names for most

of the days of the weekcome from Norse gods

• Tuesday : Tiwes - godof war

• Wednesday: Woden -god of day & night

• Thursday: Thor - god ofthunder - head god

• Friday: Frega - goddessof spring

15th centuryengraving ofdays of theweek andtheirastrologicalcounterparts

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Ancient Astronomy• Since pre-historic times, ancient

people have observed andrecorded the regular & cyclicpatterns in the sky

• To explain these motions, a dualdevelopment in human thoughtbegan:– 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

25,000 year old Ishango bone (Congo, Africa)with possible lunar phase calendar

14 Phases of the moon seen in cave paintingsin Lascaux, France (18,000 BC)