Basic Observationsof the

Night Sky

Things that shine in the Night

Looking up at a clear night sky, there are some obvious sights:

• Individual stars• Patterns, or groupings, of stars• Some extended glowing bands of light• The Moon• Planets• Sometimes ‘shooting stars’ and

occasionally comets

Patterns in the Sky• Asterisms

– Groupings of stars that seem to have a “recognizable” shape; for example,

• The Big Dipper• The Southern Cross• Orion

AsterismsLet's see how the familiar "Big Dipper" has appeared to people around the world:

In pre-classical Greece, this asterism formed a 'Great Bear'(Apianus – German ca. 1535AD)

Note: All of these 'Big Dipper' illustrations are from a book, "The New Patterns in the Sky" by Julius Staal

Asterisms

By about the 2nd Century AD, the pattern in the sky was more elaborate – this is an 1800 German illustration copied from a 2nd century drawing of Ptolemy

Asterisms

The Seven Plowing Oxen are from Classical Rome

Asterisms

Tribes in northwest europe saw a wagon being drawn by horses or a plow

Asterisms

In Babylon, this asterism was seen as a wagon

Asterisms

Whereas in North America, the Sioux saw a skunk

AsterismsIn China, the Emperor Wen-Chang was holding court for: K'uei, the Minister of Literary Affairs of the World; Chuh, Mr. Red Coat; Chin-Chia, Mr. Gold Armor and Kuan-Ti, the God of War

AsterismsFor the Aztecs, this was the evil god Tezcatlipoca

Who brought strife and problems to all things while dancing on his pegleg about the north star

Motion through the AgesDespite the Ancients idea of the ‘Fixed Stars’ they are actually moving (although very slowly from our perspective)

There are two motions:• Radial Motion - toward and away from Earth• Proper Motion – “sideways” against the pattern of the

sky

Constellations• Official boundaries marking regions of the sky• These may be asterisms by themselves or may contain

asterisms within their boundaries• The Great Bear (Ursa Major)• Orion• Cassiopea• Scorpio

• There are 88 official constellations marking regions of the sky

• Those that are positioned along the Ecliptic are known as the Zodiac– There are 12 of these according to tradition; however, in

reality, there are 13 • Everyone forgets poor Ophiuchus, (oh-fee-YOU-kus),

the Serpent Bearer

The ecliptic is the line connecting the Earth and Sun; therefore also the path the Sun apparently travels through the stars. Sometime extended to the Ecliptic Plane

Ophiuchus

Constellations

The night sky

Constellations

Adding the ecliptic line

Constellations

Adding the constellation boundaries

Adding the constellation lines

Constellations

Extended Glows of Light• Nebulae

– Fuzzy, patches of light, sometimes hard to define in shape or to resolve into stars• Andromeda• Triffid Nebula

• Zodiacal Light– A faint glow along the ecliptic

• Gegenschien– Counterglow – A faint glow opposite the

Earth from the Sun

Nebulae

Zodiacal Light

Gegenschien

Labeling the Night Sky• Constellations

– Generally Latin Names• Ursa Major• Scorpio• Lyra• Coma Berenices

• Stars– Proper names

• Aldebaran• Antares• Vega• Sirius• Altair• Polaris

Proper Names

Betelgeuse

Rigel

Bellatrix

Saiph

MintakaAlnilam

Alnitak

Orion

Labeling the Night Sky

• Stars– Catalogs

• Bayer– By descending order of brightness– Greek letter + Constellation

• Lyrae• Flamsteed

– Numbered by increasing time of appearance on the observer’s N-S axis (increasing Right Ascension)

• 3 Lyrae• Groombridge

• Nebulae– Proper Names– Catalogs

• Messier• NGC

Diurnal Motion of the Sky• Stars appear to rotate about Polaris – The

North Star. • The group of stars which never set (for a given

observer's latitude) are known as circumpolar stars

• The sky as seen from the North Pole:– Polaris is nearly at the zenith– The stars 'move' parallel to the horizon

• The sky as seen from the Equator:– Polaris is on the horizon to the north– The stars rise vertically, Looking East they move up,

looking West they move down

The Motion of the Sun• Diurnal Motion

– Daily motion of the sun– Solar Day

• Length of time between successive meridian passages.

• Can be marked out by the shadow of a gnomen (sundial)

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The Motion of the Sun• The Sun appears to rise in the East and

set in the West as seen from a vantage point on the Earth.

• If you could see it against a background of fixed stars however, the Sun moves East at about 1 per day

East West

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Apparant Motion of the Sun

• The daily movement from East to West is due to the Earth's rotation about its axis.– Earth rotates toward the east, or

counterclockwise if seen looking down on the north pole.

• This easterly motion against the stars is due to the Earth's motion about the Sun– Earth's orbit is also counterclockwise if seen

looking down on the north pole.

Apparent Yearly Solar Motion

The Siderial Day

• The Solar Day was defined as a single rotation of the Earth with respect to the Sun

• What about a single rotation of the Earth with respect to the Stars?– This is a Siderial day (or star day)– It is about 4 minutes shorter than a Solar

day*

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The Motion of the Sun

• The line connecting the earth and sun lies in a plane – The Ecliptic Plane

• This is not the same as the Equator because the Earth is tilted at 23½

N

Observed Motions of the SkyFirst some 'Navigational Aids':The Celestial Sphere

– A projection of the Earth's coordinates onto the sky

– The poles are extended to become the celestial poles

– The equator is projected to become the celestial equator

– The Latitude lines (parallels) are projected onto the celestial sphere and given the name 'Declination'

– The Longitude lines (meridians) are projected out and are now called 'Right Ascension'

The Motion of the Sun

Special points:

Winter SolsticeSummer SolsticeVernal EquinoxAutumnal Equinox

Apparent Motion of the Sun

EquinoxSummer solstice

Winter solstice

The Seasons• It is this 23½ inclination of the Earth's axis

which causes the seasons.• It is NOT because we are closer to the Sun in

the summer months; in fact, we in the northern hemisphere are actually slightly farther away from the sun during the summer.– Austrailia is slightly closer during their summer.

• In our summer, we are tilted toward the sun – as you can see in the illustration on the preceeding slide, this has the effect of making the Sun appear to rise higher in the sky and provides a longer period of daylight

• In winter, we are tilted away – lower height to the Sun and shorter days

The Seasons

And, in Summer, delivers the heat longer

Precession of the Equinoxes

• The Earth 'wobbles' like a top; this is known as precession.

• It takes 26,000 years for the Earth to make one complete cycle– This means that Polaris was not always, nor

will it remain, the North Star• Of course, if the north pole wobbles in a

circle so do the directions to the equinoxes– The vernal equinox is no longer the 'first

point in Aries'

PrecessionIn fact, superimposed on the precession, there is an additional motion, nutation, which means the wobble doesn’t draw a smooth circle, but rather a series of ‘S’ shapes about the circle.

The nutation has an 18.6 year period

Another view of Precession

Yet Another view of Precession

The Motion of the Moon• The moon orbits about the earth in

the same direction as the earth orbits the sun

It takes 27.3 days to orbit once with respect to the stars – This is it's Siderial Period

It takes 29.5 days from New Moon to New Moon-- This is it's Synodic Period

sunlight

Phases of the Moon

waning

waxing

Full

Gibbous

Gibbous

Cresent

Cresent

New

Last Quarter

First Quarter