Basic Observations of the Night Sky
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Basic Observationsof the
Night Sky
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Things that shine in the NightLooking 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
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Patterns in the Sky• Asterisms
– Groupings of stars that seem to have a “recognizable” shape; for example,
• The Big Dipper• The Southern Cross• Orion
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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
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AsterismsBy 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
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AsterismsThe Seven Plowing Oxen are from Classical Rome
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Asterisms
Tribes in northwest europe saw a wagon being drawn by horses or a plow
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AsterismsIn Babylon, this asterism was seen as a wagon
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AsterismsWhereas in North America, the Sioux saw a skunk
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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
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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
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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
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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
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Ophiuchus
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Constellations
The night sky
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Constellations
Adding the ecliptic line
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Constellations
Adding the constellation boundaries
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Adding the constellation lines
Constellations
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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
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Nebulae
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Zodiacal Light
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Gegenschien
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Labeling the Night Sky• Constellations
– Generally Latin Names• Ursa Major• Scorpio• Lyra• Coma Berenices
• Stars– Proper names
• Aldebaran• Antares• Vega• Sirius• Altair• Polaris
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Proper Names
Betelgeuse
Rigel
Bellatrix
Saiph
MintakaAlnilamAlnitak
Orion
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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
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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
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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.
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Apparent Yearly Solar Motion
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Solar and Sidereal (Star) DaysSolar Day: Time between
when Sun appears in same place in sky (24 hours on average)
Sidereal Day: Time between when a star appears in same place in sky (23 hours 56 minutes)
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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
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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'
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The Motion of the Sun
Special points:
Winter SolsticeSummer SolsticeVernal EquinoxAutumnal Equinox
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Apparent Motion of the Sun
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EquinoxSummer solstice
Winter solstice
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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
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The Seasons
And, in Summer, delivers the heat longer
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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'
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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
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Another view of Precession
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Yet Another view of Precession
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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
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sunlight
Phases of the Moonwaning
waxing
Full
Gibbous
Gibbous
Cresent
Cresent
New
Last Quarter
First Quarter