Basic Observations
of 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
Asterisms
Let'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
Asterisms
In 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
Asterisms
For 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 Ages
Despite 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
Constellations
Adding the constellation lines
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
Orion
Betelgeuse
Bellatrix
Alnilam
Mintaka
Alnitak
Rigel
Saiph
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)
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
•*
•*
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
Solar and Sidereal (Star) Days
Solar 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)
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 Sky
First 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 Solstice
Summer Solstice
Vernal Equinox
Autumnal Equinox
Apparent Motion of the Sun
Summer solstice
Winter solstice
Equinox
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
Summer
Winter
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'
Precession
In 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
Phases of the Moon
waning
Last Quarter
Cresent
sunlight
Gibbous
New
Full
Cresent
Gibbous
waxing
First Quarter