The Starry Sky
•There are roughly 6000 or so stars visible to the naked eye
Orion:
Aresince
these
stars
in the
sky or, 3000
you can
only physically
see about one half of
the sky.
related? Green = Yes, Purple = No
•The stars are so distant that they appear only as
points of light.
•We can detect color and brightness with the
naked eye – but not much more.
The Ancient Observatory of Stonehenge
Unique
Construction
•Pretend the stars
are painted on a
giant ‘bubble’
around the Earth
known as the
celestial sphere
Stars are at different
distances
The stars of a constellation only appear to be close
to one another, they may be located at very different
distances from us.
Constellations
•Stars are grouped into “easily” recognized shapes
called constellations.
•There are 88 official
constellations
•The sky is
completely mapped
by the constellations.
Early Sky Map
Lots of art work
Today, constellations are well-defined regions
on the sky, irrespective of the presence or
absence of bright stars in those regions.
Some sky maps leave out the boundaries ; size of star depends upon
magnitude (how bright).
Constellations: Three interpretations of the
Winter Constellation Orion:
Early
The Sky
Modern Maps
Star Names an example
Bayer System
 alpha Orionis
 beta Orionis
 delta Orionis
 gamma Orionis
 epsilon Orionis
 kappa Orionis
Bayers’ System
Name
Betelgeuse
Rigel
Mintaka
Bellatrix
Alnilan
Saiph
Stars are named by a
Greek letter (, , )
according to their relative
brightness within a given
constellation + the possessive form of the
name of the constellation. Catalogues HD, ASO
Sagittarius the archer ( better known as the teapot
an asterism)
Stars can be used to locate other stars and
constellations. Star hopping
To find your directions in the sky,
find the north star, put your back
to the north star and you will be
looking southward.
East will be on your left, with west
on your right.
Facing due south you are you are
looking along your meridian.
Celestial Motions
Diurnal (Daily) Motions
• The Sun , Moon , Stars and Planetsrise in east, set in the west.
• These motions are all due to Earth’s
Rotation about Its Own Axis (1 Day = 24
hours (almost).
The Moon, and Planets have their own
motion, which is from west to east moving
counter clockwise around the Sun..
Annual (Yearly) Motions
• Position of Stars, Constellations
– They rise 4 minutes earlier each night.
– Different constellations are visible at
different times of the year .
• Position of the Sun
– The position of Sun on horizon shifts back and
forth and the peak height in the sky changes.
• Seasons
– Length of a day, night, and the climate changes
over course of the year.
•Earth orbits Around the Sun, one year = 365 Days (almost)
Other Changes
Phases of the Moon (monthly)
•Moon rises about 50 minutes later each
night. Takes 29.5 days from New Moon
to the next New Moon.
Change in “Celestial Poles”
The “North Star ( Polaris)” has not
always been the North Star.
Eclipses
Sun or Moon becomes “blocked” from
view during a year.
The Horizontal System
Zenith
Local Sky
W
S
N
Horizon
E
Zenith is a point on the celestial sphere directly
above your head.
Celestial Meridian
•A line from north to south, through
your Zenith is called the Celestial
Meridian.
•When objects are on the Meridian at
their highest point in the sky, we say they
are in transit at this point.
Astronomical noon is when Sun is on the
Meridian, not necessarily straight up.
.
North Pole
Meridian
Equator
South Pole
Altitude is measured from your horizon up to the
celestial object in degrees.
Zenith
Celestial
Sphere
Altitude
Meridian
East
North
South
West
Nadir
Azimuth is measured along the horizon from a point
below the North Pole, which is zero degrees, to a point
directly below the
Zenith
Celestial
object on the horizon.
Sphere
Altitude
Meridian
East
North
South
West
Nadir
Azimuth
North is 0 deg,
East is 90 deg,
South is 180 deg
and west is 270
•For a geographic latitude of L degrees on the northern
hemisphere, you will see the celestial north pole L degrees
above the horizon.
Zenith
Your Latitude =
altitude of celestial
pole
NCP
CEq
Your
Latitude
W
S
N
Horizon
E
The
Equatorial
System
Celestial Co-ordinates
• The altitude and azimuth of a star
change all the time, and depend on
where you are.
We need co-ordinates “fixed” to the
celestial sphere.
•Declination and Right Ascension
A star’s Dec & RA will change very little
in our lifetime.
It seems as though there is a sphere of stars surrounding
the earth.The Celestial Sphere is an extension of the Earth.
North Pole
Celestial Sphere
Celestial
Equator
Earth
Earth
South Pole
South Pole
Places on the Earth are located by Latitude & Longitude
Declination (Dec) is analogous to latitude and is
measured in degrees up and down from the
celestial equator. (Max is +90 deg or –90 deg)
North Celestial Pole (NCP)
Declination (Dec)
Celestial Equator
Right Ascension is analogous to longitude, but is
measure in time units of hours, min and sec, from the zero
point located at the vernal equinox. (0 hours to 24 hours)
15 deg = 1 hr
North Celestial Pole (NCP)
Celestial Equator
(zero of Dec)
Right Ascension (RA)
(zero of RA)
Right Ascension & Declination
With these two directions, any object can be located
on the celestial sphere.
North Celestial Pole (NCP)
Declination (Dec)
Celestial Equator
(zero of Dec)
Right Ascension (RA)
(zero of RA)
Ecliptic vs. Celestial Equator
The Earth’s axis of
rotation is inclined to
its orbital plane by
23.50
The Sun does
not move, the
Earth’s
motion
makes the
Sun appear
to move.
With no tilt, the
planets would all
seem to be located
somewhere on the
Celestial Equator,
straight out from the
Earth’s equator.
The planets
are revolving
around the
Sun, but they
would appear
to be located
along the
equator.
Celestial Equator
Because the Earth is tilted 23 1/2 degrees, the Sun &
Planets move in a different location, as seen from Earth.
The Ecliptic is the
path of the Sun,
Moon & Planets
revolving around
the Sun as seen
from Earth.
0 RA where the Sun on
the Ecliptic crosses the CE
on its way upward
The position of Sun on horizon shifts back and
forth, so that the peak height in sky changes
Finding North
• The big dipper does not set from most of the USA.
• The two end stars point to Polaris which happens to be
near the celestial pole
• The big dipper acts like a big clock: It goes once round
• in 1 a day
Star Trails at our location
 Looking north you’ll see the
stars circle around Polaris.
Looking at stars rising
roughly east you’ll see them
rise upward, go to highest
point then start downward
setting in the west.
Looking toward the south
you’ll see stars rise in the
south east, go to the highest
point and set in the
southwest, a shorter arc
across the sky.
What you see depends on your latitude
NCP
Z
At the North Pole
North Pole
•If you were at the north
pole of the Earth, Polaris
would be overhead.
All stars would go round the
north celestial pole never
setting: all are circumpolar
At the Equator
Z
W
NCP
S
N
E
Equator
Stars that never set ,
and go round and
round the pole are
called circumpolar
stars.
90 deg – your latitude
is the Declination of
the limit of
circumpolar stars.
For Atlanta, 90 deg- 33 deg 45 min =
56 deg 15 min. So stars southward of this
point are not circumpolar.
The Seasons
Inclined Pole Causes Seasons
Earth’s equator
is inclined
against the
ecliptic by 23.5º.
Remember
the Sun does
not move, the
Earth moves.
The change in position of the rising sun.
March & September
June
(Equinoxes)
(Summer Solstice)
23.5
0
December
(Winter Solstice)
-23.5
1 KW/m2
1 KW/m2
1 m2
2 m2
The Tropic of Cancer & Capricorn
Measuring angles
Angular distance
Basic unit is the degree 1°
1° split into 60’ minutes of arc
1’ split into 60’’ seconds of arc
1° is 3600” seconds of arc
• The easiest way to estimate relative angular distances
on the sky without any equipment is to use your
outstretched hand like an angular ‘ruler’
Outstretched Hand as
an Angular Ruler
The Earth and its Motions
Earth’s rotation is causing the day/night cycle.
The Suns motion,
which is really
the movement of
the Earth around
the Sun, causes
the Sun to appear
to move through
the same
constellations at
certain times of
the year.
There are actually 13 constellations along the ecliptic
CONSTELLATION
TRADITIONAL
ACTUAL (2000)
DAYS
Capricornus
Dec. 22 - Jan. 21
Jan. 22 - Feb 21
Feb. 22 - Mar. 21
Mar. 22 - Apr. 21
Apr. 22 - May 21
May 22 - Jun. 21
Jun. 22 - Jul. 21
Jul. 22 - Aug. 21
Aug. 22 - Sep. 21
Sep. 22 - Oct. 21
Oct. 22 - Nov. 21
Nov. 22 - Dec. 21
Jan. 21 - Feb. 16
Feb. 16 - Mar. 11
Mar. 11 - Apr. 18
Apr. 18 - May 13
May 13 - Jun. 22
Jun. 22 - Jul. 21
Jul. 21 - Aug. 10
Aug. 10 - Sep. 16
Sep. 16 - Oct. 31
Oct. 31 - Nov. 23
Nov. 23 - Nov. 29
Nov. 29 - Dec. 18
Dec. 18 - Jan. 21
26
24
38
25
40
29
20
37
45
23
6
19
34
Aquarius
Pisces
Aries
Taurus
Gemini
Cancer
Leo
Virgo
Libra
Scorpius
Ophiuchus
Sagittarius
Precession
Gravity is pulling on a
slanted top Wobbling
around the vertical.
The Sun’s gravity is doing the same to the Earth.
The resulting “wobbling” of the Earth’s axis of
rotation around the vertical takes about 26,000
years, and is called precession.
Except for presently being the pole star, there is nothing
special about Polaris.
Because of precession, the celestial north pole
follows a circular pattern on the sky, once every
26,000 years.
It will be closest
to Polaris ~ A.D.
2100.
12,000 years
from now, it will
be close to
Vega in the
constellation
Lyra.
Messier Catalogue
• Charles Messier compiled a list of
approximately 100 diffuse objects that were
difficult to distinguish from comets.
Star Clusters
Nebula
Galaxies
Deep Sky Object Catalogs
Messier Catalog
– e.g. M1, M2, M3, etc.
New General Catalog
– e.g. NGC 7293
Index Catalog
• - e.g. IC302
•Solar day (noon-to-noon = 24 hours) is rotation period
with respect to the Sun  4 minutes longer than the true
rotation period of the Earth with respect to the stars called
the sidereal day.
•Roman Julian Calendar year = 365 d + leap yr
•Problem with Julian Calendar  average yr is 365.25 d, 11 minutes
too long !
• By the 1500’s, the time of Pope Gregory, the calendar was ahead of
astronomical time keeping by more than two weeks
Gregorian-Julian Reform: Calendar Century
years not divisible by 400 are ordinary years, not
leap years (Example: 1700 AD was not a leap year, but
2000 AD was)
SFA Star Chart 1
Thanks to the following for allowing
me to use information from their
web site :
Nick Stobel
Bill Keel
Richard Pogge
John Pratt
NASA, JPL, OSHO