Star Wheel

Donna Kubik
PHYS 162 lecture Jan 20, 2005
What are constellations?
• Constellations are patterns formed by bright stars
• The celestial sphere is divided into 88 unequal regions.
These regions are what astronomers call constellations
• Astronomers use the term to describe an entire region of
the sky and all the objects in that region
What are constellations?
• We see the projected pattern of stars of varying distances
What are constellations?
• The stars in a constellation are not even near each other!
• Stars move on geologic
timescales, so a
constellation’s pattern will
• The patterns are
unchanging in ~10,000
year timescales
• BUT, their position in sky
varies with season
2000 A.D
50,000 A.D
100,000 A.D
Different seasons,
different constellations
Winter Constellations
• Orion
Winter triangle
Summer triangle
Summer constellations
• Cygnus (The Swan)
H. A. Rey
• H. A. Rey ”The Stars, A
New Way to See Them”
• H. A. Rey also wrote the
“Curious George” books!
H. A. Rey
• “The constellations have
such intriguing names somehow we expect the
books to show us groups
of stars in the shape of a
Lion, a Whale, Twins, etc.
But they show us nothing
of the sort. This book
serves to remedy the
situation.” ~H. A. Rey
Traditional vs. H. A. Rey
• Planetarium programs often give the option to display
constellations using Rey’s or astronomical stick figures.
• The free program on Sky and Telescope’s website uses
Rey’s figures!
Stars and Planets
• Stars are very numerous.
• Planets are few in number
(5 are visible to the
unaided eye).
• Stars are “fixed” relative
to each other.
• Planets “wander” relative
to the fixed stars. So they
are not in the same
location each night nor in
the same position year to
Stars and Planets
• They produce their own
light independent of the
Sun’s location.
• Their brightness does
depend on the Sun’s
• They are very far away
• Relative to the stars, they
are near to Earth.
Stars and Planets
• Can be anywhere on the
planisphere (Star and
Planet Locator)
• Must be on (or very near)
the ecliptic (which is
indicated by a dashed line
on the planisphere)
Polaris - The North Star
• Polaris is almost directly overhead at the North Pole.
• Polaris is about halfway up from the north horizon in
• Polaris is at the north horizon at the Equator.
• Polaris is not visible south of the Equator.
• Polaris doesn’t “move” due to the Earth’s rotation.
Rising and Setting Stars
• Other stars “move” in circles about Polaris.
• It takes 1 day to complete the circle.
• The rising and setting time of a star changes with the
• Stars peak overhead (zenith) 2 hours earlier each month.
Star trails in the northern sky
• As the earth spins on its
axis, the sky seems to
rotate around us. This
motion produces the
concentric trails traced by
the stars in this time
exposure of the night sky.
The north celestial pole
(NCP) is at the center.
Star trails in the northern sky
• The very short bright trail
near the NCP was made
by Polaris, commonly
known as the North Star.
• So this is proof that
Polaris is not exactly due
Star trails in the northern sky
• More proof that Polaris is
not exactly due north.
Star trails in the southern sky
• While the bright star
Polaris lies conveniently
close to the North
Celestial Pole, no bright
star similarly marks the
pole in the South.
• Still, the South Celestial
Pole is easily identified in
the picture as the point in
the sky at the center of all
the star trail arcs.
Star trails in the northern sky
Time of exposure from star trails
• The calculation below refers to the image on the preceding
• If angle is ~20 degrees, the time of the exposure was:
Star trails at mid-latitudes
• Star trails from a lower
latitude (trails set beneath
the horizon)
Compare rising and setting at high
and low latitudes
Star and Planet Locator
(aka Planisphere)
Pole star
Celestial Sphere
• The stars can be mapped on the inside of the sphere.
• Positions correspond to points over locations on earth.
– North Pole (center)
– Equator (circle around center)
• The earth turns under the stars, so the stars turn on the
• The sun blocks star light during the day.
• The earth blocks stars too far to the south.
• The horizon is the line of the ground for an observer.
• A star finder provides a cover that act as the horizon.
– You use a different cover depending on your latitude
• The planisphere’s wheel turns to set the day and time for
the observer.
• Stars “move” East to West over the course of one Night (in
circle about the North Star)
• Stars “move” East to West by 2 hours per month and
“return” to the same position after one Year
• It’s just caused by Earth’s daily spin and yearly orbit about
the Sun
Latitude and declination
• Latitude measures the number of degrees north or south of
the Equator. DeKalb is at 42º N latitude.
• The angle of Polaris to the north horizon gives the
observer’s latitude.
• Stars have a north-south position called the declination
which is similar to the idea of latitude
• Polaris has a declination of 90º N.
• Stars over the Equator have a declination of 0º.
Longitude and Right Ascension
• Longitude measures the number of degrees east or west of
the Meridian. DeKalb is at 89º W longitude.
Right Ascension
• Stars have an east-west position called the right ascension
which corresponds to the longitude of that star.
• The vernal (spring) equinox is used to set the meridian in
the sky.
• The earth is tilted 23º on
its axis.
• The equator is not in line
with the orbit of the Earth,
sun, moon and planets.
• The Earth, sun, moon and
planets follow an
imaginary line called the
ecliptic (indicated by
dashes on a Planisphere)
• The Plane of the Ecliptic
is illustrated in this
Clementine star tracker
camera image which
reveals (from right to left)
the Moon lit by
Earthshine, the Sun's
corona rising over the
Moon's dark limb, and the
planets Saturn, Mars, and
Saturn all night long
• January 13, 2005, was a
special date for Saturn
because that's when it is
closest to Earth: only 750
million miles away,
compared to a maximum
distance of almost a
billion miles.
• This makes Saturn
unusually big and bright.
Saturn all night long
• An astronomer would say
"Saturn is at opposition"
because Saturn and the
sun are on opposite sides
of the sky. Earth and
Saturn are closest together
at opposition
• Happens every 13 months
Saturn all night long
• Where to find Saturn in
mid-January, 2005
Comet Machholz all night long
• Can see Comet Machholz
clearly with binoculars,
maybe even naked-eye!
• How to recognize the
comet (next slide)
Comet Machholz all night long
• Comet Machholz to the
right of the Pleiades star
• Notice comet’s brightness
and extent compared to
point like stars
Equatorial mount on radio telescope
Declination track
Polar axis
Right ascension track
Green Bank 140 ft
Equatorial mount on Celestron14
(as in Davis Hall Observatory, NIU)
To Polaris
To Polaris
Prof. Hedin’s Points to Remember!!
Next 2 slides:
What to Remember - NS
• Polaris (the North Star) doesn’t “move” due to the Earth’s rotation.
• The angle of Polaris to the north horizon gives the observer’s latitude
Polaris Location Latitude
directly overhead
900 N (NP)
about halfway up
420 N (DK)
on horizon
00 (Equator)
below horizon Southern hemisphere
• Other stars “move” in circles about Polaris. 1 day to complete circle.
Angle between star and Polaris gives declination (N-S location)
What to Remember - EW
• What time during the day a star rises, is overhead, and sets changes
with the seasons
• look up on Star Chart (right ascension is the East-West location)
• Changes 2 hours/month
• Only on the Equator can all stars be viewed from a single location

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