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Constellations. 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

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Donna Kubik

PHYS 162 lecture Jan 20, 2005

what are constellations
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 constellations3
What are constellations?
  • We see the projected pattern of stars of varying distances
what are constellations4
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 change
  • 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

summer triangle
Summer triangle




summer constellations
Summer constellations
  • Cygnus (The Swan)
h a rey
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 rey12
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
  • 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.

Stars are “fixed” relative to each other.


Planets are few in number (5 are visible to the unaided eye).

Planets “wander” relative to the fixed stars. So they are not in the same location each night nor in the same position year to year

Stars and Planets
stars and planets16

They produce their own light independent of the Sun’s location.

They are very far away


Their brightness does depend on the Sun’s location.

Relative to the stars, they are near to Earth.

Stars and Planets
stars and planets17

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)

Stars and Planets
polaris the north star
Polaris - The North Star
  • Polaris is almost directly overhead at the North Pole.
  • Polaris is about halfway up from the north horizon in DeKalb.
  • 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
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 seasons.
  • 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
star trails in the northern sky21
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 north.

Star trails in the northern sky
star trails in the northern sky22
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 southern sky
time of exposure from star trails
Time of exposure from star trails
  • The calculation below refers to the image on the preceding slide:
  • If angle is ~20 degrees, the time of the exposure was:


celestial sphere
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 planisphere
  • 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 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 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 Mercury.
saturn all night long
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 long37
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 long38
Saturn all night long
  • Where to find Saturn in mid-January, 2005
comet machholz all night long
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 long40
Comet Machholz all night long
  • Comet Machholz to the right of the Pleiades star cluster
  • Notice comet’s brightness and extent compared to point like stars
equatorial mount on radio telescope
Equatorial mount on radio telescope


Declination track

Right ascension track

Polar axis



Green Bank 140 ft

equatorial mount on celestron14 as in davis hall observatory niu
Equatorial mount on Celestron14 (as in Davis Hall Observatory, NIU)

To Polaris

To Polaris



what to remember ns
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 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