1. Compare and contrast between an asterism
and a constellation
2. Be able to orientate in the night sky to
celestial objects
3. Be able to measure distances and sizes of
celestial objects
celestial sphere
Circumpolar stars
Right Ascension
Angular distance
Angular size
Constellation: A recognizable pattern in the
night sky.
Examples: Orion, Ursa Major, Draco
There are 88 constellations.
12 constellations are named after the Zodiac
signs, such Libra, Leo, and Pisces..
• Asterism: recognizable patterns in the sky
that are not a constellation.
• Such as, Big dipper is an asterism in the
constellation Ursa major.
The Summer and Winter Triangles
two useful asterisms
• Summer Triangle: consists of the 3 stars
Deneb, Vega, and Altair.
• Winter Triangle: Consists of the 3 stars
Procyon, Betelgeuse, and Sirius.
The Celestial Sphere
• Celestial Sphere: an imaginary sphere that
extends out into the blackness of space in
which celestial objects appear to be placed.
• The celestial sphere is a useful fiction.
Rotation of the Celestial Sphere
• Since the earth rotates, and carries us west to
east, objects in the sky appear to rotate in the
other direction, from east to west.
• This the same reason that the sun rises in the
East and sets in the west.
Zenith and Meridian
Zenith: the highest point over head, no matter
your position on earth.
Meridian: The imaginary great circle that runs
through the celestial north and south poles.
Azumith and Altitude
• Azimuth/Right Ascension: the angle or hour
that measures the position of an object along
the horizon, in the eastern direction. (Toward
your right if your facing north)
• Any object on the meridian has an Azimuth of
0 degrees.
• Altitude/Declination: both are measured in
how many degrees an object above the
The Ecliptic
• Since our planetary system (the solar system)
was formed from the flattening of an
accretion disk all of our planets orbit on the
same plane with the sun in the middle.
• The planets of our solar system move across
the sky along the ecliptic.
Circumpolar Stars
• Circumpolar Stars: Stars that always stay
above the horizon, and are near the northern
celestial pole.
Seasonality of the Constellations
• The orbit of the earth around the sun changes
the earths location.
• For this reason the constellations change with
the season.
Angular Size and Angular Distance
• Angular Size: is the angle an object appears to
span in your field of view.
• Angular Distance: is the angle that appears to
separate two objects.
Your hands as a measuring tool
Stretched out pinky to thumb: 25 degrees
Index to pinky finger: 15 degrees
Width of your knuckles: 10 degrees
Three fingers: 5 degrees
Pinky: 1 degree

Orientation of the Night Sky