Constellations & Motions in Our Sky
(Chapter 2)
Student Learning Objectives
• List attributes of constellations
• Identify locations on the celestial sphere model
• Describe the dynamic motions of the Earth,
Moon, and Sun
What is a constellation?
 Constellations are
visible groupings of stars
that appear to have a
pattern.
 Different distances
 Different ages
 Different brightness
a, b, g, d, e, z, …
http://www.skyandtelescope.com/observing/sky-at-a-glance/
Constellations
 88 official constellations
http://astromax.org/con-page/con-88.htm
 Circumpolar constellations are always above an
observer’s horizon.
 Asterisms are visual, noticeable groupings of stars,
usually pieces of constellations.
Summer
Triangle
Asrerism
What is the Zodiac?
What does the magnitude scale measure?
 Magnitudes are a measure of brightness.
 Apparent visual magnitude is how bright a star appears
to be in comparison to other stars, as viewed from Earth,
with your eyes. Hipparchus 190-120 bc
Sun
−26.7
Full Moon
−12.6
Venus
−4.4
Vega
0
Naked Eye Limit
+6
Binoculars Limit
+10
Practice
1) Which would appear
fainter to us when
viewed from Earth with
our eyes?
a. −4 magnitude star
b. +4 magnitude star
c. 0 magnitude star
2) If two stars have the
same energy output,
what would make one
star appear fainter than
the other, when viewed
from Earth?
What is the celestial sphere?
The celestial sphere model represents our
view of motions in the sky.
 Stationary Earth at Center
 All objects orbiting Earth
What is the celestial sphere?
The celestial sphere model represents our
view of motions in the sky.
 Stationary Earth at Center
 All objects orbiting Earth
Positions in the Sky
• Zenith (above)
• Nadir (below)
• Meridian (N  Z S)
• Altitude (height)
• Azimuth (N  E)
Altitude & Azimuth
http://astro.unl.edu/classaction/animations/coordsmotion/altazimuth.html
Degrees & Arcseconds
There are 360° in a full
circle.
Ɵ
 Arcminutes: 1° = 60’
 Arcseconds: 1’ = 60”
Ɵ = 31 arcminutes
Extended fist = 10°
Practice
1) How many arcseconds?
a. 1°
b. 1/2°
c. 2°
d. 2’
2) Through how many degrees does the Earth
rotate in 1 hour?
What are the motions of the Earth, Moon, and Sun?
 All of the apparent motion
in the sky is due to the
motions of the Earth.
 Rotation (spin)
 Revolution (orbit)
 Counterclockwise
 Solar system planet orbits
are nearly circular.
What are the motions of the Earth, Moon, and Sun?
 All of the apparent motion
in the sky is due to the
motions of the Earth.
 Rotation (spin)
 Revolution (orbit)
 Counterclockwise
 Solar system planet orbits
are nearly circular.
 Precession is a “wobble”
caused by differential
gravitational tugs from the
Sun and Moon on Earth’s
equatorial bulge.
In ~13,000 years, Vega will be
the North Star.
In ~26,000 years, Polaris will
be the North Star again.
 Precession is a “wobble”
caused by differential
gravitational tugs from the
Sun and Moon on Earth’s
equatorial bulge.
In ~13,000 years, Vega will be
the North Star.
In ~26,000 years, Polaris will
be the North Star again.
The synchronous rotation of the moon results in
always seeing the same face of the Moon.
The Moon’s orbital plane is off from the Earth-Sun
plane by 5º.
Practice
1) Why do stars appear to
move east-west?
2) Does precession have
any effect on the
celestial poles or
celestial equator?
3) How many rotations
does Earth complete in
1 revolution?
4) What are the Sun’s
motions?