1. Constellations & Motions in Our Sky
(Chapter 2)

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

3. 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, …

4. Constellations 88 official constellations
Circumpolar constellations are always above an observer’s horizon.
Asterisms are visual, noticeable groupings of stars, usually pieces of constellations.

5. Summer
Triangle
Asrerism

6. What is the Zodiac?

7. 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 bc
Sun
−26.7
Full Moon
−12.6
Venus
−4.4
Vega
Naked Eye Limit +6
Binoculars Limit
+10

8. Practice
1) Which would appear fainter to us when viewed from Earth with our eyes?
−4 magnitude star
+4 magnitude star
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?

9. 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

10. 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

12. Positions in the Sky Zenith (above) Nadir (below) Meridian (N  Z S)
Altitude (height)
Azimuth (N  E)
Altitude & Azimuth

13. Degrees & Arcseconds There are 360° in a full circle.
Arcminutes: 1° = 60’
Arcseconds: 1’ = 60”
Extended fist = 10° Ɵ
Ɵ = 31 arcminutes

14. Practice How many arcseconds? a. 1° b. 1/2° c. 2° d. 2’
2) Through how many degrees does the Earth rotate in 1 hour?

15. 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.

16. 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.

17. 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.

18. 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.

19. 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.

20. Practice Why do stars appear to move east-west?
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?