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

http://www.skyandtelescope.com/observing/sky-at-a-glance/

, , , , , a b g d ez, …

88 official constellations

Circumpolar constellations are always above an observer’s horizon.

Asterisms are visual, noticeable groupings of stars, usually pieces of constellations.

http://astromax.org/con-page/con-88.htm

Constellations

SummerTriangleAsrerism

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

http://www.remi.poirier.com/astro/Chapter01celestialsphere.html

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.

Arcminutes: 1° = 60’Arcseconds: 1’ = 60”

Extended fist = 10°

Ɵ

Ɵ = 31 arcminutes

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?