The Sky

1
The Sky

• Seeds chapter 2

2
Objective

• To understand what the universe looks like seen
  from the surface of our spinning planet

3
Stars

• The night sky is the rest of the universe as seen
  from our planet
• Away from city lights, we see a few thousand
  stars in the sky

4
Constellations

• Constellations celebrate the most important
  mythical figures in each culture
• Constellations generally dont look like the
  figures they honour

5
Asterisms

• Asterisms are less formally defined groupings
• Stars in constellations and asterisms have
  similar directions but not distances

6
Names

• Common star names derived from ancient Arabic
  (Betelgeuse is from yad al-jawza armpit of
  Orion)
• Greek letters constellation ( ? Cen)

7
Brightness

• Hipparchus (2nd c BC)
• Six brightness classes
• Magnitude scale
• Apparent visual magnitudes

8
Magnitude intensity

• Each magnitude step is about a factor 2.5 times
  in brightness
• The faintest visible 6th mag. stars are about
  100x fainter than the bright 1st mag. stars

9
Celestial sphere

• Stars we see depend on location time
• Easiest to imagine a heavenly sphere
• Horizon
• Zenith
• Celestial poles
• Celestial equator

10
Celestial pole

• Apparent motion - stars appear to circle the sky
  about a fixed point above south (north)

11
Celestial equator

• Towards the celestial equator the stars move
  along gentle arcs

12
Angles on the sky

• Angular distance
• Angular diameter
• Crux 5 degrees long
• Moon 0.5 degree

13
Latitude sky

• The stars we can see change with latitude
• Height of pole is equal to your latitude
• Circumpolar constellations dont set

14
Precession

• Earths axis slowly precesses like a spinning top
• This changes the relative position of stars to
  the celestial poles and equator

15
Effect of precession

• N S celestial poles do a complete circuit each
  26,000 yrs