Astro 120 Fall 2005: Lecture 3 page 1
•Constellations
•The Celestial Sphere
•the link with terrestrial coordinate systems
•latitude and longitude
•Diurnal Motion
•Horizon (local) System
•altitude (horizon to zenith) and azimuth (East from due North)
•changes with time and position
•Celestial (Equatorial) coordinate system
•Declination (celestial equator to celestial pole)
•Right Ascension (RA) (east from vernal equinox)
•fixed to the stars
•Finding the celestial pole and equator at any longitude
Brief review of last time: Sky Positions
Reading: Bennett, Chapter 2, Sections 2.1-2.3
Polaris Project: Web course (Astro 102/103X)
Practice Quizes:! ! link on Website
Problem Set #1: !Distributed Thursday
Astro 120 Fall 2005: Lecture 3 page 2
Declination
-RA
Celestial Coordinates
declination (dec) : just like latitude
right ascension (R.A.) : measured East from vernal equinox
Astro 120 Fall 2005: Lecture 3 page 3
A simplified picture - the meridian diagram
North
Celestial
Pole
(NCP)
zenith
celestial
equator
N
S
latitude
90o - latitude
Meridian
Astro 120 Fall 2005: Lecture 3 page 4
Diurnal (Daily) Motions of the Sky
•one circle per sidereal day
•biggest circle on celestial equator (dec. = 0o)
•smaller circles nearer poles (higher declination)
stars move in circles around the celestial poles
Circumpolar regions
diurnal circle lies entirely
above the horizon
declination > 90o – latitude
