Lecture 2
Fall 2006
Celestial Sphere and Constellations
Key Concepts:
- Prehistoric Astronomy
- Celestial Sphere and Time
- Constellations
Prehistoric Astronomy
Celestial Sphere
- An imaginary sphere of the sky where all celestial
bodies are projected against
- A geocentric concept
- It is not physical -- rather a model of the heavens
- Stars, Sun and planets appear to rise in the east and set in the west because the
Earth rotates from west to east.
- The Earth's rotation appears to make the celestial sphere rotate around us
each day.
- Because we live on Earth, we only see half the celestial sphere at any one moment.
- The particular half we see depends on time, date and our location on Earth.
- The night sky changes during the night because of the Earth's rotation about its axis
- The night sky changes during the course of a year because of the Earth's rotation around the
Sun.
- Point directly overhead is the zenith
- Boundary between Earth and sky is the horizon
- Meridian: imaginary half circle, stretching from your horizon due south, through the zenith to your
horizon, due north
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- A link to the applet which lets you explore the view of the Celestial Sphere from different locations
on Earth is here.
- Angular size: degrees, minutes and seconds
- More on degrees and angular size
- On Earth, latitude and longitude
- Latitude measures positions north or south, in degrees: with 0 degrees at the equator and
the north pole being 90 degrees north.
- Longitude measures "east-west" relative to the prime meridian
- On the Celestial sphere, its right ascension (RA) and declination (dec)
- Ecliptic is the path the Sun appears to trace around the Celestial sphere
- The plane of the ecliptic is tilted at an angle of 23.5 degrees to the plane of the
celestial equator.
- As the Earth goes around the Sun, it always points in the same direction in space
- 2 points of interest on this orbit: Spring and Fall equinoxes when the Sun shines equally
on both hemispheres.
- Also, Spring equinox (roughly March 21) when the Earth goes from being tipped slightly
away from the Sun to being tipped slightly toward the Sun. The Fall equinox (Sept. 22)
is when the opposite happens.
- 2 other points of interest: Summer and Winter Solstice.
- Summer solstice around June 21 when the Northern hemisphere receives the most direct
sunlight.
- Winter solstice around Dec 21 when the Northern hemisphere receives the
least amount of direct sunlight.
- Circumpolar stars always stay above the horizon, other stars stars rise and set
- Altitude of the celestial pole in your sky is equal to your latitude
