The Celestial Sphere
- From our perspective on Earth the stars appear scattered on a two-dimensional surface. We have no sense of depth.
- One way to visualize the motions of the stars throughout a night and throughout the year is to revive the arcane concept of a sphere surrounding the Earth containing the stars - The Celestial Sphere.
- Reference points on the celestial sphere:
- Celestial Poles -- The extention of the Earth's North and South
poles onto the celestial sphere.
- Rotating the sphere about the poles causes the stars to rise
in the East and set in the West
- actually the Earth is rotating inside of the fixed
sphere -- but it looks to us as if the sphere is turning about a fixed Earth.
- Celestial Equator: A circle around the celestial sphere
which is the extention of the Earth's equatorial plane out onto the sky.
- As the celestial sphere rotates the position
of the equator remains fixed in the sky (see movie below).
- The North celestial pole (marked temporarily by the North Star -- Polaris)
lies overhead for an observer at the North Pole and on
the horizon for an observer at the Equator.
- In general the ``altitude" of the pole in the sky is
equal to the observer's latitude.
- Stars trace out circles around the celestial pole as the Earth
- Stars close to the celestial pole never set because they
never come close to the horizon. These are called circumpolar stars.
- The further north (or south) of the Equator you are, the
more circumpolar stars you see.
- Every day the Sun lies in front of a slightly different location on the
celestial sphere due to the Earth's orbital motion.
- In the course of a year the changing position of the Sun
among the stars traces out a complete circle around the sky.
- This circle is called the ecliptic and represents
the apparent path followed by the Sun around the
celestial sphere as the Earth orbits the Sun.
- Review: Can you narrarate this movie?
Updated September 14, 2005
Special thanks to Robert Knop at Vanderbilt for creating the animations (and captured still images) used on this page. See http://brahms.phy.vanderbilt.edu/~rknop/classes/a102/fall2004/handouts/index.shtml