The Celestial Sphere
External Links page.
- What is it? An imaginary sphere
surrounding the Earth on which celestial objects appear to be
located; thought to really exist in antiquity, today used as
tool to identify locations in the sky.
- Special places: Celestial Equator,
North and South Celestial Poles,
the Ecliptic; Solstices and equinoxes.
At any location on Earth, the altitude of the Celestial
Pole is the same as the latitude of that location.
- Apparent daily motion: As
the Earth rotates, the whole sky "moves backwards". As
a consequence, most objects, except for Polaris (the North Star – What
is the altitude of Polaris?) and other circumpolar ones, rise in
the East and set in the West every day.
- What can we do? Use coordinates
on the celestial sphere, as opposed to altitude and azimuth; right ascension
RA ("longitude" around the equator from the Spring equinox,
in hours, minutes, seconds) and declination ("latitude" above
the equator, in degrees).
Motions on the Celestial
- Proper motion: Subtract
the effect of Earth's daily rotation and yearly orbit, to find actual
displacements on the celestial sphere.
- The Sun and the Moon: They always move
Eastward, the Sun along the
ecliptic, the Moon on a faster orbit that produces different
- Planet: They move in more complicated,
slow paths near the ecliptic, usually West to East but sometimes with
an apparent retrograde motion (East to West); Occasionally they are
in conjunction, in opposition, or they transit the Sun.
- Other objects: Small "nearby" objects
asteroids, or even closer ones) move much faster, stars and more distant
with the naked eye.
- Telescope view: Nearby stars
show a slight back and forth motion every six months (parallax),
or move in other ways over periods of years. (Forget completely about
seeing galaxies move, although most of them are moving at millions
mph, at least!)
- What have humans made of all this?
They identified constellations and assigned meanings to their
patterns; Tried to interpret or understand the motions of planets;
Concluded that the Earth doesn't move, from evidence
including the apparent lack of stellar parallax...
page by luca bombelli <bombelli at olemiss.edu>,
modified 29 sep 2012