Celestial Equator
Celestial Equator
Each day and night as the fixed stars appear to rotate around the
earth like a giant celestial sphere, the
circumpolar stars (above the
horizon in the north) turn in circles around the north celestial pole
(Polaris, the north
star). Perpendicular to the north celestial pole is the celestial
equator, which lies directly above any observer located on the
equator of the Earth.
In their daily or diurnal
motion, stars located on the celestial equator rise due east and
set due west as seen from anywhere on Earth. Example: Mintaka, the
top star of Orion's
belt.
You can trace the course of the celestial equator even if there
are no stars visible (see below). However, in late autumn it runs
eastward from the setting Summer
Triangle through Pisces,
Cetus, Orion,
Monoceros, and
Hydra. In late
spring....
Observe
Observe in the planetarium and/or with a model celestial
sphere:
- Does the celestial equator always intersect the horizon at due
east and due west?
- Does the celestial equator lie in a plane that is always
perpendicular to the axis of the earth and the north
celestial pole?
- What is the altitude of the celestial equator on the
meridian as seen from Shawnee?
- From a constant terrestrial latitude, is there any variation
in the altitude of the celestial equator on the meridian depending
on the time of night or the season of the year?
- Does the angle between the celestial equator and the horizon
remain the same throughout the year?
- For a review of the celestial equator, and where it is located
relative to the horizon, see diurnal
motion.
Equatorial Coordinates
- Equatorial coordinates are used most often by modern
astronomers to specify the location of any star, comet, deep sky
object, etc. Tables of interesting objects and their equatorial
coordinates may be found in the current issue of Sky and
Telescope.
- What are the units of measurement by which the
celestial equator is marked off?
- Right Ascension (RA)
- Declination (d)
- Sidereal Time (ST; used to know when
to look for comets, planets, constellations, deep sky objects,
etc.)
- Given the average declination of a constellation, how could
you tell if it is visible from Shawnee, Oklahoma? (Answer)