Digital Universe Guide - Hayden Planetarium

106 3. THE MILKY WAY ATLAS
3.3.18 Equatorial Coordinates Sphere
Group Name radec
Reference —
Prepared by Carter Emmart, Brian Abbott (AMNH/Hayden)
Labels Yes
Files radec.speck
Dependencies none
Notes Also represents Earth’s radio sphere (see below).
The equatorial coordinate system is a projection of our Earth-based coordinate system of latitude
and longitude onto the “celestial sphere.” The celestial sphere is an imaginary shell that surrounds Earth
upon which all objects in the sky lie. Astronomers describe an object’s position in the sky by its right
ascension (RA) and declination (Dec). Declination is simply a projection of our latitude on Earth. The
point directly above the North Pole (the zenith point) is the north celestial pole and is located at +90 ◦
declination. If you’re standing on Earth’s equator, your zenith—that point directly overhead—would lie
on the celestial equator.
Right ascension is based on Earth’s longitude but is expressed in hours instead of degrees.
Astronomers have split the sky into 24 hours (15 ◦ per hour) measured from the vernal equinox. An
object’s location is then described in hours, arcminutes, and arcseconds. For example, the star Sirius in
the constellation Canis Major, the brightest star in the sky as seen from Earth, is located at right
ascension 6 hours, 46 arcminutes, and −16 ◦ , 45 minutes declination.
We use arcminutes and arcseconds for right ascension to remind ourselves that the length of these
units depends on your declination. Close to the pole, an hour of right ascension will be quite small, while
at the equator, it will be at its maximum. As with lines of longitude on Earth, lines of right ascension are
not parallel with one another.
Drawing the Sphere From Earth’s perspective, the wire-frame sphere perfectly represents the
equatorial coordinates. If you examine the radec.speck file, you’ll see four main sections. The first
section is the ellipsoid command that draws the main sphere:
0 0 0 ellipsoid -s line -c 1 -r 20.245 -n 24,19
The second part contains a mesh statement that draws a line around the celestial equator; this has a
color index of 2. Following this is another ellipsoid statement that draws lines every 15 arcminutes in