CELESTIAL CO-ORDINATES
NEWCOMERS to astronomy can get thrown for a loop when they first encounter declination and
right ascension. Why are the positions of stars that are light-years away in the depths of space
stated in a system that's tied to latitude and longitude on Earth?
The celestial coordinate system, which serves modern astronomy so well, is firmly grounded in the
faulty world-view of the ancients. They believed the Earth was motionless and at the center of
creation. The sky, they thought, was exactly what it looks like: a hollow hemisphere arching over
the Earth like a great dome. The stars? "They're fireflies," explains Timón in The Lion King, "stuck
to that big, uh, blue-black thing up there."
The celestial dome with its starry decorations had to be a complete celestial sphere, early
skywatchers figured out, because we never see a bottom rim as the dome tilts up and rotates
around the Earth once a day. Parts of the celestial sphere are always setting behind the western
horizon, while other parts are rising in the east. At any time half of the celestial sphere is above the
horizon, half below.
Even today this is how the cosmic setup actually looks. Never mind that we're on a moving dust
mote orbiting a star in the fringe of a galaxy. In astronomy, appearances and reality are more
different than in any other area of human experience. Perhaps for this reason, astronomers are quite
comfortable living with both -- as long as the two are kept in their proper relationship. The celestial
sphere, with its infinitely large radius, appears to turn daily around our motionless Earth, from which
we use telescopes to examine wonders painted on its inside surface. The illustration here presents
the scene.
Whenever you want to specify a point on the surface of a sphere, you'll probably use what
geometers call spherical coordinates. In the case of Earth, these are named latitude and longitude.
Imagine the lines of latitude and longitude ballooning outward from the Earth and printing themselves
on the inside of the sky sphere. They are now called, respectively, declination and right ascension.
Directly out from the Earth's equator, 0° latitude, is the celestial equator, 0° declination. If you
stand on the Earth's equator, the celestial equator passes overhead.
Stand on the North Pole, latitude 90° N, and overhead will be the north celestial pole, declination
+90°.
At any other latitude -- let's say Kansas City at 39° N -- the corresponding declination line crosses
your zenith: in this case declination +39°. (By custom, declinations north and south of the equator
are called + and - rather than N and S.) This is the declination of the bright star Vega. So once a
day, Vega passes overhead as seen from the latitude of Kansas City.
HOURS AND DEGREES
Of course Vega doesn't move; it's the Earth that's turning. But we're talking appearances here. The
celestial sphere seems to rotate around our motionless world once in about 24 hours.
This daily motion is the basis of the numbering system used in right ascension. Instead of counting in
degrees, as with longitude around the Earth, right ascension is usually counted in hours, from 0 to
24 around the sky. This is just a different way of putting dividing marks on a circle. One hour in this
scheme is 1/24 of a circle, or 15°.
The benefit of this numbering system is that as the Earth rotates, you see the sky turn by about 1
hour of right ascension for each hour of time. This makes it easy to figure out when celestial objects
will come in and out of view. The stars become a giant 24-hour clock.
Since ancient Babylonia, people have divided both degrees and hours into finer units by means of
base-60 arithmetic. In 1° there are 60 arcminutes, written 60'. One arcminute contains 60
arcseconds, written 60". A good telescope in good sky conditions can resolve details about as fine
as 1" on the surface of the celestial sphere. By comparison, 1" of latitude on Earth is about 101 feet.
So if you had a telescope at the center of a transparent Earth, you could resolve details about the
size of a house lot up on the surface.
Because declination is given in degrees, fine gradations of it are usually expressed in the Babylonian
system of arcminutes and arcseconds. For instance, Vega's exact declination (2000.0 coordinates)
is +38° 47' 01".
Hours of right ascension are divided into minutes and seconds of time, not of arc. In one hour (1h)
are, naturally enough, 60 minutes, written 60m. In one minute of right ascension are 60 seconds,
written 60s. Vega's right ascension is 18h 36m 56.3s.
Notice the different notation for the different kinds of minutes and seconds. They're truly different.
Just as 1h contains 15°, so does 1m contain 15' and 1s contain 15".
Clear skies,
