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 sky becomes 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 as big as 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 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 1m contains 15' and 1s contains 15" of right ascension.

Starting Points

Any spherical coordinate system comes with a natural, built-in zero value for its "latitude" coordinate, whether this is called latitude, declination, or something else. This zero marker is the equator. No other latitude line is like it.

But there's no such natural zero point for counting longitude — or in the sky's case, right ascension. All lines of longitude, or right ascension, are alike. So a zero point has to be picked arbitrarily.

On Earth, 0° longitude has long been defined as a line engraved on a brass plate set in the floor under a position-measuring telescope at the Old Royal Observatory in Greenwich, England. In the sky, 0h ("zero hours") right ascension is defined as where the plane of the Earth's orbit around the Sun (the ecliptic) crosses the celestial equator in Pisces. This point is called, for historical reasons, the First Point of Aries.