Although many constellation names remain the same, their actual extent may well be different, since present-day astronomers use constellation boundaries laid down in detail by the International Astronomical Union (IAU).
The IAU boundaries are laid out in coordinates, which are used for defining precisely positions in space. They are, in other words, the equivalent on the sky, or celestial sphere, of latitude and longitude on Earth. The main reference circle is the celestial equator, which lies exactly between the north celestial pole (marked approximately by Polaris) and the south celestial pole for which there is no Polaris equivalent. Crossing this circle at an angle of 23.5° (the angle of axial inclination of Earth) is the path of the Sun, known as the ecliptic.
At the crossing points, night and day are of equal length. The crossing points move slowly along the celestial equator. This is known as the 'precession of the equinoxes'; it amounts to only 50 arc seconds per year. The starting point of the equivalent of terrestrial longitude id the spring (vernal) equinox, when the Sun moves to the north of the celestial equator. It is measured eastward along the celestial equator, and is not called longitude but right ascension (RA).
The units of right ascension are hours, minutes and seconds of time, not the terrestrial system of degrees, minutes and seconds of arc (note: 1 hour of right ascension equals 15°).
The equivalent of terrestrial latitude is called declination (dec). Declinations north of the celestial equator are counted as positive and those to the south are counted as negative. Declinations are measured in degrees. Thus the coordinates of Betelgeuse are: RA 5h 55m 10.2s, dec + 7° 24' 26"; and for Rigel: RA 5h 14m 32.2s, dec - 8° 12' 06". When using celestial coordinates, 1 hour is divided up into 60 minutes, and each minutes into 60 equal seconds.
(above) The Orion constellation which is one of the easiest to recognize.
