The notion of recording the position of a celestial body with two coordinates—on the model of terrestrial longitude and latitude—gave rise to three main systems of orthogonal reference.

Tycho Brahe introduced today's equatorial coordinates. To define them, we need to establish—with respect to the stars—the positions of the celestial poles, of the celestial equator and, on the latter, of the spring equinox, i.e., the point reached by the Sun every March 21st. The right ascension is the angle that separates the body's hour circle from the spring equinox; the declination is the angle that separates the body from the celestial equator.

However, with a cycle of about 26,000 years, the precession of the equinoxes shifts the spring equinox along the ecliptic and shifts the celestial poles between the stars. The phenomenon slowly modifies both equatorial coordinates —but in a pattern that remains very complicated to compute even today. Claudius Ptolomaeus (Ptolemy) and the ancient astronomers, who already knew that the precession occurs around the poles of the ecliptic, preferred to use the ecliptic coordinates. The ecliptic longitude is the angle that separates the body's ecliptic meridian from the spring equinox; the ecliptic latitude is the angle that separates the body from the ecliptic. Over the years, precession increases only the longitude of the body; as a result, the use of ecliptic coordinates simplifies enormously the computations needed to update old data.

Between the 9th and 16th Cs, Islamic astronomers popularized altazimuth coordinates, determined by the horizon and zenith of a particular observation locality. The azimuth is the angle that separates the semicircle of the height of the body from the geographic north; the altitude is the angle that separates the body from the horizon. But the apparent rotation of the celestial sphere in 24 hours rapidly changes the body's apparent position relative to the horizon. Therefore, an observation in altazimuth coordinates must always be accompanied by a precise recording of the hour at which it was made.