10.6 Right Ascension and Declination

The globe of the heavens resembles the globe of the Earth, and positions on it are marked in a similar way, by a network of meridians stretching from pole to pole and of lines of latitude perpendicular to them, circling the sky. To study some particular galaxy, an astronomer directs the telescope to its coordinates.

On Earth, the equator is divided into 360 degrees, with the zero meridian passing Greenwich and with the longitude angle ϕ measured east or west of Greenwich, depending on where the corresponding meridian meets the equator.

In the sky, the equator is also divided into 360 degrees, but the count begins at one of the two points where the equator cuts the ecliptic — the one which the Sun reaches around March 21. It is called the vernal equinox (“vernal” means related to spring in Latin) or sometimes the first point in Aries, because in ancient times, when first observed by the Greeks, it was in the zodiac constellation of Aries, the ram. It has since then moved, as is discussed in the later section on precession (http://www.phy6.org/stargaze/Sprecess.htm).

The celestial globe, however, uses terms and notations which differ somewhat from those of the globe of the Earth. Meridians are marked by the angle α (alpha, Greek A), called right ascension, not longitude. It is measured from the vernal equinox, but only eastward, and instead of going from 0 to 360 degrees, it is specified in hours and other divisions of time, each hour equal to 15 degrees.

Similarly, where on Earth latitude goes from 90∘ north to 90∘ south (or -90∘), astronomers prefer the co-latitude, the angle from the polar axis,equal to 0∘ at the north pole, 90∘ on the equator, and 180∘ at the south pole. It is called declination and is denoted by the letter δ (delta, Greek small D). The two angles (α, δ), used in specifying (for instance) the position of a star are jointly called its celestial coordinates.

The next section tells how the stars, the Sun and accurate clocks allowed sailors to find their latitude and longitude.