Celestial coordinates, maps and magnitudes

A star map shows the celestial sphere on a flat sheet: stars as dots of different sizes by brightness, constellations joined by lines, and a grid of α and δ. A planisphere is a rotating star map: by matching the hour and date rings you can see which constellations are above the horizon at that time. Stars within a constellation are labelled by brightness with Greek letters (Bayer, 1603): for instance Vega is α Lyrae (the α of Lyra); many stars have Arabic names (Altair, Aldebaran, Betelgeuse, Rigel) because medieval Muslim astronomers recorded them. Ulugh Beg’s Zij-i Sultani (completed in the main around 1437) gives the coordinates of about 1018 stars; among modern catalogues Hipparcos (1989–1993) measured about 118,000 stars and the Gaia space telescope (launched in 2013) has measured the positions of billions.