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The celestial sphere

celestial spherecelestial equator

The duration of twilight throughout the year at Sydney and Cape Horn.

Measuring altitude and azimuth on the celestial sphere.

zenithnadirmeridiantransit

altitudeazimuth

The altitude of the South Celestial Pole equals the observer’s latitude.

The celestial sphere appears to rotate about an invisible axis, running between the North and South Celestial Poles. The location (i.e., the altitude) of the Celestial Poles depends entirely on the observer’s position on Earth or, more specifically, their latitude. The charts in this book are produced for the latitude of 35°S, so the South Celestial Pole (SCP) is 35° above the southern horizon. The fact that the SCP is fixed relative to the horizon means that all the stars within 35° of the Pole are always above the horizon and may, therefore, always be seen at night, regardless of the time of year. The southern circumpolar region is an ideal place to begin learning the sky, and ways to identify the circumpolar stars and constellations will be described shortly.

The ecliptic and the zodiac

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The Sun crossing the celestial equator at the September equinox (spring equinox in the southern hemisphere.).

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The constellations

Since ancient times, the celestial sphere has been divided into various constellations, most dating back to antiquity and usually associated with certain myths or legendary people and animals. Nowadays, the boundaries of the constellations have been fixed by international agreement and their names (in Latin) are largely derived from Greek or Roman originals. Some of the names of the most prominent stars are of Greek or Roman origin, but many are derived from Arabic names. Many bright stars have no individual names and, for many years, stars were identified by terms such as ‘the star in Hercules’ right foot’. A more sensible scheme was introduced by the German astronomer Johannes Bayer in the early seventeenth century. Following his scheme – which is still used today – most of the brightest stars are identified by a Greek letter followed by the genitive form of the constellation’s Latin name. An example is the Pole Star, also known as Polaris and α Ursae Minoris (abbreviated α UMi). The Greek alphabet is shown here and a list of all the constellations that may be seen from latitude 35°S, together with abbreviations, their genitive forms and English names is here. Other naming schemes exist for fainter stars, but are not used in this book.