Star Rising and Setting Positions

Any given star, viewed from a given place, traces the same path across the celestial dome night after night. Though it always rises and sets in the same place, the time of day when these two events occur alters constantly, getting earlier by about four minutes each night. Each of them will occur during daylight for about half of the year, and hence be invisible. So also will all or part of the star's daily passage across the sky, whenever the sun is in the sky at the same time. When people in the past observed the rising of certain stars and aligned sacred structures upon them, the event of particular significance was often the heliacal rise, the annual event when the star first rose sufficiently long before the sun to become visible in the predawn sky.

Observations of the rising and setting of stars are especially useful to the navigator wishing to set or maintain a course. Micronesian and Polynesian navigators, for example, had an excellent knowledge of the stars associated with various directions, and this served them well during their long voyages across the Pacific. In practice, such observations are hampered by extinction, the dimming of a star's light due to absorption in the intervening atmosphere. This problem is most severe when a star is close to the sea horizon because its light has had to travel through a great deal more of the earth's atmosphere before it reaches the observer than when it is higher in the sky. This is a potential headache for the navigator, as all but the brightest stars may need to be considerably above the horizon to be visible at all. However, it is less of a problem close to the equator, where the celestial bodies rise and set almost vertically.

Over a long timescale, the slow tumbling of the whole celestial sphere due to precession gradually causes the rising and setting positions of stars to drift. As a result, the place on the horizon where a star rises or sets in the present day may be several degrees away from where it rose or set at the time of interest (for example when a monumental alignment was built) several centuries or more in the past. Some stars that (for locations in the northern hemisphere) rise and set close to due south (or close to due north in the southern hemisphere) may not have appeared above the horizon at all at certain times in the past, while others visible then may since have disappeared.

Precession limits our confidence when we try to identify putative stellar alignments at ancient monuments, since there is invariably some (and often considerable) uncertainty about the date of construction and use. Extinction adds a further complication to the interpretation of such alignments, especially at high latitudes where stars rise and set at a considerable angle to the vertical, since it is difficult to be sure about exactly where a given star was when it appeared or disappeared.

See also:

Astronomical Dating; Methodology; Navigation.

Star Compasses of the Pacific.

Celestial Sphere; Declination; Extinction; Heliacal Rise; How the Sky Has Changed over the Centuries; Precession.

References and further reading

Ridpath, Ian, ed. Norton's Star Atlas and Reference Handbook (20th ed.), 5-7. New York: Pi Press, 2004.

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