Sun Motions of

On any particular day of the year, the sun occupies a certain position among the stars. As the celestial sphere turns in the course of its daily (diurnal) rotation, the sun—like all the stars—appears to trace out a line of constant "latitude" (for which the technical term is declination) on that rotating sphere. Unlike the stars, though, during the course of a year—as the sun moves gradually along its annual path through the stars, known as the ecliptic—its declination changes. It is at its farthest north at the June solstice, when its declination is about +23.5 degrees, corresponding to the latitude of the Tropic of Cancer on the earth. Conversely, it is at its farthest south at the December solstice, when its declination is about -23.5 degrees, corresponding to the terrestrial latitude of the Tropic of Capricorn. Over the years its declination varies continuously between these limits in a smooth, wave-like manner. In the same way, its rising position moves continually to and fro along the eastern horizon, reaching its limits at the solstices; and the same is true of its setting position in the west.

If we have an alignment that we suspect relates to the rising or setting position of the sun, then we only have to determine the declination of the rel evant point on the horizon in order to deduce when in the year the sun will rise or set there. There are always two such times: one when the sun is moving north (in the northern hemisphere spring or southern hemisphere autumn) and the other when it is moving south. There will be a slight uncertainty of up to a day in each case, mainly because of the leap-year cycle. It is convenient to express the resulting dates in terms of the modern Gregorian calendar, but when we interpret our alignments, we must bear in mind that the people who set them up would have used different calendars. For example, when medieval churches were constructed, the Julian calendar was in use. The other complication is that if we delve deeply into the past, we must make a slight correction (only normally significant on a time scale of millennia) to take into account an effect known as the change in the obliquity of the ecliptic.

See also:

Church Orientations; Gregorian Calendar; Julian Calendar; "Megalithic" Calendar; Solstitial Directions.

Celestial Sphere; Declination; Diurnal Motion; Ecliptic; Obliquity of the Ecliptic; Solstices.

References and further reading

Ruggles, Clive. Astronomy in Prehistoric Britain and Ireland, 24-25. New Haven: Yale University Press, 1999.

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