The movement we are most familiar with is that of our star, the sun. The sun rises at a certain point on the eastern horizon, varying each day; it makes an arc in the sky and sets at a certain point on the western horizon, varying each day, following a cycle that includes two extrema (also called standstills): a maximum one, a northernmost rising point, the summer solstice, and a minimum one, a southernmost rising point, the winter solstice. At these two rising points, the sun rises only once a year; at all the other points in between it rises twice a year. Twice a year the sun crosses the celestial equator and therefore has vanishing declination; these days are termed the spring equinox and the autumn equinox. The definition of the equinoxes in a cultural, therefore archaeoastronomical, context, is extremely delicate, however; see Ruggles (2005) for details. The motion of the rising point throughout the year is sinusoidal, very slow around the solstices and very fast around the equinoxes.
The Tropics are the two parallels, symmetrical with respect to the equator, that mark a belt outside of which the sun never passes to the zenith, that is, vertically above an observer's head at midday. Within the tropical belt, the sun passes to the zenith twice a year, the first after the spring equinox, and the second after the summer solstice; at the Tropics themselves the sun passes to the zenith on the same day as the summer solstice.
If we observe, for instance, the eastern horizon, day after day, from a fixed position, it is possible to identify and mark on the ground the directions in which the sun rises on the days of the zenith passages and the direction of the rising point at the day of the summer solstice. The naked eye is assisted in this type of measurement by "foresighting" that is, observing the star with an instrument interposed, which might be a finger, a stake, a cross-staff, or a fork. Once markers have been set out on the ground indicating the various directions on the horizon—stones, for instance—the corresponding positions of the sun can be deduced and subsequent positions can be forecast. The measurements will be more accurate if the alignments are long, and for this reason, to achieve greater precision, it is better to adjust the position of the observer in order to use, for the desired alignment, a marker located on the horizon (for example, a far-off mountain peak or a notch).
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