Diurnal Motion

Diurnal means "daily." Each day the celestial sphere turns once about an axis through two points known as the celestial poles. For an observer in the northern hemisphere, the north celestial pole is up in the sky while the south celestial pole is below the ground. The stars appear to turn around the north celestial pole, which defines the ("true") north direction. (In the southern hemisphere, the south celestial pole is up in the sky and defines the south direction.)

The diurnal motion is the most fundamental motion of the celestial bodies. For any place on earth (apart from the poles!) it defines the north-south axis known as the meridian, and hence any other direction in relation to this, regardless of the nature of the terrestrial landscape. Suppose, for example, that we find a group of monuments that are consistent in their orientation, and this has evidently been achieved over a wide area and despite a varied topography, so that it cannot be put down to prevailing winds, the local topography, or reference to distant landmarks. It follows that these orientations must be astronomical in the broadest sense, in that they can only have been achieved in relation to the diurnal motion of the celestial bodies. See also:

Cardinal Directions; Orientation. Celestial Sphere; Meridian.

References and further reading

Aveni, Anthony F. Skywatchers, 49-57. Austin: University of Texas Press, 2001.

Krupp, Edwin C. Echoes of the Ancient Skies, 3-6. Oxford: Oxford University Press, 1983.

0 0

Post a comment