Actually, the terminator for civil sunrist/sunset is not a great circle. Sunrise/sunset is defined as the time when the upper limb of the sun is on the theoretical horizon and occurs when the center of the sun is about .835 degrees below the horizon. So, the terminator line for civil sunrise/sunset is a small circle less than a degree from the great circle passing through the center of the sun. This would not be visible on the Sun Clock type presentation of the terminator line.
Similarly, the levels of twilight are defined by small circles. Civil twilight ends when the sun's center is 6 degrees below the horizon and is described as when you need artificial light to read a newspaper. Nautical twilight is when the horizon is not visible at sea and is when the sun's center is 12 degrees below the horizon. The true terminator small circle is defined as when the sun's center is 18 degrees below the horizon and there is no light from the sun at all. The 18 degree value has historical roots from Islamic astronomy. All Islamic prayer times are astronomically determined. The "al-'isha" prayer time starts at the end of twilight and ends at daybreak. A crepuscular (i.e. twilight) arc at -18 degrees solar altitude is often shown on astrolabes to mark the end of twilight. Other values such as 19, 20 and 21 degrees were used, but 18 degrees became something of a traditional standard that has endured. The level of twilight is highly subjective and depends on local air quality, cloud cover, etc. Thus, the definitions of twilight level are somewhat arbitrary, but have endured for many centuries. Best regards, Jim James E. Morrison [EMAIL PROTECTED] Astrolabe web site at astrolabes.org --------------------------------------------------- https://lists.uni-koeln.de/mailman/listinfo/sundial
