On Sunday 27 June 2010 06:07:33 John M. Steele wrote: > The above calculations use the (center of the) geometric sun and neglect > refraction of earth's atmosphere. Calculated sunrise and sunset are > calculated for 54' (arcminutes) below the horizon under the same > conditions. The 54' figure is the sum of the mean semi-diameter and mean > refraction at the horizon. These both vary slightly with conditions but on > average place the upper limb at the apparent horizon.
The center of the sun is called the cazimi. Back to topic: I had to learn (and I forgot the order of magnitude, so I just looked it up) the corrections for refraction and curvature when leveling. The correction for curvature, in millimeters, is 78.5 times the square of the distance in kilometers. The resemblance to 25π is no coincidence: it's because the circumference is 40 Mm. The correction for refraction is 11 times the square of the distance the other way. The refraction correction for a heavenly body is 2.687b/(T*tan V), where b is in kilopascals, T is in kelvins, and the result is in minutes of arc. This is also in the surveying book, but the temperature is in degrees Fahrenheit, and I did not immediately recognize the offset to rankines because I'm used to degrees Celsius. So I wrote a spreadsheet to convert the formula to various combinations of units. Pierre -- Don't buy a French car in Holland. It may be a citroen.
