Explanation of twilight. Twilight is the period of approximately one hour’s duration before the sun rises in the morning and the period of approximately one hour’s duration after the sun sets in the evening, ie between the times when the sun is 12° below the horizon and sunrise in the…
Calculating times of sunrise and sunset. The times of the visible rising and setting of the sun are contained in the daily pages of the nautical almanac. They are tabulated for the middle day of the three on that page and provide the Local Mean Time (LMT) of the rising/setting…
Polar Distance. This is the angular distance of a body from the elevated pole (the pole that is above the celestial horizon). This is the distance PX in the sketch. When the elevated pole and the declination have the same names, the polar distance is 90° – declination. When they…
Since the declination gives us the “latitude” of the heavenly body we must now find it’s “longitude”. We do this by measuring the angle between the celestial meridian passing through the body and that running through the observer’s position. The hour angle is defined as “the angle between the meridian…
Declination. Declination corresponds to terrestrial latitude and is defined as the angular distance of a heavenly body north or south of the celestial equator, measured in units of arc (ie degrees, minutes and seconds). Declination is the result of the earth’s movement/orbit around the sun and the fact that the…
Introduction. Astro-navigation is based upon the concept of the “celestial sphere”. What is the “celestial sphere”? If one looked up into the night sky one would see what would appear to be an enormous black dome with all the heavenly bodies stuck to the inside like thousands of diamonds. This…
