bartman wrote: > How 'bout another question; > when does morning start and end. ie. is sunrise when the solar disk first > breaks the horizon or when it clears the horizon? and to that respect > would someone dare to put a technical defination to the term "gloaming" if
The definition below is from the US Naval Observatory: Define sunrise as the time when the apparent altitude (H) of the upper limb of the Sun will be -50 arc minutes (34' for refraction + 16' for semidiameter). For the Moon, horizontal parallax must be ADDED to the value of H. Twilights are found for H = -6 degrees (civil), -12 degrees (nautical), and -18 degrees (astronomical). Correct for height of the observer if not at sea level. The URL for the above is: http://tycho.usno.navy.mil/srss_comp.html An interesting aside: the angle subtended by the Sun is around 30'(1/2 degree), the semidiameter being half this. The angle subtended by the Moon is around 30' too!, I've always thought it a fantastic coincidence how the two main objects in our sky happen to have very near the same angular value. I decline the call to arms to define gloaming. regards, luke
