At 05:19 PM 8/16/96 -0500, you wrote: >I must express my gratitude for all of your participation in my query! >I think it would be foolish of me to reveal the time just yet. I was so >impressed with all th responces and the dialog that I just can't cut it >off now. All of the responces were great in giving us opinions about the >event time.
After all this you should keep us informed. >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 >that is the right spelling? 1. The moment of astronomical sunrise at the given position is the time when the center of the solar disk crosses the plane of the astronomical horizon. The plane of the horizon is the plane through the center of the Earth and normal to your vertical. If you were at sea, if there were no waves, if there were no atmospheric refraction, if you kept your eyes exactly at the sea level then you would see exactly half of the solar disk. All the mentioned conditions never. You keep your eyes few meters above the sea level, there is refraction, we may disregard waves unless humungous. In such conditions the sea horizon (the one you see) is dipped below the astronomical horizon of your place. This means that at the time of astronomical sunrise the solar disk is well above the sea horizon something like 2/3 of the disk diameter. 2. Visible sunrise is defined as the moment when the solar disk starts appearing above the visible (apparent horizon). This moment precedes of course the astronomical sunrise. The time of visible sunrise depends on your position, time of the year, refraction and height of eyes above the horizon. 3. For other purposes two kinds of twilights are defined: nautical - when the Sun's altitudes are between -12 and -6 degress (below the horizon). It is still dark at this time but at sea you can see the sea horizon which enables you to measure altitudes of still visible stars. civil - the Sun's altitudes are between -6 to 0 degrees. It is really dawn preceding the sunrise. 4. The sequence of events for sunrise is as follows Sun's altitude event --------------- --------------------------------- -12 deg start of nautical twilight -6 deg end of nautical, start of civil twilight about -1 visible sunrise occurs (start of the civil day) 0 deg astronomical sunrise 5. All definitions are symmetrical for sunsets. >Thanks again for you responces the light has not yet shone > >Bart All best again - Slawek Grzechnik
