Of Lightness and Dark...
Posted July 31, 2006
While that works out pretty well near the vernal and autumnal equinoxes, the reality can be surprising at other times of year when the north pole of the earth's rotational axis is tilted significantly toward (summer) or away (winter) from the sun and the "terminator" that divides day from night, or the sunlit half of the globe from the dark side, cuts at skewed angles across latitude and longitude lines.
A good example was a question from a recent visitor to Wisconsin who noted that sunrise seemed awfully early there in relation to sunrise here in Raleigh given how much farther west he was (the times only differed by 10-15 minutes) while on the same date sunset in Wisconsin was well over an hour later than sunset around here. I checked some sunrise and sunset calculations for the two regions, and noted that six months earlier, the situation would have been reversed, with a large difference in sunrise times but sunset times fairly close to one another.
The geometry that produces this effect can be a little difficult to visualize mentally, but can be very quickly grasped using a neat online tool available from the U.S. Naval Observatory astronomical web site, where they have an Earth view generator that can quickly create a series of 5 views of the earth showing areas that are lit and those that are dark for any selected date and time (note the times are in Universal Coordinated Time - to get that you just add 4 hours to EDT, or 5 hours to EST). I generated a pair of image sets for the time of sunrise and sunset in Raleigh for July 1st, to illustrate the oddities of how the two times vary between here and other locations.
You'll note in the following sunrise examples that because of the tilt of the earth's axis, the terminator dividing night from the impending daylight runs right through Raleigh, but it doesn't run north-south. Instead, it angles toward the northwest, so that Wisconsin is about to spin into the light at almost the same time as Raleigh. This angle also results in our seeing the sun rise above the horizon not due east this time of year, but north of east.
On the other hand, at sunset in Raleigh (images below), with our city just about rotating into the darkness, the terminator now extends off toward the northeast, so that Wisconsin still has a significant period ahead during which the sun stays above the horizon.