RodneyS Posted December 7, 2012 Share Posted December 7, 2012 According to http://www.arachnoid.../index_old.html, this afternoon's sunset at DCA is forecast to occur at 4:46:50 (Click "World Location List", under "Airports" double click "DCA", click "Time/Date/Position",click "Create Year List"). The DC sunset at the winter solstice on December 21st is forecast to occur at 4:50:20 P.M. -- 3 minutes, 30 seconds later. Why? Because of a phenomenon called "the equation of time." Between about November 3rd and February 12th, a true solar day is greater each day than 24 hours for two reasons: (1) the earth is moving faster around the sun than its average speed because it is closest to the sun in early January; and (2) the earth's motion around the sun is more in a due easterly direction than on average because the earth is close to a solstice, when there is little northerly or southerly movement of the earth relative to the sun. So, for these two reasons, the earth is making relatively greater easterly progress during November 3rd and February 12th than at any other time each year, causing the period of daylight to increasingly shift from the morning to the afternoon. The net result is that the earliest sunset in DC occurs about December 7th, but the latest sunrise here does not occur until about January 4th. For more information, see http://en.wikipedia....quation_of_time Link to comment Share on other sites More sharing options...
Ian Posted December 7, 2012 Share Posted December 7, 2012 Darn my tweet the other day was wrong-- used a site that only had to the minute. Oh well, 95% of Twitter is wrong. Link to comment Share on other sites More sharing options...
RodneyS Posted December 7, 2012 Author Share Posted December 7, 2012 Darn my tweet the other day was wrong-- used a site that only had to the minute. Oh well, 95% of Twitter is wrong. Most sites show sunrise and sunset times only to the nearest minute because actual sunrise/sunset times can differ from forecast values, due to atmospheric conditions. However, showing times only to the nearest minute is misleading around each solstice because it creates the impression that there is no change at all for several days and then, suddenly, there is a one minute change. Moreover, showing times to the nearest minute does not solve the problem of incorrect table times. For example, if the forecast sunset time to the nearest second is 4:46:30 P.M. and that is rounded up to 4:47 P.M., the actual sunset time to the nearest minute is just as likely to be 4:46 as 4:47. Link to comment Share on other sites More sharing options...
nw baltimore wx Posted December 8, 2012 Share Posted December 8, 2012 Is there any relationship between the latest sunrise and earth's perihelion? Or is it just a coincidence that they occur around the same time? Link to comment Share on other sites More sharing options...
RodneyS Posted December 8, 2012 Author Share Posted December 8, 2012 Is there any relationship between the latest sunrise and earth's perihelion? Or is it just a coincidence that they occur around the same time? There is a causal relationship. As the Wikipedia article on Equation of TIme explains: "If the Earth orbited the Sun with a constant speed, in a circular orbit in a plane perpendicular to the Earth's axis, then the Sun would culminate every day at exactly the same time, and be a perfect time keeper (except for the very small effect of the slowing rotation of the Earth). But the orbit of the Earth is an ellipse not centered on the Sun, and its speed varies between 30.287 and 29.291 km/s, according to Kepler's laws of planetary motion, and its angular speed also varies, and thus the Sun appears to move faster (relative to the background stars) at perihelion (currently around January 3) and slower at aphelion a half year later. At these extreme points, this effect increases (respectively, decreases) the real solar day by 7.9 seconds from its mean. This daily difference accumulates over a period." So, perihelion -- which occurs in our era just after the December solstice -- contributes about eight seconds to the length of a solar day. Additionally, at each solstice, the tilt of the earth's orbit contributes about 21 seconds to the length of a solar day. So at the December solstice, the two factors combine to produce a solar day about 29 seconds longer than 24 hours, thus delaying both sunrise and sunset by about 29 seconds. Overall, between November 3rd and February 12th, the period of daylight is shifted by about half an hour from morning to afternoon. At the June solstice, on the other hand, the earth is nearing aphelion, and thus earth's distance from the sun is contributing a negative eight seconds to the length of a solar day. So at the June solstice, the two factors combine to produce a solar day about 13 seconds longer than 24 hours. Correspondingly, at the March and September equinoxes, the tilt of the earth's orbit is contributing a negative 21 seconds to the length of a solar day, while the earth is fairly close to its average distance from the sun. So, at each equinox, the two factors combine to produce a solar day about 21 seconds shorter than 24 hours. Link to comment Share on other sites More sharing options...
RodneyS Posted December 8, 2012 Author Share Posted December 8, 2012 Is there any relationship between the latest sunrise and earth's perihelion? Or is it just a coincidence that they occur around the same time? Reading my previous response to your question, I now realize that the first sentence was inaccurate, because I was thinking only of the Northern Hemisphere in our era. So, let me try again: The latest sunrise will always occur close to the winter solstice in each hemisphere because earth's tilt factor contributes more to the change in the length of a solar day than does earth's distance from the sun factor. In the Southern Hemisphere, the latest sunrise actually occurs around the time of the June solstice, which in our era is when the earth is approaching its furthest distance from the sun (aphelion). In Melbourne, Australia, for example, the latest sunrise in 2012 was on June 29th. So, it is just a coincidence that perihelion happens to nearly coincide with the December solstice in our era. In about 4500 years, perihelion will coincide with the March equinox, but the latest sunrise in DC will still occur about when it does now. However, because the earth's distance from the sun factor in that era will be neutral at the time of the December solstice, a solar day in late December will be only about 21 seconds longer than 24 hours, rather than 29 seconds longer as it is now. Link to comment Share on other sites More sharing options...
nw baltimore wx Posted December 9, 2012 Share Posted December 9, 2012 Mind boggling stuff. Thanks, Rodney. Link to comment Share on other sites More sharing options...
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