Re: New York Times - Today will be the earliest sunset of the year.
Thanks Bob for this link. This date may be true for NYC but the date varies with latitude. It is 7 Dec for NYC at 40.7° but 28/29 Nov for Miami at 25.7° and 10 Dec where I live in Sidney by the Sea BC at 48.6°. Some time ago I developed a spreadsheet to calculate sunrise and sunset times. Input the location, Lat and Long, and the starting date and the spreadsheet calculates the equation of time, declination rise and set times for a two month period. The results for my location are plotted in the chart attached, a 25 kb file. The spreadsheet is too big at 87 kb to attach but I would be happen to send it to anyone for their use. Helmut Sonderegger programmed the equation of time and declination algorithms in a spreadsheet that I imported into this spreadsheet for these calculations. It works well to show what happens around the solstice. Regards, Roger Bailey Walking Shadow Designs From: Robert Terwilliger Sent: Wednesday, December 07, 2016 6:34 AM To: sundial@uni-koeln.de Subject: New York Times - Today will be the earliest sunset of the year. http://www.twigsdigs.com/annex/sunset.html Bob --- https://lists.uni-koeln.de/mailman/listinfo/sundial Solstice Rise Set.doc Description: MS-Word document --- https://lists.uni-koeln.de/mailman/listinfo/sundial
New York Times - Today will be the earliest sunset of the year.
http://www.twigsdigs.com/annex/sunset.html Bob --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Earliest sunset
Is there a name for the day on which the earliest sunset occurs? I know it varies with latitude unlike the solstice. Cheers, John --- https://lists.uni-koeln.de/mailman/listinfo/sundial
Re: Earliest sunset
An excellent article on this is: Wagon, Stan. Why December 21 Is the Longest Day of the Year Mathematics Magazine, Vol. 63, No. 5, December, 1990, pp. 307-311. Mathematics Magazine is a journal of the Mathematical Association of America (MAA) and is available in most all college libraries in the US. Despite the subtitle of the article, How fundamental ideas from max-min theory explain the location of the extrema of the times of sunrise and sunset, there are no formal equations to wade through, but nice graphs and figures explaining the equation of time and effects such as offset sunrise and sunset extrema. Very readable. The title refers to the fact that since the equation of time has the greatest advance (slope) on December 22, this day is the longest in terms of time from solar noon to solar noon the next day. Also, John Shepherd is correct--the ecliptic angle is the predominant cause. The sunrise/sunset phenomenon and the general shape of the graphs of solar noon and solar day length would be the same even if the earth's orbit were a circle centered at the sun! If anyone who wants the article can't find it, send me an email with your address and I'll drop a photocopy off in the mail (post). Ron Doerfler [EMAIL PROTECTED]
Re: Earliest sunset
Earliest sunset is around 12 dec., but the latest sunrise is around 30 dec. Combining the two gives the shortest day at 21 dec. Thibaud Taudin-Chabot [EMAIL PROTECTED]
Re: Earliest sunset
Here is a copy of the letter I sent to Rory, who contacted me through my web page (http://www.uwrf.edu/sundial/), before the letter from Jack Aubert was posted. Cheers, John Rory, I am sorry to have taken so long to reply but I have a couple of deadlines to make. Your question is an interesting one and I think if you study the large photo of the Swensen Sundial you can see the answer. Many years ago a golf-playing uncle in Los Angeles asked me why it was that the shortest day of the year was the winter solstice, but the sun actually set earliest on about Dec. 13 (he had noticed that he could get in an extra hole of golf on Dec. 21 -- I'm not making this up.) After a lot of research (there was no Web then!) I realized the answer had to do with the equation of time, the difference between solar and sideral days, and mean solar time (and no doubt you know more about this than I do.) Yes you are correct. The annalema shapes arise because of the equation of time, shifting the correct positions of the hour markers. Now on my dial sun rise/sun set can be represented by a horizontal line through the base of the lower gnomon. In other words the tip of the shadows can never be higher than that point. Now I didn't cut the annalemas off there or put a horizontal line (we already have the lines of the mortar in the bricks) as I thought it would spoil the looks of the whole dial. There are some anchient dials in existence that have so many lines on them that the meaning of some of them have been lost! Anyway imagine a horizontal line through the bottom of the gnomon. Look at where that line intersects an analema, say the 5 pm or right-mostone. It is somewhere between the two upper daylines for the Gold or spring side but it is between 2nd and 3rd day lines for the Fall side. This is because in the late Fall the Sun sets before 5pm and the shadow never reaches that part of the analemma. Because of the rapid curve of the annalemas near the top of the dial the time of sun set varies rapidly becoming later as we pass the Winter Solstice. This explains the change in sunset and that the earliest sunset is not at the winter solstice. So I told my uncle this, and the family got down to accomplishing other life-tasks. Unfortunately, I recently realized that this explanation must be false. This came about by looking more closely at an analemma on a globe. If I am not mistaken, the analemma describes not only the difference between mean and clock-time (due to the eccentricity of the earth's orbit being non-zero) but also the (I guess arbitrary) dates on which our clock is set. I.e. when the official timekeepers say that the annual clock is zeroed. And here, I was shocked to see a) that this date seems to be the winter solstice; and b) that I had never noticed this before! No its a difference and when averaged over the whole year it must come out to zero. Also its not just the eccentricity of the orbit with the Earth being closest to the Sun on Jan 4th. but the larger effect (+-10 mins to +- 8mins.) is due to the 23.5 degree tilt of the Earth's Axis. The eccentricity effect is zero Jan 4th and 6months later. The tilt effect is zero Dec 21st. and every 3 months after. You see, I always knew that the earth was moving fastest around Christmas time (near perihelion) and so I figured it made sense that the clock was furthest out of whack around this time. And sure enough, consulting either an almanac or a St. Joseph's Aspirin calendar, one can see that the sun indeed sets earliest on Dec. 13, not Dec. 21. BUT HOW IS THIS POSSIBLE, IF THE CLOCK IS ZEROED ON THE SOLSTICE? Shouldn't the equation of time being equal to zero on Dec. 21 mean that the sun should set the earliest on that date, too? The equation of time which is the combination of the two effects is actualy zero close to the 25th. but this is not arbitary. I hope this helps but I realise this is complicated and drawing diagrams would help. All the best John S.
Earliest sunset
Greetings All, I've been trying to figure this one out myself since here on Guam, the sun starts setting later in November and continues rising later until February, so we don't have a shortest day. Well, that's not true, the shortest days of the year on Guam are all even numbered days between December 10 and December 30. A similar bizarre situation prevails around the summer solstice (ie. the sun starts rising earlier but also *sets* earlier long before the longest day of the year). While I realize that this probably happens because we are below the Tropic of Cancer, I'm still at a loss to explain *why* it happens. Any help anyone can give me (and I'm still trying to absorb the post about the equation of time) about why this happens, would be greatly appreciated. Pam === | Pam Eastlick | Email: [EMAIL PROTECTED] | | Planetarium Coordinator | Voice: (671) 735-2783 | | CAS/DNS | Fax : (671) 734-1299 or -4582 | | UOG Station | Location : 13.25N, 144.47E | | Mangilao, Guam USA 96923 | Time : GMT+10EST+15 | | | |LOOK UP TONIGHT, THE UNIVERSE AWAITS YOU!| ===
Re: Earliest sunset of Jack Aubert
Jack Aubert, Your question was : 'Why the earliest sunset is at 13 dec'? Only looking to the aspect of the declination of the sun in december the length of a day wil shorten so the sun will set earlier each day till about 21 december. From then the length of the days will increase. In this periode the difference in the time of the sunset each day however is smaller than one minute. From about 12 or 13 december this difference is even smaller than 30 seconds and still decreases till zero at about 21 december. (this also depends on the latitude you live) But we also have to consider the equation of time. In december the difference in the equation of time is about 30 seconds each day and this means that, only looking for this aspect, the sun sets 30 seconds later than the day before. Not the value of the equation of time is relavant, but the difference each day. When we sum these 2 aspects we see that from about 13 december the sunset will be later than the day before. For the same reason during some days after the winter begins the sun rises later than the day before. Fer de Vries. [EMAIL PROTECTED],nl
Re: Earliest sunset
On Fri, 3 May 1996, Jack Aubert wrote: I would like to forward the following enquiry to all those dialiasts who know more than I do about celestial mechanics. It seems to me that the answer has something to do with the fact that the sun appears as a disk rather than a point in space, but I don't know enough to think this through logically. The question was posed by Rory Sellers ([EMAIL PROTECTED]). What exactly does he have to do to subscribe to the sundial list? Many years ago a golf-playing uncle in Los Angeles asked me why it was that the shortest day of the year was the winter solstice, but the sun actually set earliest on about Dec. 13 (he had noticed that he could get in an extra hole of golf on Dec. 21 -- I'm not making this up.) After a lot of research (there was no Web then!) I realized the answer had to do with the equation of time, the difference between solar and sideral days, and mean solar time (and no doubt you know more about this than I do.) So I told my uncle this, and the family got down to accomplishing other life-tasks. Unfortunately, I recently realized that this explanation must be false. This came about by looking more closely at an analemma on a globe. If I am not mistaken, the analemma describes not only the difference between mean and clock-time (due to the eccentricity of the earth's orbit being non-zero) but also the (I guess arbitrary) dates on which our clock is set. I.e. when the official timekeepers say that the annual clock is zeroed. And here, I was shocked to see a) that this date seems to be the winter solstice; and b) that I had never noticed this before! You see, I always knew that the earth was moving fastest around Christmas time (near perihelion) and so I figured it made sense that the clock was furthest out of whack around this time. And sure enough, consulting either an almanac or a St. Joseph's Aspirin calendar, one can see that the sun indeed sets earliest on Dec. 13, not Dec. 21. BUT HOW IS THIS POSSIBLE, IF THE CLOCK IS ZEROED ON THE SOLSTICE? Shouldn't the equation of time being equal to zero on Dec. 21 mean that the sun should set the earliest on that date, too? Please help! A family conundrum that I thought we had settled twenty years ago is now bothering me! I believe it is the equation of time that is responsible for the difference. Let's consider the sun to be a point (taking into account the sun's semi-diameter won't affect us too much). When the sun is on the horizon (actually sunset occurs when the refracted upper limb of the sun is on the horizon, but as I said we won't worry about this detail), the local hour angle of the true sun is given by cos(H) = -tan(lat) * tan(delta) where lat is the latitude of the place and delta is the sun's declination. The hour angle of the true sun is equal to the hour angle of the mean sun plus the equation of time and the hour angle of the mean sun is equal to mean solar time minus 12 hours. So, H = H_m + E = MT -12h + E/60 where E is in minutes. So, MT of sunset, in hours, is given by MT = 12h - E/60 + acos(-tan(lat)*tan(delta))/180*12. Now, consider first a point on the equator: MT = 12h - E/60 + 6h = 18h - E/60. Notice that this equation is independent of the sun's declination, except via E. The earliest sunset will occur when E is largest. This happens around 3 November. If we re-do the calculation for Los Angles, with latitude of about 34 degrees, we find that the earliest sunset occurs around 6 December. Note that we have to take into account both the time variation of E and the sun's declination. === Richard B. Langley Internet: [EMAIL PROTECTED] or [EMAIL PROTECTED] Geodetic Research Laboratory BITnet: [EMAIL PROTECTED] or [EMAIL PROTECTED] Dept. of Geodesy and Geomatics Engineering Phone:(506) 453-5142 University of New BrunswickFAX: (506) 453-4943 Fredericton, N.B., Canada E3B 5A3 Telex:014-46202 Fredericton? Where's that? See: http://degaulle.hil.unb.ca/NB/fredericton.html ===
Re: Earliest sunset of Jack Aubert
One thing to keep in mind: the zero point of the Equation of Time is *not* arbitrary, but comes from the shape of the analemma (or equivalently from the shape of the earth's orbit and orientation of its north pole). The Equation of Time is the difference between apparent solar time (tied to the sun's right ascension, i.e. its position measured in the earth's equatorial system) and mean solar time (a uniform time scale chosen so that the average is correct). From the definition of mean solar time, the integral of the E. of T. over a year must be zero. That determines its zero point. I haven't checked this, but I'll bet the fact that the zero points of the equation of time lie near the solstices comes from the fact that the peri-/aphelion points of the earth's orbit lie (currently) near the solstice points. That is, the Earth-Sun system is almost symmetrical about the Dec 21 - June 21 axis, and so the analemma has nearly bilateral symmetry about its vertical axis. After a few thousand years of precession of the earth's pole, that won't be close to true, and the analemma's zero points will be displaced a bit further from its bottom and top.
Earliest Sunset
According to my Soda Can Dial, the earliest sunset appears to be around 11 Dec (at least for Washington, D.C.) and the latest on July 2. I don't know if that is true everywhere (in fact, I doubt it). Brad
Earliest sunset
I would like to forward the following enquiry to all those dialiasts who know more than I do about celestial mechanics. It seems to me that the answer has something to do with the fact that the sun appears as a disk rather than a point in space, but I don't know enough to think this through logically. The question was posed by Rory Sellers ([EMAIL PROTECTED]). What exactly does he have to do to subscribe to the sundial list? Many years ago a golf-playing uncle in Los Angeles asked me why it was that the shortest day of the year was the winter solstice, but the sun actually set earliest on about Dec. 13 (he had noticed that he could get in an extra hole of golf on Dec. 21 -- I'm not making this up.) After a lot of research (there was no Web then!) I realized the answer had to do with the equation of time, the difference between solar and sideral days, and mean solar time (and no doubt you know more about this than I do.) So I told my uncle this, and the family got down to accomplishing other life-tasks. Unfortunately, I recently realized that this explanation must be false. This came about by looking more closely at an analemma on a globe. If I am not mistaken, the analemma describes not only the difference between mean and clock-time (due to the eccentricity of the earth's orbit being non-zero) but also the (I guess arbitrary) dates on which our clock is set. I.e. when the official timekeepers say that the annual clock is zeroed. And here, I was shocked to see a) that this date seems to be the winter solstice; and b) that I had never noticed this before! You see, I always knew that the earth was moving fastest around Christmas time (near perihelion) and so I figured it made sense that the clock was furthest out of whack around this time. And sure enough, consulting either an almanac or a St. Joseph's Aspirin calendar, one can see that the sun indeed sets earliest on Dec. 13, not Dec. 21. BUT HOW IS THIS POSSIBLE, IF THE CLOCK IS ZEROED ON THE SOLSTICE? Shouldn't the equation of time being equal to zero on Dec. 21 mean that the sun should set the earliest on that date, too? Please help! A family conundrum that I thought we had settled twenty years ago is now bothering me! Thanks, Rory Sellers P.S. Interesting sidelights: 1. When I first tried to answer my Uncle's question, I tried calling up the chief of the Griffith Park Planetarium in Los Angeles. He not only did not know the answer to my question, he didn't even know it was true! (I.e. that the shortest day and the day on which the sun sets the earliest are not the same.) 2. The year before he died, I wrote a letter to Richard Feynmann trying to get him interested in a project I was undertaking involving interactive video and physics teaching. After reading Surely You're Joking, Mr. Feynmann I knew the only way I could get his attention and possibly get an answer to my letter was to pique his interest by announcing on the outside of the envelope that there were puzzles enclosed. It worked! Feynmann wrote back, regretted he couldn't take part in my project, and solved one of the two puzzled I had posed. Unfortunately, he chose the one OTHER THAN the one about the equation of time! (In fact, our correspondence was noticed by Gleick researching his book Genius about Feynmann who then wrote me for an explanation.) It was all very gratifying to my ego, but didn't help with the time question!! Jack Aubert E-Mail: [EMAIL PROTECTED] Homepage: http://www.cpcug.org/user/jaubert
