Sunset Down the Channel

Sunset Down the Channel

David Roseman

Sunset photo 2013

Are you sure that you chose the right date?

In 2002 the Pentwater Yacht Club began celebrating the evenings when the sun sets down the center of the channel. This, of course, happens twice a year, in May and August. The date is predicted by using 293.6 as the true bearing of the channel, and by using the US Naval Observatory data for altitude and azimuth. The chosen date is the one when the altitude of the sun approaches 0 and the azimuth is closest to 293.6.

The data for 2017 are as follows (times are EDT):

Date Time Altitude Azimuth Day
May 4, 2017 20:51 0.0 293.4 Thursday
May 5, 2017 20:52 0.0 293.8 Friday
May 6, 2017 20:53 0.0 294.2 Saturday
August 5, 2017 21:02 0.0 294.1 Saturday
August 6, 2017 21:00 0.1 293.5 Sunday
August 7, 2016 20:59 0.0 293.2 Monday

The closest Summer date this year is August 6. The official dates will be determined by the Commodore. [This is a bit like the Church determining the date for Easter.

Easter is the Sunday after the Paschal full moon, which is approximately the first full moon after the Vernal Equinox. The Paschal full moon is determined by the Church, however, and may not coincide with the actual full moon. See this for more detail ].

The first time I went through all of this (2002), I attended 10 AM coffee at the Pointe Wine and Deli. The subject came up, and Murph Schader said "Of course it's August 6. The sun always sets down the middle of the channel on that date."

Note that the May date and the August date are the same number of days from the Summer solstice. This seems pretty reasonable.

Another source of sunset/sunrise info was provided by Mike LaHaye.

When Dick Ouweneel checked the data in 2002, he found that the sun set at 19:51 on May 5, and 20:00 on August 6. Why the difference in time? This led to an interchange between Dick Ouweneel and his friend Bill Brandenburg. Here is the exchange.


It's relax and enjoy time.

A few days ago I finally got around to sending the "sun observation timing dilemma" to the friend who can and did get in to this. The following are his words from a just-received email. I do remember that in the analemma's figure-eight pattern, the lower portion is larger than the top portion. Hold that image.

May I introduce Bill Brandenburg. Dave, Bill, Bill, Dave.


The Earth's orbit around the Sun is an ellipse (not a circle) and the earth's axis-of-rotation is tilted approx 23.5 degrees to the plane of that orbit. (The Earth is nearest the Sun in Jan and farthest away in Jul). And a "year" is just a bit more than 365.24 days. All of these lead to the Sun being slow, compared with the time on your accurate wrist-watch, from late Dec thru mid-Apr and mid-Jun thru late Aug, and fast from mid-Apr thru mid-Jun and late-Aug thru late-Dec. (Remember the figure 8 -- the analemma -- on world maps and globes). Although your 5/5 observation was about 46 days before the Summer Solstice, and your 8/6 one will be about 46 days after, 5/5 was when your watch was behind the Sun and 8/6 will be when the Sun is behind your watch. (Let's ignore the atmospheric refraction that causes the "apparent" time of sunset to differ slightly from the "actual" time). I haven't (probably can't anymore) "done the calculation", but interpolating an analemma (see, what else,, it looks to me like the Sun was about 3 minutes ahead of your watch on 5/5, and will be maybe 5 minutes behind on 8/6; so, considering only the analemma, I'd guess that the 8/6 event might be 8 minutes later than that on 5/5. If the difference is actually only 3 minutes, there's probably something significant that I'm not considering (relativity?). Like hops in a Bell, we probably ought to nail down the projected vs actual and the reasons for the differences.


I should add that the "Bell" which Bill referred to is Bell's beer of Kalamazoo, such as the Oberon served at the PYC. Bill and I had a recent exchange on this subject and he is responding in the above context. As I find new beers, I send Bill a geometric figure which describes the beer to Bill. It is a language that has taken many years and other units of research to develop, but the effort has been rewarding to date and will definitely be funded in the future.


Dick Ouweneel

Bill Brandenberg also pointed to an interesting discussion of Flatland. It makes it nice that we live in a 3 dimensional (or is it 11 dimensional) world. .

There is an interesting and educational applet here.
Parallax. It all depends on your viewpoint. Some people have complained that the sun didn't really set down the center of the channel when, for example, they were standing near the door from the dining room to the docks. The "problem" is that the sun is so far away that, for practical purposes, its distance is infinite. This means that if 2 observers simultaneously sight down the north and south channel piers, both will get the same bearing to the sun. The center of the channel projects to a location on the South docks, near Chesney's boat house.

This explanation may help.

Past information.