The earliest sunset around here (Toronto) is about 10 or 11 days before the winter solstice.
On the other hand, the latest sunset occurs only three or four days after the summer solstice.
There is an a symmetry here that I wouldn’t have expected. How does it arise?
Thanks.
Just to be clear, there is an asymmetry.
The Earth’s orbit is not a perfect circle, and the perihelion and aphelion do not coincide perfectly with the solstices.
Did you take into account the Equation of Time?
But could this explain a seven day difference?
No, but again, should it introduce such a substantial asymmetry?
Maybe I need to see an orrery or something.
Here’s an explanation from our Bad Astronomer:
Several days isn’t as big an asymmetry as you thing. Around the solstices, the change in sunset (or sunrise) times and locations from one day to the next is only very slight: In fact, with naked-eye observations, it’s difficult to pin down the date of the solstice more precisely than within a week or so. So even a small change in sunset times due to the eccentricity of the Earth’s orbit can shift the date of latest/earliest sunset/rise by several days.
This. It’s not sunrise/sunset that are asymmetrical. It’s that 24:00 hours is the average length of a day, not the exact length of every day. The amount that clock time is ahead or behind solar time is what makes the difference.
The asymmetry is even more pronounced the closer you are to the equator. For example, in Miami, the earliest sunset is about 21 days before the winter solstice and the latest only 9 days after the summer solstice. I guess eccentricity effects are magnified at low latitudes (not that I can visualize it).
Just so I’m clear, does this also explain the asymmetry of the earliest sunset to the latest sunset relative to the solstices (i.e. the number of days before and after the solstices respectively)?
The effect would be bigger at the equator due to the angle that the Sun sets at. Imagine a long-exposure picture of a sunset, so that instead of a tiny dot, the Sun forms a line across the image. At high latitudes, that line will be close to horizontal, just skimming into the horizon. But in the tropics, that line will plunge nearly straight downwards.
The equator is a good example, because the day is roughly 12 hours long year-round, so all you have to consider is the Equation of Time. If you look at its graph, the asymmetry in question becomes apparent, with the maximum value occurring in early November and the minimum value in February.
Two factors cause solar day length to be a little more or a little less than 24:00:
(1) The “pendulum effect” of a tilted orbit, which makes solar days shorter near the solstices and longer near the equinoxes; and
(2) The elliptical orbit, which makes solar days shorter near perihelion (January) and longer near aphelion (July)
In winter, the two effects reenforce each other, so you get really short solar days–enough to create, at the mid-latitudes where most of us live, a 10-12 day lag between earliest sunset and the solstice.
In summer, the two effects offset. The pendulum effect predominates, so you get “somewhat short” solar days–enough to create a 4-6 day lag.
Except that I reversed “shorter” and “longer” in the above. Really long solar days in December, somewhat long ones in June.
There’s an additional reason that nobody seems to have mentioned so far.
A time zone is an hour wide, so the actual local time may be up to half an hour earlier or later than the standard time across the time zone.
At one edge of the time zone, your watch says 6:00AM, but the real local time is 6:30AM. At the other edge, your watch says 6:00AM, but the real local time is 5:30AM.
True, but that does not explain the phenomenon that the OP is asking about. In the case of Toronto, the discrepancy between standardised zone time and true local solar time is about 16 minutes (Toronto’s time zone is UTC-5, which corresponds to local time on the 75th Western meridian, and its actual longitude is 79° West). It’s as if the OP had to deduct 16 minutes to convert the time indicated by his watch into local solar time. But that discrepany is constant and applies to each day alike, and the OP is only asking about the days of the earliest and latest sunrises.
You would expect a time zone to be “an hour” (or 15 degrees) wide, but there are variations. The extreme Eastern time zone range is from Thunder Bay (Ontario) - 89.2477 degrees west, to Blanc Sablon (Quebec) - 57.1313 west. The two are only 3 degrees apart on latitude - so fairly close. The sun sets today (May 18) at 9:35 pm in Thunder Bay, and at 7:40 pm in Blanc Sablon.
The time zones aren’t really an issue, albeit confusing things in places observing Summer Time and similar. For the purposes of this question, it may be better to think in terms of local mean solar time.
If the earliest sunset comes 10-12 days before the solstice, then the latest sunrise comes about the same number of days after the solstice. As noted above it is the equation of time. The actual solar day is getting shorter as the earth approaches perihelion in early January. Look at The World Clock (extended version), find Toronto and examine the times of solar noon (when the sun crosses your local meridian) as you look through the year and you will find your explanation.
You know those figure 8 patterns you sometimes see on world globes. They mark the corrections made for solar noon at different times of year. That figure 8 pattern is called the analemma. On the campus of Williams College is a sundial with an analemma shaped pointer that is all set at such an angle that the sundial would read correctly every day of the year (but you had to know which side of the shadow to read and that changed over the course of the year). Unfortunately a tree had grown next to it and it was in the shade. Pity.
The equator is a weird place. First time I went somewhere tropical on vacation I discovered that not only was the moon broken and lying on it’s back, dusk was an on/off function rather than a dimmer. I went to the beach to watch the sunset and had to fumble my way back in the dark. Also most of the invertebrates were in the wrong sizes.