5
$\begingroup$

Why is the sunset not the latest on 21 June, but only on 27 June. Purely geometrically regarding the orbit of the two celestial bodies this should not be like this?

Is there somewhere a good source, that explains this situation ?

$\endgroup$
  • $\begingroup$ Summer has four main characteristics: early sunrises, late sunsets, large day-lengths, and great heat. But the corresponding time-periods four do not coincide: the first three are in different parts of June, and the latter is in July and/or August. (Similarly for winter: late sunrises, early sunsets, small day-lengths, and freezing temperatures). $\endgroup$ – Lucian Jul 21 '18 at 11:17
5
$\begingroup$

It's explained in this Wikipedia article. Basically, the length of a day is defined by both the Earth's rotation itself and Earth orbiting around the sun. Because the Earth's orbit around the Sun isn't a circle but an ellipse (making it moving faster when it's closer to the Sun), and the Earth rotates in a plane different than it orbits the Sun, an apparent solar day isn't always 24 hours; in June, it's slightly longer. That means every day, "noon" (the time of day when the Sun is at its highest point) happens 'later' (compared to the time on your watch), so sunset (and sunrise) are also later. The first few days after June 21st, this effect is more significant than the effect of the Sun moving south (causing the day* to become shorter and the sunrise falling earlier). The same effect causes the earliest sunrise (in your location) to be on June 15th, so before the solstice.

*: day as in day vs. night, the period between sunrise and sunset, so not 24 hours

$\endgroup$
3
$\begingroup$

Why is the sunset not the latest on 21 June, but only on 27 June?

When the latest sunset occurs depends very much on latitude. For people who live in the far north (e.g., Anchorage, Alaska or Reykjavik, Iceland), the latest sunset occurs on the day of or a few days after the solstice. For people who live just north of the equator, the latest sunset occurs in late July.

The reason the latest sunset occurs in late July just north of the equator has nothing to do with the change in the time from sunrise to sunset; there is very little variation in the portion of a day that is sunlit near the equator. The reason the latest sunset is in late July is instead because that is when solar noon is at its latest. Solar noon occurs later and later in the day in the weeks preceding late July, but then it reverses direction. There are four extrema of the timing of solar noon over the course of a year. Late July represents one of those four extrema.

The time at which solar noon occurs varies over the course of a year. This variation is a consequence of the Earth's axial tilt and the eccentricity of its orbit. Mathematically, this is described by the equation of time, depicted in the two graphs below. The first shows the cumulative effect; this is the "equation of time." The second graph shows how much local noon changes on a day-per-day basis. Negative values such as in mid May to late July indicate when solar noon occurs later than it did on the previous day.

Plot of the equation of time, showing the variance of solar noon vs clock noon over the course of a year.

Plot of the rate of change of the equation of time, showing how much solar noon changes per day. Negative values indicate that solar noon is getting later.

The length of time that a given place on the Earth north of the equator is sunlit decreases in the days after the solstice, with how much depending on latitude. Solar noon occurs later and later in the days after the solstice, and this change is the same for every location where the Sun is visible. This creates a latitudinal tension regarding when the latest sunset occurs. In far northern latitudes, the rapidly decreasing length of sunlight hours after the solstice quickly overcomes the slowly changing time at which solar noon occurs. Closer to the equator, the small seasonal changes in daylight make the latest solar noon mark the day with the latest sunset. When the latest sunset occurs is somewhere between these two extremes for intermediate northern latitudes. For your locale, the latest sunset was on June 27. Where I live, it occurred a few days later than that.

$\endgroup$
  • $\begingroup$ I know what you mean, but IMHO it's a little confusing because you use "length of a day" to refer to both the time period when the sun is up, and to the time interval from one apparent noon to the next. $\endgroup$ – PM 2Ring Jul 23 '18 at 18:41
  • $\begingroup$ @PM2Ring - I see that, and I tried to fix it. Curiously, the same confusion arises in technical literature: The term "length of day" has multiple meanings. $\endgroup$ – David Hammen Jul 23 '18 at 19:22
  • $\begingroup$ The new version seems clearer to me. Talking about time can be tricky! But I guess that's just something you learn to deal with in astronomy, with all the different types of time that are used. :) $\endgroup$ – PM 2Ring Jul 23 '18 at 19:46
  • $\begingroup$ @PM2Ring - Yes, it's tricky. It helps to have a proofreader. Thanks much! $\endgroup$ – David Hammen Jul 23 '18 at 19:51
  • $\begingroup$ I think it is helpful to visualize the analemma as it rises or sets to see why the earliest or latest sunrise or sunset do not occur on the solstices (June 21 and December 21). See math.nus.edu.sg/aslaksen/calendar/sunrise.html for a decent explanation. $\endgroup$ – JohnHoltz Jul 27 '18 at 4:08

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.