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pschroeter
2009-Dec-12, 09:49 PM
Is there a relatively simple reason why this time of year, the sunsets get latter before the solstice? I notice the sunrises are getting slightly latter and duration of daylight only increases a few days after the solstice. I'm having trouble picturing the geometry involved.

I'm using tables generated by the U. S. Naval Observatory.

grant hutchison
2009-Dec-12, 10:38 PM
It's because we're close to perihelion at this time of year, so the Earth is moving faster around the sun. So it takes a little more than 24 hours for the Earth to rotate far enough to bring the sun back into the same position in the sky. Against a strictly 24-hour day, that means the sun's position at any given clock time is drifting eastwards, delaying both sunrises and sunsets, day on day.
So in the northern hemisphere we see sunsets starting to get later before the shortest day comes. The same mechanism means that sunrises don't start to get earlier until after the shortest day has passed.

Grant Hutchison

Hornblower
2009-Dec-13, 01:33 AM
It is not only the faster motion around perihelion that causes this effect. Even with a circular orbit we would see earliest sunset a few days before the winter solstice and latest sunset a few days after the summer solstice. Latest sunrise would be a few days after the winter solstice, and earliest sunrise a few days before the summer solstice. The orbital eccentricity increases the effect in December and nearly obliterates it in June.

The variation with a hypothetical circular orbit is a geometric effect which would cause variation in the Sun's forward progress in right ascension even with a constant velocity along the ecliptic. At the equinoxes, the Sun is moving slantways to the equator and thus lags behind where it would have been in right ascension (the astronomical term for longitude with respect to the equator) had it been moving along the equator instead of the ecliptic. This gets it out of synchronization with a clock. As its trek along the ecliptic levels off around the solstices, it is in an area far from the equator in which the lines of longitude are closer together, so it makes up the lost forward progress and then some. This gets it momentarily back in sync with the clock and then out the other way. It is easy to demonstrate on a globe or a suitable chart, but often difficult to visualize from words alone.

grant hutchison
2009-Dec-13, 01:52 AM
Yeah, I was looking forward to someone taking a stab at the geometric component. :)

Grant Hutchison

Spaceman Spiff
2009-Dec-13, 03:52 PM
Several years ago I wrote this article (http://homepages.wmich.edu/%7Ekorista/sun_riseset.html) for our local astronomy club newsletter.

trinitree88
2009-Dec-13, 04:23 PM
Several years ago I wrote this article (http://homepages.wmich.edu/%7Ekorista/sun_riseset.html) for our local astronomy club newsletter.

Pretty Spiff, Spaceman. pete

forrest noble
2009-Dec-14, 02:28 AM
pschroeter,


Is there a relatively simple reason why this time of year, the sunsets get later before the solstice? I notice the sunrises are getting slightly latter and duration of daylight only increases a few days after the solstice. I'm having trouble picturing the geometry involved.


"Relatively simple" may not be the best wording. What may seem to be a relatively simple explanation to one person may not seem simple to another, even if both have similar intelligences.

As to your question:


Why (do) sunsets get later before solstice (?)

This is my attempt at a "simple" explanation.

The Earth is tilted on its axis 23 1/2 degrees relative to the plane of the solar system. On the equator of the Earth's tilt has the most direct angle and greatest exposure to the sun's direct rays and overall exposure. In the Northern hemisphere we experience the highest amount of direct rays (closest to 90 degrees) in what we call our summer time. In our winter time the southern hemisphere is receiving the most direct solar rays since it is their summer time.

Realize that in our winter the North polar region is tilted away from the sun so that the angle does not allow the sun to be directly observable for many months. For the same reason as you move away from the equator in the northern hemisphere to any location, the sun is observable for a shorter period of time than in the opposite season, the summer, where the sun is observable for the longest length of time. In the northern hemisphere the shortest day of the year is around December 21 st, which for this hemisphere is called the winter solstice.

Tobin Dax
2009-Dec-14, 03:10 AM
This is my attempt at a "simple" explanation.

The Earth is tilted on its axis 23 1/2 degrees relative to the plane of the solar system. On the equator of the Earth's tilt has the most direct angle and greatest exposure to the sun's direct rays and overall exposure. In the Northern hemisphere we experience the highest amount of direct rays (closest to 90 degrees) in what we call our summer time. In our winter time the southern hemisphere is receiving the most direct solar rays since it is their summer time.

Realize that in our winter the North polar region is tilted away from the sun so that the angle does not allow the sun to be directly observable for many months. For the same reason as you move away from the equator in the northern hemisphere to any location, the sun is observable for a shorter period of time than in the opposite season, the summer, where the sun is observable for the longest length of time. In the northern hemisphere the shortest day of the year is around December 21 st, which for this hemisphere is called the winter solstice.

That sounds like an explanation of why the sunset should get earlier as we approach the winter solstice. The question was why does sunset get later, something counterintuitive based on your explanation.

Spaceman Spiff
2009-Dec-14, 03:19 AM
Pretty Spiff, Spaceman. pete

Thanks! :)

Centaur
2009-Dec-14, 04:15 AM
Is there a relatively simple reason why this time of year, the sunsets get latter before the solstice? I notice the sunrises are getting slightly latter and duration of daylight only increases a few days after the solstice. I'm having trouble picturing the geometry involved.

I'm using tables generated by the U. S. Naval Observatory.

Relatively simple? Here's the best I can do.

The effect would not occur if we still routinely used sundial time. However, our modern standard times are related to Universal (Greenwich) Time which is based on a fictitious mean Sun that moves along the celestial equator at a constant rate. The real Sun is on the ecliptic which is tilted by more than 23 to the equator. Even if the Sun were to move at a constant rate along the ecliptic, its projection on the celestial equator would not. It would move more slowly along the equator at the times of the equinoxes because it is moving at a slant compared to the equator. At the times of the solstices it would be moving parallel to the equator but would be cutting through meridians of right ascension at a more rapid than usual pace, because the meridians converge as they move north or south from the equator. This effect of spherical geometry is the primary reason for the deviation you noticed. Its cycle is half of a year.

A lesser but significant secondary effect is due to the fact that the Earth’s orbital speed changes over the course of a year. Its cycle is one year. The two cycles together combine to form the so-called equation of time. That’s a timetable that tells how much a sundial deviates from mean local time. Its graph can be seen in this article: http://en.wikipedia.org/wiki/Equation_of_time . Once the correction for the equation of time is made to your clock, you’ll find that the earliest sunset occurs on the day of the winter solstice.

forrest noble
2009-Dec-14, 11:57 AM
Tobin Dax,


That sounds like an explanation of why the sunset should get earlier as we approach the winter solstice. The question was why does sunset get later, something counterintuitive based on your explanation.

My explanation concerned why days are shorter in winter. The exact time of day involved with sunset is based upon the seasonal orientation of the Earth as well as how clocks are synchronized accordingly in each time zone. I think the OP's question involved the geometry involved. That's what I tried to explain.

Hornblower
2009-Dec-14, 01:49 PM
Tobin Dax,



My explanation concerned why days are shorter in winter. The exact time of day involved with sunset is based upon the seasonal orientation of the Earth as well as how clocks are synchronized accordingly in each time zone. I think the OP's question involved the geometry involved. That's what I tried to explain.

Your remarks in post#7 were accurate as far as they went, but they did not address the details that are needed to answer the OP's question about the exact times of sunrise and sunset, not just the duration of daylight. Grant, Centaur and I addressed the OP's question in considerable detail.