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Saturday, 05 December 2015


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If the Earth's orbit was perfectly circular, then solar noon would always be at the exact same time, and the earliest sunset and latest sunrise would both be on or around 21 December. But it's not circular, and we're close to the sun in winter than in summer, which means that we're moving faster in winter than in summer, which means that the solar day is a bit longer in winter than in summer. Since the solar day is longerm, solar noon gets a little bit later each day, and so both sunrise and sunset also tend to get a bit later, which exacerbates the fact that sunrise is getting later anyway, but counteracts sunset getting earlier, and reverses it a few days before the actual solstice.

> ...but, although December 21st is the
> shortest day of the year here in the
> Northern hemisphere, sunset starts getting
> later a couple of weeks earlier than that.

It's called the 'equation of time.' Basically it simply is the fact that, due to the earth's orbit around the sun being an ellipse (as opposed to a circle) and the earth's axis being tilted, the time from one noon to the next, for a given point on the earth's surface, is not always the same through the year. Not only the time from sunrise to sunset varies with the seasons; so does the time from high noon to high noon.

One way of looking a this is as a very complicated geometry problem.

On the other hand, you could set it up and watch. Shine a spotlight on a globe with a properly tilted stand in a darkened room. Mark a longitudinal line on the globe with a sharpie, then watch what happens at the light/dark boundary as you rotate the stand of the globe relative to the position of the light. The angle the sharpie line makes with the line of light and dark made by the spotlight will change.

Photo therapy indeed! :-)

Well, here goes. The reason sunsets start getting earlier a couple weeks before northern hemisphere winter solstice is due to the differrence between a sidereal day and a solar day and the fact that the earth is approaching perihelion (closest approach to the sun) and therefore acelerating in its orbit. Perihelion is about two weeks after the solstice. A solar day, the time is takes for a point on earth to rotate from sunset to sunset, or noon to noon, etc, is longer than the time it takes to rotate exactly once on its axis because of the earth's orbital speed. It has to turn a bit further each day to bring the sun to the same point in the sky. As it approaches perihelion, it's orbital speed increases enough that it has to rotate even further than we usually experience, so it delays both sunrise and sunset more markedly.

Curiously, the same effect is huge on Mercury, which rotates 3 times for every two orbits. As Mercury approaches perihelion, the sun actually appears to stop, reverse and then proceed in its path across the sky.

I should have added that Mercury has a much more eccentric orbit than earth and so the acceleration as it approaches perihelion is much greater. Its orbital speed increases so much that it's rotation doesn't turn fast enough to put the sun in the same place from noon to noon, etc.

You're in good company. It's a good season to spend time under the hood of a full-spectrum color print viewing booth! (Or in the southern hemisphere.)

Do we have to call phototherapy with tungsten lights vs. LED lights analog vs. digital phototherapy?

Have you built a community of snarky commenters?

[Apparently at least one [g] --Snarky Ed.]

tl;dr: the Equation of Time (https://en.wikipedia.org/wiki/Equation_of_time)

The shortest and longest days of the year are related to the tilt of the earth's axis to its orbit. The days that these happen are not when the Earth is at its farthest or nearest point in its orbit. This difference leads to a discrepancy in where the Sun is expected to be and where it actually is, called the Equation of Time. That is why latest sunrise times and earliest sunset times don't match up with shortest day.

I believe that the disconnect between the time at sunset and the day's length is due to the Earth's precession movement. Think of it as a top (of the other kind ;) wobbling a bit about its rotation axis...

As for phototherapy, I think it's also worthwhile to mention that it's basically free, energy-wise, if you live in latitudes where home heating is required. All that light will eventually end up in the form of heat!

I lived in Boston for 21 years. For me, SAD usually kicked in around Halloween. By mid-December, I was miserable. I spent $400+ on a full-spectrum light box (made in Alaska) and doubled up on SSRIs. The light helps a lot if one is: A) a morning person; B) willing to stick to a photo therapy regimen.

I have never been a morning person, and my schedule often varies from day to day.

In April Y2K + 3, we (wife, daughter, dog, and I) moved to Florida--the sunshine state. I no longer get bogged down with SAD.

Here in Sarasota, Florida, the grass isn't greener, rather it's just a different shade of green. ... I miss Boston for its abundant intellectual, cultural, culinary, and cosmopolitan attractions. I alos miss autumn and spring.

Summers are brutal in Sarasota (6 months). Despite high heat and humidity, I'll take sunshine and sweat over darkness and chill.

Sarasota is nice. There are a lot of sharp retirees and snowbirds, hobby shops, meetup groups, cultural venues, and mom and pop restaurants.

As a kid growing up in Nebraska, I never imagined forsaking seas of corn and wheat for the Atlantic Ocean and the Gulf of Mexico.

At least much of the USA, if not all of it, still has plenty of sunlight to go around during the winter. In Europe, I couldn't stand the long dreary days of fall and winter. Hence my plan to not live north of the Alps again, if/when I move back to Europe.

Similar experience to Bob Rosinsky's. I dreaded Winter and left the Midwest for FL as soon as I could. Wish I'd gone sooner.

If you're a SAD sufferer living at high latitude your neighbors will tend to be self-selected for indifference to darkness and may not understand your problem. You have to trust your gut and do whatever you need to do to get light.

Relief from SADness was one of the biggest benefits I experienced when I moved from NY to NM. Good lighting is great, but you can't beat natural sunlight and a short walk outside to cure the winter blues.

Regarding phototherapy, I wonder if computer monitors might be a useful tool for treating SAD. We all stare at them way too much as it is, why not have them serve double duty.

You could even get a prescription and insurance coverage to cover the additional cost of specific models.

Just use +2 stops more when shooting in Dec....

It's good to know that phototherapy lights have photographic applications, though it does make sense.

Before diving in, people might want to read this page about light therapy for an overview about how it works with different kinds of depression, the slight risks involved for certain people, and alternatives. One approach, called a "dawn simulator", doesn't require a special light, has fewer risks, and may also be a more effective way to use a phototherapy light (for depression, not photography):


Oh, and I can report that at least some users of phototherapy lights use the catchier nickname "happy lamps". (I wonder if there's an emoticon for that...)

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