Treebeard's Stumper Answer 17 October 2003

Autumn Light

We were talking about our California autumn at lunch last week. Art misses the Colorado aspens, so Cynthia said "Our fall is not about color. There's something about the light that is so beautiful." We do have fall colors here in central California, but the trees are just beginning their change. I know that Cynthia is absolutely right about our special autumn light! I've been noticing it every day as I drive to and from school. Fall is a time of change. Why does the sunlight itself change so much at this time of year that we all sense that something is afoot?

I realize this stumper is vague, and I don't have a photo that shows what I mean by "special autumn light". If you don't recognize what I'm talking about, then here's another sunlight stumper that is related. (It's also a hint!) Consider these two photos of my favorite tree in the perfect light. (Click on the left photo for a better copy at my Treebeard's Photos fotolog.)

August 8, 2003 August 9, 2002

Both photos show a glowing Madrone (Arbutus menziesii) tree with the setting sun off to the left. I took both photos from my deck at home on San Marcos Pass, California. I took the photo on the left one day last summer. I waited a few days to see if the lighting would get even better, but the effect stopped altogether. I searched my archives and found the almost identical shot on the right from almost the same date a year ago. (The exposure is different and the oaks have grown a bit so they are different; photos always are.) I have another stumper about The Perfect Light for photos. But why would that perfect light happen on almost the same day in the same place in different years?

It's been too hot for autumn this week, but the changing sunlight gives away the season. We get 14+ hours of daylight in June and less than 10 hours in December. Night and day are about equal near the equinox, but the rate of change is greatest now. Summer and winter days are all about the same. Today we have two minutes less daylight than yesterday. The sun is rising and setting one full diameter further to the south every day. The light in my windows and the shadows on the mountains are different from last week. Change really is afoot!

Notes:

Every day is different, but they are more different in the fall and spring near the equinox. This thought haunts me as a photographer. Think of all the perfect views that no one sees, like those falling trees that no one hears. The sun on that Madrone tree at that particular date and time. The sun in that girl's hair across the field here and now that only I can see. She doesn't know how beautiful she is at this exact moment! Now add in changing weather and travel plans. It`s a lesson for all us photographers and artists who try to capture these glances that every view and every moment of every day is unique. I'm missing a perfect photo right now just outside my window!

My answer is that it's not the light by itself, but the rate of change in the light that makes autumn light something we notice. Sunrise and sunset times and location along the horizon are changing more now near the equinox than at any other time of the year.

I searched for software to calculate my local sunrise and sunset times, and thought about my old BASIC TBC Calendar program. Then I found an easier way using VBA workbook functions for Excel written by xxxx. I made an Excel spreadsheet that figures sunrise/sunset/azimuth/daylight/rate_of_change/ and lots more for every day of the year at any latitude and longitude. The data is a good match for NOAA. It's too big to duplicate here, but you can download my spreadsheet here.

Here are a few charts that summarize the data for my home on San Marcos Pass in the mountains behind Santa Barbara. The green dots mark the equinoxes and the yellow dots mark the solstices.

• Here in the Santa Barbara mountains, we get between 9:51 hours of daylight on the winter solstice and 14:28 hours of daylight on the summer solstice. The chart for the year (2003) looks like this:
  

• It's more interesting for my stumper to determine the rate of change of daylight hours from day to day. At the summer solstice (21 June 2003), the next day is just 4 seconds shorter. At the autumnal equinox (23 September 2003), the next day is 2:08 minutes shorter. On July 21, 30 days after the solstice, we had about 20 minutes less daylight. On October 23, 30 days after the equinox, we had more than 1 hour less daylight. Note how the rate of change stays high near the equinoxes just like daylight hours stay high near the solstices. The graph of the rate of change per day for the year (2003) looks like this:
  

• The sun rises and sets north of east in the summer and south of east in the winter. The azimuth of the rising sun is how many degrees clockwise from north the sun first appears on the horizon. This chart shows how much the azimuth is changing from day to day. The sun's angular diameter is about 1/2 degree, so it rises about one full diameter over near the equinoxes. I think this is the most noticeable daily change in autumn and vernal light. Geography can make this even more noticeable. The Santa Ynez Valley runs east-west, which exaggerates the effect as the sun rises and sets over the mountains. We live on the north side of the Santa Ynez Mountains, and we've just about lost the sun for the winter.
  

• This is cool! Plotting sunrise time against azimuth generates the mysterious Equation of Time! You've probably seen this figure printed on your globe or sundial. It's nice to derive it from a page of numbers!

I think I understand autumn light and sunrise and sunset better now, but I have a pile of new questions for future stumpers!

Here are some links for your own research:

• The U.S. Naval Observatory (USNO) Data Services has tables of solstices and equinoxes, and you can make a custom table of sunrise and sunset data for any location and year.

• I searched for ephemeris software to build a table for the year that I could work with. The most useful code I found was a VBA/Excel workbook from Greg Pelletier. I'll post my spreadsheet here as soon as I clean it up.

• Thom Hogan's Be a Calculating Photographer tells how to use ephemeris data to be a better photographer. My best solution is to always have my camera ready and take lots of photos. Check out Treebeard's Photos to see the results.

• There are more equinox links here, here, here, and here. There's info about the mysterious Equation of Time here and here.

• I have many more equinox/solstice/season/weather stumpers in the archives. The most recent is here.

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