Tuesday, September 23, 2008

Square Wave Days

The world sunlight map often shows a sort of sinuous curve along the boundary between light and darkness, but these days it has more of a binary look to it.

7 comments:

Paul said...

This is interesting!

Here's a link to a photo that I found fascinating: http://antwrp.gsfc.nasa.gov/apod/ap080922.html

I have a hill east of my house and I can approximate the date by the location of the sun at sunrise in reference to the hill.

Today we learn much that people in previous generations never learned. On the other side, people in earlier generations lived more intimately with the land and experienced things we only read about.

SimplyTim said...

Hi Laura,

I just looked at it and it was time stamped 18:32 PDT. You can see most of the U.S., South America, Africa, and Europe and parts of the Middle East. Very interesting to see the dispersal of the lights throughout those continents.

If it was a contest, the NorthEast would win hands down

Tim

arcolaura said...

Paul - that is an intriguing photo. It reminds me of a couple of sun-angle related images that I can't locate right now. One is a reversing spiral, tighter at the ends, which was created (I think) by allowing a lens to focus sunlight on a board through the day and thus burn a track on it, and then repeating the process every day throughout a whole year, and then assembling all the curved tracks end to end. The other was a diagram of how the yin-yang symbol could be created (or at least approximated) by tracing where a shadow was cast by a post through the seasons.

arcolaura said...

Tim - Isn't that interesting? I assume the night lights are a static base map with the clouds superimposed from weather satellite imagery, and I don't know what they used to create the base map. I just recently discovered that you can view the world sunlight map in different map projections, so I chose the Peters projection for another subtle dose of reality.

arcolaura said...

Tim - here's another satellite image of Earth's city lights. It may be the same one that is used in the world sunlight map.

SimplyTim said...

Laura,

That's a wonderful photo.

Re: "Originally designed to view clouds by moonlight, the OLS is also used to map the locations of permanent lights on the Earth’s surface."

I'm curious, do you think they are saying, e.g, that every night from 8 P.M. to 1 A.M. those are the lights that are on (including office buildings and homes) or are they saying that presumably the office building lights are left on all night?

Tim

arcolaura said...

I'm guessing it would be some sort of averaging of the light that is seen in the same locations over a series of nights at the times that the images are taken. I suppose a lot of it would be street lighting, yard lighting, that sort of thing, but it would be a mixture of many sources, and I assume the image resolution would be too low to distinguish much more than a general area of higher light level associated with an urban area.