Multiple Exposures Of The Sun

Stormbringer_X1 writes "This image holds many first. Called an analemma (a figure 8 loop), it is a multiple exposure of the sun, where one observes the sun at the same time of day, over the period of a year. The patience and dedication to pull this off are emphasized by the fact that there are so few in existence (7 total including the first in 1979). It is the first analemma imaged in a single calendar year, the first on the southern meridian, and the first in Greece. The author has other images from 2002 that will be available soon. So stay tuned. Here is an image from NASA archives"

Comments

It's not an easy thing to do

[smartin]

My neighbour did one of these several years ago. It is here.
It took him a year to do and required a lot of engineering including:

• Fixing the film in his camera and hacking it to do multiple exposures.
  
• Using the right filter to black out every thing but the sun. The last exposure was taken without the filter.
  
• Fixing the camera to the rad in his office.
  
• Locking the cleaning staff out of his office for the entire year.
  
• Figuring out where and when to take the picture so that the sun will not be obscured by a building at the target time.
  
• Arranging to be free to take a picture at the same time of day every two weeks for a year.
  
• Dealing with days that were cloudy.
  

It is a facinating project and there have never been that many of these taken. The building in the picture is Bell Labs by the way.

Re:What about the "wiggle"

[toybuilder]

For much of the last month or so, I've been studying techniques for accurately predicting the position (and some other parameters) of the sun at any given time of day and then setting up experiments to verify the results.

(It's not exactly fun work, especially having to get up at 5 am to prepare for the 6:30 sunrise measurements.)

The wobbles you see probably can be attributed to being off by a minute or so in taking each of the multiple exposures.

The diamter of the sun is about 0.5 degrees (31.48' according to the page accompanying the image). A one minute delay in taking the shot will mean the sun has moved (for example on 5-1-2002) about .3 degrees in azimuth, and .16 degrees in elevation. It's also possible that the camera itself was slightly mispointed (nudged? wind? who knows?)

There's 41 images taken between January 12 and December 21, all supposedly at 10:28:16.

The sun would start at (149.99,22.53) (azimuth, elevation), move upward and to the left in time until June 12 where it reaches (112.84, 60.88) and then starts to move downward until June 27, where it reaches (111.64, 60.47) and starts to swing back toward the right, until November 27 (152.86, 25.75), where it'll start swinging back to the left until his last position at (151.55, 22.72).

The sun would have reached it's "lowest" point in 2002 on 12-28 at (150.59, 22.48).

I originally thought the wobble might be due to atmospheric ("optical air mass") refraction variation due to tmperature and barometric pressure, but at those elevations, those effects turn out to be negligible.

This program from NREL will let you calculate the position of the sun and some of the properties that affect its perceived position. If you don't want to compile a program on your machine, you can check out the a href="ssd.jpl.nasa.gov/horizons">web-based "ephemeris" calculator from JPL.