Analemma with White Sands (w-0921-12)
In gallery: Photos of the month (2007)
Related photos: Astronomy
The analemma is the name for the figure-eight path that the sun follows in the sky throughout the year, when photographed at exactly the same time during the day. It is about the ultimate in multi-exposure photography, and very few people have managed to photograph the analemma on a single piece of film. Here it is shown over a yucca plant at White Sands National Monument in New Mexico (USA).
The analemma looks like a figure eight because perihelion and aphelion of Earth are somewhat out of phase with the seasons. Earth is at perihelion (closest to the sun) early January, at which time it orbits faster around the sun, and the sun appears to shift slightly faster east in the sky from day to day than it does on average. At aphelion in July the situation is reversed, and the sun still shifts eastward but at a slower rate. By looking at the sun's position in the sky each day at one particular time, the average eastward motion is subtracted, leaving the out-of-phase oscillations in right ascension (east-west) and in declination (north-south), which results in the analemma figure. The lower right of the analemma occurs around December 20th (winter in the northern hemisphere), and the upper left around June 20th. The crossover point occurs around April 12th and August 30th.
The multi-exposures of the sun were made once every ten days on average, starting 20 October 2006 and ending 10 October 2007. This multi-exposure was from one of three analemma-cameras that I had permanently mounted on the roof of the house. A computer running Linux with time-synchronized clock was kept running throughout the year. I programmed code to trigger cameras via a parallel I/O (printer) port, and this code automatically triggered all three cameras at 17 hours UTC (10:00 mountain standard time) every tenth day. Little intervention was necessary, only to adjust the list of trigger days to account for clouds. At one point I was gone for weeks while the analemma photography was tending for itself; I could control it over the internet when needed.
The sun was photographed through an ND-5 filter at 1/90 of a second at f/8. Film was Provia-100F; the cameras were Nikon FEs with 24mm lenses, mounted on modified motor winders, all put in a weatherproof enclosure.
The background exposure was made at White Sands about a month later, in exactly the same direction as the camera had been mounted. It proved difficult with those constraints to find a yucca with sand dunes and scenery that I wanted.