Solargraph

A solargraph is a photograph of the sun as it courses across the firmament. It is usually taken using a pinhole camera with exposure times varying from 1 day to 6 months. This solargraph was taken at Greenholme over the period 25 June to 21 December 2014.

25-06-14 to 21-12-14 six month solargraph

Each trace represents one day, and the highest point in each arc is 12 noon (1pm BST). So the time and duration of sunshine, and conversely of cloud cover, can be estimated for each day. The view is a full 180 degrees from East to West.

The solargraph was taken from the location of our Campbell-Stokes sunshine recorder (the globe visible at the bottom, just to the left of centre) in the weather station. The very sophisticated pinhole camera comprised an (empty!) McEwans Export beer can (disgusting stuff – not my can!) with a 1mm hole punched in the side about 2/3 from the bottom. Ordinary matt B&W 6×4 darkroom photographic paper was slotted inside opposite the pinhole. The can (now a camera) was sealed watertight (using lots of duct tape), taped to a vertical post with the pinhole pointing directly south, and left for a 6-month exposure. Processing simply involved removing the paper in a darkened room, scanning, flipping, and inverting the image (to get the negative), and increasing brightness and contrast. The original is then kept in a dark envelope in a dark drawer as any continued exposure to light will obliterate the image.

You can see the Stevenson screen blocking the early winter sun in the bottom left corner (east) and a small alder tree blocking a smaller area in the bottom right (west). The two beech trees directly south appear not to affect the number of winter hours recorded by very much. The small dark vertical line visible in the bottom brightest traces just to the left of the trees is a telegraph pole.

In a future attempt we will try locating the pinhole probably 3/4 from the bottom to see if we can include the apogee of the midsummer arcs.

In reality, of course, the sun doesn’t ‘course’ at all: the earth rotates and tilts on its axis. Extraordinary to see this effect so succinctly captured – the dynamic of the solar system in one picture. Although, what is reality?…

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