The Duration of Sunlight Recorder, also known as the Campbell-Stokes Recorder, The Stokes Sphere, or simply referred to as a Sunshine Recorder is used to concentrate sunlight into a beam hot enough to burn paper. It is one of the oldest methods for recording periods of sunlight throughout the day, and the device is relatively unchanged since it was invented in 1853. The resulting burn line in the paper (the line is created as Earth rotates) indicates periods of sunshine. Breaks in the line (no burn), indicate periods of cloud cover (during daylight hours). The paper onto which light is concentrated records only 12 hours worth of observations, as there is no need to record sunlight at night!
In polar regions where there can be 24-hours of daylight, two spheres are used, one with a south-facing card, and the other with a north facing card. The two 12-hour cards combine to record a 24-hour day.
The Stokes Sphere is simply a quartz glass sphere, 4″ (10.16 cm) in diameter, mounted on top of a 7′ (~2.2 m) stand (as shown in the photos I took above and below). As sunlight passes through the sphere, it becomes focused and burns a “line” through a piece of treated paper which is positioned beneath the quartz glass sphere.
Any break in the “burn line” is indicative of cloud cover. The treated paper is marked in such a way that the time of cloud cover occurrence can be determined. There are 3 positions in which to place the “solar cards”. One is specifically used for summer months only. Another is used only for Winter months. The 3rd position is used only during spring and fall. Different positions of the “solar cards” are to compensate for changing sunlight-to-Earth angles throughout the year. The semi-rigid cards simply slide into the appropriate channels on the mounting device and are held in place by tension created by the bow shape the cards are forced into as they are positioned around the bottom of the sphere.
In the photo above you can see four examples of treated paper. Each paper above represents one 12-hour day, beginning with morning hours on the left, and evening hours on the right.
The Roman numerals on the paper denote the local time (XII is 12 noon, III is 3pm, etc.)
Notice the burn marks. These burn lines (or dots) represent times when sunshine occurred at
our station. These are papers that have been used at our station.
The top paper was used on an overcast day. Since no sunlight shown through the overcast layer, no burn lines were created.
The second paper from the top represents a mostly overcast day with only a few instances of sunlight as shown by the burn lines; (burn lines show sunlight occurred for just a couple minutes around 9:50am, then for about 10 minutes around 10:30am, and a couple minutes around 11:35am. If you look all the way to the right on the second paper, you’ll see a small burn showing that there was sunlight for just a minute around 4:40pm.)
The third paper down from the top shows a day with scattered clouds; more sunshine occured on this day than on the day represented by the paper above it. Notice burn lines indicating sunshine from about 7:20am to 8:45am, then again for 10 minutes around 11am, again for a couple minutes around 11:30am, then again from around 12:30pm to just after 1pm, then again from about 4:10pm to about 4:40pm, and finally a minute of sunshine at about 5pm.
The bottom paper shows a cloud-free day. Notice the unbroken burn line; indicating uninhibited sunshine from about 7:45am to 4:45pm. Also notice the intensity of the burn line, indicating a more direct sun positioning overhead (most likely this was a sunny summer day). Solid burn lines during winter months tend to be thinner due to the lower angle of the sun.
Before the days of pyrometers, this device (and a human observer), was the only way to record the day’s duration of sunlight. The Duration of Sunlight Recorder was invented by John Francis Campbell in 1853 and later modified in 1879 by Sir George Gabriel Stokes with little to no modification since.
~ Steve Woodruff and Devin Lussier