COUNTRY SCIENTIST

BY FORREST M. MIMS III

How to Photograph the Solar Aureole

The purpose of the Country Scientist column is to provide projects that will encourage readers to do science. Whether you’re a student looking for a good science fair project, or an adult wanting to begin a personal science study, I hope that you’ll find this or a future project worthy of pursuing.

The Solar Aureole

the same exposure duration and f-stop for all your Dust and other kinds of particulate matter cause aureole photos. This will provide a record of the sky the sun to be surrounded by a bright glow in the without unwanted automatic adjustments by the sky known as the solar aureole. The aureole is often camera that alter or even remove the changes in sky faint or even nonexistent when the sun is viewed brightness and color caused by dust, smoke, and from a mountaintop. But it’s almost always present other forms of air pollution. at lower elevations, especially during spring, sum- The aureole is washed out if the sun is photo-mer, and fall. graphed directly. Worse, your eyes and a digital

The diameter and brightness of the aureole is camera’s image sensor can be permanently dam-related to the scattering of sunlight caused by par- aged by the focused image of the sun. Therefore, it’s ticulate matter. This means a record of solar aureole necessary to design an occulting device that blocks photographs can provide a good indication of the the direct sun when making solar aureole photos. transmission of sunlight through the atmosphere. To protect your eyes, it’s also necessary to use a The color of the sky beyond the aureole also pro- method that does not require you to look anywhere vides clues about stuff in the air. near the sun.

Since 1990 I’ve made almost daily measurements I’ve developed various methods for photograph-of the ozone layer, solar ultraviolet radiation, haze, ing the solar aureole in which you need only look at total water vapor, and other sun and sky measure- your camera. My favorite occluder rig is a simple ments from a field adjacent to the small farmhouse camera platform that keeps the sun and occluder in that serves as my South Texas office. In 1998, the same position for each photograph. This greatly I bought my first digital camera, a 1.5-megapixel simplifies the comparison of photos. Fuji MX-700. To date this camera has provided 4,465 You can design your own platform or you can try images ( 1,280× 1,024 pixels) of the solar aureole and my simple version, the Solar Photography Occluder the sky over the north horizon. While the resolution Rig, explained in the DIY on the following page. is low by today’s standards, it’s more than adequate Making a solar aureole photo with the occluder in for a record of sky images. place is simple. If the sky is not overcast, put on sun-

These solar aureole images provide important glasses and a hat and go outdoors with your camera information about my electronic sun and sky mounted on the occluder. If it’s awkward to sit in a measurements, for they quickly reveal the presence chair or on the ground, brace the occluder rig against of thin clouds or haze that might have affected a stable object such as a fence or wall.

the measurements. They also provide a visually Switch on the camera, point it toward the sky convenient way to compare the clarity of the sky away from the sun, and adjust its position so that across the seasons and years. the occluder is centered in the display. Then look at the front of the camera — not at the sun — while pointing the camera toward the sun. When the shadow of the occluder ball falls directly over the lens, press the shutter button and quickly move the camera away from the sun.

Photographing the Aureole Solar aureole photos can be made with virtually any kind of digital camera. For serious studies, you’ll want to use a camera that allows you to set

48 Make: Volume 17