166 years this week, the first correctly-exposed photograph of a total solar eclipse was made. On July 28, 1851 in the city of Königsberg, formerly a part of Prussia and now the Russian city Kaliningrad, local daguerreotypist Julius Berkowski made his famous exposure. A daguerrotype is a highly polished, silvered copper plate that was treated with iodized silver to sensitize it to light and then developed with mercury fumes. Sounds pretty toxic to 21st century ears, but chemistry got the profession on its feet and sustained it for more than 150 years until the digital revolution.
Berkowski was commissioned by the director of the Royal Prussian Observatory to take the photo through a 2.4-inch (61 mm) refracting telescope that was attached to the larger 15.8 centimeter Fraunhofer heliometer, an instrument used to measure the changing diameter of the sun as Earth’s distance varies across the year. That scope ran on a drive that tracked the sun.
It’s relatively easy to get some kind of picture of the sun’s corona in 2017. You can just whip out a mobile phone, point and shoot, but for a great eclipse photo, you still have to pay close attention to your exposure. The brightness of the corona, the sun’s atmosphere that puddles around the blackened during totality, varies drastically from its inner to outer limits. A short exposure will capture the innermost corona and small, pink flames around the moon’s limb called prominences. Longer exposures will burn out these details but record the delicate outer wisps of the sun’s atmosphere. Amateurs will often vary the exposure throughout totality to capture all aspects of the corona.
Can you imagine how tough it was to get a nice eclipse shot in 1851? Berkowksi made an 84-second time exposure, equal to more than a third of the 3½ minute totality. On the original plate, the moon measured only 0.3 inches (7.85 mm) across, yet the photo captured not only a fair bit of corona but least five prominences, too.