This post was authored by Sean Bryant, Science Reference & Research Specialist in the Science, Technology, and Business Division of the Library of Congress. He recently authored the blog posts “The Last Man on the Moon? — The Story of Eugene Cernan in Two Parts” and “An American in Orbit: The Story of John Glenn.”
In a matter of days, the Moon will slip between the Earth and Sun. This has been happening every month or so, for more than four billion years, as the Moon orbits the Earth. If you imagine a line connecting the center of the Earth and the Sun, the Moon usually passes a bit above or below the line, and what we see is a new moon, which is to say we see nothing. The Sun’s light passes over or under the Moon and reaches the Earth normally. However, every so often the Moon passes nearer or directly through the line between the Sun and the Earth, casting a shadow on the Earth and briefly blocking much of the Sun’s light from reaching the Earth. This, we call a solar eclipse.
Humans have long been fascinated by solar eclipses. We have developed myths, legends and stories around them, and celebrated them in poetry, art and, more recently, photography. Reports of occurrences appear in Chinese records from more than four thousand years ago, and come from other ancient civilizations, like the Maya, Babylonians and Greeks. Observers and astronomers of the time were able to use the reports and their own observations to predict the time and location of future eclipses.
Succeeding generations of astronomers would add their own records, refining their theories and models of celestial mechanics. As these predictions became more and more accurate, 17th and 18th century astronomers, such as Cassini and Halley, began to produce maps showing the predicted and actual paths of solar eclipses visible in their European locales. Due to the relative infrequency of solar eclipses, however, the science relating to them continued to proceed in fits and starts. By the 19th century though, astronomers were beginning to take note of more than just the Moon in their observations. With light from the center of the Sun blocked by the Moon, they were able see and measure the dimmer outer corona, a region of the Sun normally obscured by the brighter light at the heart of the Sun.
Astronomers, eager to observe more solar eclipses, began to travel farther from their European homes, traveling to far flung outposts of their colonial empires and then beyond in ever larger, more organized groups, observing from places like India, the East Indies and the Caroline Islands. They began to carry spectroscopes to search out chemical elements in the corona. New observatories and societies, such as the US Naval Observatory, US Coast Survey and the National Geographic Society, sponsored expeditions to even more remote locations and began to contribute to their observations, many of which are included in the Library’s collections.
In recent years eclipse viewing has grown to encompass the scientific community in many more countries who contribute their reports to the collective body of scientific knowledge. The Library of Congress has copies of many of these reports in the general collections. Though totality – the portion of a solar eclipse when the Moon is directly in between the Sun and the Earth, lasts for no more than a few minutes in any one place, teams, working together while spread along the path of the eclipse, can make observations that run for hours. Observations have become ever more sophisticated, even moving into outer space, where the Earth’s atmosphere doesn’t interfere with observations. We continue to learn more each time the Moon passes directly between the Earth and the Sun.
Much of the information in this post was taken from the book Celestial shadows: eclipses, transits, and occultations, by John Westfall and William Sheehan. If you would like to learn more about solar eclipses, the Library of Congress has many resources available. The Library’s Science, Technology and Business Division, in partnership with NASA’s Goddard Space Flight Center, sponsored a lecture by Dr. C. Alex Young, the Associate Director for Science in NASA Goddard’s Heliophysics Science Division which is available via webcast. Many of the items linked above are currently on display in the Science and Business Reading Room through the end of August, on the fifth floor of the Adams Building, where you are welcome to come and examine them. Many additional materials are available in the general and other collections of the Library of Congress. You can start your own expedition at www.loc.gov.