Abbot and the Solar Constant
Early Attempts to Measure Variations in the Sun's Luminosity
Aug 4, 2008
One way to measure possible variations in the solar luminosity, which is its energy output per second, is to accurately measure the solar constant, which is the amount of energy falling on one square meter of surface at the top Earth's atmosphere every second. It is also called the solar irradiance. If the solar constant changes, the Sun's luminosity changes
First Measurement of Solar Constant
In 1881 Samuel Pierpont Langley scaled Mt Whitney to get above as much of Earth's atmosphere as possible and measure the solar constant. Traveling to the thin high altitude atmosphere was an attempt to minimize the corrections needed for the amount of solar energy absorbed by the atmosphere.
Working as the secretary of the Smithsonian, Langley had, three years previously, invented a bolometer, an instrument used to very accurately measure the radiant energy at all wavelengths. Langley's bolometer was the instrument needed for this first attempt to measure the solar constant.
Abbot's Half Century on the Solar Constant
Charles Greeley Abbot (1872 - 1973) carried on Langley's pioneering efforts to accurately measure the solar constant. He spent most of his long career at the Smithsonian accurately measuring the solar constant searching for solar variability.
Abbot went to work for Langley at the Smithsonian in 1895. By that time the bolometer design had been improved for better accuracy, so Abbot again measured the solar constant. Abbot then started a program to search for solar variations. Working with his assistants Abbot repeatedly measured the solar constant from 1902 to 1957. For the greatest possible accuracy, they traveled to remote mountaintops. Correcting for the amount of energy absorbed by Earth's atmosphere, is very difficult, but Abbot did so as accurately as possible from the ground.
Abbot claimed that his data showed variations in the solar constant and therefore the solar luminosity. Abbot also claimed that when a large group of sunspots faced Earth, the Sun's luminosity was less, but the Sun was slightly more luminous during the maximum of the 11 year sunspot cycle than during the minimum. Sunspots are dark spots on the Sun's surface, and there is an 11 year cycle in the number of sunspots. The sun can be more luminous when there are more sunspots because the number of faculae also increases during sunspot maximum. Faculae are bright areas on the Sun's surface, and their cumulative effect is greater than the effect of the sunspots.
Modern satellite measurements show that Abbot had the right answer, but few scientists of the time believed Abbot's claim. They did not think his data were accurate enough to measure such small changes in the solar luminosity. The modern satellite data show that the solar constant variations are at about the limit of the accuracy in Abbot's data.
Even though many scientists of the time did not think Abbot's data were accurate enough to measure variations in the solar constant, they recognized their value. Abbot and his assistants very accurately and frequently measured the solar constant for over a half a century. It was indeed an impressive feat. The data have since been used to study the effects of phenomena such as volcanic eruptions on Earth's atmosphere.
Long term systematic data collection is often extremely valuable but underappreciated in science. Systematic accurate data are often used to answer questions not anticipated by the scientist taking the data.
Further Reading
Frazier, Kendrick. Our Turbulent Sun. Prentice-Hall, 1980.
Golub, Leon and Pasachoff, Jay M. Nearest Star The Surprising Science of Our Sun. Harvard, 2001.
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