A.E. (Andrew Ellicott) Douglass (July 5, 1867, Windsor, Vermont – March 20, 1962, Tucson, Arizona) was an American astronomer. He discovered a correlation between tree rings and the sunspot cycle.

"Douglass tracked this into past centuries by studying beams from old buildings as well as Sequoias and other long lived trees. Noting that tree rings were thinner in dry years, he reported climate effects from solar variations, particularly in connection with the 17th-century dearth of sunspots that Herschel and others had noticed. Other scientists, however, found good reason to doubt that tree rings could reveal anything beyond random regional variations. The value of tree rings for climate study was not solidly established until the 1960s."

Douglass founded the discipline of dendrochronology, which is a method of dating wood by analyzing the growth ring pattern. He started his discoveries in this field in 1894 when he was working at the Lowell Observatory. During this time he was an assistant to Percival Lowell and William Henry Pickering, but had a falling-out with them, when his experiments made him doubt the existence of artificial "canals" on Mars and visible cusps on Venus

Craters on the Moon and Mars are named in his honor.

In 1909, Clark Wissler, of the American Museum of Natural History, organized the Archer M. Huntington Survey. One objective of this survey was to determine temporal arrangement of the American Southwest’s prehistoric ruins. Wissler, who had read about Douglass’s work concerning the relationship between precipitation and tree growth, later contacted Douglass saying:

Your work suggests to me a possible help in the archaeological investigation of the Southwest…We do not know how old these ruins are, but I should be glad to have an opinion from you as to whether it might be possible to connect up with your modern and dated trees specimens [with wood specimens] from these [prehistoric] ruins by correlating the curves of growth… I shall be glad to hear from you as to whether you think it is possible for us to secure any chronological data from the examination of this material.

On June 19, 1914, the curator of the American Museum of Natural History wrote a letter to Douglass expressing his desire to begin archaeological analysis as early as possible.

In 1916, Douglass began obtaining and analyzing archaeological samples first collected during an expedition to northwest New Mexico by the University of Colorado and the American Museum of Natural History. In April 1918, Wissler asked Douglass whether or not it would be possible to assign relative dates to samples that couldn’t be dated absolutely. Although this information would not associate particular sites with exact years, it would reveal whether or not ruins were constructed within the same time period. On May 22, 1919, Douglass informed Wissler that six specimens from Aztec Ruin, New Mexico, were cut down within a two year period, and estimated that samples from Pueblo Bonito in New Mexico were possibly 25 years older than those collected at Aztec Ruin. Upon receiving this news, Wissler was certain that Douglass would make a crucial contribution to archaeology. Douglass continued comparing samples between the two sites and concluded Pueblo Bonito actually predated Aztec Ruin by 40 to 45 years. These findings led to realization that relative dating could be used on many of the other ruins in the Southwest. Although promising steps had been made in solving the mystery of the ruins in the Southwest, in 1920, the American Museum of Natural History discontinued the funding of Douglass’s research. In order to continue his dendro - archaeological research, Douglass would have to find funding elsewhere.

On January 22, 1922, Douglass was informed that the National Geographic Society could be a potential source of funding. By May of that year, the idea of a Beam Expedition funded by the National Geographic Society was conceived. Beam Expeditions, funded by the National Geographic Society took place in 1923 and 1928. These expeditions produced a floating chronology of 585 years for Southwestern ruins, and extended Douglass’s Flagstaff chronology of Ponderosa Pine, which was 500 years long in 1914, to A.D. 1260. However, these expeditions failed to bridge the gap that existed between these two chronologies.

In 1929, Douglass set out on a third Beam Expedition. This expedition explicitly targeted samples that would potentially bridge the gap between the two chronologies. Finally, on June 22, 1929, a beam labeled HH-39 was extracted at the Show Low site in Arizona. This beam took the Flagstaff chronology back to A.D. 1237. Later that day, the inner rings of HH-39 were successfully crossdated against the outer rings of Douglass’s floating chronology. Over 15 years after he began working with Clark Wissler, Douglass had bridged the gap and, as a result, had a continuous record of tree-ring data dating back to A. D. 700. For the first time in history, dates could be assigned to Southwestern ruins with certainty. Cliff dwelling at Tsegi Canyon, Mesa Verde, and Canyon de Chelly dated back to the 13th century. It was determined that Aztec Ruin was constructed during a period ranging from A. D. 1111 - A. D. 1120. Pueblo Bonito was found to be built in the latter portion of the 11th century.

Douglass formally reported his findings in the December 1929 issue of National Geographic. He wrote:

Its inner rings overlapped the late decades of the old chronology by 49 years, the final ring resting on the year 537 of that sequence; its outer ones overlapped the earliest 120 years of the new, the last one reaching to 1380. Thus the 26 years from 1260 to 1286, which belonged to both chronologies, were definitely matched and their union confirmed by HH-39, which in American archaeology is destined to hold a place comparable to Egypt’s Rosetta Stone… With careful archaeological study we shall perhaps be able to trace the movement of clans and test tribal traditions which have been so often quoted as the early history of these people.

Emil Haury, one of the men who helped extract HH-39, stated:

For the three of us [Emil W. Haury, Lyndon L. Hargrave, and Neil M. Judd], the experience was unforgettable. To be present at the instance of the celebrated breakthrough in science that set the chronological house in order for the Southwestern United States was reward enough. But beyond that, was the privilege to work for a time at the side of Douglass, the scholar, the astronomer turned archaeologist.

Douglass returned to the University of Arizona where he became the first person to formally teach classes in dendrochronology. In 1937, the science continued to develop when Douglass established the Laboratory of Tree - Ring Research at the University of Arizona.

Since Douglass’s famous discovery in the American Southwest, his dendro - archaeological techniques have been used to date structures around the world. Furthermore, dendrochronology has been applied in a number of ways. Currently, tree rings are being used to reconstruct an array of activity including: fire regimes, volcanic activity, hurricane activity, glacial movement, precipitation, mass movements, and hydrology. In many ways anthropologists, ecologists, geographers, and geologists are able to analyze the past and predict future trends. All of this is possible thanks to the archaeological achievement of A.E. Douglass.