Dendrochronology & Astronomy

Story first featured on the University of Arizona Steward Observatory’s education program “Active Galactic Videos” here.

What is the link between the trees and the Sun? They are not anywhere close to one another — the Sun is 92 million miles away and trees grow here on Earth.

Both are made out of the same basic materials fused by the universe. But the link between the Sun and the trees here on Earth runs deeper than elements from the sky.  It all leads back to one man: Andrew E. Douglass.

Taken in Spokane, Wash. by Emmalee Mauldin

Douglass was an astronomer in the early 1900’s that worked at Lowell Observatory in Flagstaff, Ariz., until he was fired from the observatory 1901 and moved to Tucson to teach Physics and Geography at the University of Arizona (UA).

During his five year break in Flagstaff, historians and astronomers believe that Douglass transformed into a forester and took his astronomical studies to the trees.

“He wanted to connect the climate history of the Earth to the solar cycle,” said Dr. Thomas Fleming, head of astronomy education at Steward Observatory at UA.

The solar cycle is an eleven-year cycle where changes in sunspots, flares, and other solar atypical manifestations appear. Every eleventh year the Sun dims slightly and Douglass’s research attempted to prove his theory about the solar cycle to the Earth’s climate. After years of research, the data he collected was inconclusive.

But through his research with trees, Douglass created an entirely new field of science: dendrochronology. Dendrochronology is the study of tree rings, which Douglass studied in order to determine precipitation, drought, fire regimes, Earth climate and climate change. By using the information he collected, Douglass attempted to connect Earth climate history back to the Sun and its solar cycle.


Taken in Spokane, Wash. by Emmalee Mauldin

Dr. Christopher Baisan, a dendrochronologist at UA’s Tree Ring Lab, explained that tree rings have a growing season and this season can be compared with other trees in a region, in order to piece together the climate history of that area. While this knowledge was not helpful in connecting the solar cycle to the trees, it continues to be very valuable in archaeology and analyzing historical sites around the globe.

This knowledge is also very valuable in studying the seasonality growth of trees and natural fire regimes (a general pattern in seasonal fires), which is data that UA graduate researchers Christopher Guiterman and Erica Bigio conduct with Baisan. Their research on utilizing tree rings to determine fire regimes has been partially funded by NASA.

“The reason tree rings were brought into [NASA] is long-term perspective,” Guiterman said.

Guiterman explained how NASA utilizes satellite data for fires that spans an entire continent, but that this data can only go back 30-40 years. However, data from dendrochronology can go back thousands of years depending upon tree ages. The older the tree, the more data that NASA can use to construct fire regimes of a region.

NASA does not often fund Earth-based research because most of these projects are funded by the National Science Foundation (NSF), whereas NASA funds mainly astronomy projects. Guiterman and Bigio explained that NASA likely funded the project to supplement their instruments in space and provide more context to the patterns discovered here on Earth.

Taken in Spokane, Wash. by Emmalee Mauldin

“[Tree rings] give you context for the present by looking into the past,” Guiterman said.

While “looking into the past” might not matter to many citizens, the decisions that humans make affect ecosystems across the world today.

“The more we know about the historical context prior to human influence,” Bigio said, “the better we can determine how much humans are influencing [climate], and then how better to manage our resources, which affects all of us.”

Andrew Douglass later founded both Steward Observatory and the Tree Ring Lab at the University of Arizona in Tucson before his death in 1962.

Douglass’s theory about tree rings and the solar cycle might not be as connected as he hoped, but for humans, seeking patterns and looking for connections is just in our nature.

“Knowledge for knowledge’s sake is important,” Fleming said, “because you never know where the knowledge will be needed.”


For more information about UA’s Steward Observatory, visit their website.

To learn more about UA’s Richard F. Caris Mirror Lab, visit their website.


Thank you for reading! Have you ever heard of dendrochronology? Have you ever heard about this connection between dendrochronology and astronomy? Let me know and like this post!




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