Teaching stellar parallax through the years

How far away are the the stars?  Answering this question constituted the major research effort of the Van Vleck Observatory for the bulk of the twentieth century. Using a technique called stellar parallax, the astronomers, plate measurers, and computers who worked at Van Vleck observed the heavens, made careful measurements, and performed calculations to determine the distances to stars.  While students were not actively involved in this process until the mid-twentieth century, the stellar parallax program was integral to astronomical instruction at Wesleyan, as we can see from the many pedagogical aids in our collection that relate to this technique.

What is stellar parallax?

Stellar parallax describes the way stars that are closer to Earth appear to be moving when measured against a background of farther-away stars. Stars appear to be moving as a result of changes in the Earth’s position: because we rotate around the sun, the Earth in January is about 186 million miles away from where it is in June. When your position of viewing changes, it appears as though things closer than you have shifted, when compared to things father away.

We can see this with a much more down-to-earth example. Put your thumb about six inches away from your face. Your left eye is the Earth in January, your right eye is the Earth in June. Close just your right eye and see where your thumb (the star) is in relation to the letters on your computer screen (which, in our example, represent more distant star). Then close just your left eye and see where your thumb is.

Your thumb appears to jump when you look at it from one eye or the other. What’s actually happening is that the angle from which you are looking at it changes. By measuring how far the star seems to move, we can find the angle we are looking at it from, and then use simple trigonometry to find out how far away the star is.

Determining the distances to stars was an important task for astronomers, because it would allow them to calculate other things, like the absolute magnitude of a star: how bright it actually is, not just how bring it appears to us from Earth.

Teaching stellar parallax

In introductory astronomy classes at Wesleyan now, students are taught stellar parallax through this intuitive, kinesthetic demonstration of parallax.  In the class I took, students were asked to stick out their thumbs and see it move against the background of the blackboard. We can imagine twentieth-century astronomy professors leading their students in the same exercise, winking and holding out their thumbs in unison.

But of course stellar parallax was also taught mathematically and in more formal ways. An introductory astronomy textbook from 1930, owned by Frederick Slocum, depicts a drawing of stellar parallax along with a formal description of it. Perhaps Slocum assigned this textbook to his students, who would have learned through reading and mathematical exercises about the observatory’s central research program.

Frederick Slocum's 1930 astronomy textbook, Astronomy by John Charles Duncan, open to a page on stellar parallax, using two diagrams. Photo by author, book from Van Vleck Observatory collections.

Frederick Slocum’s 1930 astronomy textbook, Astronomy by John Charles Duncan, open to a page on stellar parallax, using two diagrams. Photo by author, book from Van Vleck Observatory collections.

Students would also have learned about stellar parallax through other classroom methods. Nowadays, astronomy professors rely on PowerPoint presentations to supplement their lectures. In the early twentieth century, Slocum and his colleagues would have illustrated their lectures using lantern slides. These glass slides, viewed therough a projector, could enlarge photos for classroom viewing, and create serial slideshows. Many of these lantern slides survive today—we have a cabinet full of them in the observatory. Boxes of slides used for teaching include depictions of stellar parallax, solar eclipses, stars and planets, and important historical figures in astronomy. A set of teaching slides about stellar parallax illustrate the general concept, offer examples taken from Barnard’s Star, a famous red dwarf, and even include a lesson plan for the day. Images of decidedly non-astronomical subjects—such as Frederick Slocum in costume for a play he appeared in on campus, or photographs from a sailing trip he took to the Bahamas—show that lantern slides were about fun as well as teaching.

A lantern slide explaining parallax in a similar fashion to the textbook, with the author unfortunately reflected within it. Photo by author, slide from Van Vleck Observatory collections.

A lantern slide explaining parallax in a similar fashion to the textbook, with the author unfortunately reflected within it. Photo by author, slide from Van Vleck Observatory collections.

Stellar parallax has been taught at Van Vleck visually, through textbooks, and kinesthetically, all within the same four walls of the astronomy classroom. As the many teaching resources on parallax contained in our collections suggest, Frederick Slocum’s goal of uniting research and instruction under one roof was a reality from the observatory’s beginnings.

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2 thoughts on “Teaching stellar parallax through the years

  1. Pingback: Happy (Belated) Birthday, Freddy! | Under Connecticut Skies

  2. Pingback: A president, a professor, and the president’s wife go to Paris…Punchline: they come back with a telescope | Under Connecticut Skies

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