Sisters students head for the stars

Ron Thorkildson, Sisters area astronomer and meteorologist, was a guest instructor in Sisters High School teacher Rima Givot's biology and astronomy class last Tuesday afternoon, introducing a method of measuring the distance from Earth to the stars.

One aspect of preparing to go out and explore the stars is finding out how far away the nearest stars may be, and then with additional information, how much lunch to pack before starting the trip. For the students under Thorkildson's tutelage, measuring those distances began with understanding and then implementing the "Method of Parallax."

Parallax is the apparent shift of a foreground object in relation to one or more background objects as viewed from two different lines of sight.

Jacob Snyder volunteered to help Thorkildson make the demonstration by closing his right eye and looking directly at a small green star glued to a stick Thorkildson had mounted on a tripod placed in front of a whiteboard, and how it lined up with stars Ron had drawn on the whiteboard in the background.

He then told Jacob to close his left eye and look at the star on the tripod with his right eye, and note that the star's position against the background stars had shifted.

The Method of Parallax, a means of measuring the distance between Earth and stars, is also known as the "Stellar Parallax."

"Parallax shift" was greater when the tripod-mounted star was closer to the observer, and smaller when the tripod was moved farther away. Thorkildson then showed how that exercise can be used to indirectly measure how far away things are.

Givot's students correctly reasoned that the parallax effect they witnessed in the classroom was caused by the distance between their right and left eyes (about three inches on average), called the baseline. But what about determining the distance to objects farther away? It became clear that a greater baseline would be required.

Thorkildson explained that for the distance between two nearby objects in the solar system - for example, a comet or an asteroid - two observatories can observe the same object at the same time, and measure the "parallax angle" between the comet and a distant star. Knowing how far apart the two observatories are will allow you calculate the distance to the object.

Givot uses a teaching technique called kinesthetic learning, that engages her pupils physically as well as mentally, to maximize learning potential. So when Givot and Thorkildson asked the students to come up with longer baselines necessary to determine greater distances, class members began moving about the room experimenting with different lines of sight and engaging various learning aids such a globe of the earth and a picture of the solar system.

When the time for deliberation had run out, most students decided that the greatest possible baseline had to be the diameter of Earth's orbit around the sun. This led to the projection of a set of graphics demonstrating that, by making two sightings of a foreground star relative to stars farther away, conducted six months apart, will maximize the shifted angle of parallax. Then, by employing the mathematical disciplines of algebra, geometry and trigonometry, the class was shown how to calculate the distance from the earth to a measured foreground star.

Ron finished up by saying that even the nearest stars are so far away that the measured parallax angles are very small. Sirius, the brightest star in the sky, lies at 8.6 light-years from Earth, and its measured parallax is only 0.76 seconds of arc - very small indeed.

The class session ended with Givot instructing her students to write down how they would determine the distance from Earth to Sirius, using the parallax method, and why it would work.