This Instructional Activity is a part of the Lesson 12, Unit 1, Grade 8. Choosing proper units is essential for work with very large numbers, such as those involved in astronomy (e.g., astronomical distances). F.W. Bessel was the first to measure the distance to a star, 61 Cygni. Students will discover Bessel functions if they pursue STEM subjects at universities. We now know that 61 Cygni is actually a binary system consisting of two stars orbiting each other around a point called their center of gravity, but Bessel did not have a sufficiently powerful telescope to resolve the binary system.