Venus Transit and Distance from the Earth to the Sun

August 25, 2012

Transit of Venus Mauna Kea Hawaii and Maine
Photographer: John Stetson
Summary Author: John Stetson

The June 5, 2012 Transit of Venus presented the opportunity to measure the distance from Earth to the Sun as proposed by Edmund Halley in 1716 during a similar transit. This composite image of the transit was taken by Stephen Ramsden in Hawaii (red disk) and by a gathering of amateur astronomers at the L.C. Bates Museum in Hinckley, Maine (dark green disk). These observations provided a baseline for using parallax to determine the distance to the Sun. Dr. Sten Odenwald of NASA's Goddard Space Flight Center examined this image and provided the following:

Venus angular diameter = 58 arcseconds (given). Center-to-center shift = 19 arcseconds.

Baseline distance from Honolulu, Hawaii to Bangor, Maine = about 8300 km. Solar parallax = 11 arcseconds.

Corrected Venus parallax = 19 + 11 = 30 arcseconds.

Venus distance = (206265/30) x 8300 km = 57 million km. Note that one radian = 206265 arcseconds.

57 million km = 0.29 AU (the distance from the Sun to the Earth is expressed in Astronomical Units (AU) where I.0 AU = approximately 148 million km). But based on our calculations, 1.0 AU would be 197 million km. So it's evident that our calculations are a bit off, but this is still a pretty good ball-park value considering that none of the equipment we used had been calibrated.

The goal of the effort to measure the distance to the Sun with the most recent Transit of Venus was to follow Aristarchus of Samos, Edmund Halley, Friedrich Bessel, and others who sought to measure the distance to the Sun and stars. An added goal for me was to try to get my students to consider the insight it took to accomplish these feats.