This year will give us two opportunities to re-enact two classic methods to find the sun’s distance from earth: the transit of Venus and the opposition of Eros.
Throughout the telescopic era, the transit of Venus has played an important role in the determination of the solar parallax (or distance to the sun). The hazardous expeditions sent out in the eighteenth and nineteenth century to observe the transit of Venus had this determination as their main purpose. But after four observations in two centuries, employing state of the art techniques like achromatic telescopes and photography, it dawned on the astronomical community that the actual accuracy of the transit method wasn’t as high as expected. In the mean time, during the nineteenth century, other objects that could help in finding the solar parallax were discovered: astroids.
The most famous asteroid that was used to find the solar parallax is coming into view this month. 433 Eros, a near-earth asteroid discovered in August 1898, comes to opposition every 2 to 3 years. During its opposition of 1900-1901, a worldwide programme was initiated to make parallax measurements of the asteroid, from which the solar parallax could be deduced. The results were published ten years later by Arthur Hinks. Another programme was executed during a closer approach in 1930-1931 by Harold Spencer Jones. The value of 8.″790 of the solar parallax secured by this programme was held to be definitive until 1968.
The opposition of Eros this month is special, because the asteroid will come close enough to earth to be visible through a small telescope for the first time in 37 years. Its shortest distance to earth this month is comparable to that of the January 1931 opposition. This will provide an unparalleled opportunity for amateur astronomers to conduct a programme to measure the asteroid’s parallax again and compare the results with the professional campaigns of 1901 and 1931. All that is needed is to take simultaneous pictures of Eros from different places on earth, for example before sunrise from Europe and after sunset in America.
Eros moves fast enough that you can detect its motion within an hour, which makes it easy to identify. The finder chart shows the path of Eros through the constellations of Leo and Sextans during the last days of January, until its closest approach of 16.6 millions of miles on January 31. Even a small group of dedicated amateurs can make valuable observations that lead to a value of the solar parallax. How will this measurement of Eros compare to the transit of Venus later this year? If you are interested to participate in this programme, let us know! Contact us at info@transitofvenus.nl. More background information and detailed instructions can be found in this article by Michael Richmond of the Rochester Institute of Technology and me.
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