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Transit of Venus: The Unsung Heroes

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Venus Transit seen by NASA’s Sun-observing TRACE spacecraft. Credit: NASA/LMSAL

The following is a guest post by Dr. Sten Odenwald, NASA/ National Institute of Aerospace, who presented a lecture on the Transit of Venus at the Library of Congress on May 8, 2012. You can view his lecture on our webcast page and Youtube channel.

On June 5th, 2012 most people will have the opportunity to go outside and, with proper eye protection, gaze up at the Sun to see a tiny black dot leisurely pass across its bright disk. The tiny black dot is the planet Venus. This transit of Venus last occurred in 2004, and before that in 1882. The next time Venus will transit the Sun will be on a December day in the year 2117.

Many astronomers are asked about the importance of this event, and we dutifully discuss how it was used by earlier astronomers to gauge the distance between the Sun and Earth. This distance, around 93 million miles, is important to know because it gives us an idea of how big our solar system is.  Before the measurements of the transit of Venus were made in earnest back in the 1700s, the best guesses were that this distance was only a few million miles.  That made it a very crowded solar system and universe!

Astronomers also like to mention how previous societies reacted to this rare event. We look through the historical documentation to make sure that we get the facts more or less correct. When researching  who came up with the idea to use one of the ‘interior’ planets, Mercury or Venus, to figure out the Earth-Sun distance – called the Astronomical Unit– it seems we should be able to single out one individual. However, depending on your criteria, it is more likely two individuals.

James Gregory, 1638-1675

Gregory vs. Halley

In 1663 Rev. James Gregory (1638 – 1675), considered to be one of the most important mathematicians of the 17th century, proposed a method to determine the Earth-Sun distance. He published this idea in his 1668 book Geometriae pars universalis  (also in Opticae promota in 1663), and showed how this distance could be measured from observations of the transit of Venus made from various widely separated geographical locations on Earth.  It was a non-mathematical discussion, but it got all the relevant facts correct. All you needed to do was to fill in the details for how to do this in practice.

Sir Edmond Halley (1656-1742), for whom Halley Comet is named, made the same suggestion 14 years later in 1677 and published an important paper on the details of this technique in 1716.  He had originally read this paper, entitled A New Method of Determining the Parallax of the Sun, or his Distance from the Earth,’ before the Royal Society of London in 1691.  In the paper he described in detail how scientists of various nations, observing the upcoming 1761 and 1769 transits of Venus from many parts of the world, would be able to measure the Earth-Sun distance to near-perfect accuracy. Through a series of ‘back of the envelope’ estimates, he showed how observers at different locations on Earth would obtain slightly different times for the duration of the transit, and how these time differences were related to the Astronomical Unit.

Sir Edmond Halley (1656-1742)

In his paper Halley notes:

We therefore recommend again and again, to the curious investigators of the stars to whom, when our lives are over, these observations are entrusted, that they, mindful of our advice, apply themselves to the undertaking of these observations vigorously. And for them we desire and pray for all good luck, especially that they be not deprived of this coveted spectacle by the unfortunate obscuration of cloudy heavens, and that the immensities of the celestial spheres, compelled to more precise boundaries, may at last yield to their glory and eternal fame.

There was no mention whatsoever of Rev. Gregory having developed the same method. By today’s standards of attribution this is an egregious error and most modern referees of scientific papers would have accused Halley of professional sloppiness and, with a rude comment, sent the paper back for editing.

As for Rev. Gregory, as one Scottish history site about Gregory declares:

There is yet another discovery of the very highest importance to the science of astronomy, which is falsely and, we would hope, unknowingly attributed to another philosopher, whose manifold brilliant discoveries throw an additional luster over the country which gave him birth. We allude to the employment of the transits of Mercury and Venus, in the determination of the sun’s parallax, the merit of which is always ascribed to Dr Halley, even by that eminent astronomer Laplace. But it is plainly pointed out in the scholium to the 28th proposition of Gregory’s work, published many years prior to Halley’s supposed discovery.

I am not a professional historian, but as an astronomer I can scarcely ignore Gregory and give all the opprobrium to Halley, even though Halley plays a major role in the development of post-Newtonian, dynamical astronomy.   Here’s another pair of scientists you might find interesting.

Cover from Observations of the Transit of Venus, 9 December, 1874; Made at Stations in New South Wales (Sydney, C. Potter, government printer, 1892).

Simon Newcomb vs. William Harkness

You would think that the astronomer who actually determined the value for the Astronomical Unit could be singled out very clearly in the historical documents, but even this singular credit becomes murky.

Eight American expeditions were fitted out in 1874, organized by the Transit of Venus Commission, with Simon Newcomb (1835-1909) as the official Secretary of the Commission. The U. S. Congress appropriated funds totaling an astounding $177,000 for the expeditions. Although Newcomb considered the result of the 1874 observations disappointing due to inherent difficulties in the method, in 1882 at the urging of the Director of the US Naval Observatory, astronomer William Harkness (1837-1903), Congress once again appropriated some $10,000 for improving the instruments, and $75,000 for sending eight more expeditions. Following the 1882 transit, the hard work of combining the 1874 and 1882 data began in earnest, but proved to be a huge and frustrating challenge.  The data varied in quality, coverage, and location on Earth, and all they had at that time was paper and pencils to do the lengthy calculations. But the motivation was high to complete this work because it would be the last time this method could be used until 2004! In 1891, Newcomb’s refined calculation of the distance between Earth and Sun was finally published, based on all of the assembled data from several transits. He concluded that the official distance was   92,702,000 +/- 53,700 miles.

Harkness also spent a considerable part of his career analyzing the transit data and putting it in the context of other important astronomical constants. This he did in his lengthy monograph The Solar Parallax and its Related Constants, published in 1891 in Washington Observations.  Harkness obtained the value of 92,797,000 +/- 59,700 miles. In fact, Newcomb used Harkness’s value in his famous book The Elements of the Four Inner Planets and the Fundamental Constants of Astronomy (1895), but gave it a much lower statistical ‘weight’ than most other methods.

South lawn, U.S. Naval Observatory, Spring 1874. Transit of Venus teams

It is important to note that the two numbers, 92,702,000 and 92,797,000 have an average value of  92,750,000 miles, and the two estimates differ from the average by only 48,000 miles. The uncertainties in each measurement of about 56,000 miles are consistent with the fact that there is no significant statistical difference between these numbers despite Newcomb’s careful attention to weighting the quality of the data (technically the values differ from their average by only one standard deviation!).

So, why is it that Simon Newcomb tends to get all the historical credit for the final estimate? Is ‘Secretary of the Transit of Venus Commission’ a more prestigious position than ‘Director of the US Naval Observatory’? In the end, it comes down to what the international astronomical community chose to anoint as the ‘official’ number, determined through a complex process of ‘weighting’ the accuracy of the individual estimates. Presumably,  this weighting of numbers by Newcomb was not also driven by professional rivalry!  Harkness, however, did have the final say on transits by waxing poetic in his haunting comment that the next transit of Venus will occur “when the June flowers are blooming in 2004.”

There are many other unsung historical figures in astronomy. In the fullness of time and historical inquiry, I hope that they too will get their posthumous due!



  1. This was a great post! So interesting and great details!

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