1631 transit of Mercury
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| Gassendi’s diagram showing the motion of Mercury across the face of the Sun from Mercurius in sole visus & Venus invisa (1632). Credit: Todd Timberlake. Observing Transits of Mercury from 1631 to Now |
The first recorded transit of Mercury was observed on November 7, 1631, in Paris, by the French theologian, philosopher and astronomer Pierre Gassendi (1592–1655).
After completion in 1627 of the Rudolphine Tables, describing in numbers the position of the heavenly bodies using his new theory of elliptical planetary orbits, Johannes Kepler (1571–1630) predicted that both Mercury and Venus will transit over the Sun's disk in 1631 — on November 7 for Mercury and on December 6 for Venus. He wrote an "admonition" to astronomers, that was published in 1630. Kepler died that year, never witnessing his predictions.
Many astronomers in Europe attempted the observation, using a camera obscura or by projecting an image of the Sun through a telescope onto a screen. Most observers were foiled by the cloudy weather. Mercury also turned out to be much smaller than was expected and observers who were using a camera obscura could not see it. Few observers were successful and only one, Pierre Gassendi in Paris, published his observations.
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| Portrait of Pierre Gassendi, Robert Nanteuil, 1658. Credit: Rijksmuseum/Wikimedia Commons |
Gassendi used a darkened room in which the image of the sun was admitted through a simple Galilean telescope and projected onto a piece of paper. He adjusted the distance so that the projected image was about 20 cm in diameter and drew a circle with that diameter on the paper. He divided the diameter of this circle in 60 equal parts, so that each division would to 30" of arc. An assistant in the room below observed the altitude of the Sun with a quadrant to get an accurate timing of the events, each time Gassendi stamped his foot.
Because of the uncertainty in the predictions,
Gassendi began to observed two days before the expected date. On
November 5 the weather was poor, with rain all the day. The next
day he had only a glimpse of the Sun through a fog. On November
7 the Sun became visible for longer periods around 8 a.m., but
it was still hazy. Shortly before 9 a.m. Gassendi saw a small
dark spot on the Sun, but first he interpreted it as a sunspot,
because Kepler had predicted that Venus' diameter is almost a
quarter of the Sun's diameter, so presumably, Mercury should
have a tenth of the Sun's diameter:
"... I was far from suspecting that Mercury would project such a small shadow. For such was its smallness that its diameter hardly appeared to exceed half of one of the divisions marked. I thought rather that it was a spot which I had not noticed on the Sun the previous day, but which could have grown since that time, just as I had seen others do."
"...After a sensible delay between observations, and having restored the Sun again along the direction of that diameter, as before, I observed that the said distance had become four divisions greater, Thereupon, thrown into confusion, I began to think that an ordinary spot would hardly pass over that full distance in an entire day... And when the Sun shone again, and I ascertained the apparent distance to be greater by two divisions..., then at last I thought that there was good evidence that it was Mercury."
He now carefully measured Mercury's size in terms of
the Sun's diameter and found that it "could scarcely exceed
two-thirds of one division, that is the third part of a minute, or
20 seconds."
Gassendi published his observations as two letters to
Wilhelm Schickhard, the professor of Hebrew and astronomy at
Tuebingen. In Mercurius in sole visus et Venus invisa
(Paris, 1632) he discussed the smallness of Mercury in detail.
Gassendi’s observations helped to correct the long-standing error
regarding planetary sizes and also helped astronomers make slight
improvements in Kepler’s orbital theory for Mercury.
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| The Transit of Mercury which occurred on May 9th, 2016, as viewed at 14:24:01 UTC from East Grand Forks, MN. Mercury is visible as a black dot below and to the left of center, and sunspot AR2542 is prominently visible above center (as well as AR2543 below it and AR2544 on the upper left edge of the Sun). Credit: Elijah Mathews/Wikimedia Commons |
Transits of Mercury can only occur when the Earth is aligned with a node of Mercury's orbit. Currently that alignment occurs within a few days of May 8 (descending node) and November 10 (ascending node). During May transits, the apparent diameters of the Sun and Mercury are 1902 and 12 arc-seconds, respectively. Thus, Mercury appears to be 1/158 the size of the Sun. In contrast, the apparent diameters of the Sun and Mercury during November transits are 1937 and 10 arc-seconds, respectively. So then Mercury appears to be 1/194 the size of the Sun. The last four such transits occurred on May 7, 2003; November 8, 2006; May 9, 2016; and November 11, 2019. The next will occur on November 13, 2032. Mercury transits are much more frequent than transits of Venus, with about 13 or 14 per century, primarily because Mercury is closer to the Sun and orbits it more rapidly.
References:
van Helden, Albert, The Importance of the Transit of Mercury of 1631. Journal for the History of Astronomy, Vol. 7, p.1
Timberlake, Todd. Observing Transits of Mercury from 1631 to Now
Espenak, Fred. Seven century catalog of Mercury transits: 1601 CE to 2300 CE. NASA Eclipse Web Site
© 2025, Andrew Mirecki
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