The 1737 Occultation of Mercury by Venus

The Only Historical Detailed Account of a Mutual Planetary Occultation

In scanning through the Astronomy section of the
"Timetables of Science"
by Alexander Hellemans and Bryan Bunch
published by Touchstone Books

I came across the following

1737 John Bevis, at Greenwich Observatory, observes the passage of Venus in front of Mercury.

Let's look at the 1737 Venus-Mercury Occultation in detail. The path of these planets through the sky is shown below, and as you can see the paths are at quite an angle with respect to each other.

A mutual planetary occulatation is so rare because of the small probability of one distance planet's disk will cover another. The orbits of the planets are tilted with respect to each other as well as the Earth's and the chance of all three aligning exactly is small. Below is the positions of Venus, Mercury and the Earth on May 28, 1737. Note that both Venus is closer moving to inferior conjuction while Mercury is on the far side of the Sun.

In order to see what the event look like, I generated the configuration using the planetarium program GUIDE version 7.0 (Project Pluto). Then by using the programs steps in time, I created a sequence of images over a 16 minute period for an animation. In this animation, the speed of Mercury has been increased by 100 times (each step is 1 minute but lasts only 0.3 seconds in the animation. The animation lasts 5 seconds and runs five times.

[24 Kbytes]
21:42 GMT, 7 minutes before mid-occultation
Click on image to see animation......
Field of View 2.8' x 2.0' arc

Location: Greenwich, England at 0^o W, 51.5^o N
Time of Mid-occultation: 21:49 GMT(9:49 pm) May 28, 1737 (Gregorian Calendar)
Some of the details are:

ALTITUDES Sunset at Greenwich = 20:02 GMT (8:02 pm) Altitude at Mid-occultation (Greenwich) = 1.3o Elongation from the Sun = 22.2o VENUS Heliocentric Longitude of Venus = 237.7o Distance to Venus = 0.323 AU Apparent Diameter of Venus = 51.6" arc Venus Illumination = 7.5% MERCURY Heliocentric Longitude of Mecury = 173.8o Distance to Mercury = 0.979 AU Apparent Diameter of Mercury = 6.9" arc Mercury Illumination = 55%

The planets are so low on the western horizon at the time of occulation that Bevis must have had trouble observing the event from Greenwich. Observers farther west would have had a better view but we have no records of any other observers of this event .

What did John Bevis see on May 28, 1737? (from "John Bevis and a Rare Occultation" by Roger Sinnott and Jean Meuus Sky and Telescope, September 1986, p220-222) Bevis was an amateur but had access to the Greenwich Observatory. He used one of the 24 foot focal length telescopes. His observations were in the evening just at the end of astronomical twilight. At 9:44:00 he notes that Mercury is "no more distant than 1/10 part of Venus' s diameter Then Clouds cover the planets At 9:52:06 the clouds part and "Venus shines out once more brightly; Mercury is in fact entirely concealed behing Venus. But clouds catch up to Venus afresh, preventing further contemplation of this rare spectacle." It is strange that nowhere in his account does he mention how low on the horizon the two planets are. At mid-occultation Venus would have been only 1.4o above the theoretical horizon (that is about 3 apparent diameters of the Moon)!!! And this was seen at Greenwich with London to the west. NOTE: Bevis' clocks were 3 minutes 57 seconds too fast. Even in this one observation of such a rare event, Bevis was not able to see Mercury move behind Venus or emerge from the occultation. Observers farther south would not have seen Mercury disappear behind Jupiter because of the parallax.