What did Francis Baily see at Inchbonny?

In May 1836, astronomer Francis Baily travelled north in the UK to observe an annular solar eclipse. What he saw that day inspired him to write a paper which has made his name, peculiar spelling and all, part of astronomical history.

His paper, “ On a Remarkable Phenomenon that occurs in Total and Annular Eclipses of the Sun ”, published in the papers of the Royal Astronomical Society, describes what are now known as ‘Baily’s Beads’. In his own words:

For when the cusps of the Sun were about 40° asunder, a row of lucid points, like a string of bright beads, irregular in size and distance from each other, suddenly formed round that part of the circumference of the moon that was about to enter, or which might be considered as having entered, on the sun’s disc.

With this, Baily’s beads were formally described for the first time.

Inchbonny

Earlier in his paper, Baily describes his observing location for the eclipse. Having arrived in Kelso on May 13, in the Borders region of Scotland and England, two days before the eclipse, he proceeded the next morning to travel south to the town of Jedburgh:

I looked about for a convenient place for mounting my telescopes, and where I should be free from any interruption : and having heard of Mr. James Veitch, a very ingenious gentleman, residing at Inch Bonney, about half a mile to the southward of the town of Jedburgh, I proceeded thither, and was with the greatest readiness and kindness furnished by him with every convenience I required for making my observations.

Baily was not quite certain of the exact coordinates of his observing location, giving them to the best of his knowledge as “ Lat. 55° 27′ 30″ and Long. 10m 12s,0 west from Greenwich ”, i.e. 55.4583°N 2.5500°W. However, we can do better.

By remarkable coincidence, the very house named Inchbonny is still standing, and is at the time of writing listed for sale.

It’s unclear if the current owners or real estate agents are aware of the connection with Francis Baily. The brochure mentions the Veitch family, who owned the property until as recently as 2012, and speaks of visits from Sir Walter Scott and geologist James Hutton, but makes no mention of Francis Baily.

The exact coordinates of the house are in fact 55.4698°N 2.5533°W, slightly farther north than Baily’s figures.

We might assume that Baily set up his equipment near to the house for convenience but with a clear view of the Sun and Moon for the hours of the eclipse. The open gravel area to the north of the main building is a reasonable assumption.

What Baily saw

Under perfect viewing conditions (“ the sky was perfectly clear and serene ”), Baily observed his eponymous beads. The formation of the “ string of bright beads ”

indeed was so rapid that it presented the appearance of having been caused by the ignition of a fine train of gunpowder.

He continues:

as the moon pursued her course, these dark intervening spaces (which, at their origin, had the appearance of lunar mountains in high relief, and which still continued attached to the sun’s border) were stretched out into long, black, thick, parallel lines, joining the limbs of the sun and moon; when, all at once, they suddenly gave way, and left the circumferences of the sun and moon in those points, as in the rest, comparatively smooth and circular;

Baily is describing the moments around second contact, C2, when, during an annular eclipse, the Moon is first fully ‘contained’ by the Sun. He estimates that the beads observed at C2 lasted ‘about six or eight seconds, or perhaps ten at the utmost’.

Can we reproduce what he describes?

Simulating what Baily saw

Modern eclipse calculations, combined with recent digital lunar mapping missions, plus a little computer graphics wizardry allow us to simulate what Baily saw that day. The video below shows the simulation for Baily’s presumed location at Inchbonny from 10 seconds before second contact (C2), when the cusps of the Sun are indeed separated by around 40°:

The simulation does show some beading from around 10 seconds before the standard (modern) calculated time of C2, but it does not really begin in earnest until 3-4 seconds before, and continues until around 5s after C2 when the annulus (ring) is fully formed. While far from being the most spectacular Baily’s beads that have been observed, the simulator seems reasonably consistent with Baily’s description.

The one area in which it perhaps does not fully match up is in the absence of ‘long, black, thick, parallel lines’, which we must assume correspond to the so-called ‘black drop’ effect, described and illustrated so well by NASA’s Ernie Wright. The Photo Ephemeris eclipse simulator does not currently attempt to account for the potential blurring caused by Earth’s atmosphere (‘bad seeing’). It’s a potential improvement for the future. That aside, we’ve verified the simulator against a number of actual eclipse photos and videos to verify its accuracy, and it holds up very well.

Unfortunately, Baily was observing some years before the first solar eclipses were successfully photographed, so we cannot compare our results directly. However, we can be reasonably confident the simulator is a good match to what he saw.

We can check what Baily may have seen at third contact (C3) also. In his words:

After the formation of the annulus thus described, the moon preserved its usual circular outline during its progress across the sun’s disc, till its opposite limb again approached the border of the sun, and the annulus was about to be dissolved. When, all at once (the limb of the moon being at some distance from the edge of the sun) a number of long, black, thick, parallel lines, exactly similar in appearance to the former ones above mentioned, suddenly darted forward from the moon and joined the two limbs as before : and the same phenomena were thus repeated, but in an inverse order.

Here’s the simulation:

The annulus (ring) is broken at around 3.8s before the standard time of third contact (C3). Beads are prominent for around 6 seconds.

In the video, the lunar limb is shown in orange with vertical exaggeration. It’s clear from the videos that the lunar hills and valleys are more pronounced at the position angle of C2. At C3, the lunar limb, while still rough, shows fewer dramatic features. This explains the shorter duration of Baily’s beads at C3.

Baily says:

…the rapid and progressive change in the appearances, and their striking character, so riveted my attention again, that I am unable to speak more decidedly on the time occupied, than on the first occasion. The same reason also prevents me from stating the precise number of the dark lines: I should think however that they were not fewer than six nor more than ten.

By ‘black lines’, Baily means breaks in the annulus. Choosing one moment from the simulation, we too can count 6-8 breaks in the annulus:

Remember that the resolution of the lunar limb in the simulator is limited to 0.2°, which is a simplification versus reality.

Baily’s legacy

Baily acknowledged in his paper that he was not the first to observe this phenomenon, nor to mention it in accounts of solar eclipses. But he was the first to devote significant time to describing it and to surveying the literature to find the experiences of his predecessors.

Of all his astronomical work, it is for his lively account of Baily’s beads that he is best remembered.

You can explore the 1836 annular solar eclipse yourself in Photo Ephemeris Web. Were there better locations from which to observe Baily’s beads? We’ll dig into that question in another article!