Father Tachard’s Astronomy

11th October 2018

Modern astronomy in South Africa began with ships, or perhaps it would be more correct to say it began with shipwrecks. Indeed, early seafarers found the southern tip of Africa with its raging storms, gale force winds, colossal waves and deadly reefs, to be a frightening and treacherous corner of the Earth… and the coastline is littered with the wrecks of ships that met their violent end being smashed up onto a wild and alien shore.

Not only were accurate positions for southern stars simply not available (although the Southern Cross had become a well-known object by 1500), the problem lay in determining the latitude and longitude of the Cape of Good Hope. Determining latitude was relatively easy in that it could be found from the altitude of the sun at noon with the aid of a table giving the sun’s declination for the day. But without a chronometer to give Greenwich (or some other standard) time it was very difficult to determine the difference in time between noon at the Cape and noon at the place from which the voyage started. What was needed was some event which takes place in the sky and is seen simultaneously from many places on the Earth. Such events are provided by the movements of Jupiter’s satellites.

When Galileo discovered four moons of Jupiter in the early 1600s, he noticed that their motions followed Kepler’s laws. This clockwork precision meant that the Jovian system could be used as a “heavenly clock” to determine the time. One way this could be done was by observing when the moons entered-or-exited the shadow of Jupiter as they passed behind the giant planet. Even with a small telescope, one could observe a moon fade to darkness over the course of a few minutes as it entered the shadow, or gradually brighten as it left the shadow on the other side.

With a table listing the predicted times of these eclipses as seen from a particular observatory, one could know the exact local time at that observatory. By comparing that with the time that you measured your local noon to be, you could then determine the time difference between your location and the observatory. Knowing that the Earth rotates 360 degrees in a day, you could then determine your longitude relative to the observatory.

Galileo went so far as to calculate time tables for these eclipses, but they weren’t accurate enough to be very useful. Then in 1668 Giovanni Domenico Cassini published the first truly accurate table of Galilean eclipses. Cassini’s tables were so accurate that they redefined the shape of Europe. The distances between cities as given since the Roman Empire were found to be hundreds of kilometres off. However, despite its accuracy, the Galilean method of longitude the method was completely useless while at sea.


HST catches Ganymede just before it ducks behind the giant planet.

HST catches Ganymede just before it ducks behind the giant planet.


On 31 May 1685 a French ship stopped briefly at the Cape of Good Hope. On board was Father Guy Tachard, a Jesuit mathematician and missionary born on 7 April 1651, in Marthon, France. (One account of him says that he was inclined by his study of mathematics to his missionary career; I confess that I can’t see the connection.) He was accompanied by five other Jesuits (Fathers Jean de Fontanay, Francois Gerbillon, Joachim Bouvet, Louis Le Comte, and Claude de Visdelou, all scholars of repute) and they were on their way to Siam.


Father Guy

Father Guy Tachard (1651-1712), drawn by Carlo Maratta


In the  early 1680s, King Narai of Siam sent several envoys  to France  with the hopes of establishing strong political and economic relations with France (his motivation for this alliance was to deflect the growing Dutch trade and military influence in and around Siam). King Louis XIV reciprocated the honour of these special visits by sending his own embassy to Siam in 1685. His objective was not only to further commercial relations with Siam and encourage Christian conversion in the East, but also to engage in a scientific expedition of Africa, the Indies, and China. The expedition’s astronomical observations at the Cape were mainly aimed at a more accurate determination of its longitude.


The original cover of Guy Tachard's "A Relation of the Voyage to Siam. Performed by Six Jesuits, Sent by the French King, to the Indies and China, in the Year, 1685"

Tachard published an account of his 1685 voyage to Siam with five other Jesuits in 1687 – a mere five months after returning to France, and it was just as speedily published in an English translation. It was published by Orchid Press in 2006… a good read!


The expedition was equipped with the best scientific instruments of the time, provided by the Acadamie des Sciences, and Tachard described the scientific equipment of his party thus:

“They (i.e. the members of the Royal Academy of Science) gave us the Table of the satellites of Jupiter which were compiled with so much labour, and which serve at present to determine longitude. They also presented us with many large telescopes of 12, 15, 18, 25 50 and 80 feet, some of which we left at the Peking Observatory. We had made two quadrants of eighteen inches radius, the other of twenty-six, three large clocks, an instrument to find at the same time the Right Ascension and Declination of stars, and an equinoctial dial which marked hours and minutes, and which carried a compass which enabled us to find at any hour of the day the declination of the needle. All these instruments were for our astronomical observations. “

They also took with them thermometers, microscopes, barometers, astronomical tables and sea charts.

Tachard gives a tremendous description of the stars as he sailed south:

“As we approached the Line we took pleasure in noting how the stars of the North Pole sank, and those of the South Pole ascended in proportion above our heads.

“Of all the new stars we discovered in the South those which above all struck us the most were those of the Cross, so called because the four principal ones are arranged in the form of a cross. The largest of all is at 27 degrees from the Pole, and it is by this that the navigators are guided and from which they sometimes measure the altitude.

“As we advanced every day in that direction and discovered every day new stars, we had the leisure to study them and to compare this new region of the heavens with the astronomical map of Father Pardies, but we scarcely found any conformity therein. This map is in great need of correction, and we may begin with the Cross, the arms of which are more unequal in the sky than on the chart.

“We noticed the Wolf and the Centaur drawn with so little truth that it was difficult to recognise them in the sky. They make the part occupied by them extremely bright by reason of the large number of stars of which they are composed, and which make them appear as a single constellation. But it is very different in the map where the two constellations do not appear as more than mediocre ones.

“The stars of the Southern Triangle are truly placed as regards their positions relatively to one another, but badly placed with regard to other constellations.

“The stars of Taurus are not so bright as they appear in the map, but their disposition is almost the same.

“Grus, in my opinion, is the most correctly mapped constellation in these parts, and it is only necessary to see it once in the map to find it immediately in the heavens. The Bee, Toucan, and Chameleon, though small are fairly well placed.

There is also something to correct in the shape and situation of the Magellan Clouds and other southern constellations where one could find other inaccuracies by means of instruments.”

The party landed at the Cape on 31 May 1685 where they were hospitably received by Hendrik Adriaan van Rheede tot Drakenstein, the High Commissioner of the Dutch East India Company and who was inspecting the Cape Settlement, and Governor Simon van der Stel. Tachard explained to the Commissioner that “We had taken on board several mathematical instruments proper to the finding of the true longitude of places we might visit without need of an eclipse of the moon or the sun. We explained the new method of observing the satellites of Jupiter, of which the learned M. Cassini has made such fine tables. I added that we should do a great service to their navigators in giving them the certain longitude of the Cape of Good Hope which they only knew by calculation: a doubtful method and one which often deceived them very considerably.

“He said that we gave them great pleasure, and since we wished to work at this discovery, he would give us a suitable place for observations.”

Tachard and his party were given permission to set up a temporary observatory in a pavilion in the Dutch East India Company’s garden, a site where Government House was later built.


Guy Tachard's observatory at the Castle which was built in 1666 by the Dutch, alongside a photo of the Castle today, slap bang in the middle of downtown Cape Town

Guy Tachard’s observatory in the unused pavilion in the Company gardens, alongside a photo of the Castle today (built 1666) which it faced and which is now slap bang in the middle of downtown Cape Town


“On the middle of the wall, on the side which looks towards the Fort [the present Castle] there is a small pavilion which no-one inhabits; the lower story contains a vestibule, open both ends towards the garden and the Castle, which has two rooms on each side.

“Above, there is a room open on all sides between two terraces, paved and bricked, surrounded by railings, one terrace looking toward the North, the other South. This pavilion appeared to have been made expressly for our purpose, for on one side one can behold the whole North, the view of which was above all necessary for us, because at that place it corresponds to what the South is to us.

“While they were making this pavilion ready for us we returned on board to report to the Ambassador and our Fathers all that had taken place.

“The next morning the Commissioner and Commander sent on board all sorts of supplies. The officer in charge of these told us that these gentlemen had also sent us a boat to carry our mathematical instruments.

“As we had got ready during the night those of which we believed we would have need, we put these to the boat and repaired to the observatory on the 2nd June, 1685.”achard’s observations started on the night of 2 June, when the beginning of a transit of Jupiter’s brightest satellite was timed: ““A clock made at Paris by Thuret, having been set to an hour near to that which might be the correct one without our knowing it exactly, we commenced the following observations :-

“The first satellite appeared that evening elongated to a distance a little less than Jupiter’s diameter at three minutes past eleven by the clock which had not yet been corrected.

“The first satellite commenced to touch the limb of Jupiter at 11 hours 57 minutes and 30 seconds, and we could not see it any longer at 11 hours 58 minutes and 30 seconds.

“These observations were made with an excellent 12 foot telescope belonging to (or made by) the late Mr le Bas; the times are always those of the uncorrected clock.

“We continuously observed Jupiter until 2 hours and 5 minutes after midnight at which time it became hidden behind the Lion mountain, which bounded the view on the Western side, so we could not see the emersion of the first satellite of that day.”

And on the night of 4 June he recorded that the Commissioner and van der Stel wished to be eye-witnesses of his observations and came to the observatory. They then saw the Ambassador walking about in the garden (“having come incognito to walk in the garden the evening before, he had found it so agreeable that he returned today and was walking about”) and the three visitors stayed in the observatory until 10 pm. Tachard showed them the instruments which they found “very fine and curious”.

Of his observations he recorded: “After the emersion of the first satellite from the shadow of Jupiter, having compared the observations of the altitudes of the Sun taken in the morning and afternoon of the 3rd and 4th June, and having compared the difference of the times of the same altitudes of the upper and lower limbs of the Sun (as we had observed the upper limb in the morning and the lower limb in the afternoon), we found that the clock was in advance of the Sun each day by 28 minutes.”

Unfortunately, Tachard does not give details as to how he arrived at the error of the clock.

“At the emersion of the first satellite the clock showed 10hrs. 5 min. 40 sec. from which must be deducted 28 minutes. There remains the true time of the observation, 9 hrs. 37 min. 40 sec.

“The ephemerides of M. Cassini calculated to minutes gave the time of this emersion under the meridian of Paris as 8 hrs. 26 mins.; but the tables of eclipses calculated to seconds gave the same emersion as 8 hrs. 25 mins. 40 sec. This being reduced to the time observed at the Cape of Good Hope gives 9hrs. 37 mins. 40 sec.

“There remains the difference of the meridians of the Cape of Good Hope and Paris of 1 hr. 12 min. which makes 18 degrees of difference of longitude, and having supposed the longitude of Paris, taken from the first meridian that passes through the isle of Ferro (the most western of the Canaries) to be twenty two degrees and a half, the longitude of the Cape of Good Hope, taken from the same meridian will be forty degrees and a half, but little different to that which modern charts give it.

“On the morning of 5th June, at ten o’clock, the gentlemen came to the observatory and remained there until 2 p.m. to see us take the altitude and distance of Table Mountain, and to examine our instruments.

“We showed them particularly the use of the equinoctial quadrant, by means of which we again found this day the variation of the compass to be 11 degrees and a half N.W.

“We can take two advantages of these observations, the first is the variation of the compass, which we found with the astronomical circle to be 11 degrees and a half from N.W., and the second the true longitude of the Cape, which we checked by the emersion of the first satellite of Jupiter, which should have taken place at 8 hours 26 minutes at Paris, and having been observed at the Cape at 9 hours 27 minutes 40 seconds in the evening gives 1 hour 12 minutes 40 seconds of difference between the meridians of these two places, which, reduced to degrees is 18, and in consequence the charts are in error, and show the Cape too far to the East by 3 degrees, which in reality it is not.”

One really wonders how he came about his error of eight minutes six seconds for the difference between Paris and Cape Town. (I also really hope that his error had nothing to do with the 1686 mission from the King of Siam that was wrecked near Cape Agulhas, the geographic southern tip of the African continent and the beginning of the dividing line between the Atlantic and Indian Oceans).

For those of us who enjoy vicarious observing, here are Tachard’s notes from the night of 3 June:

“In the evening, having no particular observations to make, we studied different stars with the twelve foot telescope.

“The foot of the Cross as marked in Bayer is a double star; that is to say, two bright stars elongated one from the other only by about their diameter, like the more Northern of the Twins, without speaking of a third much smaller one, which one also sees there but further off from these two.” (Like most of his contemporaries, Tachard believed the stars to show definite apparent diameters rather than being virtual point sources.)

James Dunlop is officially credited with the discovery and α Crucis is number 252 in his catalogue of double stars!!



Tachard’s “an infinity of stars”. While hesplit Alpha Crucis, the glorious gem at the foot of the Southern Cross, in 1885, Father Jean Richaud split Alpha Centauri in 1689 while observing a comet from Pondicherry, India


“There are many places in the Cross, below the Milky Way, which with the telescope appear filled with an infinity of stars.

“The two Clouds, which are near the South Pole,do not appear a cluster of stars, like Praesepe Cancri, nor even a dull light like the nebula in Andromeda. One sees almost nothing with large telescopes, but without a telescope one sees them very white, especially the large cloud.

“Nothing in the heavens is so beautiful as the constellations of the Centaur and the Ship. There are no bright stars near the Pole but there are a quantity of little ones.

“Bayer and the other writers who mention them, omit many of them, and the greater part of those they do mention do not appear in the heaven in the same situation.”

On 7 June 1868 Tachard and his fellow Jesuits left Table Bay for Siam. At their departure the priests presented the governor with a microscope and a small burning glass in recognition of his hospitality. They arrived safely in Siam, and in the early morning of 12 December, 1685, they observed a total eclipse of the moon.

After spending some time in Siam, Tachard and two other members of the French embassy were sent back to France with a Siamese embassy.  In answer to the request of the king of Siam, Louis XIV sent a party of 14 Jesuit priests fitted out by the Acadamie des Sciences, two diplomats and over 600 soldiers to Siam in 1687 (the party included Father de Beze who wrote a book describing the 1688 Siamese revolution which ended the first European attempts to penetrate the kingdom). Tachard was a member of this expedition, which travelled in a fleet of six ships and stayed at the Cape from 11 to 27 June to enable more than 300 seriously ill men to recuperate.

On the voyage out Tachard records:

“I observed with the parallactic instrument the declination and R.A. of many stars towards the South Pole, which we could not observe in Siam. As all these stars are very badly charted, or are not found at all on the globes and celestial charts, which have appeared up to the present, I have resolved to make on, which  I have already commenced, and which will,if I am not mistaken, be much more exact than all the others… I dare say to you I make light of the knowledge of the stars in the situation which they are placed by all the Uranographers up to now, as regards the stars of the South, which does not give place to the North in either the number or the beauty of its stars.

“We must correct the Larger Magellanic Cloud, and still more the small one. The Cross, the Bee, the Triangle, the Centaur, the Chameleon, Grus and the Milky Way are badly charted, or have stars omitted from them. As for the ship Argo, half of the brightest stars which compose it are not even marked on the celestial charts.

“In addition to all these faults, there are many bright stars which can be seen in France, but which are not quite correctly placed, since we always see them at a great elongation and too near the horizon.”

On 11 May and he observed a partial eclipse of the Sun on 11 May while on board and gives a great description of it. (Cassini had advised him before his departure that there would be an eclipse of the Sun and it would be total at Cape Verde Islands and Guinea.)

Upon landing at Cape Town, Tachard visited governor van der Stel and obtained his permission to use the same building as before to make astronomical observations, but alas he was kept very busy with non-astronomical endeavours and thus:

“It is true that our time was so occupied and so inconveniently that it was only with incredible pains that we observed two emersions, of the first satellite of Jupiter on the 19th of June. The same satellite, having disappeared some time before, was observed to reappear at 11 hours 55 minutes in the evening. On the 21st following I observed its emersion at 6 hours 25 minutes in the evening. Combining the times of these two emersions with those made for Paris in the Ephemerides of M. Cassini viz., 10 hours and 25 minutes and 5 hours 3 minutes the difference between Paris and the Cape will be 2 degrees [an obvious misprint for 22 degrees])

However, he goes on to say – and I am sure everyone will find this as charming as I do:

“It is true that the 14 foot telescope was not altogether well stayed, and it having some movement I fear the real emersion preceded by some time the time I observed, but this could be only a minute or two at the most.” (!!)

What an adventurous astronomer he was! Over the course of his life he made more journeys to the East, and still adventuring at the age of 61, he died on 21 October 1712, in Bengal Province, India, of an infectious disease.

The shipwreck trail around Cape Point is a fascinating and moving hike; the sea- and sand-scoured skeletons an incredible testament to the early mariners, explorers and astronomers who braved this treacherous cape, the story of longitude itself, and the beginning of astronomy in South Africa.





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