READIN

In Astronomy and Geodetics the names of Charles Mason and Jeremiah Dixon are inseparably linked together. They were colleagues in making observations of the Transits of Venus in 1761 in 1769, in the survey of the famous Mason and Dixon Line in America and in the measurement of the length of a degree of latitude also in America. How they came to be at the Cape in 1761 is almost what Shakespeare would call a tragical-comical-historical-pastoral story which I regret must be considerably curtailed in this talk.

Though Gregory was the first to point out that transit of Venus over the Sun's disc afforded a means of ascertaining the solar parallax, Halley was the first to explain how it was to be accomplished. This he did in a paper read before the Royal Society in 1716 wherein he urged young astronomers who should live to observe the phenomenon to apply themselves diligently with all their might to the making of this observation so that it may redound to their immortal fame and glory. He assumed the Sun's parallax to be 2 seconds and the difference between the parallaxes of the Sun and Venus to be 31 seconds. It is impossible to go into the details of his prediction but it must be mentioned that he was careful to explain that the motions of the Nodes of Venus had not yet been discovered and could only be determined by such conjunctions of sun and planet. If the Nodes remained in the same place and if the plane of Venus's orbit were immoveable in the sphere of the fixed stars the planet would pass four minutes of arc below the Sun's centre.

He laid great stress on observtions being made either at Madras or Bencoolen on the western coast of Sumatra and also at Fort Nelson on Hudson's Bay in North America. At the first named places the whole of the transit would be visible while at Fort Nelson it would enter on the Sun's disc just before sunset and leave it immediately after sunrise. Moreover the duration of the transit would be 15 minutes 10 seconds longer at Fort Nelson that in the East Indies.

As the date of the transit approached astronomers of all nations got busy. The Royal Society raised money, including £800 from the Crown, to send Maskelyne to St. Helena and Mason and Dixon to Bencoolen. Maskelyne was unfortunate as bad weather prevented his observing the Transit. Mason and Dixon embarked on H.M.S. Seahorse but a French frigate of superior force attacked their vessel and killed 11 and wounded 38 of the crew. The Frenchmen sheered off when another English vessel hove in sight but the Seahorse had to put back to repair damages. Our astronomers had not bargained for fighting and they wrote to the Royal Society declining to proceed to Bencoolen but offering to go to Scanderoon. The reply was a letter which (let us hope) has never been equalled by that illustrious body before or since. It threatened inflexible resentment and prosecution with the utmost severity of the law. It prophesied an indelible scandal upon their characters and utter ruin, and concluded with an express command to go on board the Seahorse and enter upon the voyage be the event as it may fall out.

Fear of the Royal Society proved greater than fear of the French and the voyage began but they soon found that they could not hope to reach Bencoolen in the time, so by the advice and consent of the captain of their frigate they made for Cape Town, then of course a Dutch colony. As good patriots they could not admit to foreigners that an English ship had had a bad time from a Frenchman so with economy of truth they informed the Dutch Commander that diverse disasters in the Channel had unduly delayed them.

They asked to be allowed to spend some time here, that a site suitable for an observatory should be granted and that materials for the erection of the observatory should be supplied. All of which requests were duly complied with, and when they left our Astronomers wrote conveying their grateful appreciation of the assistance rendered to them. The site of the observatory was between John and Hope streets behind St. Mary's Cathedral. (I may mention here that thanks mainly to Mr. Wells, Capt. Cook's chief astronomer, and Sir Thomas Maclear the site of the taking of every astronomical observation made at the Cape can be ascertained with geodetic precision.) Its latitude was determined as South 33° 44' 42" and its longitude East 1hr.13m.35s.

Mason and Dixon arrived in Table Bay on April 27th, took the instruments on shore on May 2nd and set the clock going on May 4th. The body of the observatory was circular with a radius of 6 feet and the height of the wall was 5 feet. The roof which was made of board was conical in shape and was moveable. The opening was 3 feet broad and the roof was easily turned to any part of the heavens. The clock was fixed against two timbers of 10 x 8 inches section sunk 4 feet into the ground and joined by rods 1 inch in diameter. The pendulum of the clock was not altered in length. The other instruments were (1) a quadrant of one foot radius made by Bird and the property of the Earl of Macclesfield and (2) two reflecting telescopes, each 2 feet local length and magnifying 120 times, mady by Short. One at least of the telescopes had a micrometer because we find a small table for the adjustment of the nonus of the micrometer. On May 4th they fixed the quadrant satisfactorily and found by meridian observations of Procyon on the 4th, 5th and 6th that the clock has a losing rate of 2m 17s, 2m 18s, and 2 m 16s. The report says that on this date the observatory being now finished I put the clock into it, wound up the pendulum and set it to nearly sidereal time.

On the great day the sun ascended in the thick haze and immediately entered a dark cloud but in 20 minutes they obtained their first sight of Venus which of course was on the Sun's disc. Then it became first hazy and then cloudy but at 1h 18m 7s they obtained a measurement of the distance between the Sun's farthest limb and Venus's southern limb. They made 5 more similar measurements and also determined the apparent diameters of the Sun and Venus. Very soon after the transit finished the sky again became cloudy and remained so until night. Mason reports "When I saw the planet first its periphery and that of the Sun's were in a great tremour, but this vanished as the Sun rose and became well defined. Four minutes before the internal contact the Sun's disc was entirely hidden by cloud for about one minute."

During the remainder of their stay at the Cape our astronomers made many observations of stars at equal altitudes to obtain meridian passages, measured zenith distances of various stars for determining the latitude, and observed immersions and emersions of Jupiter's satellites for determining the longitude.

On September 28 they packed up their instruments and the next day put them on board the Mercury on which on October 3rd they sailed for St. Helena where they joined Maskelyne. There the clock was set going again (the length of the pendulum still not having been altered) and from Oct. 31st 1761 to Jan. 22nd 1762 they made observations for determining the rate of the clock.

They left soon afterwards and reached England in safety.

© 1951 The Astronomical Society of South Africa