Cook’s measurements of magnetic variation on the First Voyage
During the First Voyage, between May 1768 and July 1771, Cook made over 50 observations of the magnetic variation. However, it was only when sailing along the east coast of Australia, and then south of Timor, that he began to record unexpected changes in his measurements. The question arises of why Cook began to observe large changes in magnetic variation between April and September 1770?
There are four possible explanations for large and sudden changes in magnetic variation.
1.Someone in Cook’s company left something magnetic near the azimuth compass.
One would expect Cook, an experienced observer, to be well aware of such a possibility, and of the influence of the ship’s heading, later studied by Flinders.
2.There were errors in determining true north.
True north and longitude would have been based on observations of the sun, or stars, and comparison in Cook’s words with the “Assistance of the Nautical Almanac and Astronomical Ephemeris”.13 It is my understanding that an error, whether arising from the observations involved or one inherent in the astronomical tables or procedures, could not have led to an error of some three degrees in the magnetic variation.14
3.They were caused by a naturally-occurring sub-outcrop of a highly magnetic rock.
4.Cook’s measurements were affected by magnetic storms.
It has been shown that the regional magnetic surveys around Magnetic Island, offshore Townsville, did not show any large magnetic anomalies to explain Cook’s observation.15 However, if there was a geological explanation it must still be observable today. Recent enquiries amongst Australian yachtsmen has led to the following authoritative statement.
essentially there is no unusual magnetic effect [affecting navigation] from Cape Upstart, Magnetic Island or any other part of the Queensland coast.16
As to magnetic storms, there are six observations to consider. Around New Zealand, between November 1769 and May 1770, Cook mapped the gradual change in the magnetic variation, and made no mention of unexpected deviations.
On 11 April, 1770, between New Zealand and Australia, Cook noted
this 2½°E more than yesterday and expected to have it less for the observations were equally good.
On 19 May, 1770, he observed a magnetic variation 8°36’ East in evening and 8°20’ East in morning. About two or three leagues offshore Sandy Cape, Queensland.
On 5 June, 1770, between sunset and sunrise near Cape Upstart he noted a change in magnetic deviation of 3¼ degrees.
The next day, off Magnetic Island he reported that the
Compass did not traverse well near it.
Between 11 June and 4 August, 1770, Endeavour was grounded for repairs after hitting rocks on the Great Barrier Reef, and no measurements were made of the magnetic variation. However, by observing an Emersion of Jupiter’s first Satellite, Cook made an accurate calculation of longitude.
On 13 September, 1770, offshore Timor, Cook observed
1°10’ West by amplitude and 1°27’ West by azimuth
Three days later, Joseph Banks, offshore Timor, observed an Aurora Australis. This aurora was also observed by the Chinese—the first documented observations of aurora at geomagnetic conjugate points.17
Changes of three degrees in the magnetic variation only occur for short periods of time (five to ten minutes) during magnetic storms. This leaves the possibility that the change of 3¼ degrees overnight in June 1770 was caused by one of Cook’s observations being affected by a large magnetic storm. It is now recognised that, between October 1769 and October 1770, observations in Europe and Asia of large sunspot groups and a series of low-latitude aurorae reflect a period of enhanced solar activity. These widespread observations suggest that in September 1770 the world was affected by one of the largest known magnetic storms, similar to the better known “Carrington event of 1859”.18
Cook’s measurements of the influence of magnetic rocks on his compasses
When sailing north from Cape Howe, Cook and Banks between 20 and 24 April, 1770, discussed the effect of land on the variation of the Needle. Cook was aware that in several places he had visited
the land had a very remarkable effect upon the [magnetic] variation.
Cook followed up this discussion on 30 May, 1770, whilst at anchor at Thirsty Sound. He made the following comments after his climb to the top of Pier Head, a prominent 110 metre high hill.
having the Azimuth Compass with me… the Needle of which differ’d from its True position something very considerable, even above 30 degrees, in some places more, in other [places] less, for I try’d it in several places. I found it differ in itself above 2 points in the space of about 14 feet. The loose stones which lay upon the Ground had no effect upon the Needle; I therefore concluded that it must be owing to Iron Ore upon the Hill, visible signs of which appeared not only here but in several other places.19
Matthew Flinders visited Pier Head on Sunday, 5 September, 1802, and spent two days confirming Cook’s observations. He found that moving his theodolite just three yards changed bearings by two degrees. The theodolite stood four feet above the ground, whilst Cook probably placed his azimuth compass on the ground, thus explaining the differences. Pier Head is a trachyte plug, and the hill has for aeons been subject to lighting strikes, which also magnetize the surface rocks.20
Flinders also noted changes of up to four degrees in magnetic north with changes in the direction of the ship’s head, and subsequently noted how such changes might be permanently take into account. Flinders’ experiments in 1812 included procedures for magnetic compensation of compass heading errors by the strategic placing of soft iron rods near the binnacle compass, which became known as Flinders bars. This led, in 1812, to the Admiralty instructing all Royal Navy commanders to understand their onboard compass environment, and to standardise and maintain their compass binnacles, free from magnetic interferences.21
- Morris, Derek. “Joseph Banks and Geology, 1768- 1771”, in Cook’s Log, pages 21, vol. 41, no. 2 (2018).
- Halley, Edmond. “An account of the cause of the change of the variation of the magnetical needle; with an hypothesis of the structure of the internal parts of the earth”, in Philosophical Transactions of the Royal Society of London. 1692. Vol. 17. Pages 563-78.
- Thrower, Norman J. W. The Three Voyages of Edmond Halley in the Paramore, 1698-1701. Hakluyt Society. 1981. Pages 22-27.
- Thrower. op. cit. Page 35.
- ibid. Page 21, note 3.
- ibid. Page 51.
- ibid. Page 57.
- ibid. Page 140.
- ibid. Page 151.
- ibid. Page 156.
- Cook, Alan. Edmond Halley: Charting the Heavens and Seas. Oxford University Press. 1997. Pages 283-284.
- Thrower. op. cit. Pages 64, 65.
- Wharton, W.J.L. Captain Cook’s Journal During His First Voyage Round the World made in HM Bark Endeavour. Elliot Stock. 1893. Republished by the Libraries Board of South Australia. 1968. Page 176.
- Personal communication from Professor Jim Bennett, President of the Hakluyt Society.
- Morris, Derek. op. cit. Page 27.
- Personal communication from Captain Raymond Pincott in Townsville, Queensland, a licensed compass adjuster.
- Simpson, John. “The first recorded aurora australis?” in Journal of the British Astronomical Society Association. 2018. Vol. 128, no. 1. Pages 33-37. Thornton, Cliff. “Cook’s Observations of the Aurora Australis” in Cook’s Log, page 18, vol. 34, no. 2 (2011).
- Personal communication from Dr Gemma Richardson, British Geological Survey, Edinburgh. Hayakawa, Hisashi et al. “Long-Lasting Extreme Magnetic Storm Activities in 1770 Found in Historical Documents” in Astrophysical Journal Letters. 2017. Vol. 850, no. 2.
- Wharton. op. cit. Page 152.
- Morrison, Doug. “Geophysical surveying in Australia by the navigators James Cook and Matthew Flinders” in Preview, the journal of the Australian Society of Exploration Geophysics. 2017. No. 189. Pages 38-41.
- Flinders, Matthew. “Concerning the differences in the magnetic needle, on board Investigator, arising from an alteration in the direction of the ship’s head” in Philosophical Transactions of the Royal Society of London. 1805. Vol. 95. Pages 186-197.
Originally published in Cook's Log, page 32, volume 41, number 3 (2018).