An overlooked 18th-century Danish astronomer

In the late 1700s, the Dano-Norwegian monarchy decided the time was right to establish astronomical observatories throughout its colonies in Greenland, Iceland, and India. Its motivation was more practical than scientific: The monarchy wanted to know the precise latitude and longitude of its lands. At that time, before the availability of precision clocks, a fundamental problem in navigation was determining the longitude of a ship at sea. A skilled astronomer could determine local time, and thus longitude, through a measurement of the Sun’s altitude.

With a considerable grant from the king and with the help of Danish Astronomer Royal Thomas Bugge, several astronomical instruments were sent to the Danish colony of Tranquebar (what is today Tharangambadi), a small town on the southeast coast of India. A German missionary, Christopher Samuel John, was to be the first astronomer, but he backed out to visit his family in Germany. His replacement turned out to be another pastor: Henning Munch Engelhart.

Engelhart was born in in 1754 in Rendalen, Norway. Coming from a family of clergy, he was ordained to be a priest. After finishing school in Norway, he came to Denmark to continue his studies. There he was trained in astronomy under the tutelage of Bugge. Engelhart then traveled to the Danish East Indies as a ship’s priest. He wrote a Malay lexicon for the use of travelers and explored the possibilities of trade between Denmark and Malaya. Returning to Denmark, he applied for a chaplain post and was assigned to the Zion Church in Tranquebar.

From the time of his arrival in Tranquebar in 1787, Engelhart threw himself into several activities. He became the chief archivist and historian, sorting and categorizing a variety of documents from the colony. Those efforts led to The History of the Danish East Indian Establishment, a 400-page manuscript he authored detailing trade relations between the Danes and the Indians. Engelhart also established the Tranquebar Society for Dissemination of Knowledge; he served as secretary, and many prominent government officers became members. The first issue of the proceedings of the society consists of several articles by John on snakes in southern India, a piece on the establishment of a botanical garden—and a description of an astronomical observatory.

The instruments delivered for that observatory consisted of a 9-foot focal length Dollond achromatic telescope, a transit circle, a precision clock, and diverse meteorological instruments. As for where to install them, Engelhart opted for the highest point in Tranquebar: the tower of the Zion Church. Construction proved complicated, however. This was a time of turbulence in the little Danish colony. Conflicts between castes resulted in a worker shortage, making it impossible to mobilize construction. With great difficulty, Engelhart established the observatory when some workers returned.

Before construction was completed, Engelhart managed to measure the latitude of Zion Church to be 11° 1′ 20″ N, which is only some 30 meters off the current accepted value. Measurement of longitude, on the other hand, requires knowledge of the time difference between two places. To find out, Engelhart utilized his new observatory. He measured the positions of Jupiter’s satellites and used them as a celestial clock. After comparing his results with predicted times of eclipses of Jupiter’s satellites in Greenwich published in The Nautical Almanac and Astronomical Ephemeris, Engelhart determined the time difference between Tranquebar and Greenwich to be 5 hours, 18 minutes, and 58 seconds. Taking into account the statistical uncertainty of 38 seconds, Engelhart’s value agrees with the modern estimate.

Engelhart records that Michael Topping, the first official astronomer in Madras (which is today Chennai, India), paid a visit to Tranquebar in 1789; the telescope that Topping brought with him was much better in quality than the one Engelhart possessed. More than 30 years later, John Goldingham, who succeeded Topping, published an article in Philosophical Transactions of the Royal Society with measurements of the longitudes of Madras and Tranquebar. He attributed both results to Topping. However, beyond a doubt, the observations from Tranquebar were made by Engelhart.

Two years in the scorching sun and heavy monsoon rains drained the energy out of Engelhart. Due to the salt content of the air, the telescope at Zion Church deteriorated to uselessness. So in 1791 Engelhart took on a new project: a land survey of the Nicobar Islands. Because Tranquebar was sandwiched between colonies of the English and the French, its administration as a neutral territory was getting to be difficult. The Danes entertained the idea of moving the colony to the Nicobars, which required a thorough survey. Engelhart sailed at the end of January 1791 on the ketch Jutengport to Nancowry. During the voyage, it was imperative to know the latitude and the longitude.

To estimate the time difference between Tranquebar and the ship’s location, Engelhart had a chronometer manufactured by Arnold & Son showing the time at Tranquebar. The chronometer was calibrated through extensive observations of the culminations of known stars at Tranquebar. To determine the latitude and longitude at sea, Engelhart completed a spherical-trigonometry calculation (see the Quick Study by Glen Van Brummelen, Physics Today, December 2017, page 70 ). Although the formula he used was well known, his application was novel.

From the observations available at the National Archives in Copenhagen, I have been able to verify Engelhart’s estimates of latitude and longitude. Compared with the correct value for the longitude of Nancowry, Engelhart’s measurements are off by approximately 200 kilometers, which is very likely due to the inaccurate calibration of the chronometer showing the time in Tranquebar.

About four weeks after arriving in Nancowry, Engelhart died of malaria, barely 37 years old. He was buried on the island of Taraka (modern Teressa), according to a government report.

P. S. Ramanujam is professor emeritus in the department of photonics engineering at the Technical University of Denmark near Copenhagen.