Richard Dunthorne

Lore & Background

Dunthorne was born in humble circumstances in Ramsey, Cambridgeshire, and attended the free grammar school there. He attracted the notice of Roger Long, whose protégé he became. Dunthorne moved to Cambridge, where Long first appointed him as a footboy, and he received some further education, though not regular university education. He later managed a preparatory school in Coggeshall, Essex, before returning to Cambridge, where Long obtained for him an appointment as a butler at Pembroke Hall, an office he retained for life. His main activity was assisting Long in astronomical and scientific work. Dunthorne also held an appointment as superintendent of works of the Bedford Level Corporation, responsible for water management in the Fens, beginning several years before 1761 and continuing into the 1770s. In this role, he was concerned in a survey of the Fens in Cambridgeshire and supervised construction of locks near Chesterton on the River Cam. His association with Long remained lifelong, and Dunthorne acted as executor of Long's will.

Reader's Guide

Dunthorne's most significant legacy is his quantification of the Moon's apparent acceleration. Edmond Halley had suggested around 1695 that the Moon was gradually accelerating, based on comparisons of contemporary observations with ancient eclipse records. Dunthorne's computations, based in part on ancient accounts of eclipses, confirmed the apparent acceleration, and he was the first to quantify the effect at +10 arcseconds per century squared in terms of lunar longitude. This estimate is not far from later assessments, such as de Lalande's in 1786 and values from about 10 to nearly 13 arcseconds derived about a century later. Dunthorne also contributed to celestial navigation as the first Comparer of the Ephemeris for the Nautical Almanac, working as sole comparer for the first three issues (data for 1767–69) and continuing as one of several comparers until the issue for 1776. He contributed a method for clearing nautical lunar observations of the effects of refraction and parallax, for which he received a reward of £50 from the Board of Longitude in 1772. He has been credited as the first to apply trigonometrical formulae for the general spherical triangle to the reduction of lunar distances and to give auxiliary tables for that purpose. Dunthorne also planned and funded the construction of an observatory in 1765 on the Shrewsbury Gate of St. John's College, and gave astronomical instruments to the college. The crater Dunthorne on the Moon is named after him.

Did You Know?

• He was the first to quantify the Moon's apparent acceleration, estimating it at +10 arcseconds per century squared.
• Dunthorne served as the first Comparer of the Ephemeris for the Nautical Almanac, working on the first three issues for 1767–69.
• He planned and funded the construction of an observatory on the Shrewsbury Gate of St. John's College in 1765.

From Ramsey to Pembroke Hall: An Unlikely Rise

Richard Dunthorne's entry into serious scientific work was far from the conventional academic route. Born in 1711 in modest circumstances in Ramsey, Cambridgeshire, he attended the local free grammar school, where his natural aptitude drew the attention of Roger Long, who would go on to become Master of Pembroke Hall and Lowndean Professor of Astronomy and Geometry at Cambridge. Long took the young man under his protection, first bringing him to Cambridge in the lowly capacity of a footboy. Dunthorne received some further instruction there, though it was nowhere near a formal university education. He spent a period managing a preparatory school in Coggeshall, Essex, before returning to Cambridge, where Long arranged for him a post as butler at Pembroke Hall—a role Dunthorne kept for the rest of his life. It was in that domestic-sounding position that his real vocation took shape: assisting Long with astronomical and scientific research. Their partnership lasted a lifetime, and Dunthorne ultimately served as executor of Long's will. A man who never held a degree, he nonetheless reached a level of mastery that, in the words of a contemporary, would do honour to the proudest professor in any university.

Quantifying the Moon's Apparent Acceleration

Dunthorne's most enduring scientific legacy lies in his study of the Moon's subtly changing orbital speed. Edmond Halley had raised the possibility around 1695, comparing contemporary observations with ancient eclipse records and suspecting a gradual acceleration. Dunthorne took that hypothesis and gave it a numerical backbone. Drawing on historical eclipse accounts and his own comparative observations across different lunar and solar configurations, he confirmed the apparent speeding-up and, critically, became the first to assign a precise quantitative figure: +10 arcseconds per century squared in lunar longitude. His estimate proved remarkably close to later assessments, including de Lalande's 1786 determination and the range of roughly 10 to nearly 13 arcseconds derived about a century after Dunthorne's work. Neither he nor Halley grasped the true mechanism—the effect actually stems from a slow deceleration of Earth's rotation, a fact not understood until the notion of ephemeris time emerged. Dunthorne published his results in papers for the Philosophical Transactions in 1746 and 1749, securing his place in the history of lunar mechanics.

Comparing the Nautical Almanac and Taming the Lunar Distance

In July 1765, the Board of Longitude, effectively steered by Nevil Maskelyne, named Dunthorne as the first Comparer of the Ephemeris and Corrector of the Proofs for the still-unpublished Nautical Almanac and Astronomical Ephemeris. The inaugural issue, carrying data for 1767, broke new ground by furnishing mariners with the computational tools to fix longitude at sea through lunar observations. Dunthorne worked alone as comparer for the first three issues, spanning 1767 to 1769, and then continued as one of several comparers through the 1776 edition. His most practical contribution was a method for removing the distorting influences of atmospheric refraction and parallax from nautical lunar observations, a process called clearing the lunar distance. Maskelyne wove this technique into the companion table volume that accompanied the Almanac. In 1772 the Board awarded Dunthorne a reward of fifty pounds for shortening what had been tedious calculations. He is also credited as the first to apply trigonometrical formulae for the general spherical triangle to the reduction of lunar distances, complete with auxiliary tables, and an improved version of his method appeared in an 1802 edition.

Surveyor of the Fens and Benefactor of Cambridge

Dunthorne's practical side was as demanding as his theoretical one. Running concurrently with his astronomical work for decades, he served as superintendent of works for the Bedford Level Corporation from several years before 1761 into the 1770s. In that role he oversaw water management across the Fens, carried out a survey of the Cambridgeshire Fens, and supervised the building of locks near Chesterton on the River Cam. His broader scientific output was wide-ranging: a 1739 book of lunar tables constructed from Newton's theory to aid its testing, tables on Jupiter's satellite motions in 1762, observations of the Venus transits in both 1761 and 1769, and a 1751 letter on comets in the Philosophical Transactions. His generosity toward Cambridge was equally significant. In 1765 he planned and funded the construction of an observatory on the Shrewsbury Gate of St John's College and donated astronomical instruments to the institution. The observatory remained in use until its closure in 1859, a lasting testament to a man who rose from a grammar school in Ramsey to shape both astronomy and the practical infrastructure of the English Fens.