The aberration of light

From the HowStuffWorks website:

“Aberration of Light, a phenomenon in which a star or other celestial body, as viewed from the earth, appears to be slightly displaced from its true position. This phenomenon, which was first explained by the English astronomer James Bradley in 1729, occurs because (1) the earth is constantly moving in its orbit around the sun and (2) light travels through space at a finite speed (about 186,000 miles per second [300,000 km/s]). If the earth were stationary, or if light traveled through space instantaneously, the aberration phenomenon would not exist.

The traces left by raindrops on the side windows of a moving automobile provide an analogy to the aberration of light. Even if the rain is coming straight down, the traces will be at an angle.”

In The Maybe Universe: how to reclaim our heritage, J. Hacsi writes:

“The discovery that light traveled with a finite velocity…at least as measured by an observer on earth…was not universally accepted until after 1728, when James Bradley (1693-1762), an English astronomer working at Kew Observatory, discovered the aberration of stars. In observing the star Draconis, Bradley found that its position as observed from earth changed during the course of a year in a way that could not be attributed solely to the earth’s orbital movement around the sun. He deduced that this aberrational displacement was due to the finite velocity of the light streaming toward us, and computed a value for that velocity of 301,000 km/sec.

Bradley’s work established the principle of a finite velocity for light, but it was over a hundred years before attempts were made to measure this finite velocity more precisely.”

From A History of the County of Surrey: Volume 3. Originally published by Victoria County History, London, 1911:

“…Three different buildings have at various times gone by the name of Kew Palace: the one that is now standing; the house that was opposite to it until the beginning of the 19th century; and a huge embattled castle which was planned by George III, and of which a large part was built after plans by Wyatt, but never completed. The history of the other two palaces is difficult to trace with accuracy. The palace that was pulled down in 1802, and which was then a large house of plain exterior, was the more important of the two…

Lord Capell died without children in 1696 and his wife survived him twenty-five years, dying at Kew in 1721. Her husband’s great-niece Lady Elizabeth Capell was her heir. She had married in 1717 Samuel Molyneux, the astronomer, Secretary to George II, then Prince of Wales, M.P. and Privy Councillor, who arranged a private observatory in the house at Kew, from which he and James Bradley made the observations that led to the discovery by the latter of the aberration of light. The sundial in the garden marks the site of the palace, and commemorates the observations made there.

Mr. Molyneux predeceased his wife by a couple of years in 1728. Shortly after her death Kew House was leased to Frederick, Prince of Wales. He also appears to have found the observatory a source of much interest, and during the winter of 1737–8 Dr. Desagulier read lectures on astromony every day to the household. His observatory was then described as a large room at the top of the house, where he had all his mathematical and mechanical instruments at one end and a Planetarian at the other.”

(From Wikiwand:

Samuel Molyneux (1689–1728), Member of Parliament, and an amateur astronomer, who was married to Lady Elizabeth Diana Capel, the eldest daughter of the Earl of Essex, inherited Kew House on the death of Lady Capel of Tewkesbury. Molyneux set up an observatory at the house and collaborated there with James Bradley in innovative designs for reflecting telescopes. Kew House which later, as the White House, became the home of Prince Frederick and Princess Augusta, was pulled down in 1802 when George II’s short-lived gothic “castellated palace” was built.)


“…As a friend of Samuel Molyneux, who was interested in measuring the annual parallax of stars, from which it would be possible to calculate the distances to the nearest stars, Bradley was invited by Molyneux in December 1725 to his estate at Kew, a district of London, to assist him in observing the star Gamma Draconis, specifically, attempting to determine its annual parallax.

Gamma Draconis was specifically chosen since it crosses the meridian almost overhead, thus allowing observers to ignore any problems caused by refraction of light through the earth’s atmosphere. According to theory, the star would culminate at its most southerly point in December, with its point of culmination shifting northwards every day to its most northerly point in June, returning over the following six months.

What was actually observed however was that after December, Gamma Draconis continued to culminate at more southerly points, reaching its most southerly point in March, rather than December, and its most northerly point in September, rather than in June. The overall change in position over the six months was about a one hundredth of a degree.

Bradley thus, from August 1727, in an attempt to explain the cause of this movement, decided to observe a number of stars over the year. He found that they each displayed a similar motion…”

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