The Discovery of the Aberration of Starlight

By Don Selle

First published in the March 2012 issue of The GuideStar

I truly enjoy learning the history of astronomy, because I find that it is much more interesting way to learn the basics than getting them out of a textbook.  It helps me to better understanding when I am able to visualize the path taken by the astronomer, as well as the struggles and small successes that lead to the discovery of new knowledge. 

220426km_bradley.pngIt is also encouraging for me to note that in the early days of astronomical science, it was “amateurs” who were making the discoveries and developing new techniques and technologies in the process. Few stories from the history of astronomy illustrate this better than the discovery of the aberration of starlight.

The development of our understanding of astronomy has been greatly enhanced by the application of the scientific method. Observations are made to test a theory, theories are modified to match observational evidence and at times unexpected results are obtained which lead to new discoveries.  A classic demonstration of the scientific method from astronomy’s early years is the discovery and description of the aberration of starlight by James Bradley, a very dedicated amateur, driven to understand what he was observing.

In 1725 when Bradley and his partner, Samuel Molyneaux, commissioned the manufacture of a high-power zenith telescope, discovery of stellar aberration was the furthest thing from their minds. They had a grander objective –- to prove beyond doubt that the sun-centered Copernican theory of planetary motion was actually true.  In addition, they hoped to measure the distance to a nearby star.

While the Copernican theory was generally accepted by this time, there was no independent evidence to prove the model was a true representation of the solar system. Without such evidence, the Copernican theory could be just a convenient method to more accurately calculate predicted planetary positions – a mathematical model only.

To prove the Copernican theory was a true picture of the solar system, the pair planned to detect and measure the parallax of a star.  Stellar parallax is a perceived change in position of a nearby star as compared to the background stars when the star is observed from two widely separated points in the earth’s orbit. 

220426km_parallax.pngHold your thumb up at arm’s length and look at it first with your right eye only (left eye closed) then with your left eye. Your thumb will appear to shift position – that’s parallax, and the baseline is the distance between your eyes. The parallax of an object is determined by measuring the angle an object appears to move when viewed from two points on the end of a baseline.  When the target is a relatively nearby star and the length of the baseline is known, (i.e., the diameter of the earth’s orbit) the distance to the star can also be determined. 

220426km_telescope.pngIf stellar parallax were indeed detected and measured, it would prove that the earth was not the center of the universe, but, in fact, did orbit the sun.  In addition, since the length baseline (the diameter of earth’s orbit) over which the parallax was known, the distance to the star could be calculated.

The zenith telescope, which was designed for a single purpose, was about 4 inches in aperture and constructed with a tin plate tube about 24 feet long. It was hung from the vertical face of a chimney at Molyneaux’s mansion which bordered Kew Gardens in London, and holes were cut through the roof and two floors to accommodate it. It had an iron hinge at the top, and the vertical angle of the scope could be adjusted north and south with a micrometer screw. 

A fine wire cross hair had been put into the eyepiece to assist the observer to center the star as it reached its highest point overhead. A plumb bob attached near the objective hung all the way to the bottom of the tube and was used to measure how far the scope was from the vertical. Bradley determined that the telescope could produce measurements accurate to one second of arc (1/3,600 of a degree) which was far more accurate than any previous telescope could measure.

220426km_graph.pngWith the zenith telescope, Bradley and Molyneaux planned to detect and measure the change in the apparent position of the star Gama Draconis over the course of a year. If parallax was observed, the measurements, taken when the earth was at different points in its orbit, would show a distinctive pattern.  The maximum change in the apparent position of the star would occur between observations made when the earth was at opposite sides of its orbit, six months apart from each other. Based on the placement of the star relative to earth’s orbit, Bradley expected to see the star drift gradually from north to south and back again over the course of a year with the northernmost observation occurring in June and the southernmost in December. 

The star Gama Draconis was chosen because it was a relatively bright star (and therefore thought to be closer to earth than dimmer stars) and it daily passed almost exactly overhead in London. It was also bright enough that it could be observed in daylight. 

First light for the Kew telescope was in December 1725, and almost immediately, Bradley realized that the observations showed something other than the parallax of the star.  Instead of being at its southernmost point as calculated, the star kept drifting further southward. This drift continued through the spring, until it stopped in March. The star then started 220426km_aberration.pngmoving northward, returning in June to the position initially observed in December and reaching its farthest northward position in September. 

Observations taken over almost two years confirmed that the north to south drift repeated over a 365- day period.  The period, however, was shifted by exactly three months from the pattern which Bradley expected to be caused by stellar parallax. It was clear that the drift of the star had to be caused by something other than parallax

Bradley and Molyneaux considered several other possible causes, including instrumental error and, the most likely one, that they were observing a previously unknown wobble in the earth’s rotation. They then embarked on a thorough investigation of all possible causes. 

 A second zenith telescope was constructed which had a wider field of view and measurements were made on several other bright stars. All behaved as Gama Draconis had. Wobble of the earth’s rotation was ruled out as a star 180 degrees from Gama Draconis showed the same pattern of movement, not an opposite pattern which would be the result of a rotational wobble.


220426km_frame-ref.pngBradley continued to work to come up with an explanation until one day in a flash of inspiration the answer came to him. Unfortunately, Molyneaux had died before the answer was found. 

According to a history of the Royal Society, the inspiration came to Bradley when he was sailing on the Thames.  He noted that the wind vane at the top of the mast was affected by the speed and direction the boat traveled. When the boat was at rest, the vane at the top of the mast showed the actual direction of the wind. When the boat moved forward and the speed and direction of the boat had a component perpendicular to the wind direction, the vane pointed not directly to the wind direction, but shifted somewhat in the direction the boat was moving. The faster the boat moved, the farther the vane shifted toward the direction the boat traveled. This effect was due to the fact that the wind vane was affected by both the wind speed and the boat’s speed and direction.

With this insight, Bradley realized that it was the speed and direction of the earth in its orbit, relative to the speed and direction of the starlight, which was affecting the direction the telescope needed to point to center the star. He was able to develop a mathematical theory of stellar aberration that fit the observations he and Molyneaux had taken.  

In September 1728 Bradley announced his new theory of stellar aberration and presented it to the Royal Society in London the next January.  While Bradley had not, in fact, detected parallax and, thereby, proving the Copernican theory, the discovery and description of stellar aberration  which depends on the earth’s orbital speed  was sufficient to prove that Copernicus was right.  In a serendipitous bonus, knowing the earth’s orbital velocity, Bradley was able to calculate the speed of light to be approximately 183,000 miles per second which is quite close to our modern value. 

 In a further twist of irony, Bradley would later report the discovery of nutation, a wobble in the earth’s rotation which he ruled out as the cause of the much larger drift measured for Gama Draconis.   Bradley was never able to measure a parallax for that star, however, as its distance from earth would require a telescope with 15 times the accuracy of Bradley’s original zenith telescope. It would be another 110 years before a reliable stellar parallax was measured, and the scale of our Milky Way Galaxy began to be known. But that’s a story for another day. 

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