Aberration of light

James Bradley explains stellar aberration and derives the Sun–Earth light-time

Bradley looked for one effect and found another: Gamma Draconis reached its extremes roughly three months too late for parallax.

1728: a star moving on the wrong schedule

By about September 1728, James Bradley had worked out both an empirical law for the stars' annual apparent motion and an explanation for it: Earth's orbital motion combined with the finite propagation speed of light.

The work had begun as a parallax experiment with Samuel Molyneux, whose telescope at Kew was installed through the ceiling and roof of his house. Gamma Draconis did shift annually, but its maxima fell at the wrong seasons for parallax.

Bradley tested for instrumental errors and observed additional stars with a new sector at Wanstead. George Graham had built the precision zenith sectors that made shifts of a few seconds of arc measurable.

Eight minutes twelve seconds

From the aberration Bradley derived a ratio of light speed to Earth's orbital speed of 10,210 to one — equivalent, in his calculation, to light crossing the Sun–Earth distance in 8 minutes 12 seconds.

He prepared a letter to Edmond Halley setting out the observations, the calculations and the explanation. It was read at two Royal Society meetings in January 1729 by modern year reckoning, though the Society's catalogue labels the reading "9 January 1728" under contemporary British Old Style conventions.

What he did not find

Aberration turned up during a failed search for parallax, and Bradley did not measure stellar parallax. The aberration angle does not shrink with a star's distance; it was the failure to detect parallax that put the stars beyond his instruments' reach.

His 1727–1728 observations also held a residual change that aberration did not explain, but he did not yet have the long-period evidence to establish nutation of Earth's axis. He disclosed the idea privately in 1737 and published in 1748, after observing a complete lunar-node cycle.

The familiar Thames sailing-boat and wind-vane story does not appear in Bradley's own account and may be apocryphal.

Sources

Researched 24 Aug 2026 6 sources not yet audited Date disputed

How this was checked

Researched from the web into a fact sheet and rewritten from that sheet. This article has not yet had an independent model audit. How the pipeline works →

Date disputed. Sources disagree about when this happened.

What the sources leave uncertain

The securely supportable 1728 event is Bradley’s formulation of the aberration explanation. Its Royal Society presentation occurred in January 1729 by modern year numbering.
The Royal Society catalogue labels the reading “9 January 1728,” reflecting contemporary British Old Style year conventions, while modern scholarship gives January 1729.
Attaching the discovery of nutation to 1728 is misleading. Bradley observed an unexplained residual from 1727 onward, began to infer its cause over subsequent years, disclosed it privately in 1737 and published only after completing the lunar-node cycle in 1748.