September 23, 1846

Discovery of Neptune

Galle and d'Arrest identify Neptune at the Berlin Observatory

The decisive clue was not another calculation but a dot missing from a newly printed star map — checked the same night Le Verrier's letter arrived.

The night of 23 September 1846

Urbain Le Verrier's letter reached Berlin on 23 September 1846, and that night Johann Gottfried Galle turned the observatory's Fraunhofer telescope on the position Le Verrier had calculated. Heinrich Louis d'Arrest, a student collaborator, proposed comparing the field against a recently prepared Berlin star chart.

One point of light in the eyepiece was absent from the chart. It lay about one degree from Le Verrier's predicted position, and was recognized as a new planet before the session ended in the early hours of the 24th.

A planet found by arithmetic

The identification gave immediate empirical confirmation that a planet inferred from Uranus's orbital perturbations existed near the predicted location. Le Verrier had never seen it.

Galle soon notified him, and the discovery set off an international dispute over how much credit John Couch Adams should share. Modern scholarship disputes the traditional equal-credit formulation, because Adams's prediction was unpublished and did not guide the successful Berlin observation.

Seen before, recognized now

This was the first identification of Neptune as a planet, not the first time its light was recorded. Galileo saw it in 1612–1613, and the British astronomer James Challis recorded it in August 1846 without recognizing what was in his data.

Nor did an accurate predicted sky position mean the underlying orbital elements were right. What the night established was narrower and firmer: an unmapped object sat about one degree from where Le Verrier calculated a planet should be — and 17 days later, on 10 October 1846, its moon Triton was found.

Sources

Researched 23 Aug 2026 4 sources 3 audit passes

How this was checked

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What the sources leave uncertain

September 23 is the accepted discovery date, although the observing session continued into September 24.
This was the first identification of Neptune as a planet, not the first time its light was recorded. Galileo saw it in 1612–1613, and James Challis recorded it in August 1846 without recognizing it.
Modern scholarship disputes the traditional equal-credit formulation for Adams and Le Verrier because Adams's prediction was unpublished and did not guide the successful Berlin observation.