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Neptune, 1846

The Planet Found With a Pencil

Uranus kept drifting off course. Someone worked out where the culprit had to be, and pointed a telescope there.

Every planet before Found by lookingNeptune Found by calculation,then looked at
The prediction came first. The observation was a formality.

  1. Step 1 of 5

    Uranus would not stay where it was supposed to be.

    1 The predictedpath2 Where Uranusactually was3?Off by a small,stubborn amount

    After Uranus was discovered in 1781, astronomers calculated its orbit and it kept disagreeing. Sometimes it ran ahead of prediction, sometimes behind — by a tiny amount, but consistently and unmistakably.

  2. Step 2 of 5

    Either gravity was wrong, or something was pulling.

    Uranus is not where itshould beGravity itself is wrong out thereSomething unseen is pulling it
    Both were serious suggestions. One of them was right about Uranus.

    Those were the two options, and the first was a serious suggestion — perhaps Newton's law weakened at great distances. The second option was that an unseen planet further out was tugging Uranus as it passed.

  3. Step 3 of 5

    The second option can be turned into a sum.

    The measuredwobble inUranus's path Worked backwardsto a position inthe sky

    This is the hard and beautiful part. Rather than asking what a known planet would do, Urbain Le Verrier asked the question backwards: given exactly this wobble, where must the unknown planet be, and how heavy? Working it in reverse took him months.

  4. Step 4 of 5

    They found it the first night they looked.

    1 Thepredictionis posted2 Point thetelescopethere3 Neptune,within 1°of the mark4 Under anhour ofsearching

    Le Verrier posted his prediction to the Berlin Observatory. Johann Galle turned the telescope to the stated spot that same evening and found an unlisted object within one degree of the prediction. It took under an hour.

  5. Step 5 of 5

    The same trick was tried again, and failed.

    Uranus's wobble A hidden planet.Correct.Mercury's wobble No planet. Gravityneeded rewriting.

    Mercury also misbehaves, so Le Verrier predicted another hidden planet, called it Vulcan, and people searched for decades. There was nothing there. Mercury's wobble turned out to need a genuinely new idea about gravity instead.

    Two people did the calculation independently — John Couch Adams in England reached a similar answer, and nobody at Greenwich acted on it in time.

The short version

Neptune was located by working a gravitational wobble backwards, and found within an hour of anyone looking.

Try it yourself

Watch a swinging bag on someone's shoulder. You can tell it is heavy without seeing inside, purely from how it pulls them about.

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