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The missing wobble

How We Proved the Earth Moves

If Earth goes round the Sun, the nearby stars should shift. For 2,000 years nobody could see it.

What should happen Nearby stars shiftagainst distant onesWhat anyone could see Nothing. No shift atall.
The absence of the shift was the strongest argument for a still Earth.

  1. Step 1 of 5

    Hold up a thumb and close one eye, then the other.

    1 Close yourright eye2 The thumbsits here3 Now swapeyes over4 It jumpssideways
    Two viewpoints, and the near thing moves

    Your thumb jumps against the background. That jump is parallax, and it happens whenever you look at something near from two different places. It is how your two eyes judge distance.

  2. Step 2 of 5

    If Earth orbits, the whole sky should do that.

    Earth, six monthsapart A near star shouldshift against thefar ones

    Earth is on opposite sides of its orbit in summer and winter — three hundred million kilometres apart. Nearby stars should visibly shift against the far ones between those two dates. The Greeks knew this, looked, and saw nothing.

  3. Step 3 of 5

    Nothing was a genuinely powerful argument.

    No shift was ever seenSo the Earth does not moveOr the stars are unimaginably far
    The nearest star turned out to be a quarter of a million times further off than the Sun.

    No shift meant one of two things: Earth is standing still, or the stars are unimaginably far away — so far that the shift is too small to see. The second was considered absurd, because it made the universe far bigger than anyone was willing to accept.

  4. Step 4 of 5

    It took until 1838 to measure it.

    The patch of sky A shift of 0.3arcseconds — a coin at10 km

    Friedrich Bessel spent months measuring one star, 61 Cygni, against its neighbours, and found a shift of about a third of a second of arc. That is the width of a coin seen from ten kilometres away.

  5. Step 5 of 5

    And the answer to why nobody saw it is the distance.

    Distance to the Sun8 light minutesDistance to 61 Cygni11 light years

    61 Cygni turned out to be eleven light years away. The absurd explanation was the correct one. The same measurement that finally proved Earth moves also gave us the first true distance to a star.

    Both sides were reasoning well. The people who said the sky showed no shift were right about the observation and wrong about what it meant.

The short version

Earth's motion was finally proved by catching the tiny yearly shift of one nearby star — which also revealed how far away stars are.

Try it yourself

Hold a finger at arm's length and blink each eye in turn. Now bring it closer. The nearer it is, the bigger the jump — that is how the distance is calculated.

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