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Wonder Atlas

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The Tiny Shift

5 min · Eve · Quiet listening

Eve

The Tiny Shift

Eve is reading this story.

Eve is ready

How to say it

Eve can say these names for you.

Tap a name. She will say it slowly.

A child's thumb held at a night window, with one nearby star appearing twice against farther stars.
Nearby things jump. Far things almost do not. Illustration, Wonder Atlas

Your two eyes are in slightly different places. Nearby things look as if they move. Far things almost do not. That jump is called parallax. Earth has two eyes, in a way. In January we look from one side of our path around the Sun. In July we look from the other. A nearby star jumps a tiny amount against the farther stars.

How small can a jump be, and still tell the truth?

Earth in winter and Earth in summer, each sending a sightline to a nearby star that sits in two slightly different places against farther stars.
January and July. The same star. Two places on the far wall. Illustration, Wonder Atlas

We call them 61 Cygni. Bessel said the star was about ten light-years away. Today we measure a little farther, about eleven. He was already close. Two other people were racing that year, with Vega and with Alpha Centauri. Bessel's number was the first that held.

Look from here, then from there

January

Nearby things jump. Far things almost do not. The jump is how we measure far.

How sure are we?

  • We know

    A nearby object appears to jump when you change where you stand. That geometry is how stellar parallax works.

  • We know

    61 Cygni is about 11.4 light-years away. Bessel's 1838 number was already close.

  • The evidence suggests

    Gaia distances for hundreds of millions of stars are strong, with published error bars. Some stars are still hard because they are faint, crowded, or moving in complicated ways.

  • We're looking

    Stars too far for a measurable jump are placed on other rungs of the distance ladder. People are still measuring, still checking those rungs.

Try it tonight

  1. Hold your thumb at arm's length against something far: a tree, a lamp, a wall.
  2. Close one eye. Then the other. Watch the jump.
  3. Now look at a bright star. You will not see its jump. It is real, and much smaller than your thumb.
  4. If you are under a dark northern summer sky, the Swan is overhead. 61 Cygni is a faint orange pair. City light will hide it. The thumb still works.
Put it in My Sky
A little more, if you want numbers

An arcsecond is 1/3600 of a degree. Bessel measured 0.314 arcseconds for 61 Cygni. Gaia measures about 0.286. Distance in parsecs is 1 divided by that number. Space telescopes win because Earth's air does not wobble the tiny angle.

  • Bessel, F. W. (1838), determination of the parallax of 61 Cygni.
  • Gaia Collaboration, Data Release 3 (2022), 61 Cygni A 285.995 ± 0.06 mas.
  • Reid & Menten (2020), The First Stellar Parallaxes Revisited.

If you wondered