Astronomy · Rung 5 of 12

Distances and scales

Astronomy does not measure in kilometres because the numbers stop meaning anything. It measures in light-time: the Moon is 1.3 seconds away, the Sun 8.3 minutes, Neptune 4 hours, Proxima Centauri 4.2 years. Inside the Solar System we use the astronomical unit (AU), the mean Earth-Sun distance; outside it, the light year and the parsec (3.26 light years), which comes from measuring a star's apparent wobble as the Earth swaps sides of its orbit. Looking far away is looking back: Andromeda's light left two and a half million years ago.

Where this sits

Comes after
The Solar System
Opens up
The stars

Exercises from this rung

Generated by the app, from easiest to hardest

  1. What is an astronomical unit?

    • The distance light travels in a year
    • 3.26 light years
    • The radius of the Solar System
    • The mean Earth-Sun distance

    Why About 150 million kilometres, now defined as an exact value. It is the natural ruler inside the Solar System: Jupiter sits at 5.2 AU, Neptune at 30. Outside it becomes too small; that is what the light year and the parsec are for, the parsec being 3.26 light years.

    easier

  2. If the Sun is 8 light-minutes away, how far is Proxima Centauri?

    • 4.2 light years
    • 4.2 light-hours
    • 4.2 light-days
    • 42 light-minutes

    Why 4.24 light years, about 270,000 times the distance to the Sun. That jump is what makes interstellar travel so hard to discuss: Voyager 1, the fastest thing we have launched, has been flying for nearly fifty years and is 23 light-hours out, not a thousandth of the way.

    in between

  3. The parsec is not a convenience unit: it comes from a measurement. Which?

    • From how long light takes
    • From the galaxy's rotation speed
    • From the brightness of stars
    • From a star's apparent wobble

    Why From parallax. A star seen in January and in July, with the Earth on opposite sides of its orbit, appears to shift slightly. A parsec is the distance at which that shift is one arcsecond, a 3600th of a degree. No star reaches it: Proxima Centauri, the nearest, is 1.3 parsecs away.

    harder