Observing Skills · 2 of 11

Measure the Sky with Your Hand

How can your hand at arm’s length measure the sky?

Your hand at arm’s length is a ruler for the sky — a finger, a fist, a spread hand — tried out on Orion, then used to find Sirius and to count a star’s height.

You’ll be able to measure the gap between two stars, and a star’s height, with your hand at arm’s length.

7 steps · about 5 min

Where and which night

Working out when this is best placed from where you are…

Where

Which night

Date

Time

Kept for this guide only, until you change it here — your map settings, and every other guide, stay as they are.

Print, play outside or present

Present on a big screen

Scan with a phone's camera, or open

Sources

HYG stellar database v4.2 — star positions and every separation quoted here
Sky & Telescope, "How to Measure the Sky with Your Hand" — the finger, fist and spread-hand rule of thumb
Williams, Lunar fact sheet (NASA GSFC) — the Moon's apparent diameter, 29.3′ to 34.1′

← GUIDES

More below ↓

With a keyboard, focus the sky, aim it with the up and down arrow keys, and press Enter to drop the pin at the centre of the view; or press Show me.

You are here

Settings

Screen

Night mode Turns every page red, to keep your eyes adjusted to the dark.

High contrast Bolder constellation lines, larger and brighter names on the sky, and stronger text and edges everywhere. Starts on if your device asks for more contrast.

Reduce motion The camera cuts to where it is going instead of flying there, and panels and sheets appear without sliding. Starts on if your device asks for less motion.

Battery saver Draws the sky thirty times a second at most: smoothness traded for battery.

Night brightness Dims the screen below your phone’s lowest setting, down to a fifth, while night mode is on.

Clock and voice

Clock How every time is written. Until you choose, your language’s own clock decides.

D describes What D says about the view on the sky: one sentence, or in full with the field, the constellation and five names.

Sounds Quiet tones as you use the map: a tick as you drag the clock past a mark or dusk, a chime for the star you pick — higher for hotter stars — and a low swell while a figure is out of the dome. Made in your browser, and off until you turn them on.

All settings and your progress

Gaps between stars are measured in degrees: horizon to horizon is 180. You carry the ruler. Hold your hand out at arm's length: a little finger covers about 1°, three fingers about 5°, a fist 10° and a spread hand 20°. Orion has a span for each.

Start small. The line is the Belt, end to end, and its tag gives the span in degrees and in fingers. Two fingers held flat at arm's length cover all three stars. Hands and arms differ, but they grow together, so the rule holds for most people.

Now a fist. Knuckles across, thumb tucked in, arm straight: from the Belt's end to Betelgeuse is the measured line, 10°. It is the span you will use most, because it is the one a cold hand makes without thinking.

Spread your hand, thumb to little finger: about 20°. Laid from shoulder to foot, Betelgeuse to Rigel, it just about covers the whole figure: Orion is one hand tall, and the tag says so.

Now find a star with it. Sight along the Belt from Mintaka to Alnitak and keep going: two fists on, a spread hand, the line runs into the brightest star of the night sky. Every star-hop in these guides is told this way: a bright start, a line, so many fists.

Three hours earlier, Orion is lower. Heights work the same way: rest the bottom of your fist on the horizon and stack fists until you reach Betelgeuse. Each fist is 10° of altitude; the readout says how many to expect. Overhead is nine fists up.

Your ruler, on the one figure everyone on Earth sees. Try it on the Moon, then on any two stars.

Finger 1°, three fingers 5°
Fist 10°, spread hand 20°
The full Moon is half a degree