Observing Skills · 4 of 11

Dark Skies, Dark Eyes

Why do more stars appear the longer you stay outside?

How dark your sky is, how dark your eyes are, and the half hour it takes your eyes to catch up — then averted vision for what is still too faint.

You’ll be able to judge how dark your sky is, dark-adapt your eyes, and use averted vision.

8 steps · about 6 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

Bortle, "The Bortle Dark-Sky Scale", Sky & Telescope, February 2001 — the nine classes and their naked-eye limits
Falchi et al., Science Advances 2, e1600377 (2016) — the world atlas of artificial night-sky brightness
Clark, Visual Astronomy of the Deep Sky (CUP 1990) — dark adaptation curves and averted vision
Hecht, Haig & Chase, J. Gen. Physiol. 20, 831 (1937) — the dark-adaptation thresholds the chart is drawn from
HYG stellar database v4.2 — star positions and magnitudes

← 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

This is a Bortle 5 sky — suburban, and what most people actually have. The scale runs from 1, an excellent dark site, to 9, inner city. Each class up, the sky's own glow drowns another half of stars: about two in every five you could see.

↺ What a brighter sky takes, from Understanding Star Brightness

Bortle 8, city. Your limit is around magnitude 4.5 — a few hundred stars in the whole sky, the Milky Way a rumour. But the three tagged stars survive: a first-magnitude star does not care about your postcode. City astronomy is real astronomy with a different list.

Now drive to a rural sky, Bortle 3: usually 30–60 minutes, not a plane ticket. At first it looks no better than a city. Your pupils open in a minute or two, but the chemistry of night vision takes far longer. Wait.

Thirty minutes later, the same sky. Your eyes now reach about 3 magnitudes fainter — to about magnitude 7, thousands of stars, a Milky Way with structure. Where did the half hour go?

The stars you can see, minute by minute after leaving a lit room. The cones, your colour cells, see the brightest few; the rods, your night cells, bring the rest after the rod–cone break, long after your pupils opened.

Now check your phone at minute twenty. Two minutes on a bright screen and the count starts again from the bottom. Dim it right down, or use a dim red light: the rods barely see deep red.

In the ring is , a globular cluster, a ball of stars, at the edge of seeing. Stare straight at it and it fades: the centre of your eye has no rods. Look a little to one side and it returns. That is averted vision, worth up to a magnitude.

Three stars, or thousands.

Bortle 1 is dark, 9 the inner city
Give your eyes thirty minutes
For faint things, look aside