pmt.moe — A Twilight Observatory

Civil Twilight

First, the horizon dims

The sun sinks below the horizon and the brightest stars and planets begin to appear. There is still enough light to read by; the human eye has not yet surrendered to its rod-cell scotopic mode. The sky overhead retains a luminous indigo, deepening westward toward a faint amber that will not survive the next ten minutes.

In this phase the world is still legible. Color exists. Outlines are still sharp. The artificial lights of distant towns turn on one by one, and pilots are required to operate aircraft navigation lights from this moment forward.

"Twilight, the very mood of which is to suspend us between two clarities."

Solar depression0° to −6°
Sky luminance3.4 cd/m²
Visible bodiesVenus, Sirius, Vega
Duration today26 min 18 sec

The horizon becomes visible only as memory

Now the sea-horizon is barely distinguishable, and sailors traditionally took their last sextant fixes against the silhouettes of stars. The deep navy overhead hardens. The sky has shed its luminous quality and turned to a denser, glassier blue. The first proper constellations resolve themselves: Cassiopeia tilts above the northern horizon, the Pleiades rise as a faint smudge.

Eyes begin to dilate. Cone-cell color vision starts to retreat; the world enters its blue shift. This is the photographer's blue hour, and the navigator's last useful light.

Solar depression−6° to −12°

Sky luminance0.008 cd/m²

Visible bodies~50 brightest stars

Pupil diameter~5.6 mm

A near-black field, scattered with charts

The sky has reached the indigo void. Only diffuse skyglow from solar scatter remains, undetectable to the eye. Astronomers begin their long-exposure observations. The atmosphere is calmer, the seeing improves. Draco, the dragon, coils between the bears in the high northern sky.

"At the still point of the turning world, neither flesh nor fleshless."

Solar depression−12° to −18°

Sky luminance~0.0001 cd/m²

Mag. limit+5.8 naked eye

Only instrument readings remain

The descent ends at −18°. There is no longer any solar contribution to the sky. What remains is starlight, the band of the Milky Way, and the faint airglow at the upper atmosphere. The observatory becomes a small bright island of cyan instrument dials in a great void.

Polaris, alone now, sits within one degree of the celestial pole. It does not rise or set. It pulses faintly at magnitude +1.98, holding its place against the rotation of the world.

α UMi · Polaris · +1.98 mag

Solar depression> −18°

Bortle scaleClass 2 — typical truly dark