GoodSeeing
Sun below −18° · next 30 nights

True darkness at Boulder

Astronomical darkness starts only when the Sun is 18° below the horizon. These bars are calculated for the coordinates, so high-latitude nights correctly show no true darkness when twilight never ends.

Longest night10.0 h13 Oct
Nights reaching −18°30 / 30Astronomical darkness occurs
Longest window20:00–06:00Moonlight is shown separately in the planner.
Solar geometry, not a clock rule

Hours of astronomical darkness

18 Sept8.7 h54% moon
19 Sept8.7 h63% moon
20 Sept8.7 h72% moon
21 Sept8.7 h80% moon
22 Sept8.7 h87% moon
23 Sept8.7 h93% moon
24 Sept9.0 h97% moon
25 Sept9.0 h100% moon
26 Sept9.0 h100% moon
27 Sept9.0 h97% moon
28 Sept9.3 h92% moon
29 Sept9.3 h85% moon
30 Sept9.3 h76% moon
1 Oct9.3 h66% moon
2 Oct9.3 h54% moon
3 Oct9.3 h42% moon
4 Oct9.3 h31% moon
5 Oct9.3 h21% moon
6 Oct9.3 h12% moon
7 Oct9.3 h6% moon
8 Oct9.3 h2% moon
9 Oct9.3 h0% moon
10 Oct9.7 h1% moon
11 Oct9.7 h3% moon
12 Oct9.7 h8% moon
13 Oct10.0 h13% moon
14 Oct10.0 h21% moon
15 Oct10.0 h29% moon
16 Oct10.0 h37% moon
17 Oct10.0 h47% moon
10.0 hours

Use this as the outer boundary for faint-object work. Moon altitude, cloud and transparency will narrow the practical window. Add the Moon →

Keep planning

Same coordinates, different question

Moon

Moonless nights

Find the longest uninterrupted dark windows.

Core

Milky Way

See when the galactic core clears the horizon after dark.

−18°

Astronomical darkness

Track true darkness, including polar twilight.