Twilight, explained: when the sky is actually dark enough

Three stages, three angles

Twilight is defined by how far the Sun is below the horizon, measured in degrees:

• Civil twilight ends when the Sun is 6° below the horizon. Street lights come on, the brightest stars appear, and you can still read a book outside. Bright planets are already obvious.
• Nautical twilight ends at 12°. The horizon is no longer visible at sea, which is where the name comes from — sailors used the horizon and the stars together. Most of the sky looks dark but the background still glows faintly near the horizon.
• Astronomical twilight ends at 18°. The sky background is as dark as it gets. This is the point after which faint objects — nebulae, galaxies, the Milky Way from a dark site — become practical targets.

Dawn runs the same sequence in reverse. The period between the end of evening astronomical twilight and the start of morning astronomical twilight is what astronomers call astronomical night, and on our observing plan it is the shaded band on the timeline.

What it changes in practice

Planets: almost unaffected. Jupiter and Venus cut through a surprising amount of skyglow, and a bright planet is easy well before astronomical darkness. If all you want is to see Jupiter, you can start half an hour after sunset.

Deep-sky objects: completely dependent on it. A galaxy that is obvious at the end of astronomical twilight is invisible twenty minutes earlier, because you are not fighting the object's faintness, you are fighting the sky's brightness.

Satellites: the awkward middle. A pass during civil twilight happens against a bright blue sky and the station can be surprisingly hard to pick out, despite being magnitude −3. The same pass an hour later is trivial.

Why the dark window shrinks in summer

The Sun's path below the horizon depends on your latitude and the season. In summer at high latitudes the Sun does not get far enough below the horizon for astronomical twilight to end at all — the sky stays in permanent nautical twilight and astronomers call it "no astronomical night". At 55°N in late June, Moscow never reaches −18°, so the faintest objects are simply unavailable that month, however clear the weather.

This is why our city pages compute the dark window instead of quoting a generic "sunset to sunrise". A city at 1°N has roughly twelve hours of darkness year round; a city at 52°N swings from about eight hours in June to more than fourteen in December, and the usable part swings more than that.

The Moon is the other half of the equation

A full Moon raises the sky background by several magnitudes across the whole sky. An 80%-illuminated Moon is enough to erase the Milky Way for anybody within a few hundred kilometres, which is most of a continent. On our plan, the Moon's illumination is reported next to the timeline for exactly this reason: a 10-hour dark window with a 90% Moon is worse than a 6-hour window with a new Moon.

How we compute it

Rather than quoting published tables, the site walks the Sun's altitude through the night at six-minute steps and records when it crosses −6°, −12° and −18° on the way down and the way up. That is why the numbers are specific to your coordinates and tonight, and why they change gradually through the year instead of jumping at month boundaries.

The same walk produces the planet bars: an object is listed as up when it is more than 10° above the horizon, and marked as a good target only when it is both above 25° and the Sun is below −12°. Anything below 10° is behind buildings and haze for most people, whatever the geometry says.

One practical rule

If you are going out for planets, start at the end of civil twilight. If you are going out for anything faint, wait for astronomical darkness and check the Moon first. If the two conflict — bright Moon, faint target — the Moon wins, and the plan should say so.