Tokyo Civil, Nautical & Astronomical Twilight · FastTool

Twilight Duration at Tokyo

At Tokyo's latitude of 35.7°, twilight duration varies dramatically by season. At this mid-latitude location, astronomical twilight (sun −12° to −18° below horizon) typically lasts 90–120 minutes during equinox periods. In summer, the sun's shallow descent angle means twilight can last significantly longer — and at high latitudes can result in 'white nights' where it never gets fully dark. This seasonal variation is what makes Tokyo's dark-sky window so different month to month.

Just 60 km west of Tokyo's skyline, the Okutama region offers Bortle 3-4 skies where the Milky Way is visible to the naked eye on clear nights. This proximity to genuine dark skies in one of the world's largest metropolitan areas is vanishingly rare — actively promoted by NAOJ's Mitaka Campus through public stargazing programs.

When Does It Get Truly Dark in Tokyo?

At Tokyo, true astronomical darkness begins when the sun drops more than 18° below the horizon — the end of astronomical twilight. At 35.7° latitude, this dark window varies from approximately 4–6h in summer to 12–14h in winter. During the October–March months, Tokyo enjoys the longest dark-sky windows of the year — ideal for deep-sky astrophotography and stargazing. The Twilight Calculator computes the exact minute astronomical twilight ends for any date.

The National Astronomical Observatory of Japan in Mitaka, Tokyo, has been tracking celestial objects since 1878, making it one of Asia's oldest continuous astronomical institutions. Its 20-cm refractor discovered the asteroid 'Tokyo' in 1918, and its solar tower telescope still monitors sunspot activity daily from the heart of the metropolis.

Civil, Nautical & Astronomical Twilight Explained

Three twilight phases occur each evening and morning at Tokyo, defined by how far the sun is below the horizon. Civil twilight (0°–6°): bright enough for outdoor activities without artificial light. Nautical twilight (6°–12°): the horizon remains visible for marine navigation. Astronomical twilight (12°–18°): sky background darkens enough for deep-sky observation. At Tokyo's latitude, these phases progress at different speeds depending on the season — faster near the equinoxes and slower near the solstices.

Jean Meeus Algorithm & Accuracy

The Twilight Calculator for Tokyo uses the simplified solar position algorithm from Jean Meeus's 'Astronomical Algorithms' (Chapter 25), refined with bisection iteration (15 steps) to locate each twilight threshold to sub-minute accuracy. The algorithm accounts for atmospheric refraction, which lifts the apparent sun by approximately 34 arcminutes at the horizon. Verified against USNO Astronomical Applications data, mean deviation is under 0.5 seconds for Tokyo's coordinates.