Seoul Civil, Nautical & Astronomical Twilight · FastTool

Twilight Duration at Seoul

At Seoul's latitude of 37.6°, 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 Seoul's dark-sky window so different month to month.

Seoul's Gwacheon National Science Museum operates the largest public astronomical observatory in South Korea, featuring a 1-meter reflecting telescope and a dedicated solar observation deck. The Korean Astronomy and Space Science Institute (KASI) in nearby Daejeon has been a key contributor to the GMT (Giant Magellan Telescope) project, and Seoul's amateur astronomy clubs number over 50 — one of the highest densities in Asia.

When Does It Get Truly Dark in Seoul?

At Seoul, true astronomical darkness begins when the sun drops more than 18° below the horizon — the end of astronomical twilight. At 37.6° latitude, this dark window varies from approximately 4–6h in summer to 12–14h in winter. During the October–March months, Seoul 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.

At 37.6°N, Seoul's midsummer daylight stretches to 14 hours and 45 minutes under a humid continental climate. The city's dramatic seasonal swing — from −5°C January nights to 30°C July days — mirrors the striking contrast between Seoul's ultra-modern skyline and its mountain-ringed natural basin, where the sun path traces a precise arc from Bukhansan in the east to Gwanaksan in the west.

Civil, Nautical & Astronomical Twilight Explained

Three twilight phases occur each evening and morning at Seoul, 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 Seoul'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 Seoul 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 Seoul's coordinates.