Cape Town Civil, Nautical & Astronomical Twilight · FastTool

Twilight Duration at Cape Town

At Cape Town's latitude of -33.9°, 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 Cape Town's dark-sky window so different month to month.

Cape Town is home to the South African Astronomical Observatory (SAAO) in the suburb of Observatory — the southern hemisphere's oldest permanent observatory, operating since 1820. Table Mountain, rising 1,085 meters directly behind the city, creates a unique orographic cloud phenomenon called the 'tablecloth' — a layer of cloud that forms as southeast winds push moist air up the mountain face, creating dramatic sunset silhouettes visible across the city bowl.

When Does It Get Truly Dark in Cape Town?

At Cape Town, true astronomical darkness begins when the sun drops more than 18° below the horizon — the end of astronomical twilight. At -33.9° latitude, this dark window varies from approximately 4–6h in summer to 12–14h in winter. During the October–March months, Cape Town 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 33.9°S, Cape Town celebrates its December solstice with 14 hours and 25 minutes of daylight — midsummer at the southern tip of Africa. The city's position on a peninsula between two oceans (Atlantic and Indian) means the Sun rises over the Indian Ocean and sets over the Atlantic, a rare geographic privilege that only a handful of coastal cities worldwide can offer.

Civil, Nautical & Astronomical Twilight Explained

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