Paris Civil, Nautical & Astronomical Twilight · FastTool

Twilight Duration at Paris

At Paris's latitude of 48.9°, twilight duration varies dramatically by season. At this high-latitude location, astronomical twilight (sun −12° to −18° below horizon) typically lasts 2–4 hours 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 Paris's dark-sky window so different month to month.

The Paris Observatory, commissioned by Louis XIV in 1667, is the world's oldest continuously operating astronomical observatory. The Arago Medallions — 135 bronze disks embedded in Paris sidewalks along the original Paris Meridian — still mark the line that rivaled Greenwich as the world's prime meridian until 1884, when the International Meridian Conference voted to standardize on London.

When Does It Get Truly Dark in Paris?

At Paris, true astronomical darkness begins when the sun drops more than 18° below the horizon — the end of astronomical twilight. At 48.9° latitude, this dark window varies from approximately 0–3 (white nights possible)h in summer to 14–16h in winter. During the September–April months, Paris 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.

Paris at 48.9°N experiences summer days stretching to 16 hours and 10 minutes — significantly longer than the 15-hour maxima of New York or Tokyo at lower mid-latitudes. This extended summer daylight, combined with Paris's famously clear August skies, produces some of the longest and most photogenic golden hour windows of any major European capital.

Civil, Nautical & Astronomical Twilight Explained

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