Vol. Science · Astronomy · The sky’s clock

Twilight
dawn through night

The sky does not go dark at sunset. The Sun sinks through three named bands. Astronomical dawn is the center of the Sun 18° below the horizon. Nautical dawn is 12°. Civil dawn is 6°. Sunrise on this desk is the standard −0.833°: the upper limb at the horizon, with about 34′ of refraction and about 16′ of solar semi-diameter already folded into that altitude. Then the day, sunset, and the same three bands in reverse — civil dusk, nautical dusk, astronomical dusk — and night, when the Sun is below −18°. Civil twilight is bright enough for outdoor activity without lights. Nautical twilight is when the sea horizon stays visible for navigation. Astronomical twilight is the last pale sky before faint-object observing. Golden hour and blue hour are informal photography names, not almanac terms. Drawn here as approximate altitude bands: golden hour roughly −4° to +6°, blue hour roughly −6° to −4°. The clock is NOAA’s solar calculator, the Meeus equations NOAA publishes, run in this browser. The year’s swing in day length is the tilt on the seasons desk. The minutes by which the true Sun misses mean noon are the analemma. The day being divided is the mean solar day, set beside the shorter star day on the sidereal desk.

Classroom solar position. NOAA’s equations, for a level horizon and average air. NOAA says the sunrise time is good to about a minute between ±72° latitude, and can be off by more near the poles, where the Sun moves almost sideways. It is not a weather forecast, not a navigation instrument, and not a substitute for an official almanac. Refraction changes with temperature and pressure. Golden hour and blue hour are approximate labels. The seasons desk’s day-length strip leaves refraction out on purpose. This page puts it back, as the fixed −0.833°.

What each band is for

Words

Geometric altitude

The angle of the Sun’s center above a level horizon, before the air bends the ray. Negative means below the horizon. The four lines on the curve are this number: −0.833°, −6°, −12°, and −18°.

−0.833°

NOAA’s sunrise standard. 0.833° is 50′. About 34′ is refraction and about 16′ is the Sun’s apparent radius. The upper limb then sits on the horizon. Real air can shift the minute.

Solar noon

The moment the Sun is highest, on the meridian. It is not always 12:00 on the clock. The difference is the longitude inside the time zone, plus the equation of time. That sideways minute is the analemma.

Declination

How far north or south of the celestial equator the Sun sits. It runs about ±23.44° through the year, the same obliquity as the seasons desk. Day length follows from latitude and this angle.

Time zone

Presets use the place’s civil zone, including daylight time, when the browser knows that zone. A typed latitude and longitude has no zone on its own, so the clocks then use this browser’s zone and say so. “Use my location” uses the device’s zone, which is the civil time where you are standing.

What the picture leaves out

Mountains, your elevation, and the weather. A high horizon hides the Sun early. Thin air refracts less. The South Pole row uses New Zealand civil time because that is the clock Amundsen–Scott keeps. The Sun’s altitude there barely changes through a clock day. This is not a pointing aid for a telescope, and it is not the local sky chart.

Classroom clock of one geometric altitude. For Campbell, California, 37.287° N, 121.950° W, on 21 June 2024, this NOAA calculation gives sunrise 05:48 PDT and sunset 20:32 PDT, civil twilight 05:17 to 21:03, and solar noon 13:10. Those four match the U.S. Naval Observatory to the minute. Astronomical twilight that day is 03:54 to 22:25. The December pair, in Pacific Standard Time, is sunrise 07:18 and sunset 16:54. London on 21 June 2024 never reaches −18°, so the night is twilight all night. Tromsø that day is polar day. Not a forecast and not for navigation. Earthrise preview: NASA / Bill Anders (public domain).

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