Astronomical Twilight Calculator

Astronomical twilight uses the Sun at 18° below the horizon

Astronomical dawn and dusk are conventionally tied to a solar zenith of 108°, meaning the Sun is 18° below the horizon. The definition is useful for astronomy planning because the sky is normally close to fully dark beyond that boundary when light pollution and moonlight are low.

The geometric event does not account for local cloud, terrain or urban skyglow. Near the poles, astronomical twilight may persist all night or fail to occur for parts of the year because the Sun never reaches the required altitude.

When this tool is useful

Use Astronomical Twilight Calculator when the question matches the method the page actually uses. Typical workflows include:

• Answer a one-off question about astronomical twilight calculator and keep the result tied to a visible map or explicit input.
• Compare two or more scenarios involving astronomical twilight calculator online without creating an account or installing desktop software.
• Use the output as a planning or teaching reference, then move to an official or specialist source if the result affects legal, safety, engineering or financial decisions.

Worked examples

These examples are meant to show how to interpret this kind of output, not to replace the values produced from your own inputs.

• Equinox comparison — Calculated day length is about 12h 07m at the Equator and changes with latitude because of solar geometry and the sunrise/sunset convention used.
• Golden hour — The useful light window changes with latitude, season and local horizon. Treat calculated times as planning guidance, especially around obstructed horizons.
• Clock time — Solar geometry and civil clock time are different layers. Time zones and daylight-saving rules shift the clock label, not the Sun's position.

How to interpret the result

The most common errors come from using a correct result to answer the wrong geographic question. Keep these points in mind for Astronomical Twilight Calculator:

• Convert solar times to local clock times deliberately: the sun keeps astronomical time, while governments keep political time, and the gap between them is large in many countries. State both when sharing results.
• For photography, plan around sun altitude, not clock time: golden hour is an altitude band, so its clock time migrates with the seasons even at a fixed place — the calculator's windows handle that for you.

Limits & appropriate use

Solar arithmetic's limits are small but real: the official zenith encodes average refraction, not today's atmosphere; coordinates are rounded to your input's precision; and terrain is absent unless you add height yourself — a mountain rises the sun late and the formulas won't know. Time adds the political layer: zones and DST are human law, and western edges of big zones (Spain, western China) make solar noon wander hours from clock noon. Every output here therefore carries its convention and both clocks, because an unanchored sunrise time is an argument, not a fact.

Lunar and seasonal outputs carry their own gentle limits: phase from the synodic approximation is hours-precise, which outruns every planning need but not ephemeris science; day length is geometric daylight, not sunshine, and clouds belong to climate's department. Escalation is rare and well-defined — astronomical almanacs and ephemerides for science, terrain-aware apps for mountain photography, and the tz database maintainers whenever a government invents a new DST rule at 48 hours' notice, as they delight in doing.

• Science-grade ephemerides → astronomical almanacs/JPL horizons.
• Mountain sunrise/sunset → terrain-aware horizon adjustments.
• Legal daylight definitions → jurisdictional statutes (they vary!).

Data & methodology

Uses the standard solar-event algorithm at a 108° zenith (Sun 18° below the horizon). Solar times use the NOAA algorithm with the 90.833° official zenith; lunar phase uses the synodic cycle. Timezone geometry comes from the open tz database resolved locally in your browser.

For source details, licences and model limitations, see Data Sources and Methodology.

Key terms worth knowing

Civil twilight

Sun between 0° and −6°: usable outdoor light without lamps.

Synodic month

The 29.53-day new-moon-to-new-moon cycle behind phases.

Related tools

If this page answers only part of your question, these tools use a nearby but different method or dataset.