Astronomical Darkness Calculator
What Time Does Astronomical Darkness Start Tonight?
Find when the Sun falls 18° below your horizon, when astronomical darkness ends and how much dark observing time you have. Allow browser location access, or enter a city, to see calculated times and the full transition from sunset to morning.
Published November 2, 2026 · Local evening dates · Uses your location with browser permission
Astronomical Darkness Tonight
Allow browser location access for an automatic calculation, or search for a city or enter coordinates below. Astronomical darkness is different from sunset.
Choose your location
No location selected
Your browser may ask for location permission. You can also search for a city or use coordinates.
Coordinates & time zone
Browser location selects its IANA time zone from geographic boundaries. Check or change it here if needed. City search uses the city’s own time zone.
“Tonight” means the ongoing night before sunrise, or the coming evening after sunrise. Dates name local evenings. For polar cases without a sunrise, local noon separates successive observing intervals. For browser location, the IANA time zone is determined from geographic boundaries; change it under Change location → Coordinates & time zone if needed. City search uses the city’s own zone. Data are calculated on your device; no precise coordinates are saved, added to URLs or sent as custom analytics events.
Tonight’s Twilight and Darkness Timeline
Choose your location and evening date to build a proportional timeline.
All twilight events and dark intervals
| Event | Local date and time |
|---|
How Dark Will the Sky Get Tonight?
The curve follows the Sun’s geometric center. The 0° line is a geometric horizon reference; conventional sunset occurs later, using the Sun’s upper limb and standard atmospheric refraction.
The lowest solar altitude will be calculated for your selected night.
Inspecting this chart moves the same marker on the twilight timeline. The curve is sampled every ten minutes for drawing; event boundaries are found separately with numerical refinement.
Find Moonless Astronomical Darkness Tonight
Astronomical darkness depends on the Sun. This separate calculation finds its overlap with the Moon fully below a standard horizon. A bright Moon below that horizon does not directly light your local sky.
Your Moon-free windows will appear here.
Is Tonight Good for Stargazing?
Choose a location to check darkness and Moon-free overlap. Weather, skyglow, transparency and terrain are not included; a favorable astronomical window does not guarantee a view.
For a target-specific check, use Milky Way visibility tonight. To choose future observing dates, open the Milky Way Visibility Calendar.
What Is Astronomical Darkness?
Astronomical darkness is the interval with the Sun’s center more than 18° below the ideal horizon. It starts at the end of evening astronomical twilight and finishes at the start of morning astronomical twilight. These are geometric solar-position boundaries, not measurements of how black the sky looks.
What Are the Three Types of Twilight?
| Stage | Solar altitude | What it means |
|---|---|---|
| Civil twilight | After sunset to −6°; reversed at dawn | Substantial natural light remains, while the brightest celestial objects can emerge. |
| Nautical twilight | −6° to −12° | The sky becomes darker and more stars appear; visibility still depends on the atmosphere. |
| Astronomical twilight | −12° to −18° | Residual scattered sunlight fades, improving contrast for faint objects. |
| Astronomical darkness | Below −18° | The solar twilight criterion is satisfied. Moonlight, airglow and skyglow may still brighten the sky. |
When Does Astronomical Twilight End?
In the evening it ends at the descending −18° crossing. In the morning, the ascending crossing ends astronomical darkness and starts astronomical twilight. The calculator distinguishes the direction of each crossing and labels its local date, so an early-morning event is not mistaken for an evening one.
How Long Does Astronomical Darkness Last?
The duration is the elapsed time inside the dark intervals. A night often spans two calendar dates, and daylight-saving changes can shift the clock by an hour. We subtract absolute instants rather than local clock labels. If there are multiple intervals, their durations are added without counting any brighter gaps.
Why Does Astronomical Darkness Start at Different Times?
Earth’s tilted axis changes the Sun’s seasonal path. Your latitude sets how steeply that path meets the horizon, while longitude and the civil time zone affect its clock label. Twilight therefore cannot be predicted by adding a fixed 90 minutes to sunset.
Is There Astronomical Darkness Tonight?
At higher summer latitudes, the Sun may turn upward before reaching −18°. The whole night then remains in twilight. Close to the winter pole, the Sun can stay below the threshold throughout the selected interval. The calculator reports each situation explicitly instead of forcing a normal dusk-and-dawn pattern.
How Moonlight Affects Dark-Sky Observing
The Moon does not move the astronomical-darkness boundary, but its illumination and position affect contrast. A full Moon below the horizon is different from one high overhead. The Moon-free windows use its actual horizon crossings, and the readout reports illumination at a stated instant rather than assigning one phase to the entire night.
Check moonrise and moonset times, moonrise direction or the current Moon phase. For other targets, explore planets visible tonight and our Solar System Map.
How Accurate Is This Calculator?
Model, horizon conventions and limits
This page reuses Celesiq’s local Astronomy Engine 2.1.19, shared solar-event kernel and Moon/IANA helpers. The same solar-event code supplies the Milky Way calendar and the Milky Way tonight guide. At the same location and evening date, their darkness intervals use the same absolute instants.
The observer is at sea level with an ideal, unobstructed horizon. Solar altitude is the apparent topocentric direction expressed as a geometric center altitude, without a refraction correction to that altitude. Twilight thresholds are −6°, −12° and −18°. Sunrise/set use the solar disk’s upper limb plus a standard 34-arcminute refraction correction; they are not the same as a 0° center crossing. The Moon-free criterion uses the equivalent lunar upper-limb convention from the moonrise tools.
The search first locates local altitude extrema, then brackets and refines crossings. A plotting sample is never used as a reported event time. Contacts within 0.00000001° of a threshold are treated as numerical tangencies, not invented positive-duration intervals. Crossing instants are numerically refined to about a tenth of a second, but displayed values are planning estimates to the minute. This is solver resolution, not a claim of real-world subsecond accuracy.
Displayed darkness windows round their start upward and end downward to complete local-clock minutes. Durations use those complete minutes, including across daylight-saving changes. Twilight event tables use the nearest minute. A darkness window may therefore begin a displayed minute after the rounded twilight crossing. Hover readouts evaluate the selected instant directly; the drawn curve interpolates ten-minute samples.
Selectable evening dates run from 2000 through 2050. Each interval runs from local noon to the next existing local noon, using the observing site’s IANA rules. Before sunrise, Tonight selects the ongoing night; after sunrise it selects the coming evening. With no sunrise, local noon is the explicit polar fallback. A skipped civil date is rejected. Future political time-zone changes and outdated browser data can affect local clock labels.
Atmospheric refraction, mountains, buildings and observer elevation can alter actual horizon visibility. Astronomical darkness does not remove Moonlight, artificial lighting, airglow, zodiacal light or weather. There is no live weather or measured Bortle layer in this calculator. Location is processed locally; the city list is downloaded only when needed for an explicit search.
Sources
- U.S. Naval Observatory: rise, set and twilight definitions.
- Astronomy Engine: calculation methods and validation.
- Astropy/ERFA solar-system coordinates, used for independent implementation checks.
- GeoNames city-data attribution.
- Time-zone boundaries: OpenStreetMap contributors via timezone-boundary-builder (ODbL and data source).
Frequently Asked Questions
What time does astronomical darkness start tonight?
Select a city or coordinates above. The calculator finds when the Sun’s geometric center descends below −18° for that local evening. If that crossing does not occur, it explains whether darkness is absent, already underway or continuous.
What time does astronomical twilight end?
Evening astronomical twilight ends as the Sun passes below −18°. That is the start of astronomical darkness. The morning counterpart is astronomical dawn, when the Sun rises back through the same boundary.
How long after sunset is astronomical darkness?
There is no fixed delay. Latitude, season and the Sun’s path angle change the time needed to descend to −18°. At some high-latitude summer locations it never reaches that depth.
What is the difference between astronomical twilight and astronomical darkness?
Astronomical twilight spans solar center altitudes from −12° to −18°. Astronomical darkness is the interval below −18°. Evening twilight ends at darkness; morning twilight begins when darkness ends.
Is astronomical darkness the same as night?
“Night” has several everyday and astronomical meanings. This calculator uses the specific −18° boundary; the time between sunset and sunrise also includes three twilight stages.
When does astronomical darkness end?
It ends at astronomical dawn, when the Sun’s center rises through −18°. Select your evening date to see the local date and time of that morning event. Some polar intervals have no crossing.
Why does astronomical darkness start later in summer?
Longer days and a shallower solar path below the horizon can postpone or eliminate the −18° crossing. The effect is especially strong at higher latitudes and differs between hemispheres.
Is there astronomical darkness in summer?
Often yes at lower latitudes. Farther toward the summer pole, twilight can last all night. The calculator checks actual solar altitude rather than assuming a summer night must become astronomically dark.
Can astronomical darkness last all day?
Yes, sufficiently close to the winter pole the Sun can remain below −18° throughout a civil day or the selected noon-to-noon interval. The result then says continuous darkness within the interval instead of inventing a dusk time.
Is the sky completely dark at −18°?
No. Moonlight, artificial skyglow, airglow, zodiacal light and atmospheric conditions can still brighten it. The threshold describes the Sun’s position, not a measurement of sky brightness.
Can you see the Milky Way during astronomical twilight?
Some bright structure may be detectable under favorable conditions, especially photographically, but twilight reduces contrast. The bright Galactic Core also needs to be above your horizon. Use the linked Milky Way tools for that geometry.
Does moonlight affect astronomical darkness?
It does not change the Sun’s −18° definition. It can strongly affect observing contrast. The Moon-free section separately intersects astronomical darkness with the Moon’s upper limb below a standard horizon.
How do you calculate astronomical darkness?
Compute the observer’s topocentric solar altitude, find its local extrema, refine all crossings of −18°, then build the intervals below that threshold. Convert absolute times to the selected IANA zone only for display.
What is the best time for stargazing tonight?
The longest Moon-free astronomical-darkness window is a useful starting point. The target’s position, weather, transparency, terrain and local lighting still need separate checks. This tool does not guarantee a visible target.