Sky Tonight
Plan tonight with local twilight, darkness, Moon phase, and observing conditions.
How to use this tracker
Astronomical darkness begins when the Sun is 18° below the horizon. A bright Moon can reduce visibility of faint objects.
The live panel above combines your location with solar and lunar geometry so you can judge when the sky becomes dark and how strongly the Moon may affect faint-object observing. It is a planning guide, not a weather forecast.
Why your location changes the answer
Sunset, twilight, Moon altitude and the length of night vary with latitude, longitude, date and time zone. Two observers at the same clock time can have very different sky conditions. Setting your actual location therefore matters more than using a generic national forecast.
The tracker calculates the geometry for the selected place and converts it into practical observing windows. It does not infer what your local horizon looks like, so mountains, buildings and trees remain real-world constraints.
Understanding Moon phase and illumination
Moon illumination describes how much of the lunar disk appears lit from Earth. A bright Moon can wash out faint stars, galaxies and diffuse objects, especially when it is high in the sky. A crescent Moon may have little impact for much of the night, while a near-full Moon can dominate the sky.
Phase alone is not enough: Moonrise, Moonset and altitude matter too. If the Moon is below your horizon during part of the night, that interval may offer darker observing even when the lunar phase is relatively bright.
Twilight and the best observing window
After sunset, the Sun continues moving farther below your horizon. Civil, nautical and astronomical twilight are defined by solar depression angles. For the faintest targets, observers often prefer the interval after astronomical twilight ends, when the Sun is at least 18° below the horizon.
Bright planets, the Moon, the ISS and many satellites do not require fully dark conditions. Match the darkness requirement to what you want to observe rather than waiting automatically for the deepest night.
What the tool cannot know
Use the GOES satellite page for broad cloud imagery and your official local weather service for forecasts and warnings.
Useful astronomical references
For broader observing education, see NASA Universe. The calculations on this page are observer-relative astronomical geometry; values are not invented when required source data are unavailable.
Frequently asked questions
Is astronomical darkness required to see the ISS or bright planets?
No. Bright objects can be visible during twilight; astronomical darkness is more important for faint targets.
Why does Moon illumination matter?
Moonlight brightens the sky background and can hide faint stars and diffuse objects.
Why is sunset different from the start of a dark sky?
Twilight continues after the Sun drops below the horizon, so the sky darkens gradually.
Does Sky Tonight predict cloud cover?
No. Use weather data in addition to this astronomical planning tool.
How to find the darkest time tonight
Sunset is not the same as full astronomical darkness. After sunset the Sun continues moving farther below your horizon through civil, nautical and astronomical twilight. Astronomical darkness is conventionally associated with the Sun being about 18° below the horizon, although the usefulness of that darkness still depends on Moon brightness, weather and local light pollution.
The tool above calculates twilight for the coordinates you choose rather than assuming one fixed clock time for an entire country or time zone. This is important because darkness changes with longitude, latitude, season and daylight length.
How the Moon changes what you can see
A bright Moon raises the background brightness of the sky and can hide faint stars, diffuse nebulae and dim meteors. The effect depends not only on lunar phase but also on whether the Moon is above the horizon during your observing window. A new Moon is not automatically useful if clouds cover the sky, and a full Moon does not prevent observation of bright planets, the ISS or many other bright targets.
Use Sky Tonight with other live tools
For meteor observing, combine the darkness window with the Meteor Shower Tracker. For possible satellite passes, use Satellites Above Me Tonight. If geomagnetic activity is elevated, compare your darkness window with Aurora Forecast Tonight. Large-scale cloud patterns can also be checked separately on the GOES Weather Satellite page.
What this night-sky score cannot tell you
Astronomical geometry is only part of observing quality. Local cloud, smoke, humidity, haze, wind, artificial light and horizon obstructions can dominate what you actually see. Treat the calculated darkness and lunar context as planning inputs, then check a trusted local weather forecast before traveling to a dark site.
NASA publishes regular observing guidance and monthly skywatching features on its Skywatching pages.
Frequently asked questions
Is astronomical darkness always available every night?
No. At high latitudes near summer, the Sun may not descend far enough below the horizon to produce full astronomical darkness.
Does a high observing score guarantee a clear sky?
No. The score is based on astronomical conditions and does not replace a local cloud or weather forecast.