What is astronomical twilight?

Astronomical twilight is the faintest and final phase of twilight, occurring when the Sun is between 12° and 18° below the horizon. To the naked eye, the sky looks fully dark — you might mistake it for night — but a subtle glow from scattered sunlight still lingers. This glow is too weak for most people to notice, yet it's bright enough to interfere with the most sensitive astronomical observations.

During astronomical twilight, the horizon is still technically illuminated, but only the brightest stars and planets are visible without optical aid. The sky's brightness decreases steadily as the Sun sinks deeper, until the moment it passes 18° below the horizon. At that point, astronomical twilight ends, and true astronomical night begins. The duration of this phase varies depending on your latitude and the time of year.

Why astronomers care about astronomical twilight

For serious stargazers, astrophotographers, and anyone using a telescope, astronomical twilight is a critical time. Even the tiny amount of light present during this phase can wash out faint deep-sky objects like galaxies, nebulae, and star clusters. The scattered sunlight that causes the sky to glow — known as skyglow — reduces contrast, making it harder to see the fainter stars. That's why many observatories schedule their observations after astronomical twilight ends.

If you're planning a night of stargazing, it's worth checking exactly when astronomical twilight ends for your location. On sunset.now, every city page lists the precise times for all twilight phases, so you can plan your session down to the minute. Remember that even a sliver of moonlight can affect your view, so also consider how the moon affects the night sky.

When does true darkness begin?

True astronomical night begins the moment the Sun drops more than 18° below the horizon. At that point, scattered sunlight no longer contributes to the sky's glow, and the sky reaches its natural darkness (aside from artificial light pollution). For most locations in the mid-latitudes, this happens about 90 minutes to 2 hours after sunset, depending on the season. However, this timing is not uniform — your latitude plays a huge role.

In summer at high latitudes — like in Alaska, Scandinavia, or northern Canada — the Sun may never dip 18° below the horizon. During these months, astronomical night simply never arrives. Instead, you get the famous 'white nights,' a milder cousin of the midnight sun. These are bright, lingering twilights that allow outdoor activities late into the night but make deep-sky observing impossible.

Astronomical twilight vs. civil and nautical twilight

Twilight is divided into three phases based on the Sun's angle below the horizon: civil, nautical, and astronomical. Civil twilight (0°–6° below) is the brightest, with enough light for most outdoor activities. Nautical twilight (6°–12° below) is darker, when the horizon becomes indistinct. Astronomical twilight (12°–18° below) is the darkest phase, when the sky is nearly black to the naked eye.

Understanding these distinctions is useful not only for astronomers but also for photographers. The golden hour occurs during civil twilight, while blue hour spans both civil and nautical twilight. If you're shooting the night sky, wait until astronomical twilight ends. If you're capturing cityscapes at dusk, the best time of day for photos often falls within nautical or civil twilight.

How to check astronomical twilight times for your city

Finding the exact start and end times of astronomical twilight is simple on sunset.now. Each city page shows the current day's twilight schedule, including when astronomical twilight begins (at sunset, when civil twilight starts) and when it ends (when the Sun reaches 18° below). You can also see how these times shift throughout the year, especially around the summer solstice and winter solstice.

For example, if you're in Chicago, you'll notice that summer evenings have a shorter astronomical twilight window because the Sun sets at a shallower angle. In winter, the twilight phases are generally longer. Knowing these patterns helps you plan everything from stargazing to night photography. Bookmark your city's page to never miss true dark sky conditions.

Practical tips for enjoying astronomical twilight

If you're not an astronomer, astronomical twilight still has its charms. Because the sky is nearly dark, it's a great time for watching the Moon rise or spotting bright planets like Venus and Jupiter. The air tends to be calm, and the fading light creates a serene atmosphere perfect for reflection. Just be cautious when walking or hiking in low light — consider watching the sunset safely and bringing a red flashlight to preserve your night vision.

For astrophotographers, this phase offers a unique window: the sky is dark enough for long exposures, but a faint glow can add drama to a landscape shot. Experiment with long exposures during the last minutes of astronomical twilight to capture the Milky Way or star trails with a hint of twilight color on the horizon. The best time to photograph sunset might actually be a few minutes later.

Frequently asked questions

How long does astronomical twilight last?

Depending on your latitude and the time of year, astronomical twilight typically lasts between 30 minutes and 1.5 hours. It's longer near the poles and shorter near the equator.

Can you see stars during astronomical twilight?

Yes, you can see the brightest stars and planets, but faint deep-sky objects like nebulae and galaxies are harder to spot due to the residual skyglow. For the best views, wait until astronomical twilight ends.

Does astronomical twilight happen everywhere on Earth?

Almost everywhere, but not at high latitudes during summer. In locations near the Arctic or Antarctic Circle, the Sun may never dip 18° below the horizon, so astronomical night does not occur during parts of the summer.

How is astronomical twilight different from nautical twilight?

Nautical twilight occurs when the Sun is 6° to 12° below the horizon, and the horizon is still visible at sea. Astronomical twilight is darker (12° to 18° below), and the horizon is no longer distinguishable to the naked eye.

What is astronomical twilight? — sunset.now guide

What is astronomical twilight?