Although we often use the terms “twilight”, “dusk,” and “dawn” interchangeably, each of the terms has an exact astronomical definition. Moreover, there are three different types of twilight, dusk, and dawn. Let’s deal with these terms once and for all — light pollution explanation included!
Twilight is a time of the day when the Sun is below the horizon and not visible directly but partially illuminates the sky. We can see twilight because the Earth has an atmosphere, where the light partially scatters and illuminates our planet even when the Sun is gone.
Types of twilight
There are three types of twilight — civil, nautical, and astronomical. They follow each other and occur in the same order all over the Earth; the twilight types vary depending on the Sun’s center position below the horizon:
- Civil twilight: 0–6°
- Nautical twilight: 6–12°
- Astronomical twilight: 12–18°
The time after astronomical twilight when the Sun goes 18° or farther down below the horizon is called the night.
How long is twilight?
The twilight duration depends on the latitude, time of year, and weather conditions. The shortest twilight is observed at the equator, where darkness falls 24 minutes after sunset; the longest one can last about six weeks at the poles.
Twilight vs. Dusk and Dawn
Dusk and dawn occur when the Sun’s center is at the exact point below the horizon; the twilight occurs between these points. For example, when the Sun is exactly 18° below the horizon — it’s called astronomical dusk (or dawn), but the interval from 12 to 18° is astronomical twilight.
Together with twilight, there are three types of dusk and dawn. Here is how low beneath the horizon the Sun’s center during each phase is:
- Civil dusk / dawn: 6°
- Nautical dusk / dawn: 12°
- Astronomical dusk / dawn: 18°
Times of astronomical twilight
The twilight duration differs in different locations, so there is no universal time for it. But how to figure out the astronomical twilight time? Use a website or an app. For example, Time and Date provides detailed information about sunrise and sunset for your location.
There are several tools like Photopills, Sun Surveyor, the Photographer’s Ephemeris when it comes to apps. They will be especially useful for photographers in terms of planning and visualizing images. Among our apps, you’ll also find a convenient photo planner for outdoor photographers — Ephemeris. This app shows exact times for civil, nautical, astronomical twilight, calculates the time and duration of gold and blue hours, contains detailed information about the Milky Way visibility, and more.
What can I photograph during twilight?
The civil twilight is the brightest phase; most people refer to this type when talking about twilight. It starts right before sunrise or after sunset when the Sun is just below the horizon. The horizon should be well seen during this time, and only the brightest stars and planets are visible. Laws in many countries require to turn on street lights and car headlights during this phase.
Right after sunset, the sky is very bright and full of rapid colors. Try to catch the beginning of civil twilight in the evening and take some landscape pictures to reveal different shades of the skies.
The nautical twilight is the second phase when the horizon becomes challenging to distinguish. The term itself came from the times when sailors used the stars to navigate at sea. During nautical twilight, many of the stars are visible, making it possible to navigate according to the position of the stars.
Artificial lights fully illuminate cities during this interval, so it’s good for urban photography. But if we mean astrophotography, consider capturing the Full Moon at the horizon. You can create a beautiful silhouette against the background of the Moon.
When the darkness is almost complete, and the horizon isn’t discernible, it means that the astronomical twilight is here. The Milky Way starts to appear, and the faintest stars and planets can be observed with the naked eye unless the Moon illuminates the sky. However, galaxies, nebulae, and globular clusters require full darkness when the Sun is more than 18° under the horizon.
This is the perfect time for photographing and observing celestial objects. If the Milky Way is visible in your sites (check with Ephemeris for its visibility time), try to capture this breathtaking galaxy as well.
What do astronomers mean by light pollution?
Generally, light pollution is excessive or unwanted artificial light. The most common examples are:
- Street lights;
- Car lights;
- Advertising boards.
There are various negative effects of light pollution; in terms of astronomy observations, the main problem is the reduction of celestial objects’ visibility. You can barely see the brightest stars in the middle of a city, not to mention the whole constellations or planets. Just imagine that the Milky Way is no longer visible to more than one-third of the world’s population due to artificial lighting!
This is why stargazing guides actively advise getting away from the city lights as far as possible for better observations — a typical city sky is up to 10 times brighter than the natural background at midnight.
While light pollution is a general term, there is an exact reason behind the reduction in night sky visibility — it’s called the “skyglow.” The skyglow is an upward-directed light from poorly designed, incorrectly directed light fixtures that shines into the sky. This light is scattered and reflected by solid or liquid particles in the atmosphere and then returns to our eyes, obliterating our view of the night sky. The effect of skyglow is not necessarily localized; it can be observed even far away from the primary source.
Let’s summarize: twilight is an interval, dusk and dawn are moments. Civil twilight isn’t the best time for night sky observations, but many stars become visible during nautical twilight. The best option is to start looking for the stars and planets during astronomical twilight and keep searching for deep-sky objects at night. And find a place without light pollution; otherwise, you’ll be able to see only the Moon and the brightest stars.
Our stargazing apps will guide you across the night sky, so don’t forget to get the one that meets your needs the most. Also, always use the night mode — it will preserve your night vision.
Wishing you clear skies and happy observations!