How Do Twilight and Dusk Differ?

As the sun sets, the sky gradually darkens. Some people refer to this moment as 'dusk,' while others use the term 'twilight.' However, in the field of astronomy, these terms have precise definitions. In fact, there are three types of twilight, dusk, and dawn that astronomers use. Here's what they are and how to distinguish between them:

The Three Stages of Twilight

There are three distinct types of twilight: civil, nautical, and astronomical. Civil twilight is the brightest phase, while astronomical twilight is the darkest and occurs right before or after nightfall.

Twilight phases happen in the same order across the globe, but their duration depends on your specific location on Earth.

The duration of each twilight phase varies depending on the season and your geographical location. In areas near the equator, twilight phases can last as little as 24 minutes, whereas near the poles during summer or winter, they can last several weeks.

How Do the Different Types of Twilight Compare?

When people refer to twilight, they are usually talking about civil twilight, which is the first phase occurring just before sunrise or after sunset. At this time, the sun is just below the horizon, and there's still plenty of light in the sky. Many countries base regulations on this phase, such as the requirement for streetlights and car headlights to be turned on.

There's no strict rule for distinguishing between each type of twilight, but here are some helpful guidelines:

One method is to assess how well you can discern shapes in the dimming light. We know that civil twilight occurs just before sunrise or after sunset when the sun is barely below the horizon. As the horizon itself becomes hard to define, you're entering nautical twilight. This term comes from sailors who used to navigate by the stars—when the sun dips beyond 12 degrees, they couldn't distinguish the sea from the sky. By astronomical twilight, the sky is nearly pitch black, and it becomes difficult to identify any shapes unless the moon is visible.

Another technique is to observe the stars that appear. During civil twilight, only the brightest stars and visible planets are visible. As the twilight shifts to nautical, spotting stars becomes easier until, during astronomical twilight, even the faintest stars become visible. It's also the phase when you can first glimpse the Milky Way.

Twilight vs. Dawn or Dusk

Although we often interchangeably use the terms "dawn," "dusk," and "twilight" based on the time of day, both dawn and dusk hold precise astronomical definitions. There are, in fact, three distinct types of both dawn and dusk.

Similarly, civil, nautical, and astronomical dawn happen when the sun is at those respective degrees below the horizon just before sunrise. In other words, dawn and dusk take place when the sun crosses a specific threshold beneath the horizon (for example, 18 degrees), whereas twilight refers to the period between those thresholds (like between 12 and 18 degrees).

Next time you find yourself witnessing a stunning sunrise or sunset, take a moment to think about which of the three phases of dawn, dusk, or twilight you might be experiencing!