Astronomical Twilight – what does it mean?

If you’re interested in astronomy, then you’ll likely have heard of twilight. However, there are different names for twilight, including astronomical twilight, civil twilight and nautical twilight. But how do these twilights differ to one another?

Well, the answer is actually quite simple. Let’s look more at astronomical twilight and what it means.

Astronomical Twilight – What does it mean?

Astronomical twilight is the darkest phase of twilight, darker than both civil twilight and nautical twilight. It is defined as when the sun is between 12° and 18° below the horizon. For astronomers, this is the best phase of twilight as stars are easily visible and there are no colors in the sky.

So, astronomical twilight is just a phase of twilight, and many of us will just refer to it as twilight

What other twilights are there?

In general, astronomers will tend to categorized twilight into three different periods. When the Sun is between 0° and 6° in the sky, we refer to this period as civil twilight.

During civil twilight, we can still see clearly and it’s the time where you might just flick your car headlights on to ensure that others can see you. However, things are still clearly visible. After this, the Sun ascends or descends to between 6° and 12° in the sky, which we call nautical twilight.

astronomical-twilight-stages
Image credit: auroraborealisobservatory.com

Nautical twilight is when the sky is too dark for us to see objects is the distance difficultly, but we can make out the horizon and trees in the distance due to the remaining brightness in the night sky. We can star to see stars in the night sky clearly. When there are no traces of glow and light in the sky, we know that nautical twilight has transcended into astronomical twilight.

Common Questions about Astronomical Twilight

Can you see during astronomical twilight?

Yes, for astronomers this is a good time to look in the night sky. The majority of stars and objects in the night sky are visible during this time. However, there will still be some parts of the night sky that will be hidden, as the Sun is still less than 18° below the horizon.

What is the difference between astronomical twilight and nautical twilight?

The only different between these two stages of twilight is the Sun’s position in relevance to the horizon. Nautical twilight is when the Sun is between 6° and 12° below the horizon, and astronomical twilight is when the Sun is between 12° and 18° below the horizon.

Why is Twilight shorter at the equator?

Yes, twilight is shorter on the equator than it is if you’re closer to the North and South poles. This is all down to the motion of the Sun in relevance to it’s viewed angle from the equator, as at the equator it declines or rises faster vertically. However, if you’re at the North pole, it rises in a slower and more diagonal fashion, meaning that twilight lasts longer at the poles.

Does Twilight last longer in summer?

Yes, twilight will last for a longer period of time in the summer than it does in the winter. This is because of the Earth’s inclination towards the Sun in the summer, and away from the Sun during the winter.

Why is it called nautical twilight?

The reason why we refer to the period of time when the Sun is between 6° and 12° below the horizon as nautical twilight is that before technology advanced, this time was valuable to navigators at sea. We can use the stars positioning in the sky during this time to help navigate.

Conclusion

All in all, I hope that this has cleared up any questions that you might have about astronomical twilight. it’s just a short period in time where the night sky is free of solar glow, which makes it a good name for astronomers (hence it’s name!).

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