If you’re interested in astronomy, then you’ll likely have heard of twilight. It’s the time in the evening that occurs when the Sun has started to descend below the horizon. 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.
If you can get to a location with no artificial lighting, then astronomical twilight is the best time of night. It is still just a phase of twilight, and many of us will just refer to it as twilight – but there is a difference between the times.
What other twilights are there? Astronomical Twilight vs Civil Twilight vs Nautical Twilight
In general, astronomers will tend to categorized twilight into three different periods. This is the same as in the morning, except the Sun rises above the horizon as opposed to falling beneath it. Still, the stages for each part remain the same as either civil, nautical or astronomical.
When the Sun is between 0° and 6° in the sky, we refer to this period as civil twilight, or it’s sometimes referred to as morning twilight when the Sun is rising too. During evening 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. It is the evening equivalent of civil dusk, which is essentially just a morning civil twilight.
When it’s morning, civil twilight begins. After this, the Sun ascends or descends to between 6° and 12° in the sky, which we call nautical twilight. Even at this time, only the brightest stars in the night sky are going to be clearly visible, as the Sun still has an effect on what you can see with the naked eye.
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.
At this point, you will want to think about getting the telescope out, as it’s the best time to see stars (especially if there’s no artificial lighting in the sky). When the sun disappears beneath the horizon, it’s the perfect time to start trying to find the brightest planets in the night sky.
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.
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!).