Institute of Astronomy

Definition of Twilight

Published on 07/11/2012 
Question: 

As you may know the Holy month of Ramadan is approaching and as a Muslim I am going to observe the fasts in this one month. However Similar to last year there is many conflicting opinions on when the fast should close.
For this reason I have decided to make my own enquiries and get external information.

We have to close our fast when there is some light in the sky. Some Scholars are saying that is when the sun is below the horizon at 18degrees and others are saying 12 or 15degrees. This is causing great confusion
Can you please clarify this
Also During the summer months in the UK,(May-July) is it very difficult to differentiate between the two twilights? Meaning the twilight of the night, and the twilight of the morning?

The confusion I'm afraid is because unlike sunrise and sunset it is difficult to properly define 'twilight'.  There are three rough 'bands' of twilight that are generally agreed upon, civil twilight - the sun is less than 6 degrees below the horizon, nautical twilight - the sun is between 6 and 12 degrees below the horizon, and astronomical twilight - the sun is between 12 and 18 degrees below the horizon.  Civil twilight is what most people would think of as 'twilight', and at the start of which is when you would see the characteristic reflected red-orange glow from high clouds, there is generally still enough light to see by and the horizon is clearly visible.  Most people would likely think of nautical twilight as being 'darkness', in that bright stars will be visible and it would be difficult or impossible to tell which direction is East or West simply by looking for the glow of the sun below the horizon, at the start/end of nautical twilight when the sun is 12 degrees below the horizon even the horizon itself will be indistinct and for almost all intents it is completely dark.  Even after this though there will still be enough scattered sunlight around that it is not ideal for observing faint, diffuse, objects like nebulae until the sun is more than 18 degrees below the horizon.

I expect that for your purposes nautical twilight would be quite sufficient and that civil twilight is probably adequate, but that is something that you should decide for yourself.  Wikipedia has well written, detailed, articles about twilight http://en.wikipedia.org/wiki/Twilight and the brightness of the night sky http://en.wikipedia.org/wiki/Sky_brightness which have a variety of other links that you can check.

You are also quite correct that in a similar way to the fact that there are days when the sun does not rise or set within the Arctic and Antarctic circles there are concentric regions around the poles where, although the sun does rise and set, it will for a period around the summer solstice (21st June in the Northern hemisphere), never be darker than civil twilight, nautical twilight, or astronomical twilight.  It is easy to work out the lowest the sun will be below the horizon at any given location on the summer solstice from Earth's axial tilt of 23.4 degrees.  If you are at a latitude of X degrees then at the summer solstice the sun will be no more than 66.6-N degrees below the horizon, so in London (51.5 degrees North) on 21st June the sun was never more than 16.1 degrees below the horizon, so it was never darker than astronomical twilight.  In Edinburgh however at 56 degrees North the sun was never more than 10.6 degrees below the horizon and so it was never darker than nautical twilight.  It will always be darkest at midnight however (though note that since British Summer Time is one hour ahead midnight is actually 1am).

It is also worth noting that aside from the effects of the tilt of the Earth light pollution in large cities means that it may never seem completely dark.

Page last updated: 7 November 2012 at 15:33