Dawn, dusk and twilight
How do we calculate the precise moment of sunrise or sunset? And exactly when is twilight?
Sunrise and sunset
We all know that sunrise or dawn is when the sun comes up and sunset or dusk is when it goes down but what if we need a more precise time to refer to?
The published times of sunrise and sunset refer to the moment when the Sun's upper limb, as affected by refraction, is on the true horizon of an observer at sea-level. This occurs when the Sun's centre is 50 arcminutes below the true horizon, the upper limb then being 34 arcminutes (just more than the Sun's apparent diameter) below the true horizon.
There are three different definitions of twilight:
- Civil twilight – when the Sun's centre is 6° below the horizon, is roughly equivalent to Lighting-up Time. In the UK, it is between 30 and 60 minutes after sunset. The brightest stars are visible and at sea the horizon is clearly defined.
- Nautical twilight – when the Sun's centre is 12° below the horizon, is to all intents and purposes the time when it is dark. For nautical purposes it is that time when the horizon ceases to be clearly visible and it is impossible to determine altitudes with reference to the horizon.
- Astronomical twilight – when the Sun's centre is 18° below the horizon, is when it is truly dark and no remnant of the Sun's afterglow can be seen.
Why isn’t the earliest sunrise on the longest day and the latest sunrise on the shortest?
The longest and shortest days of the year are those with the greatest and least amount of daytime. They occur at the time of the solstices, either on or around 21 June and 21 December in the UK and most of the northern hemisphere.
Many people notice that the time of sunrise continues to get later after the winter solstice. The reason for this has to do with the slight variation in the length of 'natural' days throughout the year (the longest natural day is about 51 seconds longer than the shortest).
For a clock to work, all days measured need to have a fixed, equal length. Each is therefore fixed at the average length of a natural day (this is where the ‘mean’ in Greenwich Mean Time comes from). By averaging out the length of each day like this, the clock time at which the sun reaches its highest point slowly drifts backwards and forwards as the months progress.
There is a knock-on effect on the times of sunrise and sunset. The earliest sunrise occurs a number of days before the longest day and the latest a number of days after the shortest.