It is around this time of year that I begin to feel the nights closing in, encroaching on my evenings and generally making me feel that the summer is slipping away.
The change in the number of hours of daylight in the UK is dramatic, falling from a peak of over 16.5 hours to less than 8 hours. If it wasn’t so familiar we would be amazed.
At this time of year ‘day length’, the difference between sunrise (when the Sun just pops over the horizon) and sunset (when it just disappears from view) changes by more than 3.5 minutes each day – or 25 minutes each week.
But of course it does not get dark immediately after the the sun sets. Even when it is below the horizon the sun illuminates the sky which scatters light downwards giving rise to ‘twilight’.
Google poetically informs me that twilight is:
the soft glowing light from the sky when the sun is below the horizon, caused by the reflection of the sun’s rays from the atmosphere.
Wikipedia tells me there are three definitions of twilight based on how far the Sun is below the horizon.
- ‘Civil‘ twilight is defined by the time that the Sun is less than 6º below the horizon.
- Wikipedia describes this as “the limit at which twilight illumination is sufficient, under clear weather conditions, for terrestrial objects to be clearly distinguished“
- ‘Nautical‘ twilight is defined by the time that the Sun is less than 12º below the horizon
- Wikipedia describes this as “when there is a visible horizon (at sea) for reference“.
- ‘Astronomical‘ twilight is defined by the time that the Sun is less than 18º below the horizon.
- Wikipedia describes this as “when the dimmest stars ever visible to the naked eye become visible”.
The chart below shows the times of Sunrise, Sunset and the three twilight times compiled from data on this wonderful web site.
This chart tells us what we already knew, that in the winter, not only is the day length shorter, but so is twilight.
In the chart above I have removed the anomaly of British Summer Time and so all the times shown are Greenwich Mean Time.
If we leave in this anomaly, then (looking only at sunset and sunrise) the effect of changing to British Summer Time can be seen below.
This shows that when we switch the clocks, instead of giving ourselves extra hours of daylight early in the morning, we choose to give ourselves an extra hour of daylight during summer evenings.
I think this makes sense. Alternatively, we could all just choose to get up a little earlier? Mmmmm.
Tags: Day length