The change from one day to the next in the length of daylight durind 2011. Graph and calculation Nick Lomb
The length of the day is the interval between sunrise and sunset. We have discussed in a previous post how that varies during the year. Briefly, days are longest at the time of the summer solstice in December and the shortest at the winter solstice in June. At the two equinoxes in March and September the length of the day is about 12 hours, a mean value for the year.
Let us now ask what is the change in the day length from day-to-day? You may have noticed that the time of sunset since 2 January has not changed so far and will not change for a few more days until Friday 14 January 2011. The time of sunrise has been becoming later by about a minute each day so that the length of the day has been shortening accordingly.
The diagram above makes things clearer. The change from one day to the next at the time of the summer solstice in late December is around zero. Similarly, there is little change from day-to-day at the time of the winter solstice in June. This is not surprising as solstice means ‘the day the Sun stood still’, so at those two times of the year we expect little change from day-to-day.
There is more change at the time of the equinoxes – autumn and spring – in March and September respectively. At those times the changes day-to-day can be up to three minutes.
The curve above is surprisingly messy and not the smooth curve that would be expected. The probable explanation is that it is due to the rounding to the nearest minute in the calculation of sunrise and set times. If they were calculated to the nearest second then the curves calculated from them would be smoother. However, there is no point in giving sunrise and set times to any higher precision than a minute as atmospheric conditions each day can make the actual times vary slightly from the calculated times.