Sunset at Sydney Observatory on 29 March 2008, image Nick Lomb
The summer solstice or the longest day for 2008 is on Sunday 21 December 2008. The instant of solstice when the Sun is at its furthest south for the year is at 11:04 pm Australian eastern summer time. The summer solstice is also known as the longest day as that is the day the Sun is in the sky for the longest time for the year. In Sydney on the solstice the Sun is above the horizon for 14 hours and 25 minutes.
Many people assume that the longest day is the day on which we have the earliest sunrise and the latest sunset for the year. Some even get upset and write letters to the newspapers when they discover that that is not the case.
The 2009 Australian Sky Guide indicates that in Sydney from 1 December to 10 December 2008 the Sun rises at 5:37 am, the earliest for the year. The latest sunset is listed as 20:10 from 2 January to 12 January 2009. Why are things so complex? Why don’t these events occur at the solstice?
The explanation lies in that esoteric phenomenon the equation of time. This has been discussed in more detail earlier in this blog. Basically, the equation of time is graph indicating when the Sun reaches its highest point each day during the year. It is a fairly complicated curve with two maximum and two minimum points. In December it is climbing steeply so that the time when the Sun is at its highest point or due north is about 30 seconds later each day.
The change in the length of daylight from day to day around the the time of the solstice is small and of the order of a few seconds. This small change is thus overwhelmed by the larger change due to the equation of time. It is the equation of time that is making sunset later in early January until the daily day length change is sufficiently large to dominate. Similarly for the earliest sunrise.
Wouldn’t it be boring if things were not a little bit complicated?