Why the sun is running late
IWAS delighted to receive a letter recently from a Sunday Tasmanian reader about the rising and setting times of the sun, which appear to show a lack of balance, or symmetry, as the days get longer and shorter.
On December 21 we had the longest day of the year, the day of the summer solstice. (Depending on the year, it can occur on December 21 or 22.) However, as the reader pointed out, the times of sunset continue to get later until early January, and from the second week of December both sunrise and sunset were getting later.
Even so, the difference in time between sunrise and sunset reached a maximum on December 21, with sunrise at 5.28am and sunset at 8.49pm (summer time) from Hobart.
To digress for a moment, the definition of sunrise and sunset is a little complicated. It is the moment when the angle between the vertical and the sun’s centre (which is by then below the horizon) is 90.83 degrees. This extra 0.83 degree is because sunset is the moment of disappearance or reappearance of the sun’s upper edge at a level horizon, and also because Earth’s atmosphere bends the sun’s light, making it appear to be higher up than it really is. That effect is most pronounced near the horizon.
Returning to the apparently odd way in which the sunrise and sunset times change, the reason for the lack of symmetry is a quantity known as the equation of time. One obvious manifestation of this is that a sundial can run either fast or slow, depending on the date.
It is sometimes stated that noon is when the sun is due north (or due south when seen from the northern hemisphere). With the solar day being 24 hours long, you might expect the sun to return to due north after each 24-hour period. However, Earth’s orbit is elliptical (oval-shaped, not circular), so sometimes it takes a little more than 24 hours for the same place on Earth to face towards the sun again, and sometimes less. So we more correctly call the 24-hour period the “mean solar day”.
When combined with the tilt of Earth’s axis, this means that overall the sun can appear to run fast by up to about 16 minutes (around the beginning of November) or slow by up to about 14 minutes (in mid-February).
The effect is even more complex than that, with smaller extremes of running about three minutes fast in May and six minutes slow in July. This difference is the abovementioned equation of time.
A good sundial should include a table of the equation of time corrections, so the user can tell the correct time.
So, how does this relate to the curious sunrise and sunset times?
To use the recent summer solstice period as an example, the equation of time is changing quite rapidly then, and is “dragging” the sunrise-sunset pair with it, resulting in the sunrise time starting to get later before December 21, and the sunset time starting to get earlier after December 21.
However, as I have mentioned, the summer solstice day is the longest day. The equation of time does not change that.
Confused? Well, all you really need to remember is that the sun can appear to run a little slow or a little fast, and that makes the sunrise and sunset times sometimes seem a little odd.
The equation of time, therefore, has the effect that at noon, the sun only averages due north. Its changing height in the sky at noon, and its pattern of running slow and fast, forms a lovely distorted figure-eight pattern called an analemma.
There is an additional correction for locations that are not on a standard time meridian, which in our part of the world is the longitude of 150 degrees east.
Hobart’s longitude is 147.3 degrees east, so superimposed on the everchanging equation of time correction, the sun runs an additional 11 minutes slow from Hobart!