Throwing some light on the subject

IN THESE northerly parts December 21, the winter solstice, is the shortest day of the year. But somewhat bafflingly, the earliest sunset always occurs several days earlier.

It seems paradoxical — logic would strongly suggest that the shortest day should have the earliest sunset and latest sunrise.

Not so. On December 8 this year the sun will set at 4.06pm in Dublin, and it will hold steady at this sunset time until December 16. From the next day, December 17, the sun will sink in the sky progressively later (by a minute every couple of days or so). But between December 8 and December 16 the nights don’t draw in any further.

In London, it being further east, sunset is earlier — at 3.51pm on Friday, December 8. The sun continues to set at this time until December 16, when it sets at 3.52, and thence forward sunsets become progressively later.

But whether you're in Dublin or London or elsewhere in these islands, no date between December 8 and December 16 is the shortest day. That is December 21. It’s just that the sun has been rising progressively later in the morning.

This occurs because of the discrepancy between the clock and the sun. A clock ticks off exactly 24 hours from one noon to the next. But an actual day – as measured by the spin of the Earth – rarely equals 24 hours exactly. For instance, December 25 this year will be 24 hours, 0 minutes, 0.0010531 seconds long.  That’s right — you'll have an an extra 0.0010531 seconds of Christmas cheer. In fact, only four days in 2017 were exactly 24 hours long.

The reason for this anomaly is the earth’s imperfect orbit, and the fact that the tilt of our planet is not constant. This means that throughout the year the actual length of a solar day varies slightly. The shortening of the day as winter approaches is not a constant curve.  It's the reason that, almost suddenly, there seems to be a stretch in the evenings from the beginning of January. In actual fact the evenings (but not the mornings) have been drawing out from around December 16 onwards.