Nudging the winter solstice to and fro
A solstice is one of the two instants every year when the sun is at its greatest distance either north or south of the equator.
In the northern hemisphere, the summer solstice is when the sun is at its greatest distance north of the equator, and occurs around June 21st or 22nd; the winter solstice is when the sun is at its greatest distance south, and this year it occurs at 1 p.m. today, December 21st. But this is no ordinary winter solstice: it is the earliest by the calendar that we have had for 100 years.
Although the solstice, almost by definition, comes around in an astronomical sense at exactly yearly intervals, our odd habit of inserting a leap day into the annual cycle now and then means that it oscillates backwards and forwards on our calendar.
Indeed its date may vary even from place to place. If, for example, it falls late in the day on December 21st in Ireland, in Japan it will be early morning December 22nd, local time, and so there the solstice will be recorded as falling on this latter date.
But the date of the solstice is also affected by the inconvenient length of the astronomical year. This "tropical" or "equinoctial" year, as it is variously called, is just 11 minutes short of 365-1/4 days. Because of the quarter, it follows that if a winter solstice one year is in the late evening of December 21st, it will be nearly six hours later the following year, putting it early hours of December 22nd.
But this trend for the event to occur later and later each year is disallowed; every four years the extra leap day hauls it backwards by a day - according to the calendar. Because of the 11 minutes already referred to, however, the additional day in a leap year brings the solstice 44 minutes too far back, and the error increases as each century progresses. For this reason, therefore, the 2000 winter solstice is earlier than at any time for the past century.
To remedy this backward creep of the winter solstice - and of course the summer one as well - the last year of each century is in most cases not a leap year. The omission of the extra day every turn of century, however, would push the solstice six hours too far forward again; the ultimate remedy is to retain the leap day in one centennial year in four, the one where its number is divisible by 400. And this, of course, is why we had a February 29th this year.