A milestone that has been noted through the centuries is set to recur this afternoon: the winter solstice, start of the coldest season.

Today is the shortest day of the year, the one with the least sunlight. But with the arrival of winter of 6:14 p.m. comes what a Gilbert and Sullivan operetta calls a "promise of merry sunshine."

As of Sunday morning, the daylight hours begin to lengthen. The sun rides higher in the sky each day, the minutes of sunshine grow daily, and Earth eventually warms. The process of lengthening days continues for six months, until the start of summer next June.

"Things are reversed in the southern hemisphere," noted Patrick Wiggins, NASA's solar system ambassador to Utah. There, "they see the December solstice as the beginning of summer."

Many holiday traditions we enjoy at this time of the year are thought to have their beginnings in solstice ceremonies held thousands of years ago.

"An example of this is our use of festive lights," Wiggins said.

In earlier eras people paid close attention to what the sun was doing, he said. "At this time of year it was easy to see that the sun was getting lower and lower in the sky each day and that if something wasn't done, it would soon disappear altogether."

Ancient people would build great bonfires to encourage the sun to stay, he noted. "Sure enough, every year their efforts paid off and the sun would start to gradually creep higher and higher into the sky."