When is the first day of winter?
Posted December 20, 2008
MIKE MOSS SAYS: Bill, The standard definition we use for the first day of winter by way of an astronomical marker is the occurrence of the winter solstice, which happens this year at 7:04 am EST on Sunday, December 21st. The winter solstice is the point in earth's orbit around the sun at which the north pole is tilted most directly away from the sun. This results in the shortest "day" of the year in terms of the time difference between sunrise and sunset, although the combination of earth's slightly elliptical orbit and tilted rotational axis results in sunset times that have already been getting later for about a week and a half, while the sunrise times will continue to get later for about the same amount of time following the solstice. The difference between the two, however, reaches a minimum on the solstice itself, with the day to day chances in day length quite small for a few weeks either side of the solstice. The rate of day length change from one day to the next maximizes, on the other hand, in the weeks surrounding the equinoxes in March and September.
Note that in climate records and for most meteorological purposes, we consider "winter" to be the entire months of December through February, "Spring" to be March through May, and so on...