The Sun in winter.
Image: Frey Micklethwait
Source: Museum Victoria
Question: After reading last week’s question about the variation in the dates and times of the solstices and the equinoxes, I was just wondering, why do the seasons start on the first of the month and not on the day that it actually starts, for example, winter starting on June 21st?
Answer: One thing to note is that there is not really an "official start date" for seasons at all. When we talk about a season starting on a certain day it is only by convention. There is no government act or universally accepted scientific definition for what constitutes a season.
While most countries popularly use the equinox and solstice dates to "start the seasons" there is no particular reason why this convention needs to be followed. Meteorologists around the world, for example, start counting the seasons in the same way that Australians do - on March 1, June 1, September 1, and December 1.
Part of the difficulty with this issue is that there are two ways in which we perceive the seasons and while these two are related it is not in a simple manner.
Firstly, seasons are marked by the weather. This is the oldest way in which seasons have been understood. Summer is the warmest time of year and winter the coldest.
Secondly, seasons are marked by the solstice and equinox dates. Midsummers day - the summer solstice - is the longest day of the year and Midwinters day - the winter solstice - is the shortest. These times can be measured accurately by astronomers. They occur because the axis of the Earth is tilted to the plane of its orbit around the Sun.
Although the changing seasonal weather patterns are basically caused by the tilt of the Earth's axis there is a time-lag in when these effects appear. The ground (and the oceans) take a long time to heat up or to cool down. Although the ground is heated most effectively on Midsummers day, this time-lag means that it continues warming up for some time afterwards. Thus the hottest day of the year is around a month after Midsummers day. Similarly the coldest day is around a month after Midwinters day.