Start of Autumn: Cool wind arrives

February 8th, 2012
by Jo Law

In this pentad, the sun arrives at 315º on the ecliptic. By the reckoning of the almanac, autumn begins. Presently, the weather and the general environment appear to give little clue to the imminent seasonal changes, although preceptibly, minute by minute, the sun rises later and sets earlier each day. The first term has commenced in primary and secondary schools across all states. Parliamentarians of Australia are sitting their Autumn session. The first semester at Universities is about to begin.

The almanac is based on the Chinese solar calendar which is reckoned using astronomical observations. Solstices and equinoxes, known as quarter days, are determined by the sun’s position on the ecliptic. The Chinese solar calendar locates the start of seasons at midpoints between these quarter days (known as cross-quarter days). This astronomical model squarely puts the middle of the seasons at the corresponding solstices or equinoxes so that mid-winter’s day, for example, is also the shortest day when the sun is the lowest in the sky. This does not occur in the more prevalent 4-seasons model which uses meteorological reckoning (that is assigning the hottest 3 months as summer and the coldest 3 months as winter).

Another meteorological reckoning of seasons based on thermal criteria is used in higher latitudes or northern temperate regions. Seasons are defined when surface air temperature reaches a certain degree and stay within that range for a certain amount of time. For example, in Finland winter begins when daily mean temperature falls below 0ºC for 7 consecutive days; spring is marked by temperatures staying above 0ºC; summer has temperatures ranging from above 10ºC; and autumn’s daily mean temperature falls below 10ºC. This schema assigns no definitive dates for the beginning and end of seasons prior to their actual occurrences, which vary from year to year. It highlights how different geographic locations experience seasonal changes at different times.

Simiarly, an ecological definition of the seasons, practised by different groups of Australian indigenous people, is based on observations of botanical occurrences and animal behaviour in their local geographical areas. This, too, places emphasis on the relationship between seasons and locality.

One Response to “Start of Autumn: Cool wind arrives”

  1. 1 Louise null Curham
    February 9th, 2012 at 23:01

    Today in Canberra at 3pm in Parkes it started to rain and it absolutely poured until about 4.05 pm. There was thunder. The water on the bike paths was midway up my calves and at one point I almost had to walk because the current from the runoff was too strong to pedal against. It was cold enough for my fingers and toes to go numb. Yet in 2009 it was average daily temp of something ridiculously high like 32. I feel quite sorry for our friend coming from Norway who is counting on the tail end of summer to smooth her passage for life in Canberra. It is quite a distinct change in the way the darkness is falling at the moment – very quickly with much shorter twilight yet only two weeks ago it was light well past 8.