THE SEASONS

"Spring has arrived" - (Editorial. CANOPUS Sept 1999). Arguments about when the Seasons begin and end are often more emotional than logical, but there is more to the subject than one might think. The mathematical treatment of it is given in "GENERAL ASTRONOMY" by Sir Harold Spencer Jones, late Astronomer Royal. The main factors concerned are:

1. The Earth moves around the Sun in an elliptical orbit at a varying velocity, greatest at Perihelion (about 2 January), when it is closest to the Sun and slowest at Aphelion, when it is furthest from the Sun (about 4 July).

2. The Earth’s axis is tilted at 23,5 degrees so the Southern Hemisphere faces towards the Sun in summer, when the Earth is near Perihelion and travelling fastest. In winter the Southern Hemisphere faces away from the Sun and the Earth is near Aphelion and travelling slowest.

3.    The result is that, THEORETICALLY, the Southern Hemisphere has short hot summers and long cold winters as follows:

So if we assume that the Equinoxes and Solstices mark the midpoints of the Seasons and the above Periods be equally divided about them, then we can say that THEORETICALLY:

    Spring begins on 8Aug, mid-Spring =2lSep (Equinox), ends on 5 Nov

    Summer         6Nov, mid-Summer =2lDec (Solstice), 2 Feb

    Autumn                 3Feb, mid-Autumn =2lMar (Equinox), 5 May

    Winter                 6May, mid-winter =21Jun (Solstice), 7 Aug

However, another factor is that there is a delay in the time that the Earth absorbs heat from the Sun in Summer and radiates it out in Winter. But the ruling factor is climatic conditions, which are generally unpredictable and often overrule all the above.

It must be remembered that the Equinoxes and Solstices do not always fall on the 21st of the month and can vary between the 20th and the 23rd of the month from year to Year.

Incidentally, the above dates agree very closely with those given in the Chinese Calendar which has been in use for some 3 400 years. The beginnings of the seasons are marked by specific names and dates (for the Northern Hemisphere) viz:

Lichun beginning of Spring:        4 February

Lixia        beginning of Summer: 6 May

Liqiu        beginning of Autumn:         6 August

Lidong beginning of Winter:        6 November

Richard Overy