R E Smart, of Wanganui, asks :-
How is it that our seasons change exactly on time by calendar whether the Earth is moving in orbit away from the Sun or towards it?
Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.
They don't. The dates that the news media trumpet as the starts of seasons are just calendar dates.
There are no 'official' dates for seasons but most developed countries use the solstices and equinoxes as starting dates. These correspond reasonably well to the changes in temperature. So our summer should have begun on December 22nd, the solstice when the sun is furthest south. On this convention the 2013 autumn begins at the equinox on March 21st; winter at the solstice on June 21st; spring at the September 23rd equinox and summer on December 22. These dates can slip a day over the four-year leap-year cycle.
New Zealand's news media, alone in the developed world as far as I know, have dumbed-down the season starts to the beginnings of the months. So summer begins at December 1st, etc. That three-week shift can make quite a difference to the temperatures in some years. Still the old joke applies that "We don't have a climate, we just have weather".
The seasons have no relation to Earth's distance from the sun. They are caused by the tilt of the Earth on its axis. The solstices are when the Earth's axis is at maximum tilt toward or away from the sun. The equinoxes are when Earth's axis is side-on to to the sun.
It so happens that, just now, Earth is closest to the sun in January and furthest away in June. As earth moves faster in its orbit when close to the sun that makes the September-March equinoxes closer together than the March- September equinoxes. The intervals are around 178 days and 187 days. So our spring and summer are nine days shorter than the northern hemisphere's. But we are 4% closer to the sun in January so we get 7% more sunlight. Swings and roundabouts.