Max Continuous,
My terminology may be a little "old fashioned" as I am relying on facts studied nearly 50 years ago and the modern terms may be different.
We all know that the Earth orbits around the Sun although, to an observer on Earth, the sun apparently revolves around the Earth. The Apparent Solar day would be the time between two successive transits through the meridian of the observer and has been divided into 24 hours. In fact the Earth's angular speed of orbit varies and is not uniform so in arriving at the Mean Solar Day which is then divided into 24 hours the successive transits over a period of years was measured and then averaged to be divided into the 24 hour day we know. So the time between successive transits of the Sun is contantly varying and is not exactly 24 hours. The points where the apparent path of the Sun (The Ecliptic) intersect the Earth's Equatorial plane (The Celestial Equator)are about March 21st (First Point of Aries) with the Sun moving South to North and about 23rd September(First Point of Libra) with the Sun moving North to South.
According to Kepler's Law about orbital motion orbital speed increases when two bodies are closest and reduces with increased distance apart. If you calculate the days from March 21st to September 23rd you get 186 days and from September 23rd to March 21st you get 179 days. A difference of 7 days. So we have a cycle of slowing and then increasing orbital speed but not at a constant rate. This is easily confirmed as you would expect because the Earth is at Perihelion (closest to the Sun) about Jan 2nd and orbital speed is greatest, whereas it is at Aphelion (furthest from the Sun) about July 2nd and orbital speed slowest.
And the Solstices are the mid points on the Ecliptic halfway between the two points of intersection of the Ecliptic and the Celestial Equator.
Take half of the 186 days between 21st March and 23rd September...93 days after that is 22nd December; the Northern winter Solstice.
Take half of the 179 days from 23rd September to 21st March.....say 90 days after that is 22nd June the summer Solstice.
The usual typos!
[ 21 December 2001: Message edited by: pterodactyl ]</p>