This is an extract -
A quick look at sunrise and sunset times on successive days in March and September will show that the above dates do not coincide precisely with the times of equal day and night length. There is another factor to be considered which is that the orbit of the Earth around the Sun is an ellipse and not a circle. By Kepler's Law the Earth moves fastest when it is closest to the Sun (approximately 3 January each year) and slowest when it is furthest away (approximately 4 July ). This 'unequal motion' causes variations in the length of the solar day and in the times of sunrise and sunset. -
from this
Equinoxes and solstices : Time & timekeeping : Fact files : Learning : NMM
I'm not much wiser from reading it either
Happy Yule and Solstice folks
