[EMAIL PROTECTED] wrote: > > On 21 Dec 2007, at 09:13, David Sullivan wrote: > > Bruce wrote on Dec 20: > >> and it's not even the shortest day 'til tomorrow. > > > > Umm. The solstice is AFAIAA on Dec 22 this year. > > As for sunrise and sunset times, the sunset was at its > > earliest (15:54) where I live from Dec 11th to Dec 15th > > (it's 15:56 today) and the sunrise is STILL getting later - > > it was 08:06 today and it will be at its latest (08:08) > > between Jan 5th and Jan 7th. > > > > This has been a public service announcement. You may > > return to your grog now. > Not until you have explained why this is so.
Basically because the length of the days using natural solar time (as measured by the position of the sun - when it's due South and at its highest it's noon) varies slightly around the year. Whereas our clocks use 'mean time' where we average them out to the same length. Although the variation is only a few seconds a day, at some times of year they add up to the point where our clocks are something like fifteen minutes different from solar time. And in the autumn, our clocks are slow relative to the sun, whereas in winter and early spring they're ahead. So when we measure our time by GMT it seems like the evenings are darker in the autumn, and the mornings are darker in the first part of the year. And that's enough to slew the dates of the latest sunrise and the earliest sunset by a few days. The effect is greater the nearer the equator you are, because the variation in the length of daylight is less. I'm guessing Dave lives somewhere in the south of England. The reason for the variation in the length of the days (measure by solar time) is due to two factors, and they're both related to the fact that a day isn't just one revolution of the earth, it also includes a small component representing the amount extra that the earth has to turn each day to face the sun, because it's travelled a little bit further around its orbit so the sun isn't in the same apparent position any more: (1) the earth's orbit is an ellipse, and its speed around the orbit varies with its distance from the sun as described in Keplers laws. (2) The earth's axis is tilted relative to its orbit. This means that the sun's apparent movement against the background stars isn't a circle, but climbs and falls at different times of year. When it's climbing and falling, its speed of movement when extrapolated down to the horizon is slightly slower than when its travelling horizontally at its peak and trough. I think. There are websites that explain it better. Try searching on 'analemma'.... Martin L
