Equinoxes in Canberra

I noticed a discrepancy in Canberra equinoxes that I cannot explain.  Perhaps one of you will be able to shed some light on this puzzle.



At an equinox, day and night have the same duration and, if you are in the middle of your [theoretical] timezone, you see the Sun rise at 0600 and set at 1800.

Canberra is almost in the middle of its timezone, but not exactly.  It would be smack in the middle if it were located at a latitude of 150° East, but it is only 149° 8’ east of Greenwich.  As 24h correspond to 360°, that degree of difference means that sunrise and sunset at the equinoxes are theoretically delayed by approximately four minutes.  Consequently, on March 23rd and September 20th, the Sun should rise at 0604 (plus daylight saving in March) and set at 1804.

What I cannot explain is that the delay, instead of the expected four minutes, is between nine and ten minutes in March and between two and three minutes in September.  The discrepancy in September could perhaps be explained with roundings and with the fact that equinoxes actually happen at different times of the day.  But, considering that the times of sunrise and sunset only change very slowly, at a rate of around a minute per day, the difference of five to six minutes in March remains unexplained.

Any idea?

Just for fun, I calculated how far from its current position should Canberra be in order to be exactly 10 h ahead of Greenwich.  The length of the equator is 40,075 km.  As Canberra’s longitude is 35° 15’ South, it means that the parallel passing through Canberra ihas a length of approximately 40075 * cos(35° 15’) = 32,727 km.  Therefore, to be in the middle of the +10 time zone, Canberra should be 32727 / 360 ≈ 91 km more to the East.  But, I suppose, that didn’t figure very high on the founders’ priority list.  In any case, it would have been on a mountain range.  Not easily accessible...