Does anybody know the claimed accuracy of the USNO or Handbook values? I’ve had a quick look and could not see anything.
My point is this, is the two minute difference actually significant? If the USNO figure is accurate to 10 seconds then yes, if accurate to 10 minutes then no. Same with the Handbook value.
To assess the actual difference I have done a rough calculation which may well be wrong – I am not a mathematical astronomer by any means so feel free to fault my approach. What I did was as follows:
- From the Handbook calculate the rate of change in declination by taking values either side of the equinox.
- Pro-rata this to arrive a shift in the two minute difference of roughly 0.2 arcsec.
- Using the polar radius of the Earth, 0.2 arcsec translates into 61 metres.
- So in the two minute difference the position on the Earth’s surface where the Sun is directly overhead moves northward by just 61 metres.
Are the Earth’s orbit, the planet’s shape and axial tilt sufficiently well defined to give this level of accuracy?