Re: [Phys-L] analemma and equation of time

[John Denker <jsd@av8n.com>]

On 9/15/20 7:31 AM, David Bowman wrote:

> It should be noted that the particular shape of an analemma is not a permanent or fixed feature,

Yes! No kidding. That's not an idle technicality.

The effect would be big enough to ruin the diagram we've been discussing,
if you're not careful.
   https://www.av8n.com/physics/analemma.htm

Skyfield uses the International Celestial Reference System (ICRS) by default.
That's very similar to J2000. That's just fine if used consistently ...
but if you ask Skyfield what things look like "now" it will dutifully do so,
and you may be in for a surprise.

At this point you will discover that the earth's axis has precessed by a
nontrivial amount since the year 2000. The ICRS declination of the zenith
is not equal to your latitude, and the ICRS right ascension of the zenith
is not equal to the RA of the southern horizon (azimuth 180°), which is
just nuts. The meridian really should be a great circle of constant RA.
The discrepancy is not super-huge, less than a minute of arc, but it would
show up in the analemma diagram. Seriously wrong and ugly. Guess how I know.

Once you figure out what's going on you override the default by specifying
"epoch=t" in a few places, and then everything works fine. The zenith and
horizon calculations agree within a few parts in 10^14.

================

The reference for zero longitude was arranged to run through the town of
Greenwich, more specifically through the Royal Observatory, and even more
specifically through the very middle of the transit instrument (aka the
meridian circle instrument). That was supported by massive stone pillars,
and locked to the meridian. It was used to measure the position of the
sun and stars super accurately.

The interesting thing is that you can't just lock it to the pillars and
forget about it. Otherwise you come back in couple of years and find that
the whole thing is cockeyed, not aligned north/south anymore.

Even the prosaic task of surveying a precise north/south line is nontrivial.
You come back in a few years and find that it's cockeyed.