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I wrote:
> Dave Walden wrote:
>
>> lat seems to effect geocentric LHA?

Although the answer is no, the question got me thinking about a related
matter: local apparent time. I decided to redesign my implementation and
release a new version of Tinyac.

One problem was that I failed to document a subtle feature in the
original local apparent time scale. When you selected it, the resulting
time scale was measured by the *topocentric* (affected by geocentric
parallax and dinural aberration, but not refraction) apparent place of
the Sun on the celestial sphere, relative to observer's geodetic meridian.

By contrast, if Greenwich apparent time was selected, the apparent
*geocentric* position of the Sun was used.

This difference between GAT and LAT was intentional. My rationale was
that if you need to enter a time in geocentric LAT, you can easily and
precisely substitute GAT plus east longitude. On the other hand, if a
topocentric apparent time scale isn't provided, the fix is less easy.

But how often is the latter time scale needed? Hardly ever, I think.
When a navigator makes a time sight, a correction for parallax in
altitude is applied, thereby reducing the observation to the geocenter.
So in this new release, the difference between LAT and GAT is purely due
to longitude. I think this is a more useful and less confusing design,
though a bit less flexible.

Anyway, you can always read topocentric apparent time by selecting
"terrestrial (LHA)" as the frame for Sun coordinates, and setting hours
as the theta unit of measure. If you switch the origin between
geocentric and topocentric, there's a small change in the coordinates.
That's parallax and dinural aberration at work.

The new Tinyac version is 1.0.1.0. Uninstall the old version via Windows
Control Panel, download the updated .msi installer file, and launch it.
The new program is the same size. Installation takes only a few seconds.
Get the installer at the Tinyac page, as before:

http://home.earthlink.net/~s543t-24dst/tinyac/index.html

Note that some intermediate versions between the 1.0.0.0 initial release
and the 1.0.1.0 current release were on my site for a few hours today.
They don't incorporate all the updates. I've fixed the documentation to
explain the GAT and LAT time scales better. Also, there's a little
improvement when you open a leap second file. The dialog box shows only
*.txt files by default now, getting rid of the clutter of irrelevant files.

Speaking of the leap second file, in a text editor you'll see that it
shows no leap seconds through the June 2011. That's not official, just
my guess based on the small UT1-UTC difference predicted at the end of
this year.

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