On Apr 19, 9:05*pm, brian whatcott wrote:
Darryl Ramm wrote:
* /snip/
You can see how this could vary when the horizon is the Rockies, versus
five miles West where it might be 5000 ft lower...

Brian W

Stop tying to make this hard, there is one proper definition to worry
about for gliding in the USA. That is the table generated by the US
Naval Observatory. The FARs *define sunset times as published in the
Air Alamanac which is produced by the US Naval Observatory, but for
practical purposes the official data is the USNO tables /snip/
Darryl

I have no problem with competitive sporting endeavors adopting a uniform
definition of anything they choose to. * * But supposing that the
Official Definition of Sunset from the USNO and the ACTUAL sunset time,
as described by the USNO, always coincide for a particular place and
time would be an over simplification.

At times, a LETHAL over-simplification.

Take this NTSB accident investigation from April 15th, 2002 for example.

http://www.ntsb.gov/recs/letters/2002/A02_08.pdf

Here an aircraft landing in the Rockies failed to note an intervening
summit. It was subject to a LOCAL sunset that was 25 minutes earlier
than the official sunset time as pointed out by the USNO in a footnote,
mentioned in this report.

Brian W

Again the OP was a question about official sunset times. The official
definition of Sunset by the USNO is what they use to generate the
table. There is no conflict between USNO definition and their own
tables, do you really think there would be? This is not an arbitrary
SSA sporting definition, the SSA is ensuring glider badge and record
flights comply with the strict sunset flight requirements described in
the FARs (unless you can show you are legal to fly to civil twilight).

USNO Definition: "Sunrise and sunset. For computational purposes,
sunrise or sunset is defined to occur when the geometric zenith
distance of center of the Sun is 90.8333 degrees. That is, the center
of the Sun is geometrically 50 arcminutes below a horizontal plane.
For an observer at sea level with a level, unobstructed horizon, under
average atmospheric conditions, the upper limb of the Sun will then
appear to be tangent to the horizon. The 50-arcminute geometric
depression of the Sun's center used for the computations is obtained
by adding the average apparent radius of the Sun (16 arcminutes) to
the [So presumably by incorporation via the Air Almanac reference,
this is pretty much the technical description of Sunset referred to in
the FARs]

I hope pilots get this is a numerical model, they should especially
when presented with a relatively crude table. They need to take into
effect local conditions that affect safety, whether it be terrain,
weather etc. that can all affect local light. But again that was not
the question being asked, which was about official sunset times.

Darryl