NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Sun sights during an eclipse: "bad limb" calculation
From: Paul Hirose
Date: 2023 Oct 12, 14:03 -0700
From: Paul Hirose
Date: 2023 Oct 12, 14:03 -0700
On 10/11/2023 9:16 PM, NavList Community wrote:
> 2023-Oct-14 15:36:08.0 = First touch of Moon with the Sun. (Standard
> Refraction)
> 2023-Oct-14 15:36:11.5 = First touch of Moon with the Sun. (NO Refraction)
>
> 2023-Oct-14 18:51:10.5 = Last touch of Moon with the Sun. (Standard
> Refraction)
> 2023-Oct-14 18:51:08.5 = Last touch of Moon with the Sun. (NO Refraction)
>
> So Chuck, you raise a good point, when comparing Eclipse times, is
> refraction accounted for?
I don't think it makes a difference to the contact times. It's true that
refraction alters the apparent distance between Sun and Moon centers, as
well as their semidiameters. However, if you draw a diagram of two
tangent circles and view it in a fun-house mirror, distances are
distorted but the circles are still tangent.
My time for first contact (refracted):
date 2023-10-14
15:36:11.300 UTC
15:36:11.300 UT1
15:37:20.487 TT
-90°00'00.0" +28°00'00.0" east lon, north lat
0.0 meters (0 feet) above ellipsoid
15.0 C (59.0 F)
1013.3 mb (29.92″ Hg) air pressure
15:36:11.3 UTC 1st contact
0°31'14.0" center to center, unrefracted
0'01.1" refraction
0°31'12.9" center to center, refracted
-15'11.4" Moon near limb refracted SD
-16'01.5" Sun near limb refracted SD
0°00'00.0" computed Moon to Sun
Re-calculate with air pressure near zero (my software doesn't allow
exactly zero.)
15.0 C (59.0 F)
1.0 mb ( 0.03″ Hg) air pressure
0°31'14.0" center to center, unrefracted
0'00.0" refraction
0°31'14.0" center to center, refracted
-15'12.0" Moon near limb refracted SD
-16'02.0" Sun near limb refracted SD
0°00'00.0" computed Moon to Sun
Apparent distance and semidiameters have increased, but the discs are
still tangent.
My results, independent of refraction:
15:36:11 UTC first contact
18:51:09 UTC last contact
The Astronomical Almanac Online used to have eclipse predictions for
selected places. I think the contact times were calculated to one second
of time. It was a good resource for software testing. But that part of
the HMNAO site has been down for months.
--
Paul Hirose
sofajpl.com
