[guide-user] Lunar eclipse on Guide

Paul Schlyter Jun 21, 2000

> Hi Guiders.
>
> I found little mystery about lunar eclipse of 16 July.
>
> Astronomical Ephemeris tells as :
> Prnumbra enterd : 10h46m4 (UT)
> Umbra entered : 11h57m0
> Totality begins : 13h01m8
>
> Guide's [Tables] -> [Lunar eclipses] tells as :
> Prnumbra enterd : 10h47m (UT)
> Umbra entered : 11h56m
> Totality begins : 13h01m
>
> I displayed lunar eclipse on Guide Lebel-12, animating 5-sec steps.
> Prnumbra enterd : 10h51m20s (UT)
> Umbra entered : 12h00m20s
> Totality begins : 13h04m50s
>
> It seems Earth's shadow is little small.
> I turned lunar displaying 'Bitmap' to 'Shaded' .
> I found Guide has 8 steps Penumbra.
> And no--8 Penumbra is edge of Umbra Astronomical Ephemeris use to
> calculate.

There are different ways to compute the size of the Earht's shadow.
The problem is the Earht's atmosphere which enlarges the shadow, but
by how much? That really depends on weather/etc.

The Astronomical Almanac assumes that the radius of both the umbra and
the penumra is enlarged by 2% by the Earth's atmosphere.

The French "Connaisance de Temps" uses a different model: instead
of adding 2%, it adds a constant value in kilometers (don't remember
the value right now). Jean Meeus promotes the French method, and uses
it in his publications.

Bill -- how do you compute the enlargening of the Earth's shadow?

----------------------------------------------------------------
Paul Schlyter, Swedish Amateur Astronomer's Society (SAAF)
Grev Turegatan 40, S-114 38 Stockholm, SWEDEN
e-mail: pausch at saaf dot se or paul.schlyter at ausys dot se
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