Re: Contact times Occultation by Saturn rings and Moon eclipse times

Bernd Brinkmann Nov 13, 2003

Hello Bill,

> Hi Oliver, Bernd,
>
> (Bernd, Oliver mentioned your interest in the lunar
> eclipse problem. So I am copying this to you.)

thanks for your answer of a question I just wanted to ask you. I had a
discussion about this topic with Alfons Gabel (and he with Oliver) on a german
ccd-list. At
http://www.sternwarte-herne.de/index.html
you can find my images of the last total lunar eclipse, which were the reason
for this discussion.

> In Guide, I assume that the earth's atmosphere adds 2% to its
> radius. This is probably the most common assumption; in fact,
> I'm a little surprised that NASA didn't follow that convention,
> but I don't know where their figures came from.
>
> The actual addition varies a bit from eclipse to eclipse,
> which is why people bother to time when certain lunar features
> enter or leave the umbra;

I have made this timings at my early astronomy days in the 1980's...

> such timings can help pin down just
> how much the earth's atmosphere is affecting the eclipse. (The
> variations in actual eclipse times also mean that there's not
> much point in giving them to an exact second. Much as with
> rise/set times, Guide gives them to the nearest minute; giving
> times to the second would be misleading.
>
> The difference is greatest when the moon hits the shadow
> at a grazing angle. It did so when entering/leaving totality;
> that's why you see the greatest time difference there.

Yes, this was really the case with this lunar eclipse, when there was a strong
brightness gradient even at mid totality.

> The difference in the "visual" appearance comes about because
> of a recent change I made in how Guide shows the moon and planets.
> They aren't shown as actual spheres, but as faceted objects. I
> had not thought through the implications for eclipse timing... I
> don't consider this a really serious problem, given that (as I
> wrote above) the times of eclipse are not so solidly determined;
> but I'm glad you pointed it out to me.

So this effect can also affect the timings of lunar occultations of stars? What
happens, when I use only the outer line of the moon (symbol, no bitmap)?
With the star SAO 93170 = HIP 17509 the reappearance seems to be late for
about 15 to 30 sec even with "symbol".

Clear skies
Bernd


Bernd Brinkmann

Sternwarte Herne, #A18
Herne, Germany

e-mail: info@...
http://www.sternwarte-herne.de