Hi everybody,
On november 3rd an occultation of Saturn by the Moon occured for european
observers. In our club somebody watched the times of the immersion and
emersion of the rings and the globe. I was curious to compare them with
those that Guide would give. So I ran an animation of the occultation with
a high zoom level (2 arcminutes). My intention was to progress step by
step and to note the times in the simulation when the limb of the Moon
comes in contact with the outer edge of the A ring, with the Cassini
division, and so on. Having not a very fast computer, I switched to "line
figure" for the drawing of the Moon, but I used the bitmap for Saturn.
Then I noticed that the line for the limb of the Moon is not a curve, but
a polygon (very close to the real limb, I guess). When the Moon was partly
over Saturn, I could see that in fact Guide draws that line and also a
vast disk filled with black, just a little smaller than the polygon line
and inside it, but with its edge not maching it. Eventually, just at the
moment the saturnian globe was completely covered by the Moon, the part of
the rings that remained visible disappeared also. So I was unable to
simulate the end of the disappearance of the rings.
So far, I see O-C ranging from +1 to -10 s (a rather high dispersion), but
I am not sure I used Guide properly to obtain good theoretical times...
and the accuracy of the observed timings was not reported (unluckily). I
will care next december 1st : another occultation of Saturn occurs and it
will be visible both from Europe and US (watch in Guide ;-)
My questions are :
- What is closest to the limb of the Moon: the line or the edge of the
black disk ? (I mean the average limb, not the real profile.)
- Is it possible let Guide draw the rest of the rings even when the globe
of Saturn has completely disappeared (or has not yet reappeared) ?
- What is the accuracy of the times of contacts obtained from that kind of
simulation ? I used a time interval of 1s; is it meaningful to slow down
to 0,1s ?
By the way :
Next week an astronomy day takes place in Belgium. For that occasion I
used Guide and its wonder-power-ful feature "Make a PS file" to produce a
map of the sky for that night. I imported the PS file into an image
processing software at a high resolution (600 dpi) and I added some color
and text and cropped the map to the horizon. I have put a lower resolution
of that map at
http://users.skynet.be/zmn/cab/tempo/JNA2001.htm.
Sincerely
Laurent Zimmermann
http://users.skynet.be/zmn/cab