Observability of solar sate

7 May 1997 17:13:25 -0800

jean monseur <jmonseur@mail.cpod.fr> wrote:

> ... how do you intimately explain that a satellite can be seen in
>front of the sun while it could not against the simple sky
>during daylight ?

To which Bruno Tilgner replied:

> ... the contrast between a satellite and the daylight sky is not
> strong enough, but between a satellite and the sun it is. In addition,
> when we see a satellite against the sun, we see its DARK side
> whilst elsewhere on the sky the satellite is more or less
> illuminated by the sun.

I would add that transits of Mercury and Venus across the sun are easily
visible in a telescope.  In fact, it was this very phenomenon that encouraged
me to add a satellite transit search capability to SkyMap.

And another point about contrast.  Consider sun spots.  Most people interested
in astronomy have seen them at some point in their life. But the spots
themselves are not black -- they're incandescent.  They just happen to be
regions of the sun's surface that are ~slightly~ cooler than the rest.

Robert Sheaffer mentioned the GIF file that I sent him showing the transit
path of Cosmos 1763 R/B (#16864) across the sun as predicted for his location
last Friday morning (8:55:54 PDT May 2).  If anyone is interested, I'll be
happy to email it to them.  The file is small -- it's just a screen capture of
SkyMap's plot of the time-tagged track across the solar disk, with a grid and
subtitles.  --Rob