Observability of solar satellite transits
Bruno Tilgner (Bruno_Tilgner@compuserve.com)
Wed, 7 May 1997 11:04:58 -0400
jean monseur <jmonseur@mail.cpod.fr> wrote:
>Some questions about this most interesting debate on satellites
>solar transit...
>Also the rope of TiPS can be seen on the dark sky while it is much
>smaller than a human hair at that scale, because it is violently lit
>by the lateral sun.
For a BRIGHT object against a DARK background two conditions must be met:
1. The receptors in the human eye must receive a sufficient energy flux,
i.e. photons per second, to trigger the chemical process which the brain
interprets as "light".
2. Neighbouring receptors must NOT be stimulated, i.e. the background must
be sufficiently dark compared with the source of light. This explains why
the limiting magnitude of faint stars just visible in the suburbs of a
large city is not the same as in the open countryside, although the
amout of energy per second from the stars falling into the eye is the same.
For TIPS this condition is met. In the case of TIPS there is an additional
effect: the human brain recognises LINEAR structures much better than
pointlike ones. Two examples: 1) You can recognise a high-voltage power
line which spans a considerable fraction of the field of view much better
than a short piece of wire of the same diameter. 2) It is said, but I am
not sure if it is true, that the only sign of human activity which
astro/cosmonauts have seen from space are stretches of the Big Wall in
China, which runs as a straight line over large distances.
>But 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 ?
By the same reasoning: 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.
>Please, explain the role of filters.<
If you mean solar filters: their role is to attenuate the light intensity of
the sun so that the eye is not completely blindfolded. The satellite, which
is anyway dark, can only get darker.
>Rob Matson sent me a GIF file of the transit path of Cosmos 1763 R/B
>(#16864) across the sun, predicted for 15:55:54 UT here Friday morning.
>Dowloadable ? Uploadable ?<
This question is for Robert Sheaffer to answer. But in case you are interested:
I too can produce drawings which show the path of a satellite across the
sun and I can also do it for the moon with the most prominent features on
the moon (maria in particular) shown in the correct orientation (i.e.
parallactic angle and rotation axis of the moon). However, these are binary
files and sending them via the Internet is inconvenient (uuencoding/decoding
required). Sometimes I send them by fax, but the result is not satisfactory.
Despite all our high-tech gadgetry there seems to be no practical method
to send high-resolution images electronically.
Bruno Tilgner
100533.2016@compuserve.com
48.85N, 2.02E, 90m, UT+2