Sattelite solar transits and Mercury

Bruno Tilgner (
Thu, 8 May 1997 12:43:58 -0400

Rob Matson wrote:

>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.

This is an interesting argument. During the last 10 years, two such transits
of Mercury took place: on 13 Nov 1986 and on 6 Nov 1993 (the next one will be
on 15 Nov 1999).

For a description of the 1986 transit see SKY & TELESCOPE Nov 86, p. 498
and in particular the pictures in S&T Feb 87, p. 223.

The 1993 transit is described in S&T Nov 93, p. 69 with pictures in S&T
Feb 94, p. 112.

In both cases Mercury's disk was 10 arcseconds across. I strongly suggest
to have a look at these pictures and then to make your own judgement what
it takes to spot a satellite which is typically an order of magnitude
smaller but moves within 1 to 3 seconds across the Sun.

Two of the photos of the 1986 transit were taken very shortly after sunrise
with the Sun not much above the horizon. The Sun's edge appears very jagged
and the small spot of the planet nearly blurred out. This teaches a lesson
about satellite solar transits observing when the seeing is bad.

Bruno Tilgner
48.85N 2.02E UT+2