Solar Transit Observations

Bruno Tilgner (
Sun, 4 May 1997 12:17:53 -0400

Recently, several possible solar transit observations have been reported here.

Five unsuccessful attempts for such observations, undertaken in the last few
days by a couple of friends and myself lead me to the following conclusions:

1. A satellite or rocket body in the 6m to 10m size class at typical distances
between 1000 and 2000 km has an angular size in the order of 1 arcsecond,
give or take 25%.

2. A rule of the thumb for the resolving power of a telescope objective lens
is "13.8 arcseconds/objective diameter in cm". To resolve objects 1 arcsec
apart, the minimum size of the objective lens is thus around 5 inches.

This assumes perfectly collimated optics and does not account for the effects
of bad seeing which in the case of solar transits is inevitable due to air

3. Even if the object can be resolved by the telecsope optics, the human eye
possibly cannot. Another common rule of the thumb says the eye's resolving
power in arcseconds is "720/eye pupil's diameter in mm". The eye pupils'
diameter is usually assumed to be 6 mm (depending on age), and this leads to
the figure of 120 arcseconds often found in the literature.

However, 6 mm corresponds to a dark-adapted eye. Under bright illumination,
which can certainly be assumed for the observation of the sun (through a filter
or by projection), the eye's pupil is closer to 2 mm in diameter. The eye can 
then only resolve objects of at least 360 arcseconds size.

4. The typical satellite/rocket body at a typical distance, observed through
a telescope of suitable opening, must therefore be magnified at least 360 times.

At this magnification it is exceedingly difficult to aim the telescope at a
certain point on the solar disk, assuming it can be calculated at all which
region of it will be crossed by the object. Magnifications which still show
the entire solar disk in the field of view (i.e. in the order of 50 to 80
times) are clearly not sufficient no matter the size of the telescope aperture.

5. When staring for a minute or more at a bright surface like the sun or
the moon in a telescope, strange objects tend to appear, all of which are
inside the eye. Medical doctors call them "mouches volantes". Most of them
cannot possibly be confused with satellites because of their thread-like
appearance, but sometimes a more round or square object crosses the field of

6. In view of all the arguments above I have reluctantly conluded that the
observation of solar transits of anything below the size of Mir is not
feasible with amateur equipment. 

As regards the moon, the basic constraints are the same, except that the
seeing will be much better and the eye's pupil probably somewhat larger.

Can anybody prove me wrong?

Bruno Tilgner