Re: Observability of solar satellite transits

Bruno Tilgner (
Fri, 9 May 1997 18:03:07 -0400

Jean Monseur wrote:

>In my opinion this seems only partially convincing and does not lead into the
>heart of the question.

Perhaps the following discussion leads somewhat more into it.

>Do solar filters only attenuate the light intensity, or do they work as
>wavelenghts selectors ?

They work as wavelength selectors in the sense that a good solar filter is
supposed to block UV and IR entirely, but to attenuate uniformly the light
intensity over the visible spectrum (390 .. 780 nm). They are not selective
in the sense of for instance a H-alpha filter or any other filter with a
narrow passband.

>Is there a relation between visibility and wavelenght ?

Yes, there is. During normal daylight seeing, when the CONES are the active
receptors, the eye is most sensitive at 555 nm (green). A dark-adapted eye,
in which the RODS are active, is most sensitive at a shorter wavelength
of 510 nm. However, the eye then cannot distinguish colours any more. As an
aside: At the center of the retina, where the eye lens produces the sharpest
image, there is a small (0.3 mm) area, the fovea centralis, which has no rods.
This is why averted seeing can show more detail than viewing a faint object

>When calculating the minimum surface of the would-be-seen satellite, will 
>you input the wavelenght in the formula ?

No, because the reflective properties of a satellite are normally not
known. Moreover, for the visibility of a satellite in front of the Sun
its brightness does not matter much. The contrast with the Sun will always
be extremely high.

>Can you estimate or calculate the diffraction of the light which could,
>to some extend, light up the dark side of the satellite ?

No, I cannot, although I am sure there is no unsurmountable difficulty.
But again, this if of no practical consequence for the visibility of a

>Is really Io transit of Jupiter comparable to Mir transit of the sun ?

If we assume for simplicity the orbits of Jupiter, the Earth and Io as
circular, then Jupiter is at a distance of 778.3E6 km from the Sun, and
the Earth at 149.6E6 km. The radius of Io's orbit is 0.422E6 km. At
opposition, the minimum distance between the Earth and Io is 628.3E6 km. 
With Io's diameter of 3642 km this gives an apparent size of 5.796E-6 rad
or 1.2 arcseconds. By comparison, the apparent size of Mir is between 5
and 7 arcseconds, depending on its distance.

But the main difference is this: an object (satellite or planet) passing in
front of the Sun always shows us its dark side, wheres a Jovian satellite
passing in front of Jupiter's disk is always illuminated. There are no phases
(like the Moon's or those of Venus and Mercury) for the outer planets and
their moons. The transit of Io (or any other moon) in front of Jupiter's
disk is, therefore, not observable, at least not with amateur equipment.

They can be observed with sophisticated space equipment. I have nice pictures
of Io crossing Jupiter which were taken by Voyager 1 in February 1979. I
suppose there are also pictures of that kind taken by the Hubble Space 

But we were talking anyway about transits of Mercury and Venus in front of
the Sun. Just to complete an earlier message: Venus transits occur about
every 120 years in pairs, separated by 8 years. The next events will be on
8 June 2004 and then on 6 June 2012. 

>Since the most important diffraction in space and in the atmosphere is that 
>of the blue and u.v. wavelenghts , would you be  surprised if you were
>told that the main role of solar filters is to stop blue and u.v. ?

Yes, I would be surprised and I would not look through such a filter.
Infrared is even more dangerous and must be filtered out. UV does not
penetrate very deep into the eye, except with people who had their eye
lens removed because of a cataract. IR is extremely dangerous. I know of a
case where someone looked into the beam of an IR laser without noticing it
until it was too late.

Bruno Tilgner
48.85N 2.02E UT+2