Venus (and therefore Iridium) visibility in Daylight

Willie Koorts (
Mon, 24 Nov 1997 09:38:03 +0200 (GMT+0200)

Hi Folks

Before about a week of rain set in (in the Cape which is not supposed to be a
summer rainfall area) I experimented a bit with spotting Venus naked eye in

I "cheated" by first finding it in an 80mm refractor with plastic protractor
"setting circles" and my old ASTRO program.  By looking along the scope I
next found it naked eye.  I would then stand away (and look away) and then
try and find it again on just the blue sky background.  This was quite
difficult.  I then used a tree-top and put it near Venus as a guide and found
that is was very easy to spot (and re-spot) it like this. 

I then used me sun, wife and friends as guinea-pigs and got them to first try
and spot it on the blue sky by pointing it out.  The success-rate was not
very high with only few of the adults and more of the kids spotting it.  When
I asked the ones who got it to look away and find it again, it took them all
some time to find it again.  When put next to a treetop everybody saw it
almost immediately and could not understand why is was so hard just now. 
Re-finding it after looking away was just as quick. 

My conclusion is thus that it is cannot be focus as someone suggested before
that is the problem because the treetop is noticably out of focus once your
eyes lock onto Venus, but rather just giving your eyes something to keep
still by giving them something to latch onto,  therby getting the retina 
to "integrate" for long enough to form an image the brain can recognise. 

I hope these "findings" can be applied to spotting Iridiums in daytime.


         Willie Koorts

       Cape Town,  Observatory   33d 56' 03"S   18d 28' 36"E   GMT + 2h
       Wellington, South Africa  33d 38' 56"S   19d 00' 52"E   GMT + 2h