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Re: Sirius



Some excited students called me after midnight wanting to know what the
bright object in the sky was that was flashing different colours. I went out
to have a look, and concluded that the object is Sirius, but there certainly
is something odd about it: it's twinkling very strongly and with the
twinkling come very distinct coloured flashes. All sorts of colours, just
for an instant. The effect is stronger through binoculars. The other
brighter stars are higher up from the horizon, in Orion, and they look
normal. I've tried looking for the same effect in other stars round the same
elevation as Sirius (assuming it is atmospheric) but there's nothing bright
that is not beind a cloud or lost in the sodium haze of Trieste. Is this
effect known to anyone? Ice crystals in the sky or something?

This sounds like ordinary twinkling to me. Another star which can be
relied upon to put on a good display here is Capella. At our latitude
(49 degrees north) Capella speds a lot more time in appropriate
altitudes for twinkling than Sirius does. Farther south, say from Kitt
Peak AZ (32 degrees north latitude) Canopus is also a good twinkler.

All that is necessary for twinkling is a bright star so color sensors
(cones) in the retina can respond. These sensors are less sensitive
than the color insensitive rods. The other requirement is that the
atmosphere must be unsteady. Twinkling suggests that astronomical
seeing will not be good.

Leigh