The Return of Venus

by Paul Downing

SOUNDS
like the title of a romantic novel, doesn’t it? However, this is definitely
astronomy. Has anyone noticed the bright star that has suddenly appeared in the
evening sky after sunset? If you look
just after sunset as the sky is darkening you will see it, and you may be
surprised to learn this is not a star at all but the planet Venus.

About
the same size as Earth, Venus is our nearest neighbour in space apart from the
moon, and at times, when our orbits around the sun bring us close to each other
it can be "only" 41 million kilometres away. Right now, though, it is
almost as far away from us as it can get – approximately 250 million kilometres.
This is because it has just become visible in our night sky as its orbit brings
it around the far side of the sun from us.
As the weeks go by you will see this "new star" rise higher
and higher in the sky after sunset. It will also become even brighter, mostly
because it is getting closer to us as it chases us in our own orbit around the
sun.

By
late May it will be as high in our night sky as it ever gets because it reaches
a position where it is at its maximum apparent distance from the sun. After
this point it will appear to start moving back towards the sun, sinking quite
quickly lower and lower until, by late July, it will once again be lost in the
sun’s glare, to return as a morning star rising before the sun.

It
is interesting to note as Venus drops towards the horizon during July it will
appear significantly larger and a good pair of binoculars, held steady on a
wall or tripod can show it as a tiny crescent. PLEASE do not attempt this until
the sun has properly set for fear of accidentally looking at the sun through
the binoculars. You can easily be blinded for life in a second.

Venus
is so bright because its surface is completely covered by thick, white clouds
which reflect back into space a large proportion of the sunlight falling on
them. We say that its albedo is very high. The clouds
consist of corrosive sulphuric acid vapour and other nasty things and surface
temperatures are high enough to melt lead due to a runaway greenhouse effect. So
why does Venus show phases, like the moon? To understand this I have attached a
diagram for you which demonstrates how the phases are
produced. You can conduct an interesting experiment in your own kitchen to see
how this works, using a torch and two round objects (representing the Earth and
Venus). Venus and Mercury are the only two planets to show phases because they
orbit inside our own path around the sun. All the other planets orbit further
out than we do and so do not show significant phases.

This article was printed in January 2007

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