Page 24 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

SEEING AND

ATMOSPHERIC

TRANSPARENCY

HOW THE ATMOSPHERE CAN AFFECT YOUR ABILITY TO
OBSERVE STARS AND PLANETS TO A SURPRISING EXTENT

The weather is generally considered to be the biggest images of stars. This is thanks to the layers of moving air
hindrance to astronomy. What’s the betting that the night between you and the object you’re looking at, the effects of
you decide to use your new telescope is the night that spell which are magnifi ed by your telescope. On the other hand,
of fi ne weather changes for the worse? So you’d have thought deep-sky objects like galaxies and nebulae aren’t as badly
that when the skies fi nally clear, your problems would be over. affected by bad seeing.
Surprisingly, though, even a clear night may not be the best In the atmosphere, air at different temperatures is always
time to go out and observe. moving around and mixing together. Light travels through
The issue is the ‘seeing’. In astronomy, this doesn’t mean hot and cold air at different speeds, so it is continually bent
how you look at something. It’s a term that describes how this way and that before it fi nally arrives at your scope all
much the view you see through your telescope is disturbed by shaken and stirred. Sometimes there are very few moments
what’s going on in the atmosphere above you. of clarity. One of the best ways to see this distortion is to
At times of good seeing, you’ll get sharp, steady views watch the Sun setting on a clear horizon. It will have a jagged
through your telescope. But bad seeing produces turbulent, appearance, thanks to the sunlight moving through layers of
unstable telescope views of the Moon and shuddering, shaky turbulent air.

GEORGE D. LEPP/SCIENCE PHOTO LIBRARY, PETE LAWRENCE X 2 The setting Sun appears to

have a jagged edge because
you see its light through a
turbulent atmosphere

24 skyatnightmagazine.com 2012