Problems caused by the atmosphere to astronomers
Astronomers are hindered by the atmosphere in the following ways: Light pollution reduces the appearance of night sky objects – especially
towards the horizon.
Power Blackout Shows the Night Sky Once Again
A blackout across the eastern side of Canada in August 2003 created perfect
viewing conditions for Astronomy. The two pictures below show the town of
Goodwood, Ontario on successive nights – showing just how bad skyglow is
from our towns:-
Picture credit : Todd Carlson, SkyNews magazine and the International Dark-Sky Association
Power Cut
August 2003 The Milky Way can be seen clearly
14th
Normal Lighting
15th August 2003
Skyglow ‘washes out’ the stars
 As light from the stars passes through the atmosphere, there is a reduction
in brightness of the starlight. The more atmosphere that the starlight
travels through, the greater the reduction in brightness of the starlight (as
seen nearer the horizon).
 Movements within the atmosphere affect the quality of photographs and
observations.
More moist air at lower altitude creates the effect of
‘bubbling’ in the atmosphere, creating the twinkling, or scintillation, of
stars that can look very attractive at night – but not to astronomers!
 Chemical pollution makes the atmosphere more opaque to light.
 The atmosphere blocks out some electromagnetic radiation, making
observations at certain wavelengths impossible eg UV and X-rays.
 The weather! As well as clouds, a windy night can cause problems.
 The apparent position of stars is altered by the refraction of light.
Refraction of light in the atmosphere can cause a slight change in the actual
position of an object seen in the sky. As light ‘bends’ when you look into
water and the depth of water seems less than it really is, so too does light
‘bend’ as it passes through air – more refraction occurs in damp air and
also increases as more air is looked through eg looking towards the horizon
compared to looking up to the zenith (the point directly overhead).
TRUE POSITION OF
VENUS
(EXACT LOCATION)
APPARENT HIGHER
POSITION OF MERCURY
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TRUE POSITION OF
MERCURY
(LOWER IN SKY)
In the photograph of Venus and Mercury, Venus is directly up from the
position shown and no refraction takes place – Venus is seen exactly where
it is expected to be. Mercury is at a low position in the sky, nearer to the
horizon. As a result, light rays from the planet pass through more of the
atmosphere and are refracted downwards to the observer’s position (shown
by the arrowed line in the direction towards the observer). Following the
dotted line up, the observer sees Mercury apparently higher in the sky than
it really is.