Page 25 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

NEED TO KNOW

IMPROVE YOUR SEEING

You can’t do anything about air above it for longer.
‘high-level seeing’ – the air ❯3 Air currents tend to stay
currents far above you – but low to the ground, so it can
you can infl uence the ‘low- be a good idea to raise up THE ANTONIADI
level seeing’ to create steadier your scope on a platform. SCALE
air conditions immediately ❯4 If you build an
around you and your scope. observatory, make it using It’s very useful to note
Here’s how: thin materials like wood that down what the seeing is
❯1 Leave your scope outside can cool quickly. when you’re observing.
to cool to the ambient ❯5 The geography of your Many astronomers use
temperature, eliminating any observing site affects how air the Antoniadi Scale as
air currents in the tube. behaves. Being near the sea a measure of what the
❯2 Observe on grass rather gives you calmer air than if atmosphere is up to.
than concrete. Concrete you’re near a range of hills, It’s a fi ve-point scale
absorbs more heat from the where air is forced upwards, Placing your setup on grass can reduce air using Roman numerals.
Sun and radiates it out to the causing turbulence. turbulence around your scope I indicates the best
conditions, while V
describes the worst.
The other factor that affects observing A good way to think of it is to imagine a I Perfect seeing, without
conditions is the transparency of the night – just swimming pool with a penny coin on the bottom. any quiver of turbulence
how clear the sky is. After it’s been raining, the The water represents our atmosphere and the whatsoever.
II Slight shimmers;
sky is completely transparent because the rain coin the starry object you’re looking at. Through
moments of stillness last
clears away particles of dust and smog from the air. completely still water with no currents, the coin
several seconds.
However, when it’s been raining it also tends to be looks still, crisp and clear. In this case the seeing is III Average seeing;
windy, which means that the seeing is bad. You’ll perfect and so is the transparency. If the water is larger air tremors blur
notice that the stars are twinkling because of this. made to move – causing ripples – the coin’s image the view.
Transparent conditions are however good for large, will shake around; the transparency is still good IV Poor views, with
constant troublesome
faint objects like nebulae and galaxies, which really but the seeing is bad. And if some milk is spilt in
undulations of the image.
benefi t from the better contrast. Poor transparency the pool so you can’t see the coin very clearly, the
V Bad views with severe
generally means the air is steady with good seeing, transparency will be reduced.
undulations; so unstable
but dust and particles are sitting in the atmosphere It goes to show that you’re at the mercy of the that even quick sketches
because the air is still. These conditions are good atmosphere… and that moments of clarity are a are out of the question.
for looking at the Moon and stars. wonderful thing.

14 FEBRUARY, 9PM HOW
5.0
FAINT
4.2 7.1
6.5 2.0
6.7 6.3 CAN YOU
5.6
6.4
5.9 SEE?
5.2
4.7
4.4 Atmospheric conditions have an
impact on the faintness of the stars
you can observe. Use the chart here
to check the faintest stars you can see
by looking at Ursa Minor on a very
4.8 clear night to work out your limiting
magnitude. This is the faintest star
4.2 magnitude, or brightness, that you
6.4
4.3 can see from your location – higher
numbers mean fainter stars.
4.3 5.2
2.1
5.6
Work out your limiting magnitude by
5.5 fi nding the dimmest stars you can see
5.0
6.4 in Ursa Minor, as described below.
5.0 = magnitude 3.1 Under a perfect sky you should be
N able to spot mag +6.5 stars
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