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III. OBSERVING THE SKY
1) Sky Conditions
Sky conditions are usually defined by two atmospheric characteristics, seeing, or the steadiness of
the air, and transparency, light scattering due to the amount of water vapour and particulate
material in the air. When you observe the Moon and the planets, and they appear as though
water is running over them, you probably have bad "seeing" because you are observing through
turbulent air. In conditions of good "seeing", the stars appear steady, without twinkling, when you
look at them with unassisted eyes (without a telescope). Ideal "transparency" is when the sky is
inky black and the air is unpolluted.
2) Selecting an observing site
Travel to the best site that is reasonably accessible. It should be away from city lights, and upwind
from any source of air pollution. Always choose as high an elevation as possible; this will get you
above some of the lights and pollution and will ensure that you aren't in any ground fog.
Sometimes low fog banks help to block light pollution if you get above them. Try to have a dark,
unobstructed view of the horizon, especially the southern horizon if you are in the Northern
Hemisphere and vice versa. However, remember that the darkest sky is usually at the
"Zenith", directly above your head. It is the shortest path through the atmosphere. Do not try to
observe any object when the light path passes near any protrusion on the ground. Even extremely
light winds can cause major air turbulence as they flow over the top of a building or wall. If you
try to observe on any structure, or even a sidewalk, movements you make may cause the
telescope to vibrate. Pavement and concrete can also radiate stored heat which will affect
observing.
Observing through a window is not recommended because the window glass will distort images
considerably. And an open window can be even worse, because warmer indoor air will
escape out the window, causing turbulence which also affects images. Astronomy is an outdoor
activity.
3) Choosing the best time to observe
The best conditions will have still air, and obviously, a clear view of the sky. It is not necessary that
the sky be cloud-free. Often broken cloud conditions provide excellent seeing. Do not view
immediately after sunset. After the sun goes down, the Earth is still cooling, causing air
turbulence. As the night goes on, not only will seeing improve, but air pollution and ground lights
will often diminish. Some of the best observing time is often in the early morning hours. Objects are
best observed as they cross the meridian, which is an imaginary line that runs through the Zenith,
due North-South. This is the point at which objects reach their highest points in the sky. Observing
at this time reduces bad atmospheric effects. When observing near the horizon, you look through
lots of atmosphere, complete with turbulence, dust particles and increased light pollution.
4) Adapting your eyes
Do not expose your eyes to anything except red light for 30 minutes prior to observing. This
allows your pupils to expand to their maximum diameter and build up the levels of optical
pigments, which are rapidly lost if exposed to bright light. It is important to observe with both
eyes open. This avoids fatigue at the eyepiece. If you find this too distracting, cover the non-
used eye with your hand or an eye patch. Use averted vision on faint objects: The center of
your eye is the least sensitive to low light levels. When viewing a faint object, don't look directly at
it. Instead, look slightly to the side, and the object will appear brighter.
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