URSA MINOR, THE "LITTLE DIPPER"
There are two things to note when using this chart, one: you should use
averted vision to determine the faintest star visible, and two: you should
add a tenth of a magnitude to the figure so determined to actually figure
out how dark the sky is at the zenith (i.e. if 5.6 is viewable at Polaris¡¯
altitude, 5.7 is visible at the zenith).
Obviously the darker the sky is, the fainter the objects that can be
seen. A Dark Site is considered one where stars of magnitude 6.2 or fainter
can be seen.
Even worse for the urban observer, every magnitude lost due to the brightness
of the night sky loses more than a magnitude of light grasp in the telescope
because of contrast reduction. Since each magnitude is 2.512 times as
bright as the next one, this means that you have to increase the light
gathering area of your telescope by more than that (actually closer to
2.9 times!) to compensate for one magnitude of light pollution in your
area. It¡¯s multiplicative, too¡ªtwo magnitudes of light pollution would
require a telescope almost 8.5X as large in area to see the same as a
smaller telescope at a dark site.
The good news is that the opposite is true¡ªgaining a magnitude by having
the sky get darker (as when traveling away from a city toward a darker
place) has the effect of increasing the size of your scope¡¯s area by nearly
2.9X. That¡¯s a VERY good reason to travel to the dark sites frequented
by your local astronomy clubs, and a VERY good reason to take your telescope
with you when going on vacation to the National Parks. Sometimes just
traveling a few miles away makes a big difference in what can be seen.
It¡¯s good that amateur telescopes are all quite transportable.
Our night skies weren¡¯t always this bright. In the US, the population
has approximately doubled since 1950, yet the use of outdoor lighting
has increased more than 10,000 times! It was once possible to see the
night sky in the middle of our cities but today we can only make out the
Moon, the planets, and a hundred or so of the very brightest stars. No
doubt this growth was due to the lower prices of energy in the past. But
energy isn¡¯t cheap anymore. Additionally, this surplus lighting is injurious
to our safety and even health.
Today, there are organizations like the International Dark-Sky Association
(http://www.darksky.org/ida/index.html) that are working with cities and
states to revise our lighting to provide the same light on the ground
(for our safety) without sending light upward where it is not needed.
The Win-Win situation that will come about by the use of intelligent lighting
is that we will light our cities with less than 50% as much power, provide
the same (or better) light on the ground, and at the same time darken
our skies once again so that we will go back to seeing the stars in the
sky at night.
Many states and cities have already gotten in line with the idea of intelligent
lighting--you can read all about it at the link provided.