Expert: Tom Whiting - 3/8/2005

Question
Why does the sky seem more populated with stars one night, and the next there seem to be far fewer? Thank you for your time, I would greatly appreciate your feedback.

Answer
Hi Ashley.....
Easy question, but a very good one.
Well Ashley,  first realize that there is the same number of
stars in our night sky pretty much every night over our
short lifetimes.  In fact, there is the same number in our
daytime sky too, but we can't see them because of the sunlight blocking them from our view.

And that same reasoning applies at night.....to see ALL  the
stars available to our human eyes, we need a perfectly
clear sky and TOTAL darkness....both.

So if there is the slightest haze, perhaps up at a high altitude,
or there is the slightest moon, even a crescent moon,
or the slightest light pollution from your neighbor's
security light being on, or the slightest fog in the air, or
perhaps a northern lights display going on right over your
northern horizon which you can't see, but it's giving you
what we call "air-glow", then it's not totally dark outside, and
not all the fainter stars are visible on that night.

In astronomy, contrast is everything....we need and want
that totally clear, black sky at night.  If it's any less than
that (for the reasons given above) then it's not a perfect
black sky, and not as many stars will be seen.
The worst light polluter is the glow of city lights around you....to see all the stars, you have to get out into the
country and away from all the light-glow of the cities around
you.  That's why we travel for hundreds of miles to get into
totally black dark skies for astronomy, and get away from
city glow...we call that light pollution.  It is the very worst
thing for preventing us from seeing all the stars.

Also, your eyes count too.  Your eyes have to be fully dark
adapted to see all the stars, and this takes up to 30 minutes
in total darkness to do this.  You just can't run outside
from a brightly lit room and expect to see a lot of stars.
Dark adaptation is a 2-step operation...the first 5-10 minutes
your pupils are getting bigger and this is 90% of the dark
adaptation process.  Then for the next 20 minutes, a light
sensitive chemical called rhodopsin  (row-DOPT-sin) is
coating the retina of the eye.  So it takes 30 minutes to get
fully dark-adapted.....and one white light into your eye, like a white flashlight or headlight of a car, and you have to start all over.  That's why us astronomers use red flashlights to read our star-charts at night...red does not affect dark-adaptation very much.

Also, if you've been outside in bright sunlight all afternoon,
playing, or in the garden, that bright sunlight has washed out
and destroyed all the rhodopsin in your eye, so you have
to wait 12-24 hours to get that chemical back.  On evenings
when I know that I am going out observing with the
telescope, I don't get into bright sunlight for very long during
that daytime. Because I know that it will affect my excellent
night vision.  Also, other factors hurt your night vision....
any drug, smoking, drinking alcohol, etc. will detract from
your night vision, so I avoid those too on nights of
observing.

So it's not the stars that determine how many we see;
it's the sky conditions, the light pollution, (how dark the
background sky is) and our own eyes that determine how many stars we see on a particular night.

Hope all this helps,
Clear Skies,
Tom Whiting
Erie, PA