Seeing Stars
Activity 1F: Sky Glow
Objective
To perform experiments to determine the visual qualities of the night
sky and to predict the best local observing locations for astronomy.
Ask students where they would put an observatory in their part of the
county, state, or nation. Discuss where the darkest skies might be. Ask
students to describe the darkest, starriest night they can ever remember
seeing. (It may even be the scene in Steven Spielberg's Close Encounters
of the Third Kind where special effects create a view of the Milky Way
that matches the power the night sky once had for people living before
the Industrial Age!) Ask students to comment on why some areas are better
for star gazing than others.
Explain to students that light pollution has washed out the night sky
for most people. Also tell students that they are going to calculate the
best place to go star gazing in their area and to learn a way to determine
objectively just how clear their night sky is.
Materials:
- copy of activity 1F, page 25
- pencil
- ruler
- road map for their state or region
Procedure: Distribute the activity sheets and discuss the first
paragraph as a class. Assist students, if necessary, in making the light
pollution graph. Spread out a map of your area. Note the location of cities
and towns. Make circles around each city at a radius determined by the
population. Shade in the circles. Then look for a place that is not in
one of the circles. That's a prime location for star gazing.
The Pleiades star cluster can be found close to the constellation Taurus
the Bull. If necessary, students can use the star chart on page 27 to
find the Pleiades and Taurus. The Pleiades form a tiny dipper that is
so distinctive that students may well be able to find it, even without
a star chart. It rises in the east around sunset and is overhead by midnight
in the Fall. Students will count how many stars they can see in the Pleiades
cluster. The number seen is a good indicator of the darkness of the night
sky.
Interdisciplinary Connection
Light pollution isn' just a problem for astronomers: it wastes energy
and therefore money. Ask students to think about ways to save this money
and also give people a better view of the night sky. Several communities
around the nation have tried to do just this: have students research how
it's worked out: Hint--think about San Diego, California, which is down
the road from Mount Palomar, one of Earth's premier observatories. Student
suggestions might include less bright lights (yellow sodium vapor instead
of the blue-white mercury vapor), lamps with housings that aim the light
down, or lights that turn on with a motion detector, so they are only
used when needed.
If students determine there's a significant problem with light pollution
in their region, have them research and write a letter to the Mayor or
Governor suggesting what steps might be taken.
Before any letters are sent, however, stage a debate between those who
think it's important to see stars better, and those who feel that powerful
urban illumination, evening baseball, longer working hours and greater
security are more important. There are good arguments on both sides, and
the discussion might well be more important than any specific "winner"
or "loser" in the debate. Think of similar debates that might
form part of wrap-up and reflection activities after the KAO flights.
Remember to check out the "Star Census" area on-line.
KAO Connection
The KAO flies with its telescope looking out into space, well above
the light pollution of cities. If all goes well, we'll try and have the
KAO astronomers or Telescope Operator report on the number of stars they
can see in the Pleiades cluster from the airplane.
Sky Glow
The quality of the night sky depends on your altitude, the weather,
and the amount of light in the sky. During a full moon, for instance,
it's much harder to see faint objects. Cities create a sky glow that's
even worse than the full moon. The effect of urban light pollution depends
upon your distance from the city, the size of the city, and your city's
lighting practices. Some cities are more wasteful than others, but on
average a city produces between 500 and 1,000 lumens of light per person.
Porch lights, car lights, shopping centers, sports arenas and office buildings
all contribute to the lighting. Sky glow begins disrupting astronomical
observations when it's 10% more intense than the natural sky brightness,
half way up in the sky in the city's direction.
Dark Sky Challenge
This activity tests your vision and the quality of your night sky. For
the best results you need to be far from city lights, on a clear moonless
night with good "seeing".
Locate the Pleiades star cluster near Taurus the Bull. According to
legend, there were 7 Pleiads. See if you agree. With your naked eye, look
at the cluster and draw what you see.
Compare your drawing with this star chart. Match the 6 bright stars
that form a dipper shape. Now with this chart as a guide, see if you can
find some of the fainter stars.
The numbers beside each star indicate how bright the star is. (This
is called visual magnititude.) Lower numbers indicate brighter stars.
To see a faint Pleiad, the sky must be clear and dark. To see a Pleiad
that is very close to another Pleiad, the "seeing" must be good.
You can use this test to measure the quality of your viewing on any night
when the Pleiades are high in the sky.
Population of City Distance to sky glow
3,160 people 10 km (6.2 miles)
31,250 people 25 km (16.5 miles)
177,000 people 50 km (31 miles)
1,000,000 people 100 km (62 miles)
5,600,000 people 200 km (124 miles)
a. Plot these points on a graph with Distance in KM's on the vertical
axis and Millions in Population on the horizontal axis and connect them.
b. Look up the population of cities close to you.
c. Using your graph, find the distance associated with each population.
d. On a map draw a circle around each city at the distance where sky
glow should end.
e. On your map identify the best location to go star gazing in your
area with your parents, class group or Boy or Girl Scouts.
f. Use the star counting activity to test your sky glow prediction.
g. Make sky counts in the direction of the city from different distances
away.
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