Seeing Stars

Activity 1F: Sky Glow


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.


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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