Light Pollution and Effect on Observing the Night Sky

The following questions highlight the effect of light pollution while observing the night sky and correspond with the set of six tactiles in the book “Light Pollution: Urban vs Rural Skies”. The authors’ desire is for you to do this as a family. Take it as a challenge to see how many you get right! On the answer sheet, answers are highlighted in yellow.

An answer sheet is available here for download.

The cover image for the Light Pollution book


Most have heard of pollution in our environment. Pollution is often categorized or grouped based on what it affects the most. For example, water pollution and air pollution are two common types that affect water and air. What many don’t realize is that there are other forms of pollution. Pollution is essentially anything that interferes with, alters or destroys something in the environment from its “natural” state of being. Noise, soil contamination, litter, thermal, radioactive are other forms of pollution.

Light pollution is another major form of pollution, and is attributed to any type of light introduced to the environment that is night natural. Long ago, the night sky could easily be seen from nearly any location. With the advent of artificial light sources, the natural night sky as been altered. Cities, street lights, businesses, bill boards and more contribute to the light pollution that prevents humans from enjoying the beautiful night sky.

Now that we have a basic understanding of light pollution, let’s discover the effect light pollution has on the natural night sky. Observing the night sky can be a fun and entertaining way of reconnecting with the universe around us. If you have ever been out on a clear night and marveled at the grandeur of the stars, planets, moon or other solar system objects, you have participated in one of the oldest forms of philosophy, science and rigorous observation known to humans. But how does light pollution affect the night sky? Observers long ago, kept vigilant records of the movement of the heavens and were able to suggest grand explanations of our local region and the universe in general. Some of these explanations were right and some were wrong, but one thing that did not deter them in their observations was the ability to access a dark sky. Today, lights of civilization shroud the night sky. Many historic observatories have been recommissioned as points of interest at worst and for observing only the brightest of objects at best.

In this activity, you will get a feel for how light pollution affects the stars in 2 significant constellations, one from the summer sky and the other observed during the winter. A constellation is simply a group of stars in the sky that form a shape or figure of a person or thing. All people groups throughout history have had their own set of “constellations” to help them navigate through the night sky and even “navigate” locations and times here on Earth!

As the Earth orbits around the Sun (a term called revolution), the Sun appears to move in the sky through various constellations along its path (called the ecliptic). Earth’s revolution provides its inhabitants with a view of different constellations during different times of the year. A prominent wintertime constellation is Orion (oh-RYE-un). According to Greek mythology, Orion was a mighty hunter and is easily spotted by finding the 3 stars in a row that make up his belt. Tactile 1 shows Orion in a dark sky at the time it is found nearly due south (about 11:30p.m. towards the end of December; about 7:30p.m. early March).

To get a better feel for Orion, answer the following questions:

1. Count all the stars that make up Orion. How many are there? Note: not ALL the stars on this tactile are labeled.

2. Are all the stars the same size?

3. How many different sizes can you find? (hint: there are less than 8)

On this tactile representation we have used different sizes to represent the magnitude of each star, NOT its relative size. Magnitude is another way of signifying the brightness of a star as observed here on Earth. The larger the “bump” the brighter the star is. Although the stars in Orion vary widely in brightness, we have purposely reduced them into 4 levels to simplify the diagram. Two of the stars in Orion are particularly bright (or large). One of the stars is called Betelgeuse (Beetle-juice) and forms the right shoulder of Orion! Kitty-corner, through the belt of Orion, you will find another bright (or large) star called Rigel (RY-gel). Rigel forms the left leg, or left knee of Orion. Notice that 5 of the stars in Orion are labeled with corresponding magnitudes.

4. What magnitude is associated with the star Rigel?

5. What magnitude is associated with the star Mintaka (min-TA-ka)?

6. Which star, Rigel or Mintaka, has the “bigger” dot, and hence is the brightest?

7. What does that tell you about the magnitude scale? Hint, complete the sentence… The brighter a star is the __________ (lower/higher) the magnitude number is. In math, this is called an inverse relationship!

8. What do you think will happen to the number of stars you can see, as we “increase” light pollution?

9. Which stars will “disappear” as we increase light pollution (the larger/hence brighter or the smaller/hence fainter stars)?

Now explore Tactile 2: Orion in Moderate Light Pollution. At this time, it will help if you take Tactiles 1 – 3 apart and lay them side-by-side.

10. How many different levels of brightness are now able to be seen? (hint: there are less than 5)

11. Compare Rigel and Betelgeuse on Tactile 2 to the same stars on Tactile 1. How have they changed? What does that imply?

It is important to note that the MAGNITUDE of the stars is NOT changing, but rather our ability to see them as bright is diminishing.

Now explore Tactile 3: Orion in Heavily Light Polluted Area.

12. How many different levels of brightness are now able to be seen?

13. Compare Rigel and Betelgeuse on Tactile 3 to the same stars on Tactile 1 and Tactile 2. How have they changed? What does that imply?

14. Did the actual Magnitude of Rigel or Betelgeuse change?

15. Based on Tactile 3, can you easily “see” the Hunter Orion now? What shape or image do you think it looks like now? What would you name this constellation as it appears here?

Orion the Hunter is found in the winter sky after sunset from December through May. From June through November, a constellation called Cygnus (CIG-nus) can be found in the night sky. According to Greek mythology, Cygnus is a swan. The brightest star in Cygnus is Deneb and has a magnitude of 1.3. Deneb is a bright star that happens to be very far away. In fact, Deneb is about 1400 light years away. Tactile 4 reveals Cygnus the swan during the summer months in a dark sky when it is found nearly due south (about 3:00a.m. towards the end of June; about 7:30p.m towards the end of October).

Explore Cygnus the swan on Tactile 4: Cygnus in a Dark Sky.

16. How many stars make up the constellation of Cygnus?

17. How many different levels of “brightness” are there in Cygnus?

18. What are the 3 stars that are labeled and their corresponding magnitudes?

19. Which of those 3 stars is the brightest (hence largest “dot”)?

Tactile 5: Cygnus in Moderate Light Pollution reveals the constellation of Cygnus after increasing the amount of light pollution level to the same amount it was increased in Tactile 2 above. At this time it is helpful to lay Tactile 4 and Tactile 5 side-by-side.

20. How many of the stars of Cygnus are now NOT visible with the unaided eye? (hint: you may want to recount the stars in Cygnus)

21. What is the general shape of the “swan” now? Is there another shape that can describe Cygnus at this time? (hint: eliminate the 2 upper-right stars and the 1 lower-left star… it makes a familiar religious shape)

Cygnus the Swan is also known as the Northern Cross. The Northern Cross is what is called an asterism. An asterism is a modern-day representation of an ancient constellation. Another common asterism is the “Big Dipper” found in the constellation Ursa Major.

Now suppose the level of light pollution is increased to the same amount that it was for Tactile 3 above (Orion in Heavily Light Polluted Area). Tactile 6: Cygnus in Heavily Light Polluted Area reveals what the swan would look like with that level of light pollution. Let’s look again at the Northern Cross.

22. How many stars are now visible?

23. Can you see the “swan” now?

24. What effect does light pollution have on the ability of humans to observe the night sky?

25. Based on this activity, where would you suggest you and your family go to see a dark sky?