Measuring Light Pollution - Globe at Night
Doing Science with Stars – Measuring Light Pollution
The Globe at Night project
By Dr Noorali Jiwaji
Email: ntjiwaji at yahoo dot com
For the next two weeks you have an opportunity to learn how science is done as well as contribute to science knowledge while enjoying the night stars. Between 3rd and 16th March, hundreds of thousands of star watchers all over the world will be turning their heads upwards as soon as the night falls. The observations are reported to a central data centre where all the data will be collected and compiled.
They will all be seeking out the famous constellation of Orion (the Hunter) to find out how easily it is visible. Everyone, including those who are not familiar with the skies can try their hand at this star watching activity. Orion is directly overhead in the early evening skies at the moment, close to the dense belt of stars that marks our Milky Way galaxy. This is the fifth year that these world wide measurements are being made under the Globe at Night project. However, very few results are being reported from Africa.
The aim of this world wide activity is to determine what parts of the world’s skies are affected by light pollution and by how much. With more than half of the world’s population now living in cities, billions of people are not able to see the sparkling dark skies due to light pollution of the atmosphere. City lights that throw their light upwards brighten the skies and cause light pollution. In fact more than 60 percent of the people living in developed countries cannot see the stars from their doorstep. You can contribute to our understanding of the state of this problem and to determine if various actions being taken are having any effect.
We in Tanzania are blessed with dark skies in most parts of the country. So we are in a position to attract astro-tourists in search for the wonders of the night skies which they miss where they live in densely lighted developed parts of the world.
With sufficient data from all over Tanzania, we can show scientifically how much of our country is available for this tourist potential as well as to mount telescopes for ground-based astronomy research on the stars in our skies. We will collect all the data collected by observers in Tanzania so that we an determine the extent and amount of light pollution in our country. We will describe how these measurements are to be reported.
The Orion constellation is the most easy to recognize in the night skies and lies directly overhead in the night skies at the moment. A constellation is a collection of stars forming a certain shape (by joining the stars with imaginary lines like in connecting dots). The Orion constellation forms the shape of a hunter if all the stars are seen. However, it is easier to notice the large rectangle or trapezium that is formed by the brightest stars at the four corners of the constellation. One of the four corners has a bright reddish star called Betelgeuse. It is a red giant star a thousand times bigger than our Sun. The opposite corner is a bright bluish star called Rigel. The remaining two corners stars are relatively faint.
Even more prominent in this Orion constellation are three stars that are close together and forming a straight line called the “Belt of Orion”. The line is slightly slanted within the rectangle that forms the outer edges of the constellation. To the left (or right or top or bottom, depending on how you are facing when viewing this constellation) lies a patchy cloudy area which is called “Orion’s Sword”. Just outside the large rectangle on its short side is a faint collection of stars which forms the “Head of Orion”
I have given this detailed description of the constellation so that you can match what you see in the sky with the picture which shows Orion’s stars as seen under various amounts of light pollution.
From the city you may just be able to make out the four stars making up the large rectangle and the eye-catching three stars close together in a line slanted in the middle of the rectangle. For this you will need to look for a place where your eyes are blinded by any bright lights. If you cannot avoid the bright lights, then shade your eyes from this brightness and wait a few minutes to allow your eyes to adjust to the darkness.
From very dark areas in villages, forests and national parks it can be a bit confusing to locate constellations since thousands of stars are visible. But if you watch around 8 pm look at the stars that are directly overhead and locate the large rectangle and the three close stars in its middle that are in a line. From this dark location, you should be able to see far more cloudy patches in the region since Orion is very close to the Milky Way.
By comparing what you observe with the chart that is shown, you can write down the degree of light pollution by giving its magnitude, for example it will be magnitude 1 (highest) in the centre of cities, while it will be the lowest (that is, magnitude 7) when the skies and surroundings are completely dark. Away from the cities the skies can still be polluted by light from the distant city and it is most noticeable in the direction that the city lies. For example when coming into Dar es Salaam from Chalinze, he eastern skies are quite bright and most of the stars in that region cannot be seen. In contrast, even if you are in Dar es Salaam, the skies facing the seas are quite dark and you can enjoy many more stars since there is no light pollution coming from the Indian Ocean.
Since this is the time of the rainy season to commence and in fact we are already experiencing some heavy rains, you should be able to get at least one opportunity in the two weeks over which this activity will be conducted upto 16th March. Also, if possible try to make comparisons at several locations within your reach, at least a few kilometers apart.
The measurements that you do can be reported directly to the central data centre if you have access to the Internet. The main website is http://www.globeatnight.org/ and the reporting cab be done by entering your results in the forms fount at this page http://edcommunity.esri.com/gan/2010/report.cfm. You can compare the analysis of the results from various sites world wide at this webpage http://www.globeatnight.org/analyze.html.
If you are not familiar with the Internet or you are not able to find some of the information that is required, such as your latitude and longitude, you can send me your results and I can enter it in the database. You will need to send me the following information: 1. Note the name of the town or city and area where it was observed, 2. Your latitude and longitude, if you can. 3. Look for a place away from direct lights and as dark as you can get around your area. 4. Find Orion by going outside an hour after sunset (at about 7-10pm local time it will be well up in the overhead sky). 5. Match your nighttime sky to one of the magnitude charts shown below. 6. Note the date and time of your measurement. 7. Note the sky condition (hazy, smoke, dust, fog, rain). 8. Note the cloud condition (clear or almost clear, quarter cloudy, half cloudy, or more than half cloudy, fully cloudy). 9. Note any other related matters. 10. Send the information to me on ntjiwaji at yahoo dot com, or send it by text message or by calling me on 0713517009.
Your contribution to science is much appreciated and hopefully this activity will raise your interest in supporting the development of science in Tanzania as well as protecting our dark skies and hence promote astro-tourism in our country.
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