Category

The Effect of Light Pollution

Introduction

Find out how easily the stars are visible in your local area

　In so-called "night sky environmental research", a special investigation instrument is used to count the number of stars visible in a specified area looking north, south, east, west and overhead. The extent to which the night sky is affected by light from streetlights, outdoor lamps etc. is determined by the brightness of the faintest stars visible.
　In this teaching aid, you will watch the night sky using an investigation instrument made from a milk carton, and count the number of stars which are visible inside its field of view.
　The results will tell you the faintest stars visible from a given location, and this will enable you to think about how the visibility of the night sky is affected by light pollution.

Measuring Instrument Construction

Are there places where the stars aren't even visible at night?!

　Depending on the brightness of streetlights and outdoor lights, the night sky becomes brighter, making the stars more difficult to see. What is the night sky environment like in your local area? What location will be best for observing the stars?
　Using an easily-made instrument, determine the night sky environment.

Things to Prepare

　1-litre carton (milk carton or similar), 2x 100mm lengths of bookbinding tape, scissors, compass

Table below shows the relationship between Angle of visibility of the investigation instrument and tube length.

Method of Examination

Preparation

　Record paper; Writing materials; Investigation instrument; Star Atlas; Flashlight; Clock
　Before going out to observe, write headings on the record paper for the five observing directions (north, south, east, west and overhead), ready to note the number of stars visible in each, together with the observing site and the date and time.
PDF File
　Prepare a star atlas for the observing date and time. An atlas showing the constellation outlines, such as that produced by the Astroarts Corporation, is suitable for this purpose.
　Astroarts Corporation Homepage: http://www.astroarts.co.jp/
　When you have prepared a star chart for your observing data and time, label it with each of the five directions you will be observing.

The figure shows standard stars [Night sky on 2003.10.15／20:00]

●The stars labelled like this represent the centre of the observer's field of view for each observing direction: North●Polaris; East●Capella; South●Mars; West●Altair; Zenith●Deneb.

Observing

Thing below is done with several positions which we would like to compare.

1. After going to the observing site, look at the night sky for about 10 minutes to allow your eyes to become dark adapted to the darkness.
2. Having determined the direction of north from the observing site, the observer faces in the required observing direction.
3. From the star map, the standard star in a given observing direction is identified with ● this symbol.
4. When the investigation instrument made from a milk carton is centred on a given observing direction, the standard star will be obscured by the central crosshairs.



　　　Milk carton aligned with zenith observing direction.


5. How many stars will be visible within the milk carton? Note this number on the record paper, together with details of any cloud cover.
6. Repeat the 3~5 process for each of the observing directions

Determination of Result

Compilation in Conversion Table

　Because the number of stars theoretically visible in a given region varies across the sky, it is important to study the night sky conditions in a region where there is a very large number of stars. Next, we need to translate the number of stars visible in a given field into the faintest stars visible, which is done using a conversion table.
　For example, with the map below we would count the number of stars visible within 40° of Deneb (labelled ●) and then use the conversion table to find the faintest star visible on a given evening.

　So, if you observe 4 stars in the zenith region, is the limiting magnitude 2? Think about this.
　Prepare a chart like this for each observing direction.
　With these charts, knowing the number of stars visible in a given region of the sky, it is possible to determine brightness of the faintest star visible ("limiting magnitude").

Discussion

　What was the condition of the night sky in your home town?
Did it vary with observing direction?
How did this compare with your expectations?
　Using these results, we can now try to construct a night sky light pollution map.

Sample Light Pollution Chart

Compilation of Light Pollution Map

　This collates your observations, and plots them on a map.
　Data from each of the observing sites, i.e. "limiting magnitudes" of 1, 2, 3 or 4, are represented by different symbols, which are plotted on the map.
　When the data is plotted in this way, it is easy to see how the night sky conditions vary with location.
　Try to compare the results with your prior expectations. Do they agree?
　If the results are different from expectations, think about why this might be.

Example of Light Pollution Chart

Dark Sky Meter Project / ITO, Yoshiharu

Dark Sky Meter Project / ITO, Yoshiharu