Light Pollution: Chapter Two   

An article on Light Pollution experiments in and around the St Austell area, by Tara Hill

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Methodology

Equipment

Star Maps Site Map of St Austell Torch with a red light Pencil and Recording Chart Car Warm Clothing

Star Maps

For use in the field the maps provided on the Redshift 4 CD ROM were easy to read in the dark.

Initially the survey was to include constellations looking North, South, East and West. This was not possible, as the directions could not be matched with constellations on the website. Therefore, the altitudes of the constellations were chosen instead.

Various constellations were chosen and referenced with Redshift CD ROM to ensure their suitability for observing from the St Austell area (Longitude 4 47 W and Latitude 50 20 N). Two practice sessions were performed which resulted in the following constellations to be used: Gemini; Orion; Ursa Minor and Leo. They were chosen because their altitudes were close to 20 degrees, 40 degrees and 60 degrees at the time of the survey, these different altitudes gave a good even range over the sky.

Other constellations such as Pegasus were considered but on observing them, I realised the lowest reference star would be setting quite early in the observing session. Gemini stayed quite close to the 60 degree mark most of the session as did Orion at the 20 degree mark, however two constellations, Ursa Minor and Leo were used for 40 degrees as Ursa Minor moved quite a distance from the 40 degree mark and it was felt it would not give an accurate reading to use it after 2130 GMT. This movement of the stars including the Sun is known as sidereal rotation and is not the actual stars moving but the Earth rotating on it's axis every 24 hours.

	Stars seen in Triangle Limiting Magnitude 5 4.3 6 4.4 7 4.8 8 4.8 9 5.5
Table 1a	Table 1b

Explanation of Data Collection

Star charts were prepared before the observing session. To explain this method it is necessary to study the star charts showing the constellation Orion, (Tables 1a and 1b above) which is used as an example. Three stars are used as corner indicators for the triangle - Rigel (constellation Orion), Nihal (constellation Lepus) and Scetum (constellation Eridanus). The stars which are visible within the triangle (including the corner stars) are counted. The number of stars can then be used to determine the limiting magnitude of that area of sky, at that site. For example, if 6 stars were observed a limiting magnitude of 4.4 would be the result for that site. 

Torch

A torch with a red beam was used as this does not disturb the dark adaptation of the eyes as much as a white beamed torch would. Dark adaptation is necessary for this experiment and on the website 30 minutes was recommended. This would have been okay if only one observation was taking place but due to the nature of the survey and six sites it was felt that 15 minutes would be adequate. It was still quite difficult to find good sites due to car headlights and security lights coming on halfway through the dark adaptation period.

Systematic Sampling

A map of St Austell was produced, and a circle of a 4 mile radius was marked on the map with St Austell railway station being the central point. It was at first thought eight sites could be used for sampling, but after some calculations this would have taken 41/2 to 5 hours to complete. Instead six sites were used taken just 31/2 hours. The results from this showed extinction of star light was taking place, this was known at the time of observing (15.02.02) as skyglow was evident. Having proved that light pollution existed it was decided to try four more sites closer to the town centre, at a radius of two miles. Depending on the site and orientation it could possibly distinguish how much extinction was lost due to other sources other than St Austell town centre itself.

Map Two shows St Austell and surrounding area. On it are the observing sites marked A to J. A,B,C,D,E and F represent the sites that were observed from on the 15.02.02. These sites are at a distance of 4 miles ( where possible) from St Austell railway station. G,H,I and J represent the sites that were observed from on the 16.02.02 at a distance of 2 miles from St Austell railway station.

The sites do not form perfect circles as St Austell is a coastal town and in some areas it was impossible to observe the full distance from St Austell railway station due to the sea. Other sites show irregularities due to the view being obscured by lights, buildings or trees.

Data Collection and Recording

Table 2a

Results of observations taken on 15, February 2002. Humidity 93%

Table 2b

Results of observations taken on 16, February 2002. Humidity 93%

The results were recorded on a chart designed on Microsoft Works Office, spreadsheet program. Using the titles: date; humidity; time (GMT); constellation; number of stars and site; limiting magnitude was also on the sheet but not necessary for the observing session. 

Explanation of Data Entry

The following steps explain the tables 2a and 2b.

Step 1

Number of Stars were counted

Step 2

Limiting magnitude was calculated using star charts and information from tables 1a and 1b

Step 3

Altitude needs to be taken into account. The reason for this is the closer your view to the horizon, the thicker the atmosphere. More scattering of light occurs nearer the horizon as more water droplets are present. The altitude was calculated by using Redshift 4. This was done by taking the lowest star altitude from the highest, therefore giving the midway point of the survey area.

Step 4

Natural extinction of starlight does happen on a limited scale( Interview - Brian Sheen 10, Feb 2002). The following information was used to account for the natural extinction.

Altitude Degrees Dimming in Magnitudes 

Above 45 degrees the amount of natural extinction is negligible. (Patrick Moore, 1964).

Step 5

Corrected limiting magnitude was calculated by adding the natural extinction figure to the limiting magnitude.

Step 6

The average limiting magnitude is what a person with good eyesight would expect to see from a dark observing site.

Step 7

Magnitude lost was calculated by taking the corrected limiting magnitude from the average limiting magnitude.

Other Equipment

A pencil was used to record the results. Pens are not useful with this type of work as they usually stop working in very cold conditions.

A car was necessary for this survey, as 45 miles was covered on the first night alone.

Warm clothing for this survey especially in February is advisable, as the discomfort of the cold will affect the observers results.

Other Factors to be Taken into Account

Phase of the Moon

The phase of the Moon is an important consideration. When the Moon is full it produces a lot of light drowning out the fainter stars in it's glow. Therefore, the survey had to take place a week either side of New Moon when it was only a crescent. In February the New Moon was 12th February, on the 15th and 16th the Moon was a small crescent setting early in the survey.

Weather

The sky had to be clear of clouds for an accurate result. On both observing nights the humidity was 93% (Ref: www.weather.co.uk 15.02.02 and 16.02.02), which seemed quite high. The observing took place with clear skies, however sea mist was apparent on coastal sites.

Forward to Chapter Three