Light Pollution: Chapter Two
An article on Light Pollution experiments in and around the St
Austell area, by Tara Hill
Back to Chapter One
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%
| TIme |
Site |
Con |
No
of Stars |
Lim
Mag |
Altitude |
Natural
Extinction |
Corrected
Lim Mag |
Ave
Lim Mag |
Mag
Lost |
|
|
|
|
|
|
|
|
|
|
| 1950 |
Nanpean (A) |
Orion |
7 |
4.50 |
26 |
0.30 |
4.80 |
6.00 |
1.20 |
| 1950 |
Nanpean (A) |
U.Minor |
3 |
3.90 |
35 |
0.20 |
4.10 |
6.00 |
1.90 |
| 1950 |
Nanpean (A) |
Gemini |
6 |
5.00 |
55 |
0.00 |
5.00 |
6.00 |
1.00 |
| 2030 |
Bugle (B) |
Orion |
5 |
4.30 |
24 |
0.35 |
4.65 |
6.00 |
1.35 |
| 2030 |
Bugle (B) |
U Minor |
3 |
3.90 |
37 |
0.15 |
4.05 |
6.00 |
1.95 |
| 2030 |
Bugle (B) |
Gemini |
7 |
5.10 |
61 |
0.00 |
5.10 |
6.00 |
0.90 |
| 2100 |
St Blazey (C) |
Orion |
6 |
4.40 |
23 |
0.35 |
4.75 |
6.00 |
1.25 |
| 2100 |
St Blazey (C) |
U
Minor |
4 |
5.20 |
40 |
0.15 |
5.35 |
6.00 |
0.65 |
| 2100 |
St Blazey (C) |
Gemini |
7 |
5.10 |
65 |
0.00 |
5.10 |
6.00 |
0.90 |
| 2130 |
Porthpean (D) |
Orion |
8 |
4.80 |
22 |
0.40 |
5.20 |
6.00 |
0.80 |
| 2130 |
Porthpean (D) |
U Minor |
3 |
3.90 |
42 |
0.10 |
4.00 |
6.00 |
2.00 |
| 2130 |
Porthpean (D) |
Gemini |
7 |
5.10 |
67 |
0.00 |
5.10 |
6.00 |
0.90 |
| 2200 |
Pentewan (E) |
Orion |
8 |
4.80 |
19 |
0.50 |
5.30 |
6.00 |
0.70 |
| 2200 |
Pentewan (E) |
Leo |
7 |
4.40 |
34 |
0.20 |
4.60 |
6.00 |
1.40 |
| 2200 |
Pentewan (E) |
Gemini |
8 |
5.30 |
69 |
0.00 |
5.30 |
6.00 |
0.70 |
| 2230 |
Hewaswater (F) |
Orion |
7 |
4.50 |
16 |
0.70 |
5.20 |
6.00 |
0.80 |
| 2230 |
Hewaswater (F) |
Leo |
8 |
5.00 |
39 |
0.10 |
5.10 |
6.00 |
0.90 |
| 2230 |
Hewaswater (F) |
Gemini |
8 |
5.30 |
69 |
0.00 |
5.30 |
6.00 |
0.70 |
Table 2b
Results of observations taken on 16, February 2002. Humidity
93%
| Time |
Site |
Con |
No
of Stars |
Lim
Mag |
Alt |
Natural
Extinction |
Corrected
Lim mag |
Avg
Lim Mag |
Mag |
|
|
|
|
|
|
|
|
|
|
| 2015 |
Charlestown (G) |
Orion |
6 |
4.40 |
26 |
0.30 |
4.70 |
6.00 |
1.30 |
| 2015 |
Charlestown (G) |
U Minor |
3 |
3.90 |
37 |
0.15 |
4.05 |
6.00 |
1.95 |
| 2015 |
Charlestown (G) |
Gemini |
6 |
5.00 |
60 |
0.00 |
5.00 |
6.00 |
1.00 |
| 2035 |
London Ap (H) |
Orion |
7 |
4.50 |
26 |
0.30 |
4.80 |
6.00 |
1.20 |
| 2035 |
London Ap (H) |
U Minor |
3 |
3.90 |
38 |
0.15 |
4.05 |
6.00 |
1.95 |
| 2035 |
London Ap (H) |
Gemini |
6 |
5.00 |
62 |
0.00 |
5.00 |
6.00 |
1.00 |
| 2055 |
Trewoon (I) |
Orion |
6 |
4.40 |
25 |
0.30 |
4.70 |
6.00 |
1.30 |
| 2055 |
Trewoon (I) |
U Minor |
3 |
3.90 |
40 |
0.15 |
4.05 |
6.00 |
1.95 |
| 2055 |
Trewoon (I) |
Gemini |
7 |
5.10 |
65 |
0.00 |
5.10 |
6.00 |
0.90 |
| 2130 |
Trethurgy (J) |
Orion |
4 |
4.30 |
23 |
0.35 |
4.65 |
6.00 |
1.35 |
| 2130 |
Trethurgy (J) |
U Minor |
3 |
3.90 |
42 |
0.10 |
4.00 |
6.00 |
2.00 |
| 2130 |
Trethurgy (J) |
Gemini |
7 |
5.10 |
67 |
0.00 |
5.10 |
6.00 |
0.90 |
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
Altitude
Degrees
|
Dimming in
Magnitudes
|
1
|
2
|
2
|
2.5
|
4
|
2
|
10
|
1
|
13
|
0.8
|
15
|
0.7
|
17
|
0.6
|
21
|
0.4
|
26
|
0.3
|
32
|
0.2
|
43
|
0.1
|
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
