For analysis of the light pollution by the cometary method were used observations of comets made by the Polish amateur astronomers, included in the archive of the Comet Observers Section (SOK) of the Polish Amateur Astronomers Society (PTMA) for the years 1994-2004, as well as the observations from the period 2005-2009 obtained from the Comet Observations Center (COK). These observations, sent regularly to the Smithsonian Astrophysical Observatory (Cambridge, Massacusetts, USA), are stored in the database of the International Comet Quarterly (ICQ), as well as published in this publication [81].

In the years 1994-2009 in the above mentioned archives were recorded a total of 10,428 observations of comets, with the annual average of 652 observations. Richest in this respect was the year 1996 (1907 observations), the poorest 1994 and 2009 (92 and 155 observations).

Another variable was the number of the observed comets. It depended not so much on the number of comets actually in the sky during the year (this number is almost constant and equal to the average of 12, but of course there are exceptions), but above all on the prevailing weather conditions of a given year (comet observations require very good weather conditions ). This is evident in the considered period, when in 2008 as many as 33 comets were observed, while in 1995 and 1997 there were only 5 comets.

A very important factor influencing the opportunity to study the changes of light pollution in Poland was also a number of regular observers of comets. About 20 observers observe the comet in Poland continuously. Of course, when there is a bright comet in the sky, this number could rise to several (in 1996, there were as many as 74 observers), but for the purposes of our research the essential were those regular observers, watching the weak and difficult to observe comets.

Due to the nature of the studied phenomenon, for further analysis were selected only observations of comets weaker than 7 mag, and especially those where the coma brightness distribution was as flat as possible (the degree of condensation DC from 0 to 2). In order to avoid the effect of lightening the horizon, as well as associated with dawn or dusk, were selected comets observed high above the horizon, near the local midnight, at the moonless nights (except for the analysis of the sky surface brightness dependence on the phase of the Moon). In the years 1994-2009 a total 451 observarions meets the above conditions, and thus were selected for further analysis.

When analyzed the changes in the level of light pollution at a certain observation point, so when were used the observations made by the specific observer, in order to avoid the effect of random and subjective observations were analysed only observations of those comets for which the observer made at least 5 estimations at the same time, wherein the two extreme estimations were rejected.

To analyze the problem of the light pollution state in Poland and its changes in the period 1994-2009 were selected the comet observations according to the above principles. It should be noted that these observations were not dedicated to the discussed problem, so observers certainly could not suggest any established presuppositions. Criteria for the selection of comets relevant to the analysis allowed for the determination of the highest S_a value recorded in the place of observation, which means the darkest possible sky at this point.

For comparison purposes were used the base of comet observations made in the years 1984-2009 in Germany and in 1995-2004 in the UK, containing a total of over 20,000 observations.

The measurements were carried out in two ways:

1. measurements on the measurement positions (fixed measurements)
2. touring measurements

Fixed measurements were carried out on the ten positions. Within the city the position closest to the city-center was located on a Vistula boulevards at the foot of Wawel Hill (KCE - Cracow-center). Three positions were located on large housing estates: the Podwawelskie Estate almost at the city center (KPO), the Red Pradnik Estate in the northern part of Cracow (KPC) and the Nowy Bieżanów Estate at the eastern edge of Cracow, nesr the Wieliczka Node (KBI). Another position within the city were located in the allotments near Bronowice Nowe Estate in the north-western part of the city (KBR) and at the tram-base in the Krakow-Lagiewniki, heavily estate near the Cracow-Zakopane route (KLA). Two positions were located beyond the administrative borders of Cracow: in Wieliczka city east of Cracow (WIE) and in Mogilany village south of Cracow, near the Cracow-Zakopane route (MOG). The last two, more distant positions, were: in Łuczyce village, 14.6 km northeast of the city center (LUC) and the farthest, in Jerzmanowice village, 21.5 km north west of the center, along the Cracow-Olkusz route (JER).

Measurements were carried out, as far as possible, every day, in all weather conditions, around the time of local midnight . Measuring person records the measuring time, the temperature, estimated the cloudiness (scale: clear, single clouds, clouds torn apart, breaks in the clouds, clouds) and haze (scale: no haze, light mist, mist, fog) and also noted the other factors that may affect the measurement (eg, phase and position of the Moon, the state of snow cover, cloud type, etc.).

The measurement was performed with the SQM directed toward the zenith at the such measurement point, where there was no light tresspass from the neighbouring light sources and where the part of the sky, from which the light was collected (the angle up to 20 ° from the zenith), was not obscured by any objects (eg crowns of trees, walls, buildings, etc.).

Each result consisted of a series of five measurements, of which the average was recorded to the nearest 0.1 mag/arcsec². Each measurement was repeated in the same manner, obscuring the speculum of the SQM by the IDA-LPS filter.

At the two sites (KPO and MOG) more experienced observers estimated the percentage of cloudiness and the snow cover. At the position KŁA was also measured the humidity, while at the position MOG was esimated the haze, basing on the visibility range.

Outside these, fixed points, were done a few comparative measurement series at the selected points in Poland: at the Roztoki Górna in the Bieszczady Mountains, an area free from light pollution, and in a hamlet on the slopes of the Łysina Hill summit at the Beskid Makowski Mountains (south of Dobczyce water reservoir, at the edge of the light island of Cracow), and also in Lublin and the Karkonosze Mountains.

Within the instrumental measurements using the SQM, were done a series of touring measurements, including Cracow and its surroundings, in order to determine the range of the area in which artificial sky glow exceeds the natural sky glow. This area is called the "light island".

Series of touring measurements, going all the night, had to be carried out with homogeneous weather conditions. Therefore, were selected only the moonless nights when the sky was devoid of any cloudiness. The condition was also a lack of any noticeable haze. The measurements raised in directions: south-west, west, northwest, north, north-east and east, to the previously assumed distance of 50 km from the city center.

In Cracow were also performed the luxometer measurements of the ground lighting, in order to determine the relationship between this quantity and the brightness of sky. For this purpose, were selected the open areas, not wooded areas. One measurement was carried out in the middle of the Blonia Meadow in Cracow, close to the city center and the other in the open areas, on the outskirts of the city, between the settlement Rżąka and the Baranówka hill (where the fort 50 "Prokocim" is situated). The measurements were carried out in the complete cloud cover, with the full snow cover - this one on the Błonia Meadow was old and flooded, near the Rżąka Estate the snow cover was fresh.

During the measurements at the same time were recordec the SQM and the luxometer readings. Measurements were carried out from the collapse of civil twilight to the total darkness (the end of astronomical twilight).

Were also done the continuous measurements by the SQM-LE, computer-controlled meter. The measurements were performed at intervals of five minutes. One measurement was made for a cloudless sky from dusk to dawn, the second measurement was performed in conditions of the complete cloud cover, starting at dusk. This measurement, due to the rain stopped before dawn. The measurements were designed to test the stability of the measured values over the time .

Measurement at the fixed positions have been done from mid-November 2008 to early January 2010. The majority of the measurement positions exceeds the number of measurements 300, and only the two does not exceed the 200. In total, the research project carried out the 2890 measurements of this type.