The first dark sky map of Chiang Rai: three decades of change affecting Astro-tourism

Abstract

We present the first Chiang Rai dark sky map, measurements of the calibrated zenith artificial night sky brightness at sea level, which covers data since 1992-2021. The Data were collected from two US satellites including the Defense Meteorological Satellite Program with the Operational Line Scan System (DMSP-OLS) and the Suomi National Polar-orbiting Partnership with the Visible Infrared Imaging Radiometer Suite (NPP-VIIRS). Images containing ratios of artificial light to natural light were originally excluded from sunlight, solar glare, moon light, lunar illuminance, lightning features from the aurora, clouds, fires and background noise. Based on the correlation between data from the overlapped year 2013, the result showed the great matching relationship between DMSP-OLS and NPP-VIIRS data (R2 = 0.92) which revealed the mathematical model was feasible and reliable. The number of dark sky areas over Chiang Rai in the past 3 decades was classified in 8 classes based on the sky brightness in Bortle scale colour code and in magnitude: 1. Excellent dark sky sites (>21.9 mag, black), 2. Dark sky (21.9-21.5 mag, grey), 3. Rural sky (21.5-21.3 mag, blue), 4. Suburban/Rural transition sky (21.3-20.1 mag, green), 5. Suburban sky (20.1-19.1 mag, yellow), 6. Bright suburban sky (19.1-19.5 mag, orange), 7. City/suburban transition sky (19.5-18.0 mag, red)and 8. City sky (<18.0 mag, white). It was found that 14% of area in Chiang Rai are covering by the bright sky with Bortle class 7 and 8. Within this only 14% of area dwell more than 33% of population that have now already lost naked eye visibility of the Milky Way. In addition, a risk assessment model for dark sky conservation revealed that dark sky area are dramatically becoming extinction. It was suggested that Chiang Rai will not have an area suitable for stargazing by 2040 if there is no designated area for conservation. Our map could ultimately encourage land owners of such dark sky sites to register their properties in the IAU dark sky reserve database to elevate Astro-tourism in Chiang Rai.