The starry sky has been a source of inspiration throughout human history. Astronomy has been a common element in all cultures and civilizations, being used to establish calendars, navigate and discover new lands, and drive numerous scientific and technical breakthroughs. This Review discusses how it is becoming increasingly difficult for professional and amateur astronomers to observe the night sky because of light pollution. Artificial light at night, radio interference, and the deployment of satellite constellations are all rapidly increasing and are having adverse impacts on astronomical observations, limiting scientific discoveries, cultural connections to the night sky, and opportunities presented by astrotourism. Potential mitigation strategies to preserve the night sky are discussed.

The main goal of this work is to analyse new approaches to the study of the properties of astronomical sites. In particular, satellite data measuring

The Observatorio del Roque de los Muchachos (ORM), in the Canary Islands (Spain), was one of the candidates to host the future European Extremely Large Telescope (E-ELT) and is the site of the Gran Telescopio Canarias (GTC), the largest optical infrared facility to date. Sky transparency is a key parameter as it defines the quality of the photometry to be acquired in the astronomical observations. We present a study of the atmosphere extinction at the ORM, carried out after analysis of a database spanning more than 20 yr, to our knowledge, the longest and most complete and homogeneous in situ database available for any observatory. It is based on photometric measurements in the V band and r' band (transformed to the V -band extinction coefficient kV) using the Carlsberg Meridian Telescope (CMT). Clear seasonal variations that repeat yearly are observed. The median value of kV is 0.13 mag/airmass; the mean value has a maximum in the summer months (June- September), corresponding to the season with maximum frequency of nights affected by dust or cirrus (~29% in summer, but only ~13% during the rest of the year). Two volcanic eruptions took place during the database baseline, which has enabled the study of the impact of volcanoes on the global atmosphere extinction. For the 5 yr of available information, we have estimated the average monthly weather downtime from the CMT data log, obtaining a result (20.7%) in reasonable agreement with earlier studies. The main conclusion of our study is that there is no significant evidence from the CMT data for any secular changes in kV over the 20 yr database baseline.

The main goal of this work is the analysis of new approaches to the study of the properties of astronomical sites. In particular, satellite data measuring aerosols have recently been proposed as a useful technique for site characterization and searching for new sites to host future very large telescopes. Nevertheless, these data need to be critically considered and interpreted in accordance with the spatial resolution and spectroscopic channels used. In this paper we have explored and retrieved measurements from satellites with high spatial and temporal resolutions and concentrated on channels of astronomical interest. The selected datasets are OMI on board the NASA Aura satellite and MODIS on board the NASA Terra and Aqua satellites. A comparison of remote sensing and in situ techniques is discussed. As a result, we find that aerosol data provided by satellites up to now are not reliable enough for aerosol site characterization, and in situ data are required.