Too many lights, the stars are almost invisible (also because of the LEDs) – Corriere.it

And then we went out NOT to see the stars again. Maybe today Dante he would write this verse at the end of canto XXXIV of the Inferno. The sky appears less and less populated by stars, especially for those who live in a city, due to the lights and luminosity (skyglow) which is released into the atmosphere and that prevents you from observing the stars. So much so that a child born today in a place where it is still possible to see 250 bright dots in the sky, when he turns 18 and looks up to the firmament in the year 2040, will see only a hundred.

citizen science

the result of one of the best examples of citizen science ever made, one study that lasted eleven years between 2011 and 2022 in which 51,351 people distributed in 19,262 locations worldwide responded through the Globe at Night program on the visibility of the stars with the naked eye. The result was analyzed by an international group headed by Christopher Kyba and published on Science
.

Lights on the ground, few stars in the sky

It has been known for some years that the sky is less and less suitable for observing the stars and all astronomers can testify to it, even the amateur ones, but the study shows that the situation worse than that which appears from satellite analysis. Until now, in fact, it was believed that the brightness of the surface due to artificial lights increased by about 2% per year. Instead this new analysis, carried out on the ground and not from above with satellites, shows that the increase is 9.6% per year.

More lights

The increase, according to analysts, is not due only to the increase in artificial lights but to at least two other factors. Instruments aboard satellites are most sensitive to lights pointing directly up, but they are there horizontal lights are mainly responsible for the diffused luminosity (skyglow). Horizontal lights have increased with the increase in the number and luminous power of advertisements, shop windows, signs and the light of houses that filters through the windows.

The LED factor

Another important fact was the transition from sodium vapor incandescent lamps that emit a yellow-orange light, to LEDs that emit a bluish-white light. Led lamps allow great energy savings and last longer, but they emit light at a wavelength between 400 and 500 nanometres. This wavelength, however, is the one that is most widely scattered by the atmosphere and one of the reasons why skyglow. The human eye is most sensitive to these wavelengths at night, Kyba says, and could be one reason for the discrepancy between what satellites measure and what ground-based observers perceive.

Problems also for astronomical observatories

On December 20, an international group led by Fabio Falchi of the Institute of Science and Technology of Light Pollution of Thiene (Vicenza) published a study on Monthly Notices of the Royal Astronomical Society
on light pollution which is beginning to represent a problem also for the 28 largest astronomical observatories. Of these, only seven have pristine natural zenithal visibility. What then is the best point to observe the sky? It is located in Namibia in a desert lodge, says Falchi, who also makes seven telescopes available to lovers of observations and photographs of the night sky.