A child is born where 250 stars are visible tonight. How many will they see on their 18th birthday?

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A child's 250 visible stars fall to about 100 by age 18, because the sky in front of them gets brighter. Artificial light scatters off air and dust until the whole sky glows faintly, a haze astronomers call skyglow. A faint star is simply lost in it, the same reason none are visible in daylight. Skyglow grew 9.6% brighter a year between 2011 and 2022.

Cities have been swapping orange sodium lamps for white LEDs, which emit far more blue light, and blue scatters through the atmosphere much more readily than orange. Researchers think that color shift is the main reason the sky keeps brightening even where a city uses less light than before.

A child born where 250 stars are visible tonight will see only 100 of them on their 18th birthday.

Source: Kyba et al., Science · checked Jan 2023