When you go outside on a clear, moonless night ... When they look at sky brightness—what they call the sky background—they use a unit of brightness per area of the sky: how much light is ...

black-and-white images of the planet’s barren, speckled surface at sunrise. The spacecraft, BepiColombo, gave scientists their first clear view of Mercury’s south pole. It also captured ...

WE constantly find allusions in ancient classical authors, to lightning and thunder occurring in a clear sky. The former is often explained as referring to the phenomenon commonly known as ...