Why Is It Dark at Night?

This is one of the first questions children ask when embarking upon the difficult path to knowledge. So, why exactly is it dark at night? Because the Sun isn’t in the sky, of course!

A Simple but Incorrect Answer

This question has a “grown-up” name — the photometric paradox. More than one generation of scientists has wracked their brains trying to find its answer. We’ll start by explaining in simple terms: the stars are extremely far away, so their light weakens as it travels towards us. Illumination (the amount of luminous flux per unit area), referred to in astronomy as apparent stellar magnitude, decreases inversely to the distance squared. Therefore, if a star like our Sun, a yellow dwarf of spectral class G2, is pushed back by 1000 astronomical units, it will illuminate the Earth a million times less intensely. By cosmic standards, this distance is nothing (less than 140 light-hours), however, the decrease in brightness is quite noticeable.

Inverse Square Law

Red lines indicate the radiation flux from the source. The total number of lines remains unchanged with increasing distance.

Note the density of the lines (quantity per unit area): the closer to the source, the higher it is. In other words, the density of flow lines is inversely proportional to the square of the distance from the source. This is due to the fact that the surface area of the sphere increases in proportion to the square of the radius.

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An astronaut floating in space above the earth, surrounded by particles and positrons.

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