why is there a border between light and darko... ~~

There is a sharp-looking “border” between light and dark on a moon or planet because only the half facing the Sun gets illuminated, and the other half stays in shadow, creating a moving line called the terminator.

What that border actually is

• The object (Moon, Earth, etc.) is (roughly) spherical, so sunlight can only hit one side at a time.

• The side facing the Sun is lit; the side facing away is dark, since space itself doesn’t provide light and the Moon doesn’t shine on its own.

• The terminator is the dividing line between the sunlit hemisphere and the dark hemisphere, which is what you see as a “border.”

A simple way to picture it: shine a flashlight on a ball in a dark room—half the ball is bright, half is dark, and the curve between them is that same kind of terminator line.

Why the border moves and changes

• As the Moon or Earth rotates and orbits, the angle of sunlight changes, so the terminator slowly slides across the surface.

• On Earth, this moving line is literally where it’s sunrise on one side of the line and sunset on the other at the same time.

• This motion is what gives us lunar phases (on the Moon) and day–night cycles (on Earth), as different regions move in and out of the lit half.

Along that line, craters and mountains on the Moon often look extra sharp and dramatic because the low-angle sunlight casts long shadows right at the border.

Extra angle: symbolic “border between light and dark”

People also like to use “the line between light and darkness” as a metaphor—for example, in religious writing or fantasy and sci‑fi stories where light and dark represent good and evil, or two opposing cosmic forces. In that sense, the “border” can mean a moral or philosophical threshold, where choices or perspectives decide which side you stand on.

TL;DR: The border between light and dark is real and physical—a terminator caused by sunlight only lighting one half of a spherical body at a time—and it also inspires symbolic “light vs. dark” ideas in stories and philosophy.

Information gathered from public forums or data available on the internet and portrayed here.