The famed science fiction author Isaac Asimov called them Ribbon Worlds.

Planets forced to always show one face to their parent star.

The star side would be locked in perpetual day, its sun never dipping below the

horizon. Indeed, its sun never moving at all, fixed in place as if time itself stood still.

The far side would be trapped in perpetual night, a sky blazing with the light of a thousand stars, never

knowing the warmth of its own parent. And in between the two extremes, a special place.

A Terminator line, the boundary between night and day, a region of infinite twilight, or, depending on your perspective,

perpetual dusk, caught between the two poles of heat and cold, this ribbon that stretches

like a girdle around a planet might, might, just be a home for life, neither too hot in the

never-ceasing glare of the star, or too cold in its

infinite night. At the time that Asimov coined that term in the 1950s, astronomers still

believed that Mercury, the innermost and smallest planet of the solar system, was in such a state.

In speculation abounded about whether it could possibly support a strange form of life

in the narrow ribbon of its own Terminator.

But soon observations revealed that Mercury does indeed rotate just extremely slowly.

There are worlds in our solar system that are indeed locked this way, but they are

all satellites. Our own moon is like this, always presenting the same face to observers on

the surface of the Earth. Indeed, despite our intimacy with our nearest of cosmic neighbors,

it wasn't until the 1950s that we humans got our first

hazy glance at the far side of the moon. This frozen face is given a technical term, because

of course it is, and it's known as tidal locking. The source of tidal locking is, well, we actually have it a little easy here

because this is a rare case of the technical jargon term actually describing the physics.

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