rotating liquid mercury telescope mirror cost tilt

A spinning mercury telescope mirror must always face upward

Spinning liquid mercury naturally forms the curved shape needed for a telescope mirror. Such mirrors can be far cheaper than large solid ones. But liquid cannot be tilted without spilling or changing shape, so the telescope must always look straight up.

Europeans once commonly slept in two shifts

In preindustrial Europe, many people slept for a few hours, woke for a period in the middle of the night, and then slept again. As artificial lighting made nighttime activity easier, this pattern faded. By the 1920s, the idea of a "first" and "second" sleep had largely disappeared.

Sandy Island stayed on maps after being declared nonexistent

Sandy Island, in the Coral Sea, was declared nonexistent after a search in the 1970s. Yet it remained in mapping datasets into the early 21st century, until its nonexistence was confirmed again in 2012.

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rotating liquid mercury telescope mirror cost tilt

A spinning mercury telescope mirror must always face upward

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