The Maya built a year into Mexico's El Castillo pyramid
El Castillo at Chichén Itzá in Mexico has 91 steps on each side plus a top platform, making 365, one for each day of the solar year.
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Gaia, parked at L2, still fired thrusters to stay put
At a Lagrange point the pulls of two bodies balance, so a probe stays near it on little fuel. Gaia's controllers still planned a correcting burn every month.
Pluto failed one of three tests, and lost its planet status
On August 24, 2006, astronomers in Prague voted on a definition of a planet. Pluto passed two of its three tests and became a dwarf planet.
Gaia's star map: the position and motion of two billion stars
Gaia measured the position, distance, motion, brightness, and colour of two billion stars, over more than a decade of observing.
Two astronomers spent seven years measuring out the metre
In 1792 two French astronomers set out to measure the meridian to fix the metre. It came out 0.2mm short, and Méchain hid readings that did not agree.
Why a star vanishes at once when the Moon passes in front
The Moon has a very thin atmosphere and a star is tiny, so an occultation ends its light at once. A star that vanishes in stages in a graze is a hidden pair.
Webb's bigger mirror catches light too faint for Hubble
Webb's 6.5-metre mirror and infrared cameras see light Hubble could not. It found galaxies near Milky Way size sooner than standard models allow.
The Antikythera mechanism's rebuilt front still needs proof
A 2021 model reconstructs the Antikythera mechanism's lost front gearing, but nobody has shown the ancient Greeks could have built it.
What the Large Magellanic Cloud gives astronomers
The Large Magellanic Cloud sits close enough for astronomers to resolve single stars, and far enough to double as a ruler for the universe.
A comet's tail points away from the sun, whichever way it flies
Sunlight and the solar wind, not a comet's own motion, decide which way its tail streams as the comet swings past the sun.