How Long is a Day on Venus? Astronomers Make Their Best Measurement Yet

There’s a problem with Venus. We don’t know how fast it rotates. For a space-faring civilization like ours, that’s a problem.

Measuring the length of day, or rotation rate, of most bodies is pretty straightforward. Mark a prominent surface feature and time how long it takes to rotate 360 degrees. But Venus is blanketed in thick clouds. Those clouds give it its reflectivity, and make it bright and noticeable in the sky, but they make it hard to measure Venus’ day length.

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