How Long is a Day on Venus? We Finally Know the Exact Answer

Venus, aka. Earth’s “Sister Planet,” has always been shrouded in mystery for astronomers. Despite being planet Earth’s closest neighbor, scientists remained ignorant of what Venus’ surface even looked like for well into the 20th century, thanks to its incredibly dense and opaque atmosphere. Even in the age of robotic space exploration, its surface has been all but inaccessible to probes and landers.

And so the mysteries of Venus have endured, not the least of which has to do with some of its most basic characteristics – like its internal mass distribution and variations in the length of a day. Thanks to observations conducted by a team led from UCLA, who repeatedly bounced radar off the planet’s surface for the past 15 years, scientists now know the precise length of a day on Venus, the tilt of its axis, and the size of its core.

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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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