What’s the Slowest-Spinning Planet?

Venus, Saturn, and Molasses

Apr 6, 2018
Solar System

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Space is famous for zippy activity, like streaking meteors that sail through the night sky. But space has another face—a slow-and-easy side. In reality, Venus, the Moon, and many of our most beloved sky objects are as slow as molasses. What’s the slowest-moving planet? Slowest-spinning star? Read on for some fun space facts.

What is the Slowest Planet

Venus, which is floating higher each evening in twilight, low in the west, is the slowest-spinning body in the known universe. If you walked along a bike path that circles its equator, you’d only need to go four miles an hour to keep night from ever falling on Venus.

While Earth takes 24 hours to complete one spine, our day on Venus would last 5,832 hours. Why so slow?  One reason is the friction caused by Venus’ thick atmosphere and hurricane-speed winds. With a toxic blanket of carbon dioxide, the surface pressure on Venus is 90 times what we experience on Earth at sea level.

The slowest naked-eye planet is Saturn, which is nicely up just before dawn. Its very name is used to epitomize sluggishness, when we say that something or someone is saturnine.

The Moon’s Slow Spin

The Moon spins slowly too, at just 10 miles an hour. Because it’s so slow, the Moon does not seem to be spinning but appears to observers from Earth to be keeping almost perfectly still.

(However, the Moon has “synchronous rotation,” meaning it’s rotation period is the same as its period of revolution, so we always see the same side of the Moon from Earth.)

What is the Slowest Spinning Star

In all the sky, nothing is slower than the North star, Polaris. It appears glued in place, and if you see it hovering above some neighbor’s chimney, it will always reliably be in that spot even 50 years from now. Shakespeare put a positive spin on it when he had Julius Caesar boast that, “I am constant as the Northern Star.

Slow as Molasses

Here on Earth, one substance epitomizes lethargy above all others. It’s molasses. Slow as molasses.

Okay, so how slow is molasses? And what is it, anyway?

Molasses actually has three forms, which result from sugar refining. You crush the sugar cane and boil the juice, dry the sucrose, and the leftover liquid is first-grade molasses. If you then reboil it to extract more sugar you get second molasses  and I hope you’re taking notes on all this. A third syrup boiling creates blackstrap molasses, a low calorie product containing lots of minerals. But we don’t really care about all this. What matters is how slow it is.

Viscosity is the thickness of a liquid or gas in terms of how resistant it is to flowing. If water is rated at around “1” on the scientific viscosity scale, then blood officially rates a 3 ½. So blood reallyis thicker than water! Sulfuric acid is 24. Did you know that that scary acid is so syrupy?

Enough fooling around. The all-time slowest flowing fluids are:

  • Olive Oil: with a viscosity of 81
  • Honey, at  2,000
  • Molasses at 5,000
  • Ketchup at 50,000
  • Peanut Butter, which is 250,000

So, forget molasses. “Slow as peanut butter” would best epitomize lethargy throughout the cosmos. Whether you can find a jar of Skippy in any exo-planet’s supermarket is a matter best left to future researchers.

Read more about Venus, the Planet of Paradox.

About This Blog

Welcome to “This Week’s Amazing Sky,” the Almanac’s blog on stargazing and astronomy. Bob Berman, longtime and famous astronomer for The Old Farmer’s Almanac, will help bring alive the wonders of our universe. From the beautiful stars and planets to magical auroras and eclipses, he covers everything under the Sun (and Moon)! Bob, the world’s mostly widely read astronomer, also has a new weekly podcast, Astounding Universe

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Viscosity of molasses in January?

My grandma used to say "slow as molasses in January".

the art of learning

I will never look at peanut butter the same way again--loved Bob Berman

Viscosity of Molasses

Very interesting perspectives on movement in space and on earth. Makes me wonder: is there a viscosity number for a glacier?

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