You may have heard it has been said that if our planet were shrunk down to the size of a billiard ball, it would be smoother than it. In other words, the Earth is smoother than a billiard ball. Is that true?
Back in 2008, on the “Bad Astronomy” blog on discovermagazine.com, in the article titled “Ten things you don’t know about the Earth“, Phil Plait wrote about that, and he said “…according to the World Pool-Billiard Association, a pool ball is 2.25 inches in diameter and has a tolerance of +/- 0.005 inches.” and after making some calculations, he concluded that “… the urban legend is correct. If you shrank the Earth down to the size of a billiard ball, it would be smoother.”
Even the famous American astrophysicist, author and science communicator Neil deGrasse Tyson once tweeted about that, saying “If shrunk to a few inches across, Earth would feel as smooth as a billiard-hall cue ball.”.
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Smoothness vs roundness
In fact, the Earth is much smoother than one might think. It definitely would NOT look like this without water, for example. Yes, there are big mountains like the Himalayas and big trench under the oceans like the Mariana Trench. The highest point on Earth is the top of Mount Everest, at 8.85 km. The deepest point on Earth is the Mariana Trench, at about 11 km deep. But even those are very small compared to the Earth’s diameter which is about 12,735 kilometers (on average).
According to the World Pool-Billiard Association, “All balls must be composed of cast phenolic resin plastic and measure 2 1/4 (±.005) inches [5.715 cm (± .127 mm)] in diameter”.
So, if we could shrink the Earth to the size of a billiard ball, the height of Mount Everest would be only 0.04 millimeters. The depth of the Mariana Trench would be only 0.045 millimeters. These measurements are inside the tolerance of 0.127 mm or 0.005 inches, no pits or bumps more than that, so the Earth is smoother than a billiard ball, right?
First of all, the specifications of
Let’s we assume that we produced a billiard ball and covered its surface with medium sandpaper (grit particle size of 0.005 in, for more about grit sizes of a sandpaper see the Grit size table on the Wikipedia entry of sandpaper). By the definition of smoothness used by Phil Plait of Discover Magazine and Neil deGrasse Tyson, that billiard ball would also be “smooth” – which is obviously ridiculous.
The billiard-ball sized Earth’s smoothness would be equivalent to that of 320 grit sandpaper. Still not quite smooth, right?
So, it’s obvious that 0.005 inches (0.127 mm) official tolerance is for shape, for roundness, not for smoothness.
Human fingertips are very sensitive
According to a 2013 study titled “Feeling Small: Exploring the Tactile Perception Limits” published on Nature, a human finger can feel wrinkles as small as 10nm (nanometers), or 0.00001 millimeters, demonstrating that human tactile discrimination extends to the nanoscale. So, if the Earth were shrunk down to the size of a billiard ball, you would definitely feel Mount Everest, which would be 0.04 millimeters high.
Dr. James O’Donoghue’s explanation
JAXA (Japan Aerospace Exploration Agency) scientist Dr. James O’Donoghue also tweeted about this. You can see the thread below.
Earth is as round as a billiard ball
Speaking of roundness, is Earth as round as a billiard ball?
Earth’s equatorial diameter is 7,926 miles (12,756 km), but from pole to pole, the diameter is 7,898 miles (12,714 km) – a difference of only 28 miles (42 km).
If we take the bigger diameter and shrink it down, the difference would be 0.0049 inches (0.0125 mm). If we take the smaller diameter, the difference would be very slightly bigger, but almost the same. So yes, the Earth is as round as a billiard ball. But it’s almost at the limit.
- Is Earth as smooth as a billiard ball? Answer: No.
- Is Earth as round as a billiard ball? Answer: Yes.
You can also watch Vsauce’s great video titled How Much of the Earth Can You See at Once?, which also covers this very subject.
As you can see in the video above, as microscopic photography shown, imperfections on a billiard ball are only 1/100,000 inches, or about 0.5 μm deep and high. Scaled down to the size of a billiard ball, Earth’s Mariana trench would be 49 μm deep.
Let’s think it in another way: If a billiard ball was scaled up to Earth size, the difference between the highest peak and lowest point would be about just 223 meters (732 feet) at maximum. A billiard ball is way smoother than the Earth.
- Ten things you don’t know about the Earth, “Bad Astronomy” blog by Phil Plait on discovermagazine.com
- World Pool-Billiard Association Equipment Specifications on wpapool.com
- Is Earth as smooth as a billiard ball? on skeptics.stackexchange.com
- Is the Earth Like a Billiard Ball Or Not? on Possibly Wrong blog