How far away is the Moon?

Bad Astronomy
The entire universe in blog form
Feb. 24 2011 7:00 AM

How far away is the Moon?

I'm sometimes asked what's the one thing I wish people would understand better about the Universe. My answer is always the same: scale. We humans have a miserable sense of just how big space is, and I've spent a lot of time over the years working out ways to express it better.

Most people don't really grasp just how far away the Moon is, and it's the closest astronomical object in the sky! So I'm glad this video came out, and is actually getting spread around the web a bit:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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When I see something like this, my first reaction is: I'd better check that math. So, first question: is the basketball/tennis ball size ratio the same as for the Earth/Moon? IN other words, if the Earth is a basketball, does a tennis ball get the size of the Moon right?

The Earth is 12,740 km (7900 miles) across, and the Moon 3474 km (2150 miles) in diameter, for a ratio of 3.7.

A standard NBA basketball is 24 cm (9.4 inches) in diameter, and a tennis ball 6.7 cm (2.6 inches), for a ratio of 3.6. Pretty good! I'll have to remember that; it's pretty useful.

So how far away would a tennis ball Moon have to be from the basketball Earth to be to scale? On average the moon is 380,000 km (235,000 miles) from the Earth, a distance of about 110 times its own diameter. A tennis ball would then have to be 110 x 6.7 cm = 7.37 meters (about 24 feet) from the basketball. That's a lot farther than most people would think!

So let's ramp that up a bit. On this same scale, how far away do you think the Sun is? Don't cheat! The answer is below the fold.

The average distance to the Sun from the Earth is about 150 million km, or 11,750 times the Earth's diameter. Since our basketball is 24 cm across the Sun would be a staggering 2.8 km (1.75 miles) away! To scale the Sun would be 26 meters across, the size of a house. A big one.

And the nearest star? It would be -- to scale, mind you -- 800,000 km (480,000 miles) away: twice as far as the real Moon is from the real Earth!

Space is big. Very, very big.

Tip o' the scale bar to reddit.

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