The cosmic basics: Fourteen key numbers

(Photo by Janet Ramsden)

(Photo by Janet Ramsden)

Many are bothered that Pluto is no longer classified as a planet. I think the annoyance runs a bit deeper. Astronomy is very number-heavy. Its textbooks are crammed with countless numerals. Yet everybody knew the number “nine”: nine planets. Most folks could not recite any other celestial fact; but everyone could rattle off the names of all the planets, and knew that there were nine. Then in 2006, some council took that away from them.

Me, I’m a moron when it comes to remembering faces and names, but numbers have always stuck. As a teenager I wanted always to remember that Jupiter, say, spins in nine hours and 50 minutes, and ended up memorizing thousands of physics and astronomy numbers. Of course, some are more important than others.

So what are the most important figures in the cosmos? Ignoring physics and staying with astronomy, here are the top 14, in my opinion. All educated people should know these cold. We’ll use the most meaningful unit for Americans: miles rather than kilometers.

Start with the distance to the Moon and to the Sun: a quarter-million miles, and 93 million miles. That latter figure is actually pretty well-known. It was the first winning $1,000,000 answer on the TV show Who Wants to Be a Millionaire?

The size of the Earth, Moon and Sun: Rounding things off, our planet is 8,000 miles wide. The Moon’s diameter is 2,000, while the Sun is nearly a million miles across. Alternatively, in terms of proportion, the Moon is one-quarter our diameter, while the Sun’s width equals about 100 Earths. In three dimensions, using volume, one million Earths would fit inside a hollow Sun. It’s important to grasp the sun’s immensity fully.

The speed of light and the speed of sound: In one second, a noise will travel four city blocks. In that same time, a flash of light could go more than eight times around the world. The famous light stat is 186,282 miles per second. Round it to 186 thousand.

The number of stars: Our Milky Way galaxy has 400 billion. In the observable universe there are 200 billion galaxies.

Cosmic distances demand that you know three numbers. Our unit is the light-year, which is how far a flash of light travels in a year. The average naked-eye star is 200 light-years away.  The nearest spiral galaxy, Andromeda, is 2 ½ million light-years away. The farthest visible galaxies are about 12 billion light-years away.

These make you astro-savvy. If you know these few numbers, you won’t make the common mistake of thinking that the night’s stars are “millions” of light-years away, for example. Even if you don’t memorize those 14 numbers, letting them in and feeling what they convey gives an important, fundamental sense of the visible universe.


Want to know more? To read Bob’s previous “Night Sky” columns, visit our Almanac Weekly website at



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