Taming mind boggling celestial distances
When you start to consider the size of the cosmos, the numbers can get so large and so mind boggling that when you try to wrap your brain around them your head could explode! That’s a really bad scene! To avoid making a mess of your mind, you can make use of some handy units of measurements designed for astronomy that ease the strain on your brain. First, let me start with a unit we all know and love, the mile. To give you some perspective the diameter of the Earth is about 8,000 miles, and the circumference of the Earth at the equator is just under 25,000 miles.
The closest celestial object to us in the heavens, although human made, is the International Space Station that circles our world once every 90 minutes at a height of around 250 miles. If it happens to be passing above your locale you can’t miss it. It’s easy to see, no binoculars or telescopes needed. I think a lot of folks mistake it for a high-flying aircraft. It resembles a super bright star generally pushing from west to east at various heights across the celestial dome. The best website to keep up with the space station’s comings and goings is www.heavensabove.com. Just set your location with the database provided and you’re good to go.
The next closest celestial body is the moon, at an average distance of about 238,000 miles from us. Now I can deal with hundreds of thousands of miles, but when it comes to millions of miles, the distance of the planets in our solar system, that’s when I need help! For example, there are two bright planets in the early morning sky now. Just before morning twilight you can easily see Jupiter and Saturn in the low south to southeast sky. Without a doubt they are the brightest starlike objects in that part of the early morning sky. Jupiter is the brightest one on the right. An easier way to express distances in our solar system is in astronomical units, and they’re really simple. One astronomical unit (AU) equals 93 million miles, the average distance between the Earth and the Sun. That would put Venus at 5.3 AUs away, which equals 492 million miles, and Saturn at 10 AUs, which would be 930 million miles.
For stellar distances it be would absolutely silly to talk about them in terms of miles. The next closest star to our sun is Proxima Centauri. That star is nearly 25 trillion miles away or in longhand, 25,000,000,000,000 miles. As humongous of a number as that is, that’s celestial chickenfeed compared to distances of other stars, many of which we can see every night with just our naked eyes. That’s why it’s best to express stellar distance in light-years. A light- year is defined as the distance a beam of light travels one year’s time. Using the speed of light, which is 186,300 miles per second, one light-year computes out to be 5.8 trillion miles. That would put Proxima Centauri at about 4.3 light-years away.
For example, the next brightest actual star, just to the right of Jupiter in the low predawn southern sky, is Antares, which is 553 light-years away. You certainly wouldn’t want to express that in miles. There are some stars you can easily see with just your eyes that are thousands of light years away. In fact, there are whole other galaxies of stars that are over 10 billion light-years away!
Light-years are cool because not only do they express distance, but also time. Since a light-year is the distance light travels in one year, the light that you see from a star that’s 10 light-years away takes 10 years to reach your eyes. The light you see from a star that’s 100 light-years away would take a century to reach you. If a star is 1,000 light-years away, it would take a millennium for the light to reach you. So when you’re stargazing on these warm summer nights keep in mind that when you look into the heavens, not only are you looking at incredibly far away places, you’re also peering back in time. Don’t let your head explode!
Mike Lynch is an amateur astronomer and professional broadcast meteorologist for WCCO Radio in Minneapolis/St. Paul. He is also the author of “Stars: a Month by Month Tour of the Constellations,” published by Adventure Publications and available at bookstores and at adventurepublications.net.