Up above the world so high, like a diamond in the sky

A few days ago we got acquainted with how much distance one light year measures and learned it was about six trillion miles. That only takes us a quarter the way to Alpha Centauri, the nearest star system beyond the sun. How do astronomers measure such enormous distances? Well, it all starts with our own two eyes.

Stand in front of a window and do a "thumb’s up" while sticking your arm straight out in front of you. Now open and close your right and then your left eye in a back and forth blinking pattern. As you do, you’ll see your thumb appear to jump back and forth across the more distant background. Because your eyes are separated by about three inches, you see your thumb from a slightly different perspective as you blink. This makes it appear to shift against the distant scene. The shift is called parallax (PAIR-uh-lacks), and astronomers use the very same concept to measure star distances.


Because our eyes are separated by several inches, we see things along two different lines of sight, allowing us to sense depth and distance. If you know the distance between your eyes and then measure how much an object shifts in front of you by blinking, you can find its distance. Astronomers apply the same concept to measuring star distances using the diameter of Earth’s orbit — the equivalent of two eyes separated by 180 million miles. Photo: photos.com

If you measured the distance between your eyes and calculated the shift in the angle of your dancing thumb, you could determine exactly how far away your thumb was from your head using simple trigonometry.

Now place your thumb right in front of your face and do the same thing. Notice how it jumps even more? The closer an object is to you, the larger its parallax. You can also make your thumb jump through a bigger angle if you could somehow increase the distance between your eyes. Either way works.

We use parallax for depth perception and seeing the world in 3-D. Unfortunately, stars are much too far away to see even the closest ones shift against the background of those more distant by blinking. Even distant objects right here on Earth show no shift when we "blink" them.


Astronomers uses the "eyes" of the Earth in January and then again in July to measure a nearby star’s parallax or shift against the more distant background stars. See below. For an excellent, interactive animation of parallax, click here. Illustration: Bob King

Astronomers need a set of eyes that are many miles apart to measure the shift of nearby stars against those in the remote distance. Luckily, Earth’s orbit provides just the ticket. Our planet is about 93 million miles from the sun, which adds up to about 180 million miles from one side of our orbit to the other. The brightest stars are generally the closest. To find the distance to one, astronomers photograph a star through a powerful telescope in, say January, and carefully measure its position in relation to the fainter stars around it. Then they wait six months and photograph the star again in July. During that time, our planet has traveled all the way around to the other side of its orbit, providing us with two different lines of sight. Careful measurement of the the second photo will reveal a tiny shift in position, enough to calculate a distance. 


In this demonstration, you can see how our star has shifted against the background ones when photographed through a high powered, professional telescope on opposite sides of Earth’s orbit. Illustration: Bob King

Tiny is an understatement. Alpha Centauri moves 0.77 arc seconds against the background sky over six months — that’s the size of a quarter seen from over three miles away. The bright star Sirius, below and east of Orion, shifts only half as much or about the size of a dime at the same distance. The Earth’s orbit allow us to measure star distances to about 160 light years. After that, the atmosphere blurs the stars too much to measure any tinier angles.
To escape the effects of our tubulent atmosphere, the European Space Agency launched the Hipparcos satellite in 1989. During its four years in orbit, Hipparcos measured parallaxes of 118,000 stars to 20 times more precisely than possible from the ground and one million stars to good precision. We now have good estimates of star distances out to about 800 light years.

That’s still a small area when you consider the 100,000 light-year-wide diameter of the Milky Way galaxy alone. There are other methods that take us out farther, but they’re all ultimately based on the parallaxes we’ve gleaned from our yearly orbit around the sun and the work accomplished by the Hipparcos Mission.


The moon will be near two star clusters Sunday night. Take a look in the west around 9 o’clock. Created with Stellarium.

Tonight we’ll enjoy a banana moon lined up with the Seven Sisters (Pleiades) and Hyades star clusters. The International Space Station will pass from west to east across the southern sky beginning at 8:53 p.m. Watch for it to first glide beneath the moon and then above Orion.

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