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February 8th: Astronomy Word of the Week : Aberration


Date: February 8, 2012

Title: Astronomy Word of the Week : Aberration

Podcasters: Dr. Christopher Crockett

Organization: United States Naval Observatory

Links: http://astrowow.wordpress.com/

Description: This week we introduce a new feature: the Astronomy Word of the Week! For the inaugural episode we introduce the word “aberration” and learn about the first observations to definitely demonstrate that the Earth orbited around the Sun. There’s also something about umbrellas.

Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.


You’d be surprised at how much umbrellas and telescopes have in common.

In 1725, British astronomer James Bradley attempted to measure the motion of the Earth around the Sun. Since Copernicus had reintroduced the idea that the Sun, and not the Earth, sat immovable at the center of all things, each succeeding generation of astronomers tried in vain to directly detect that movement. The trick was to measure stars shifting back and forth in the sky as the Earth looped around the Sun.

To get a better understand of what, exactly, astronomers were after, look across the room where you are sitting. Now rock gently from side to side. You’ll notice that objects sitting close to you – perhaps the computer on which you listening to this – appear to move as you do. As you rock to the left, the computer drifts to the right in your field of view (and vice versa). The more distant objects appear to move less. As the Earth orbits the Sun, the same effect should be apparent in the stars. As the Earth hurdles through space in one direction, the nearest stars should appear to shift ever so slightly in the opposite direction. As the Earth comes back around the other side of its orbit, the stars come back to where they started. Much like the difference between your computer and the trees, the closest stars should appear to wobble the most whereas the most distant stars will hardly move at all. Measuring this shift – the stellar “parallax” – not only allows astronomers to measure the distance to the nearest stars, but is also definitive proof that the Earth orbits the Sun.

Unfortunately, because the size of this effect is vanishingly small, such a measurement eluded astronomers for thousands of years. Even the nearest star, a dim red dwarf called Proxima Centauri, only shifts by an amount equal to the width of a human hair held 70 feet away. Even with a telescope, how do you measure the position of a star with that level of accuracy?

Bradley had come up with a simple solution to this problem. Sawing holes in the floors of his home and through its roof, he strapped a long telescope to the interior wall of his chimney thus converting his residence into a makeshift observatory. The telescope’s limited range of motion constrained it to only point at stars that passed directly overhead. With the help of a plumb bob, Bradley could center a star in the telescope’s eyepiece and measure how far over the telescope was tipped. The idea was to track one star over the course of the year and use this setup to make precise measurements of the star’s relative position in the sky. This honor went to the star gamma Draconis.

Bradley began his observations in December 1725 and, sure enough, he began to record shifts in the position of the star! If the motion of the star was the result of the Earth’s motion around the Sun, then gamma Draconis should be at its most southerly position in December. It should then start drifting north through the winter and spring reaching its most northerly position in June at which point it would reverse course. Unfortunately the motion was in the opposite direction of what was predicted! Over one year, the total amount of this drift was roughly 40 seconds of arc. That’s about 1% of one degree or the thickness of a piece of paper seen from just under 2 feet away.

The cause of this motion remained a mystery. The mystery deepened when Bradley checked his measurements with other stars. Each star he measured moved in the exact same way and by the same amount! This puzzle remained unsolved for several years until one afternoon when Bradley was enjoying an afternoon sail down the Thames. He noticed that each time the boat shifted direction, the boat’s wind vane changed direction as well. And in that simple observation, Bradley had his answer to the mystery of gamma Draconis.

Imagine standing outside in the rain on a day with no wind. The rain is falling straight down on your head. To keep dry, you open an umbrella and hold it over you. Now start moving forward. To stay dry, you have to angle the umbrella slightly forward as you walk in to the rain. The faster you move, the more you have to tilt the umbrella. From your perspective, the rain is now falling at an angle and you must adjust your umbrella accordingly.

The same thing can happen with light. Imagine pointing a telescope at a distant star straight over head. If the Earth were stationary, then the light would enter the telescope and fall straight through to your eye. But now put the telescope on a moving Earth. The light enters the front end of the telescope as before, but as the light moves through the telescope, the Earth moves the telescope. The telescope shifts, but the light doesn’t, and therefore the light eventually hits the side of the telescope. To get the light to move unobstructed through the telescope tube, you’d have to angle the telescope slightly in the direction of movement – just like you angle your umbrella forward in the rain. What’s more, the amount you’d have to move the telescope is completely independant of how far away the star is. It’s dependent entirely on the speed of light and the speed of the Earth.

How a telescope is like an umbrella. The motion of the Earth forces us to tip our telescopes forward to get the light to fall through to the eyepiece.

Bradley had discovered the effect of stellar aberration. The shifting angles he measured were much like someone with an umbrella running back and forth in the rain. Every time that person turned around, she would have to change where the umbrella was pointed. Likewise, Bradley had to continually shift his telescope as the Earth went around the Sun. He hadn’t discovered parallax – that would take another 113 years – but had definitely proved the motion of the Earth around the Sun!

Think of that the next time your running to catch a bus in the rain!

End of podcast:

365 Days of Astronomy
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