Parallax

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The first time I went to Australia was in tenth grade for a school trip. I have less than ten pictures from that time because my camera got wet while we were canoeing. Apparently I did not seal the waterproof bag correctly. Either way, my camera could not have captured the most memorable moment of the trip.

In the middle of one night, one of the girls I shared the cabin with woke the other three of us up. She wanted to go to the bathroom but was afraid to go on her own in the darkness. It was a remote campsite and the booth was outside. We accompanied her, and, while waiting, I glanced up. Now, this was well before I considered doing astronomy, and living in big cities such as Beijing and Singapore meant that seeing a dozen stars during the night was pretty impressive. So, what I saw in the Western Australia wilderness was the most number of stars I'd ever seen. I was unaware, however, of that fact that the stars were different. I didn't know that the stars you see in the Northern hemisphere differed from those in the Southern hemisphere. On the other hand, Singapore, where I went for high school, lies near the equator and would offer a view of the entire celestial sphere instead of just part of it, if not for light pollution.  

Alan Hirshfeld had a similar experience when he went to Arecibo for a summer during college. Unlike me, he knew that he would be seeing completely different constellations. Years later, he wrote a book, a book my astronomy professor recommended so highly that I bought it to read over winter break - Parallax: The Race to Measure the Cosmos.

The title itself was cool enough for me to want to pick it up.

post 8 - 1 - book cover.JPG

Source - Barnes & Noble

What is parallax? I first encountered the term when I started adding planets for the EOD. A field that belonged to the stellar parameters, parallax describes, in Hirshfeld's words, "the apparent shift in position of an object when viewed alternately from different vantage points," and it can tell us the distance to the star.

post 8 - 2 - parallax.gif

Source - ESA

A few posts back, I mentioned the upcoming transit of Venus. Over two hundred years ago, Edmund Halley proposed using the pair of transits to determine the size of the solar system. Hirshfeld describes the united efforts of many astronomers to do that, stating that "150 observations of Venus's transits were recorded worldwide." Considering how communication back then was nowhere as convenient as it is today, such collaboration really shows dedication.

post 8 - 3 - venus.png

Source - Wikimedia

But if effort of that collaboration is likened to the size of the solar system, I think the effort of astronomers to detecting stellar parallax would be like the size of the galaxy in comparison. 

Over two thousand years ago, when Aristarchus, Greek astronomer, suggested that, instead of being the center of the universe, the Earth went around the sun. If the Earth were at the center, we would use the diameter of the Earth as the largest baseline for detecting parallax. If the Earth orbited the sun, then that baseline would, as you can see above in the picture, extend to the diameter of the Earth's orbit - much, much larger. But people back then detected no shift, and, until the 1800s, people still did not. 

Just think - the basic unit of stellar parallax is "pc," or parsec (parallax second), and a distance of 1pc is 206265AU (astronomical units; distance from Earth to Sun), or 3.26 light-years. One parsec is equivalent of a parallax of 1" (arcsecond), which is 1/206265 of a radian. Such a small angle needs excellent telescope and technique to detect. To comprehend the difficulty, I give you one of Hirshfeld's analogies: the parallax angle of the star 61 Cygni, first found to be 0.314" (compare with today's 0.287" value), is "equivalent to the apparent size of a Manhattan taxi cab as viewed from Mexico city." 

When reading about the result of the first couple of parallax measurements done by Friedrich Bessel (for 61 Cygni), Friedrich Streuve (for Vega), and Thomas Henderson (for Alpha Centauri), you can't forget about build-up of attempts at measuring parallax as well as advancement of technology and instruments.  

Without knowing the details of someone's work, it is hard to appreciate their result. I enjoyed learning about background and quirks of scientists I once viewed as impressive figures featured in text books. The next time you go on a long bus ride or flight, or just have a few hours to spare, I hope you will read this book. 

Once you finish reading about the 2000-year-old quest for stellar distances, you can't help but wonder - for your passion or your desire for certain answers, how far are you willing to go? 

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4 Comments

i love how inspiring your blog posts are! I now want to read that book myself!

Thanks Rowena :) You should check it out from the library one day! It's not technical at all so it'll be easy to understand.

Cool entry, though I seriously don't understand anything. Not surprising since I find Astro 001 to be slightly challenging without a textbook, LOL. I would rather think of how pretty and sparkly the stars are than all that geometry and physics. True words of a science failure. :p

It always blows my mind the time, effort, genius, etc. that had to be given by any of the early pioneers in any field really. I generally contemplate this in conjunction with the Natural Historians and their intense and national/global explorations with such primitive gear/technology/transportation. But my husband and I often star-gaze and contemplate all the early scientists and we wonder how in the world they figured any of it out (even if we're just talking about as far back as the 1800's!!). The time, dedication, and passion it must have taken to work out all those things we now use as the most basic of building blocks and what we take for granted due to how far our technology has come, is indeed mind-boggling!

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