Stellar parallax

Something else should be said about the impact of accommodation- When the (general) theory concerned is strongly supported independently of the fact at issue, the accommodation of some fact, even in this ad hoc way, may well still supply the best explanation that science can currently supply for that fact. So, for example, the best explanation in, say, 1700 for the observation of no stellar parallax was surely the Copemican one—that there must in fact be an apparent parallactic motion but that even the nearest stars are so far away as to make the effect too small to be detected by even the best available telescopes. (Here, as before with Ptolemy and with scientific creationism, we use the phenomenon—no observed parallax—to fix (in this case in a rather loose way) an otherwise free parameter in the theory (distance to the nearest star).)... 

Parsec A unit of distance resulting from a stellar parallax of 1 arcsecond, equal to 3.26... 

Bob shakes his head. Mr. Plex, for the next few days I will teach you everything I know about the wonderful stars in our Universe. Today I want to start by having you help determine the distance of Sirius from Earth using stellar parallax, a simple, elegant method for understanding our place in the Universe. Sounds delightful. Shall I get Miss Muxdroozol ... 

That s right. The distance you traveled was not enough to demonstrate stellar parallax. The best way to make you remember this was to send you out and have you return to the ship. I m sure you will never forget this lesson. ... 

Figure 1.2 Stellar parallax. Sirius appears to be in two different locations against the backdrop of distant stars depending on where Mr. Plex is when he measures the location from Earth. (The small parallax angle is exaggerated in the figure to make the concept easier to visualize.)...

If there is no stellar parallax, the Earth does not orbit the Sun. 

Figure 1.3 Galileo Galilei (1564-1642). Would the Church have reprimanded Galileo if he was able to demonstrate stellar parallax (Drawing by K. Llewellyn Blakeslee.)...

One wonders how the Church and the Inquisition would have reacted if Galileo were to find evidence of stellar parallax. Would they have shrugged it off as easily as they did other scientific evidence or would this additional piece of evidence be so strong as to cause them to transcend their literal interpretation of the Bible ... 

After Bessel s initial measurements, the number of stellar parallax studies gradually increased. Before 1900, astronomers had estimated the distances to about 60 stars using the parallax approach. The relative scarcity of good telescopes made it difficult for many researchers to accurately measure the small changes in star positions. In 1903, the American astronomer Frank Schlesinger (1871-1943) improved the accuracy of stellar parallax calculation by comparing photographs of stars taken at different times. A few years later,... 

Here d is the distance from Earth in parsecs, and the distance modulus is simply m-M. Sometimes you see this expressed in a related way M=m + 5-5 log d. Astronomers can measure the apparent magnitude m. Sometimes we can estimate a star s absolute magnitude M from its stellar spectrum, and therefore we can compute the star s distance from us. This can be useful if we want to estimate how far away a distant star is. Remember that stellar parallax is difficult to use when measuring distances to faraway stars. ... 

Sir, is the Cepheid blinking better for determining distances than stellar parallax, which you already told us about. ... 

Stellar parallax is measured most reliably by photographic methods. The most extensive set of early measurements were conducted from the Johannesburg station. The position of a star of interest is measured relative to a number of more distant reference stars over a period of months. The method has a serious limitation - for stars beyond a certain maximum distance the paralactic angle becomes too small to measure. 

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