Close your left eye and align your upright index finger against some distant object. Then open your left eye and close your right eye. Your finger has moved, relative to the background object. This is the phenomenon known as parallax, and it arises whenever you look at an object from two spatially separated vantage points.
The importance of parallax is that it enables you to calculate distances. Since you know the distance between your eyes, and you can measure the angle through which the object has appeared to move, the application of some simple trigonometry gives the distance to your finger. Of course, figuring out the distance from your eyes to your finger isn’t a big deal – but precisely the same logic allows you to figure out the distance to nearby stars.
The Earth moves around the Sun, so when we look at a nearby star in January and in July we look at it from different vantage points: the nearby star appears to move relative to the background of the distant “fixed” stars. In other words, the nearby stars exhibit a so-called annual parallax. Since we know the distance between Earth and Sun (this is one of the earliest rungs on the cosmological distance ladder) and we can measure the angle through which the star appears to move over the course of a year, the application of some simple trigonometry gives us the distance to the star.
In Measuring the Universe I devoted a lot of space to a discussion of parallax, since it was the first technique that enabled astronomers to obtain accurate distances to nearby stars – the first successful measurement of an annual stellar parallax was made by Bessel in 1838 for the star 61 Cygni. However, there’s a difficulty in trying to use parallax as method of distance measurement in astronomy: the angles involved are so tiny. Even the nearest star has an annual parallax of less than 1 second of arc. (The parsec is defined as the the distance at which an object will possess an annual parallax of 1 second of arc; it corresponds to 3.26 light years. The Centauri system, of course, is 4.37 light years distant.) As time goes by, however, the accuracy with which astronomers can make measurements increases. Indeed, the other reason I spent so long discussing parallax was that, during the writing of the book, the results of the Hipparcos mission were being disseminated – Hipparcos (“High precision parallax collecting satellite”) represented a step-change in the field of astrometry.
Hipparcos was an ESA mission, which was launched in 1989 and ran until 1993. It was the first space experiment devoted to astrometry – the accurate measurement of the position of stars. The final Hipparcos Catalogue contained information on the parallaxes of about 120,000 stars – with a median accuracy of better than 0.001 seconds of arc.
In Measuring the Universe I wrote that ESA were hoping to develop a successor mission to Hipparcos. The planned mission, called Gaia, would map not 100,000 stars but a billion stars. And the positional accuracy would not be measured in milliarcseconds but in microarcseconds (20 microarcseconds at a stellar magnitude of 15, and 200 microarcseconds at a magnitude of 20). Gaia would measure the distances of 20 million stars to a precision of 1%, and of 200 million stars to better than 10%. Such a mission would inevitably impact on many other fields (such as extrasolar planet determination, the testing of general relativity, quasar discovery…)
When I wrote the book, the Gaia mission was so far in the future that I found it difficult to imagine that it would ever fly; I thought it would be lost in the maze of technological, computational and political obstacles that were in its way. But Gaia is on its way! It launched successfully today, 19 December 2013, at 09:12GMT and in a month or so it will be at its new home at the Earth-Sun L2 point.
The plan is for Gaia to observe the sky for five years; astronomers will be analysing the Gaia data for much longer than that. This new astrometric mission is going to have a huge impact on all aspects of astronomy.