Unsung Heroes of Astronomy

To judge the scientific contributions of Hipparcos, we start by rec­ognizing that measuring the positions of stars is both fundamental and unglamorous. It’s fundamental because it’s the key to measur­ing the physical properties of celestial objects. Positions are the keys to the trigonometric determination of distance, and distance is needed to calculate the size, mass, and intrinsic brightness of any planet, star, or galaxy.28 Without distances we’re stuck with the appearance of stars in the sky, and a star that’s far away and lumi­nous can appear to be the same brightness as a star that’s nearby and dim. That ambiguity is fatal to any reliable understanding of the denizens of the night sky. It’s unglamorous because measuring a position is the simplest and most obvious way to characterize a star: there’s no image, just two angles to identify a unique spot on the sky, with no units. Needless to say, the people who do such prosaic work don’t always get their due.

It was not always that way. On the spinning Earth, the measure­ment of star positions is critical for keeping time and navigating. Early cultures noticed and tracked star positions as if their lives depended on it, which they did! In the third century BC, Timo – charis and Aristillus produced the first star catalog in the Western world while working for the Great Library at Alexandria. About a century later, Hipparchus extended their work, generating a cata­log with 850 star positions. He also divided the stars into intervals of logarithmic brightness that form the basis for a system that as­tronomers still use. This was a natural way to classify brightness since the eye has a nonlinear or logarithmic response to light. Ptol­emy increased the catalog to 1,022 stars.29 These star catalogs are among the most impressive intellectual achievements of antiquity; later generations of admiring astronomers called Ptolemy’s stellar compendium Almagest, which means “greatest” in Arabic.30

As in many other aspects of astronomy, the torch for mapping stars was then taken up by Arabs for a millennium. Around AD 964, the Persian Abd al-Rahman al-Sufi wrote his Book of Fixed Stars, which depicted the constellations in glorious, natural color. Al-Sufi was the first to catalog the Large Magellanic Cloud and the Andromeda Nebula, two distant star systems whose true na­ture would not be fully understood until the 1930s. The pinnacle of pre-telescopic observations was reached by Tycho Brahe in the sixteenth century. Through relentless attention to detail and the control of systematic errors, he improved on the positional er­rors of earlier catalogs by a factor of fifty. His reputation didn’t suffer from doing these mundane measurements; Brahe was cel­ebrated in his lifetime and is considered the greatest observer before Galileo.

The big prize in astrometry was its use to measure the distance to a star. Stars are so far away that the apparent seasonal shift of a nearby star with respect to more distant stars—the effect called parallax—was not observable for the first two centuries of use of the telescope. Friedrich Bessel won the race to detect parallax by showing that 61 Cygni, one of the closest stars, was nearly 10 light – years away, or a staggering 60 trillion miles.31 Bessel didn’t have a university education, but his meticulous calculations elevated him to fame as one of the most noted scientists and mathematicians of the nineteenth century. The parallax shift is extremely subtle, and far more difficult to detect than the large-scale migration of constellations through the night sky as the Earth spins on its axis and orbits the Sun (figure 8.4). Almost all stars have parallax shifts over the course of a year of about one second of arc or less, and most stars visible to the naked eye have parallax shifts smaller than 0.1 second of arc. For comparison, each of the letters on an eye chart that defines 20/20 vision spans an angle of five minutes of arc, a 3,000 times larger angle.

Thereafter astrometry lapsed into the status of a worthy but dull aspect of astronomy. In part, this was because it was so chal­lenging to measure parallax; in the half century after Bessel’s mea­surement, new star distances were only added at the rate of about one per year. Through the twentieth century, photographic plates made it easier to capture and measure star positions, and Her – schel’s project to map the Milky Way galaxy was carried out by researchers in Europe and the United States. But the air blurs out the light of all stars to about 1 arc second in diameter (1/3600 of a degree), larger than the size of the angle that had to be measured to detect parallax. Refraction and telescope flexure also complicate a parallax measurement. Astronomers were bumping up against the limitations of the atmosphere and the only solution was to go into the pristine environment of space.