Astrometry

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Astrometry, literally star-measure, is perhaps the oldest field of study of the oldest science. Astrometry is the measurement of the positions and motions of objects in the sky. In ancient times, even before the great civilizations got organized, sky watchers noticed that the stars seemed to remain fixed on the celestial sphere, except for the Sun, Moon, and five planetes (wanderers – the planets Mercury, Venus, Mars, Jupiter, and Saturn). Even these seven didn’t seem to venture out of a band on the sky, now called the zodiac.

Jacob’s staffs (cross staffs, Figure 1), astrolabes (Figures 2 and 3), and armillary spheres (Figure 4) were among the first instruments used to measure the positions of celestial objects. They were used by some of the ancient civilizations on the Mediterranean Sea and permitted measurements of position that were accurate to about one quarter of the diameter of the full moon. More than 15 centuries later, these instruments, as well as large quadrants (Figure 5, permitting measurements of position equal to the angular sizes of the diameters of some of the more prominent craters on the Moon), were still being used by astronomers to measure star positions with their unaided eyes. With astrolabes and quadrants, the observer looked from a rear sight to a foresight to the star and positions were recorded. Additional stars were observed and their positions recorded. The analysis of the data involved determining the angles separating the stars and recording their positions based on a grid, like latitude and longitude, but imagined in the sky. (Latitude’s celestial equivalent is called Declination. Longitude’s equivalent is called Right Ascension.)

Fig. 1. A Jacob's staff (called that when used for astronomy, otherwise a cross-staff when used for navigation), from John Sellers' Practical Navigation (1672).

Fig. 2. An astrolabe is used by measuring the altitude of a star above the horizon. Source: Pearson Scott Foresman.

Fig. 3. Planispheric astrolabe from Al-Andalus (Islamic Iberia), made in Toledo (Spain). Author's name and year of carrying out (459 of Hijra / 1067 AD) are shown in the Arabic inscriptions of the plates. Photograph by Luis García (Zaqarbal)

Fig. 4. Armillary sphere with astronomical clock made by Joost Bürgi and Antonius Eisenhoit, Kassel, 1585. Constructed from bronze, steel and ebony. Photograph by Chris Bainbridge.

Fig. 5. Scaled reconstruction of Tycho's Augsburger Quadrant in Römerturm in Göggingen. The celestial object is sighted along the upper right-hand straight edge and its elevation read off where the plumb-line crosses the curved vernier scale. Photograph by Johannes "Jodo" Dosch 2008, licensed under GFDL 1.2.

Astronomers couldn’t do this without a telescope, but the appearance of the supernova of 1572 in Aristotle’s supposedly unchanging heavenly sphere put a crack in the crystal. Measurements of the position of the Great Comet of 1577 also showed no parallax, thus placing the comet well beyond the crystalline sphere carrying the Moon.

The application of telescopes to studying objects in the sky started in the 17th century and the development of precision telescope mounts to support them followed. This led to the desired breakthrough, the first parallax of a star (named 61 Cygni), finally, in the 19th century. Telescope mounts had to have grooves inscribed very precisely on their “setting circles” so that angular measurements of positions, in the two coordinates of the celestial grid, could be made (Figure 6). (Angles could be measured to the width of canyons on the Moon.) Later, telescopes dedicated exclusively to position measurements – transit circles (Figures 7 and 8) – would be used with precision clocks to ascertain positions in right ascension, though declination was still measured with a setting circle.

Fig. 6. A U.S. Naval Observatory 5” Alvan Clark refracting telescope (the long brass tube, with a short brass finder telescope mounted on top). The declination setting circle (edge in view) is the brass disk above the counterweight, to the upper left of the descriptive sign in the picture. The right ascension setting circle is hard to see, but placed just to the right of a black arc beneath the left end of the brass telescope tube.

Fig. 7. The transit circle of astronomer Stephen Groombridge, from A Catalogue of Circumpolar Stars, 1838. The telescope is visible from lower right to upper left between the spoked wheels supported by the two sturdy piers.

Fig. 8. The modern Flagstaff Astrometric Scanning Transit Telescope (FASTT) is a completely automated transit telescope utilizing scan-mode CCD cameras to measure the positions of hundreds of thousands of stars with much less bias and much greater accuracy in the measurements than are possible for a human observer.

The development of photography later in the 19th century meant that star images could be recorded and preserved and re-measured multiple times if desired. The images were measured with precision “measuring engines” kept in laboratory settings.

Electronic detectors started replacing photography in the 20th century. Special screens were developed to improve the precision of measurements of many stars in a telescope’s field of view. Digital detectors, similar to those in digital cameras, are now in common use (one example is Figure 8).

The last quarter of the 20th century saw the development of new methods for making astrometric measurements. The European Space Agency (ESA) lofted a satellite, Hipparcos, a homonym for the ancient Greek astronomer Hipparchus who produced a catalog of 1080 star positions. Hipparcos, the satellite (Figure 9), made measurements of over 2.5 million star positions to unprecedented precision around the whole sky. It used a pair of telescopes feeding images of widely separated areas of the sky to a detector. Computer analysis sorted out which stars belonged where, very accurately compared to ground-based astrometry because widely separated areas were measured at the same time. Angles could be measured that were the equivalent of a person’s height standing on the Moon as seen from Earth.

Fig. 9. The European Space Agency’s Hipparcos was the first space mission for measuring the positions, distances, motions, brightness, and colors of stars. Its two telescopes recorded images of two widely spaced patches of the sky to better measure star positions.

Now in design and assembly, ESA’s Gaia spacecraft will improve on the work of Hipparchos. In a few years, it will be making measurements 10 to 125 times more precisely than Hipparchos.

Even though electronics and computers are used for making and analyzing the measurements, SIM Lite’s results still come down to what Hipparchus and other early astronomers did, albeit much more precisely now: determining the angles separating the stars and recording their positions for additional analysis, study, and interpretation.