Instruments and Observatories

The oldest observatories involved backsites and foresites and appear to be nearly as numerous in ancient Britain in megalithic times as town square clocks are in Europe today. This, at least, is the impression gained from a plot of the locations of those sites (Figure 6.1). The builders were concerned with the solar and lunar rising and/or setting azimuths. We will show in a later chapter that these types of observations could have been made for calendric purposes but also possibly for eclipse prediction.

Among the instruments used by astronomers in ancient Egypt (§8.1) were sighting instruments, shadow boards, and other types of shadow clocks, some very elaborate. The boards were leveled by plumb-bobs. Sundials were created with gnomons, and with a leveling device known as a merkhet. Essentially a string-supported plumb-bob, the device served as a portable sundial, the string of which provided the shadow. Instructions for constructing a shadow clock are provided in the funerary text on a cenotaph of the pharaoh Seti I in Abydos. The benben pillars, including later obelisks, were associated with the sun and could have been used as large gnomons or sighting devices. Unfortunately, we have no descriptive evidence suggesting such a use. Water clocks, of both "inflow" and "outflow" types, were used by astronomers and frequently have constellation designs. The so-called Ramesside star clocks show transit stars "measured" relative to a figure of a seated individual against a grid of 8 x 13 squares. These show a clear concept of a gridded star map, but the use of the grid seems crude and never seems to have been extended to the whole sky.

For an observer responsible for the measurement of time, a decan must have functioned as a star almanac entry, indicating the appropriate time of night in a given season when a particular asterism arose. Star clock tables from the tombs of Rameses VI, VII, and IX, kings of the 20th dynasty (~1500 b.c.), seem to use a scheme different from the successive risings implied by the decans (Neugebauer and Parker 1969, Vol. II, p. 74). Each table is associated with a seated man; vertical lines traversing the figure apparently mark the time that a star passes by that point. These star clock tables could not have been very precise: The references are to "on the shoulder," "on the Right (or left) ear," "opposite the heart" (the origin, which Neugebauer and Parker envisage to be on the celestial meridian), "on the right (or left) eye," apparent references to the seated figure who had to remain motionless for the entire night. If the figure was a statue, such a scheme would still not be very precise because a sentient observer had to mark the hours! Nor could this reckoning be very accurate, given the march of the seasons. Neugebauer and Parker (1969, Vol. I, pp. 107-113) find evidence of attempts to repair the decan scheme (in the asterisms that operated in the five epagom-enal days) as the errors of the calender accumulated.

Nevertheless, as Neugebauer and Parker remark, the scheme was impressive enough to the ancient Egyptians to be enshrined forever in the Ramesside tombs. The purpose of including such depictions on the wall of a sealed tomb can only have had meaning for the soul of the pharaoh. The telling of the hours of the night must have been a necessary part of the dealing with the dangers of the underworld before being reborn at dawn with the rising sun. See §8.1 for detailed discussion of the background for, and the astronomy of, ancient Egypt.

Ptolemy mentions several instruments in the Almagest (cf. Toomer 1984, 61ff). He describes the construction of a meridian circle with which the noon zenith distance can be measured. He cites writings of Hipparchus, who mentioned a bronze ring located in the "Square Stoa" section of Alexandria. With this ring, which was accurately aligned in the plane of the celestial equator, Hipparchus said that it was possible to indicate the dates of the equinoxes by noticing when the face that is illuminated by the Sun changes from top to bottom (Fall) or bottom to top (Spring). Ptolemy describes the construction of what he calls an "astrolabon" instrument. The object Ptolemy describes resembles what we would call an armillary sphere, a series of nested, graduated rings, one set in the ecliptic, another in the equatorial plane (and perhaps a third in the plane of the horizon). The result is a means to measure the position of a heavenly object in any of several coordinate systems. The ecliptic longitude and latitude could be read off the graduated rings directly. What we today would call an astrolabe would have been referred to as a "small astrolabe," in Ptolemy's day, according to Theon of Alexandria (~375 a.d.).

The astrolabe, like our modern planisphere, was a two-dimensional representation of the celestial sphere, with the ecliptic projected onto it, and retaining a circular shape. By selecting the date, the time of night was then revealed by what stars were visible on the meridian; it served also as an instrument for the observation of both the Sun and the stars. It was equipped with a rotatable marker (called an alidade) that was fitted with sights. A measurement of the solar altitude could give the time from meridian passage for a given latitude (once the date had been dialed in). The measurement of the altitude of a star could be used similarly; alternatively, measurement of the altitude of the Sun or that of a known star when on the celestial meridian could reveal the latitude of the observer [refer to (3.34)]. A filigree metal star chart called a rete was included on the face of the astrolabe, and beneath this, visible through the filigree, was one of several plates (which could be interchanged for locales of different latitudes) marked with projections of altitude circles (almucantars or almucanturs), as well as the equator and tropics, and sometimes other circles as well. The almu-cantars are circles of equal altitude that are projected stereoscopically18 onto to a plane parallel to the equator.

The almucantars remain circles (this is a characteristic of stereoscopic projection), but their centers shift along the line joining the projection of the north celestial pole and the zenith. The separation of the zenith and NCP depends on the latitude, and so the family of almucantars differs in placement with latitude also; hence, the need for more than one plate, if the user of the astrolabe was given to travel.

After a sighting on the Sun, the rete would be rotated until the Sun's position coincided with the appropriate altitude circle, and the hour of the day could then be read off. Its small size made the astrolabe popular among travelers, and it was used up to the mid-18th century when it was replaced by a forerunner of the modern sextant. Figure 3.19 illustrates a German astrolabe, constructed in 1537 by George Hartman.

There were four plates, each inscribed on both sides with the stereographically projected altitude circles appropriate for a particular latitude: 39°, 42°, 45°, 48°, 51°, and 54° are explicitly marked. The center of the astrolabe represents the NCP, and the tropics of Cancer and Capricorn flank the equator with which they are concentric. Around the outside of the body are the Latin names of the winds. Within these are the 24 equinoctial hours, in Roman numerals of 1 to 12, repeated; and a ring of four sets of altitudes in 1° intervals over the range 0° to 90°. The rete contains the ecliptic, marked with the zodiacal signs and subdivisions of 5°, and the locations of particular stars marked by perforated pointers. The index arm that pivots about the center (the NCP projection) marks off the location of the Sun on the ecliptic for a particular time of year. The intersection of this arm with the body of the astrolabe indicates the hour of the day (the Roman numerals). The arm is inscribed with degrees of declination, north and south of the celestial equator. The back view shows the alidade with collapsible sighting plates. On the body, proceeding from the rim inward, are altitudes, degrees (in Roman numerals) of the zodiacal signs, and the days of the months. The dates corresponding to the boundaries of the signs indicate that the first point of Aries occurred on March 10. Even more elaborate astrolabes are known. See, for example, Gibbs and Saliba (1984) for several interesting examples.

Ptolemy's description of the large "astrolabe" is in the context of his discussion of the lunar "anomalies"; i.e., the Moon's departures from Ptolemy's model. He was using the device to measure the longitude of the Moon relative to the Sun. One of the difficulties in measuring the position of the Moon is the presence of a large amount of parallax (see Figure 3.6). For this reason, Ptolemy built what he called a "parallactic instrument" (Toomer 1984, Fig. G, pp. 244ff),19 with which he measured the zenith distance of the center of the Moon's disk at meridian passage.

First, he established the actual celestial latitude variation of the Moon by observing it when it was at its highest above the ecliptic (at such time it would have been close to the

18 The focus of the projection is the south celestial pole. Thus, each projected point is the intercept of the equatorial plane with the line joining the SCP and the point of interest on the celestial sphere.

19 According to Toomer, during the Middle Ages, this instrument was called a triquetrum, because it consisted essentially of three main components (see Figure 3.20).

Figure 3.19. An astrolabe: (a) Front and (b) back views of a German 16th-century astrolabe, No. 262 [Smithsonian Catalog No. 33617] discussed by Gibbs and Saliba (1984). Such devices were two-dimensional representations of the celestial sphere that provided a practical method of navigation, or, alternatively, local date and time determination. Like the modern planisphere, it is a two-dimensional representation of the celestial sphere, with which the time of night on a given date is revealed by what stars are visible on the meridian. In addition, it permitted observations of the altitudes of the Sun and stars with a rotatable alidade, which was equipped with sights. The solar alti-

Figure 3.19. An astrolabe: (a) Front and (b) back views of a German 16th-century astrolabe, No. 262 [Smithsonian Catalog No. 33617] discussed by Gibbs and Saliba (1984). Such devices were two-dimensional representations of the celestial sphere that provided a practical method of navigation, or, alternatively, local date and time determination. Like the modern planisphere, it is a two-dimensional representation of the celestial sphere, with which the time of night on a given date is revealed by what stars are visible on the meridian. In addition, it permitted observations of the altitudes of the Sun and stars with a rotatable alidade, which was equipped with sights. The solar alti-

tude provided the time of day (given the date), and the altitude of a star on the celestial meridian revealed the latitude of the observer. Beneath the filigree metal star chart (rete) on the face of the astrolabe is a site-specific plate on which altitude circles (almucantars) are marked. After a sighting on the Sun, the rete would be rotated until the Sun's position coincided with the appropriate altitude circle, and the hour of the day could then be read off. From Gibbs and Saliba [1984, Figure 12, pages 9, and 97, page 148, Smithsonian Prints #79-1769 and #82-8299] and reproduced here by permission of the Smithsonian Institution, Washington, D.C.

zenith at Alexandria) and subtracted z = 21/8° from the latitude, f = 30°97 to get 5max = 28°85, and, with e = 23°85, found i = 5°0. He then measured the Moon's lowest z at the opposite solstice20 and found the parallactic shift (z2 = 50°92 vs.

20 These conditions are referred to as major and minor standstills, respectively, from the effect that the celestial latitude variations has on the declination variations of the Moon during the month and consequently on the amplitude of lunar rise and set azimuths. The evidence for the megalithic studies of the Moon is mainly from alignments to distant foresights allegedly marking the standstills (see §6.2).

2e + z1 = 49°83). Ptolemy thus deduced a lunar parallax of ~1°07', not too different from the modern value (57'). See Toomer (1984, pp 246-251) for more detail.

Graduated scales on quadrants were in use for centuries, culminating in the work of Tycho Brahe. This great 16th-century observer used massive instruments at the royal observatory on the island of Hveen in Denmark to measure the precise relative positions of the planets. He used a clever vernier scheme to read the scales: transversals, lines of ten uniformly spaced dots, were placed along lines angled upward from each side of every other base scale tick mark, to zenith to object (Moon)

to zenith

/z

'pivot

. hinge

Figure 3.20. A rough sketch of the triquetrum, as it was called in the Middle Ages, consisted essentially of three main components; it is closely similar to the instrument described by Ptolemy as a "parallactic instrument," which was used to measure the meridian zenith distance of the Moon at each solstice (Toomer 1984, Fig. G, p. 245). The rod anchored at the top of the vertical rod was lined up with the object in the sky, and the position on the lower rod read off; in combination with the vertical length, it yielded the equivalent of the zenith angle. Drawing by E.F. Milone, after Pannekoek (1961/1989, pp. 154, 181).

Figure 3.20. A rough sketch of the triquetrum, as it was called in the Middle Ages, consisted essentially of three main components; it is closely similar to the instrument described by Ptolemy as a "parallactic instrument," which was used to measure the meridian zenith distance of the Moon at each solstice (Toomer 1984, Fig. G, p. 245). The rod anchored at the top of the vertical rod was lined up with the object in the sky, and the position on the lower rod read off; in combination with the vertical length, it yielded the equivalent of the zenith angle. Drawing by E.F. Milone, after Pannekoek (1961/1989, pp. 154, 181).

forming a rough triangle with the base. A line-of-sight indicator extended into the triangle region would intersect the line of dots at the decimal fraction of the distance between the base marks. The leverage thus gained created an improved precision of angular measurement (Thurston 1994, Fig. 10.7, p. 215). Thurston (1994, p. 215) notes that the inventer of this scheme was not Tycho Brahe but Johann Hommel [1518-1562]; but Brahe used it to great effect.

In addition to the astrolabe, another instrument in widespread use for measuring angles was the cross-staff, sometimes called a Jacob's staff in the Middle Ages. Basically, this simple hand-held instrument consisted of two perpendicular sticks, both graduated, with the cross piece able to slide along the main shaft so that the angular extension of the cross-piece (or some part of it) could be made to match the angular separation of objects. Thus, the separation between a planet and a star could be measured, and from these, with the help of spherical trigonometry, the differential right ascension and declination could be obtained. It can also be used to find the altitude of an object. The construction and operation of the cross-staff is described in Schlosser et al. (1991, Appendix C, Ch. 1). The angle to be determined, a, can be obtained from the expression w = 2 x l tan—, 2

where w is the width of the cross piece that spans the angular distance in question and € is the distance from the cross piece to the eye. A finely elaborated version, with several cross pieces can be found in the David M. Stewart Museum Collection in Montreal, a photograph of which has been published by Levy (1990, p. 120). See §7.6 for further discussion of astronomy in the Middle Ages.

In Mesoamerica, there are certain sites from which alignment observations could have been made. The Caracol in the Mayan city of Chichen Itza on the Yucatan peninsula is partially in ruins today, but from the existing windows on the upper floor of the remaining part of the structure, the inner and outer edges of the window casements define narrow slitted areas through which the amplitudes of the Moon and of Venus could have been observed (see §12.22). Venus and the Moon are both tied into a calendrical system that was used in Mesoamerica during the period of time in which Chichen Itza flourished. The city also contains a pyramid named for the god Kukulcan, and called by the Spaniards, El Castillo. A bas relief of a ruler who was named after the god and may have been regarded as identical with him is depicted inside a room at the top of the pyramid. Kukulcan has been associated with Venus, although Kelley (§12.6) has argued for an identification with Mercury. Solar alignments have also been claimed for certain window casement structures in North America, principally, at Casa Grande in Arizona and at Casa Rinconada in New Mexico, although it is less clear that the latter structures were exclusively used as observatories (§13.1).

In Asia, we have some of the oldest observatories still standing (cf. §10.2). The oldest structure known at the time of writing is the Chomsongdae or Star Tower (see Figure 10.9) at Kyungju (Chhing-Chow) in the ancient Silla kingdom (now part of South Korea) during the reign of Queen Sondok (632 to 647 a.d.). It has a single window facing south and could have supported a platform on the top holding an armillary sphere. The armillary sphere is a device that recreates the basic frame of the celestial sphere. Typically, it contained the celestial equator, the celestial meridian, the horizon, and perhaps the ecliptic. With it, observations could be made of the altitudes of celestial objects on the celestial meridian, as well as at other azimuths, and of declinations. If the ecliptic was included, ecliptic coordinates could be obtained. Azimuths and hour angles could have been measured also with such devices, with, however, limited precision. There are many existing armillary spheres and other devices, such as quadrants, that were used for altitudes and declinations, in China. See Figure 3.21. A 15th-century version of an equatorial armilla said to be designed by Kuo Shou-Ching (~1276) is now located in the courtyard of the Old Beijing Observatory (Figure 3.21a). A horizon circle is supported by dragons; the celestial equator and ecliptic can also be seen. Hour circles permit the measurement of angles around the celestial equator. An "abridged armilla" also from the 15th century, and also located in the courtyard, is seen in Figure 3.21b. Notice that this instrument is actually a collection of instruments, including a sundial, horizon and azimuth circles, and an equatorial circle. See §2 for details on the coordinates and how they are measured.

The tower of Chou Kung at Kao-chheng near the important ancient imperial site of Loyang permitted measurement of the length of the Sun's shadow as it transited the meridian (Needham 1959, pp. 296-297, Figs. 115-117 from Tung Tso-Pin et al. 1939). This was therefore an observatory for the study of time and solar date (attested), and it was an ancient analog of such institutions as the Royal Greenwich

Figure 3.21. Chinese armillary spheres said to be from the 4th year of the reign of the Ming emperor Zhengtou, ~1439 a.d., and now located in the "Old Beijing Observatory," Beijing,

Figure 3.21. Chinese armillary spheres said to be from the 4th year of the reign of the Ming emperor Zhengtou, ~1439 a.d., and now located in the "Old Beijing Observatory," Beijing,

Observatory and the U.S. Naval Observatory. Figure 3.22 is a sketch of the structure of the tower with an opening providing a clear view of a graduated shadow scale 28 ft (81/2m) below, on which the length of the shadow cast by a 40-ft (~12 m) high gnomon indicated a solar calendar date. Rooms in this structure contained an armillary sphere and a water clock.

The Old Beijing Observatory houses a copy of an instrument known as a chien i, "simplified instrument," and apparently a basic type of equatorial torquetum, designed by Kuo Shou-Ching. A torquetum is a device equipped with graduated disks in both ecliptic and equatorial coordinates to facilitate conversion between the systems. The torquetum was probably invented by the astronomer Jabir ibn Aflah (b. ~1130; Spain) and possibly brought to China from the Maragha Observatory in Persia by Jamal al Din ibn Muhammed al-Najjarl in 1267 (Needham 1959, Fig. 164; Needham/Ronan 1981, Figs. 106-109, pp. 174-178). Kuo Shou-Ching's device, however, lacks the ecliptic disks, and it would have been used only to measure stars' right ascension positions relative to the Xiu, which are marked on the equatorial circle along with 12 double hours, and declinations (graduated in degrees and minutes of arc); Needham describes it as "the precursor of all equatorial telescope mountings" (Needham/Ronan, Fig. 107).

The Mongol prince Ulugh Beg [1394-1449], grandson of Tamerlane, was a gifted mathematician who founded an observatory at Samarkand, now part of Uzbekistan, in 1424. Much of the instrumentation was copied from Maragha, an older, Persian observatory, but it included the largest instrument of the day, a meridian circle of 40-m radius. Piini (1986) discusses how the instrument could have been used and illustrates another instrument from this observatory, a par-allactic ruler. Figure 3.23 shows the site and part of the now entombed meridian circle.

China: (a) Based on the equatorial armilla of Kuo Shou-Ching (~1276) and (b) the "abridged armilla." Photos by E.F. Milone.

Chou Kung Tower

Chou Kung Tower

Figure 3.22. The tower of Chou Kung at Kao-chheng near the important ancient imperial site of Loyang: View from the north end of the "Sky Measuring Scale," a 120-ft (36.5 m) low wall on which graduations permitted the shadow length cast by the noon Sun to be read off. Drawing by E.F. Milone, after Needham (1959, Plate XXXII, Fig. 116, p. 296).

Figure 3.22. The tower of Chou Kung at Kao-chheng near the important ancient imperial site of Loyang: View from the north end of the "Sky Measuring Scale," a 120-ft (36.5 m) low wall on which graduations permitted the shadow length cast by the noon Sun to be read off. Drawing by E.F. Milone, after Needham (1959, Plate XXXII, Fig. 116, p. 296).

References to early Indian instruments are cited by Sub-barayappa and Sarma (1985). The earliest reference to such instruments is in the work of the astronomer Aryabhata (b. 476) and concerns various kinds of shadow clocks, water clocks, vertical circles (for solar altitude measurements), and a device using a plumb-bob and a water level to establish the horizon and zenith. More elaborate instruments include an armillary sphere, described by Lalla (b. 768) and in the Suryasiddhanta (cf. Sen 1966). Subbarayappa and Sarma mention an observatory at Mahodayapuram in the state of Kerala, dating from 860. The 18th-century observatories of

(c) Dietrich Wegener.

Maharaja Jai Singh [1699-1743] at Jaipur and at Delhi are well known to most of the world, because many of the instruments are still in place and tours are frequently conducted to them. They represent a recreated heritage—an attempt to build classic instruments to demonstrate the way observations were likely made in ancient India. At the Jantar Mantar in Delhi, there were four kinds of instruments, one of which is alleged to have had four purposes (Nath, undated, <1986):

(1) Samrat Yantra, a large triangular structure and two quadrantal arcs, acted as a giant sundial, which was used to measure hour angles and declinations of the Sun.

(2) Rama Yantra, two large circular structures, was used to measure altitudes and azimuths. According to Nath (undated), this instrument (as well as the Dakshinobhitti described below) may have required strings as site lines.

(3) Jayaprakash Yantra, two hemispherical bowls, to measure altitudes, azimuths, hour angles and declinations.

(4) Misra Yantra, an inverted heart-shaped structure, which was composed of

(a) a Samrat Yantra;

(b) Niyat Chakra Yantra, four semicircular arcs, onto which a gnomon could cast a shadow—with this structure, the declination of the Sun could be measured at four specified times of day, corresponding to noon at each of four observatories spread around the world;21

(c) Dakshinobhitti with which meridian altitude or zenith distances could be measured; and

(d) Kark Rashivala, which provided the zodiacal sign.

At least some of these instruments can be seen in Figure 3.24, which shows the Jaipur site as it looked in November 1985. See Figure 9.10 for details of the instruments at Jaipur and at the Majarajah's other observatory, the Jantar Mantar in Delhi. These instruments are hardly "ancient," but they are naked-eye instruments and provide us with valuable insight into how data could be obtained with precision by practitioners in early times (cf. §9.1.5).

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