Babylonian observations (1500 BC?) recorded solar and lunar eclipses as well as planetary observations using merkets and waterclocks.

Macedonian philosopher **Thales** of Miletus (575-532 BC?) predicted a
solar eclipse using Babylonian observations and mapped out constellations
to aid navigation.

Alexandrian astronomer **Eratosthenes** (260-201 BC?) measured the
circumference of the Earth
using comparative shadow rod measurements in two
places and knowledge of the distance between them.

Alexandrian astronomers **Aristillus** and **Timocharis** charted the
positions of the brighter stars (284 BC), producing the first star catalog
using a Cross-staff.

**Aristarchus** of Samos (250 BC?) calculated the distance of the
Sun from
the Earth and the Moon
and Sun's sizes relative to Earth by observations
during solar and lunar eclipses and at first quarter Moon using a
Cross-staff.

**Eratosthenes** (204 BC) catalogued more than seven hundred stars using
one, and possibly two, armillary spheres.

**Hipparchus** of Rhodes (c. 150 - 125 BC) used the equatorially mounted
Armillary Sphere for a variety of measurements. He determined the distance
from the Earth to the Sun by two methods (129 BC). He established a new
star catalog complete for his latitude and to the limit of human vision
when a "new star" appeared in the sky in 134 AD; containing some 8000
entries, this catalog remained the most complete stellar catalog until
Edmund Halley undertook in 1677 observations of the southern skies from an
observatory in St. Helena that yielded a catalog of 341 stars.

He discovered the precession of the equinoxes by comparing his star catalogs with two earlier ones (125 BC). This remained the most precise calculation of equinoctal precession until it was redetermined at Samarkand by the Arabic astronomer Ulugh Begh in 1450.

He measured the lunar orbit, determining lunar orbital precession and orbital eccentricity (125 BC?).

He measured the solar orbit, proposing the Sun did not move in a circular path around the Earth (125 BC?).

Alexandrian astronomer **Ptolemy's**
(c. 100- c. 178 AD) preferred the
Quadrant to a complete circle because of easier construction and use. He
designed one but never built it. To simplify altitude measurements further,
Ptolemy designed the Triquetrum (also called "Ptolemy's Rules") which was
easier to construct and more portable than the Quadrant.

Arabic astronomer **Abu Abdullah al Battani** (Albategnius) invented the
Mural Quadrant, used in a 20-foot version by Abul Wefa at Baghdad; also
used was a 56 foot stone sextant (900-1050 AD?).

Arabic astronomer **Nasir ed Din al Tusi** (1005-1072?) began his career
as an astronomer at Baghdad, became the captive astrologer of Shaikj al
Jebel, from whose imprisonment he was freed by Hulagu, grandson of Genghis
Khan. al Tusi then worked in an observatory started by Hulagu at Meragha,
close to modern Tabriz. Here al Tusi invented the Azimuth Quadrant and the
Torquetum. The Azimuth Quadrant allowed simultaneous measurements of an
object's altitude and azimuth. The Torquetum was an astronomical
calculator with the ability to measure simultaneously an object's right
ascension and declination from a single measurement of its position on the
sky, once the observer's latitude was calibrated into the machine.

Uzbekh astronomer **Ulugh Begh** at Samarkand (1450 AD) constructed
large masonry quadrants with which he determined the obliquity of the
ecliptic and data for the construction of new solar tables.

Ptolemy designed the astrolabe (c. 150 AD) as an armillary ring, held vertically, on which was fit a diametrical cord with one sight at each of its ends. Arabic craftsmen (9th-11th centuries) improved the astrolabe to determine time from stellar or solar observations without use of tables, though it was limited to one century and one latitude. English poet Geoffrey Chaucer describes an astrolabe in detail in his Treatise on the Astrolabe (1381).

German astronomer **Georg Purbach** (14XX-14XX?) invented each of these
instruments. The Geometrical Square was designed to measure altitudes like
a quadrant; unlike a quadrant, this device had a square shape and was
marked off at arbitrary intervals rather than in degrees of arc. The
"Jacob's Staff" was a modification of Ptolemy's Triquetrum. The Regula was
designed for measuring the altitudes of the Sun and Moon. So great were
the instrumental and observational errors in these devices that combining
their results produced a position estimate whose uncertainties were larger
than 6 degrees of arc.

Polish astronomer **Nicolas Copernicus** (1473-1543), or Nikolaus
Koppernick, built an eight-foot-long triquetrum. Despite the accuracy
possible with it, Copernicus did not pursue planetary measurements long
enough to discover the significant errors in predictions based on Ptolemy's
model for the solar system. So great was Copernicus' trust in Ptolemy's
observations that he quoted Ptolemy's incorrect value for the horizontal
parallax of the Sun of 3 arcminutes rather than remeasure it. Had he done
so, Copernicus would have discovered the value too small to measure with
his instruments; today we know the value to be 20 times smaller than
Ptolemy's value -- only 8.8 arcseconds.

Danish astronomer **Tycho Brahe** (1546-1601) improved on all of these
instruments. His first azimuthal quadrant was 19 feet in radius. Built
for his friend Peter Hainzel, mayor of Augsburg, Germany, in 1568, it
featured divisions to one arcminute; destruction by fire only five years
later prevented this instrument from making important observations. Just
as a "new star" in 134 BC prompted Hipparchus to create the world's first
vision-limited star catalog, so a "new star" appearing in Cassiopeia in
1572 -- whose parallax measurements by different observers were so
discordant that Michael Mästlin's measurements in Tübingen, made with only
a piece of thread and which revealed no parallax, were considered the most
trustworthy -- prompted Tycho Brahe to devote his life to improving
astronomical instruments and to making the best measurements with them.

Tycho's observations of the nova of 1572, published in 1573, and lectures on astronomy he delivered in 1574 in Copenhagen, convinced the Danish king Fredrick II to grant Tycho an island on which to build an observatory, money to construct the needed instruments and a lifelong pension to run it. The facility he constructed was equivalent in function to a modern observatory.

Tycho used equatorial armillary spheres for determining declinations and hour angles of objects anywhere in the sky. He also employed large azimuthal quadrants for a similar purpose.

Tycho used an early form of vernier scales -- called transversals -- and compound sights on his instruments to measure objects to an accuracy of three arcminutes, better than his contemporaries by factors of 10 to 50. He further reduced this error by averaging the several results of each of several observers and by correcting these measures for atmospheric refraction, the first astronomer to do so. His smallest correction for refraction was one arcminute (one-sixtieth of a degree).

Bill Arnett); last updated: 2003 Jun 25