Ancient Astronomical Instruments

Ancient societies, including the Babylonians, Greeks, Egyptians, Indians, Chinese, and Mayans, took stargazing seriously. Within the limits of naked eye astronomy, they used a variety of basic observational techniques and later very simple instruments to define, monitor, and predict the motion of celestial objects. As revealed by archaeoastronomy, the alignment of many special ancient buildings or monuments corresponded to the locally observed position of the Sun at equinox or solstice. Other ancient structures, such as the great Egyptian pyramids, were aligned to highlight the appearance of certain conspicuous (bright) stars.

The ancient Greeks developed instruments such as the armillary sphere to track the movement of objects in the plane of the celestial equator against the annual motion of the Sun. This basic device consisted of a set of graduated rings that represented important circles on the celestial sphere, such as the horizon, the celestial equator, the ecliptic, and the meridian. These rings formed a skeletal celestial sphere. A movable sighting arrangement allowed early astronomers to observe a celestial object and then read off its position using the markings on the relevant circles.

The cross staff was another tool widely used by astronomers and navigators before the invention of the telescope. The device consisted of a main staff with a perpendicular crosspiece attached at its middle to the staff and able to slide up and down along it. The cross staff allowed medieval astronomers to measure the angle between the directions of two stars.

Ptolemy and other Greek astronomers used the quadrant, a graduated quarter of a circle constructed to allow an observer to measure the altitude of celestial objects above the horizon. An astronomer would sight a target celestial object along one arm of the quadrant and then read off its elevation from a scale (from 0 to 90 degrees) with the help of a plumb line suspended from the center of the quarter circle. With this arrangement a celestial object just on the horizon would have an elevation of 0 degrees, while an object at zenith would have an elevation of 90 degrees.

The astrolabe is the most famous ancient astronomical instrument. Essentially an early mechanical computer, this versatile device helped astronomers and navigators solve problems concerning time and the position of the Sun and stars. The planispheric astrolabe was the most popular design. A planisphere is a two-dimensional map projection that is centered on the northern or southern pole of the celestial sphere. The planispheric astrolabe had the celestial sphere projected onto the equatorial plane. The instrument, usually made of brass, had a ring so it could easily hang suspended in a vertical plane. It also had a movable sighting device that pivoted at the center of the disk. The typical astrolabe often came with several removable sky map plates to accommodate use at various terrestrial latitudes

Another basic and commonly used ancient astronomical instrument was the sundial. In its simplest form, the sundial indicates local solar time by the position of the Sun’s shadow as cast by the indicator.

Finally, the triquetrum was a medieval instrument used to measure elevation angles. It consisted of a calibrated sighting device supported between vertical and horizontal spars. Astronomers calibrated the horizontal spar in degrees.

Questions to Ponder

• How did Aristotle predict the radius of the Earth?
• When was the first telescope invented? By Who?