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How the equatorial mount changed astronomy: an innovative design that allowed telescopes to track the sky made it the star of the 19th century--and today
Astronomy. 39.2 (Feb. 2011): p58.
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Most historians rate the telescope as one of the top inventions ever. It literally opened up the universe for study. But the telescope would have had limited application if not for one specialized accessory that let astronomers master the ever-improving instrument: the equatorial mount.

This mechanical wonder provided a steady platform from which observers could launch increasingly sophisticated research by producing long-exposure photographs and spectra.

This early 19th-century development catapulted the telescope from a device for enthusiasts to an engine of great discovery. The story of how the equatorial mount came to be--and how it ruled supreme for almost two centuries--is the story of how one man used technology to halt the effects of Earth's rotation.

Before the equatorial mount

In the early days of the telescope--the first half of the 17th century--optical tubes and mounts were simple affairs. Galileo Galilei's first scopes were handheld or attached to a pole to add some stability. Even as telescopes grew larger and more difficult to use, the mounts remained nothing more than flagpoles with ropes and pulleys to swing the great apparatuses toward celestial targets.

The complexity became extreme with the long-focal-length refractors used near the end of the 17th century. Astronomers such as Christiaan Huygens in Holland and Johannes Hevelius in Poland were making observations with telescopes up to about 150 feet (46 meters) long.

They and other observers used hired help to man the ropes and pulleys. That they made any meaningful observations is a testament to their determination.

Throughout the 18th century, the situation for astronomers improved only slightly. The quality of optical glass and new lens designs made for shorter telescopes with better optics. But the apertures of many research telescopes were still quite small by today's standards.

One of the most successful designs was the Gregorian reflector, which folded the light path to make a shorter and more manageable instrument. Astronomers such as French comet-hunter Charles Messier did incredible work with such telescopes. All of them sat on alt-azimuth stands for tabletop use.

The alt-azimuth system is both intuitive and simple. Imagine standing on a flat plain with a low, even horizon. As you turn to face different points on the horizon, you are moving in azimuth. Azimuth starts at due north, which is 0[degrees] on a giant compass. As you rotate to your right and face due east, you have moved to 90[degrees] azimuth, and so on. Altitude is measured from the horizon up. The horizon marks 0[degrees] altitude while 90[degrees] is directly overhead (the zenith).

A problem arises, however, because celestial objects move in arcs across the sky. With an alt-azimuth mount, you need to move the telescope in a stair-step fashion to track a star. If you watch an object for several minutes, you need to nudge the scope over and up (or down) for as long as the observation takes.

The situation really got out of hand in the late 18th century when German-born English astronomer Sir William Herschel began making reflecting telescopes. His

telescopes were as much artistry as engineering. A typical Herschel telescope held between a 6- and 8-inch mirror mounted in mahogany octagonal tubes. The mount, however, was little more than a short crank and jack mounted in a framework with wheels. This was an altazimuth mount on the go! Like all astronomers, Herschel wanted bigger mirrors, so the tubes and mounts continued to grow.

Herschel's jewel was a reflecting telescope with a 48-inchdiameter (1.22m) mirror and a focal length of 40 feet (12.2m). This monster rotated on a huge azimuth platform and operated via ropes and pulleys. There was a lot of grunting and groaning during a night of observing. Herschel eventually gave up on this monster and returned to using more manageable telescopes.

Enter Fraunhofer

The big break for telescopic astronomy occurred in Germany July 21, 1801, when a building collapsed around 14-year-old Joseph von Fraunhofer. The child was an apprentice to a rather cruel Bavarian glassmaker, Philipp Weichselberger.

As luck would have it, Maximilian IV Joseph, the prince elector of Bavaria, was riding by when the building fell. He actually assisted in the rescue efforts, finding the young Fraunhofer alive and mostly unhurt. The prince was so amazed at the seeming miracle that he became Fraunhofer's patron.

Fraunhofer excelled in practical problem-solving and soon went to work for the Optical Institute in the municipality of Benediktbeuern. He was soon turning out the finest glass in Europe. Fraunhofer also designed and built incredibly accurate instruments, such as the spectroscope, which he invented.

His greatest achievement was a refracting telescope with a 9-pouce lens. Also known as the Paris inch, 1 pouce equals 1.066 inches. The lens, therefore, measured 9.6 inches in diameter--the world's largest at the time.

Fraunhofer's lens also displayed the highest quality of any telescope optics then available. It provided excellent images over a field of view greater than 2[degrees] and had good "color," a term actually used to denote the lack of false color in an astronomical image. Historians now regard it as being the first achromatic lens. In the early 19th century, astronomers were obsessed with binary stars--pairs of suns that revolve around each other. By studying the orbits of two gravitationally bound stars, astronomers could test Isaac Newton's theory of gravity to see if it applied everywhere. To do this, however, observers needed telescopes that delivered high-precision views.

One such astronomer, Friedrich Georg Wilhelm von Struve of the Dorpat (now Tartu) Observatory in what is now Estonia, ordered what would become the most celebrated telescope of the 19th century--the Dorpat refractor, sometimes called Fraunhofer's refractor. With this telescope, Struve examined some 120,000 stars and discovered more than 2,000 previously unknown double stars.

But the optics were only half of the story. To match their quality, Fraunhofer designed a new mount, which we now call the German equatorial mount. It was this mount that changed the way astronomers studied the sky.

'Round and 'round

As Earth rotates, celestial objects appear to rise in the east, arc across the sky, and set in the west. Depending on the observer's latitude, a number of stars toward the north are circumpolar and, like Polaris the North Star, never rise or set.

Fraunhofer realized that placing the optical tube on a mount parallel to Earth's axis would allow it to track celestial objects with one smooth motion. An equatorial mount has two axes that correspond to the astronomical coordinate system of right ascension and declination. The axis parallel to Earth's polar axis corresponds to right ascension. The other axis moves the tube up and down, thus matching declination.

Astronomers need to align the telescopes in professional observatories only once. If the telescope is portable, however, the user must align it to the latitude of the observing site by pointing the mount's polar axis to the North Celestial Pole (or the South Celestial Pole in the Southern Hemisphere).

The equatorial mount greatly simplified the telescope's motions. But Fraunhofer didn't stop there. He added a weight-driven clock drive to free the observer (or a servant) from continually adjusting the telescope. In Fraunhofer's honor, astronomers even today call his design the German equatorial mount.

The idea of such a mount did not originate with Fraunhofer. In the early 17th century, German astronomer Christopher Scheiner had used a similar mount for his small refracting telescope. But Fraunhofer's design was more elegant, held a much larger telescope, and added a clock drive.

The success the Dorpat refractor enjoyed because of its innovative mount changed the world of astronomy. From then on, huge telescopes sat on sleek German mounts. And because optical glass continued getting better, the 19th century became the age of the great refractors.


Observatories with German equatorial mounts and high-precision optics began to spring up across Europe and America in the 19th century. At Konigsberg, German astronomer Friedrich Bessel used a Fraunhofer-designed 6.3-inch heliometer to measure the parallax (apparent displacement due to Earth's revolution) of the star 61 Cygni in 1838. This seminal step in astronomy opened up a way to measure cosmic distances.

At the same observatory in 1851, astronomers used a 2.4-inch Fraunhofer refractor to take the first photograph of a solar eclipse.

Yet another Fraunhofer telescope, this one with a 9.6-inch lens like the Dorpat refractor, allowed German astronomer Johann Gottfried Galle at the Berlin Observatory to discover Neptune in 1846, right at its predicted position.

Since 1897, the world's largest refractor has been the 40-inch at Yerkes Observatory in Williams Bay, Wisconsin. Weighing in at 6 tons, the tube is 62 feet (18.9m) long. The massive tube sits on a 43-foot-high (13.1m) pier topped by an incredible equatorial mount.

While Yerkes marks the end of an era, if Fraunhofer could step out onto the observatory floor, he would recognize his mount and most of the other equipment.

For more than a century, the equatorial mount dominated astronomy. With the advent of microprocessors and global positioning satellites, the need for equatorial mounts has all but disappeared. Still, manufacturers continue to equip small telescopes with this mount, demonstrating the value of Fraunhofer's elegant design.

Raymond Shubinski is a contributing editor of Astronomy who is partial to refractors.


Please note: Illustration(s) are not available due to copyright restrictions.

Source Citation   (MLA 8th Edition)
Shubinski, Raymond. "How the equatorial mount changed astronomy: an innovative design that allowed telescopes to track the sky made it the star of the 19th century--and today." Astronomy, Feb. 2011, p. 58. Academic OneFile, Accessed 22 July 2018.

Gale Document Number: GALE|A245805864