Sky and Telescope : Deep, clear photographic history of astronomical observation 3

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sky and telescope

Introduction : Astronomical observation

Astronomical observation has continued since primitive man. Since ancient times, man has been associated with two elements, sky and telescope. Man has been observing the sky only with his eyes. Astronomical observations are being carried out to find out how all things in the sky affect human life and how to use some of the important things to improve life.

The Merkhet : An ancient Egyptian astronomical instrument

Sky and telescope

The Egyptians were astrologers rather than astronomers. They use the The merkhet  the surveying and timekeeping instrument.The merkhet the oldest known astronomical tool develop in around 600 BCE. It involved the use of a bar with a plumb line, attached to a wooden handle. A pair of Merkhet was used to establish a north-south line (or meridian) by aligning them with the Pole Star.

They could then be used to mark off nighttime hours by determining when certain other stars crossed the meridian. The Merkhet was basically a piece of wood which was placed horizontally and from which a plumb-line was hung. Its call star clock also. It is useful to measure the time at night.   The Egyptians timed the apparently regular movements of the stars across the sky using an equal interval timer such as a water clock, and this set the basis for the Egyptian time-keeping system.

Regiomontanus torquetum circa 1470

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The Arabs encouraged astronomical learning. They translated Greek scientific works and gave special attention to Ptolemy’s Mathematical Symposium or Almagest, as his great work was called. Observatories were established at Baghdad, Cairo, Damascus, and at other important centers.

Ptolemy’s idea of using a quadrant instead of a complete circle for measuring the altitudes of stars was put into practice, and impressive masonry instruments had in size and bulk what they lacked in accuracy. Besides fixed instruments, Arabian craftsmen provided a variety of portable quadrants, dials, astrolabes, hour-glasses, and armillary spheres. After they had conquered Spain in the eleventh century, the Arabs established observatories at new centers of learning, so that observational astronomy, at least, passed without break into Europe.

The city of Baghdad became an important astronomical centre, and here Kalif al Mumun, a patron of art and science, erected an observatory which he furnished with Ptolemaic instruments. Nasir ed-din el-Tusi, first astronomer at Meragha, introduced the azimuth quadrant.

Two quadrants rotated on a common vertical axis over a graduated horizontal or azimuth circle and so enabled two observers to measure the angular separation of two stars at one and the same time. In addition to the quadrants, there was an 11-foot meridian circle, a large azimuth circle, and armillae fitted with alidades and graduated down to minutes of arc. 52

Nasir ed-din el-Tusi is believed to have introduced the turquet or torquetumša an instrument that became very popular in the fifteenth and sixteenth centuries. It was a kind of portable equatorial and altazimuth. To a base plate was hinged an inclinable plate which could be set in the plane of the celestial equator by adjusting the length of a graduated arm or stylus. At right angles to the inclinable plate was a polar axis carrying two circles.

A movable alidade indicated declinations on the upper circle, while the equatorial circle, in the plane of the inclinable plate, indicated right ascensions. Regiomontanus ascribes this instru- ment to Geber (twelfth century), whose the odolite was of similar construction, but with a vertical moving alidade instead of a full vertical circle and alidade. In any case, the torquetum appears to have been in regular use in the fourteenth and fifteenth centuries.

The last Arabian observatory of importance was that which Ulugh Beigh established at Samarkand in the fifteenth century. Here he erected large masonry

Sky and telescope : Tycho Brahe's observatory

sky and telescope

Tycho Brahe became interested in astronomy after observing a conjunction of Jupiter and Saturn, the cause, he thought, of the great plague in 1563.  He found that planetary tables were in error by several days, and there and then decided to make their improvement his prime concern.

The opportunity came in 1576, when the King of Denmark, Frederick II, granted him the island of Hveen (between Copenhagen and Elsinore) and sufficient money to build and maintain an observatory. Tycho called the observatory Uraniborg . In its scope and plan it was similar to a modern establishment, with its own garden, living-rooms, printing-presses, workshops, library, and four different buildings filled with a varied array of instruments.

Tycho saw that one way of improving observations would be to construct instruments of great size, a plan which he put into practice when he was only twenty-two by designing a 19-foot quadrant for his friend, Paul Hainzel.” This great instrument, constructed by the best craftsmen in Augsburg, was so heavy that it took twenty men to erect it on a hill in Hainzel’s estate at Göggingen.

The massive body was made almost entirely of oak beams, with the exception of iron braces and a brass strip along the limb divided into 5400 minute spaces. The entire quadrant rotated on a large oak pillar set in masonry and was made accessible for low-altitude stars by steps. When not in use, it was covered by a roof, but this and its massive proportions did not save it from destruction during a violent storm some five years later.

1535 Nurnberg edition of Viteello’s treatise on optics

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Euclid, in the third century B.C., was the first to write about the refraction and reflection of light and to mention that light travelled in straight lines.” Ptolemy’s knowledge was equally restricted and is just a summary of the ideas of his predecessors. Alhazen, an Arabian writer of the tenth to the eleventh centuries A.D., was the first to experiment with different media in the hope of finding a working theory of reflection and refraction.

He correctly explained the apparent change in the shape of the sun and moon as they approach the horizon and showed the necessity of allowing for atmospheric refraction in astronomical observations. He was also aware of the magnifying effects of spherical glass segments, but did not mention using them as aids to vision. In Opticae Thesaurus Alhazeni Aribis, the 1572 Latin translation of Alhazen’s works, there is also reference to plane, spherical, and parabolic mirrors.

Alhazen’s disciple, Vitello, a Pole who spent most of his life in Italy, also wrote a book on optics and tried to establish the laws of refraction. He showed that the scintillation of stars is due to moving air currents and pointed out that the effect is intensified when stars are viewed through running water.

The Galilean telescope

Two Galileo telescopes Institute and Museum of 1

Galileo Galilei first heard of Lippershey’s invention in May, 1609. At the time he was forty-five years of age, professor of mathematics at Padua and in some disfavour with his professional colleagues because of his outspoken anti-Aristotelian views. He was staying in Venice when he heard the rumour that’a Dutchman had constructed a telescope, by the aid of which visible objects, although at a great distance from the eye of the observer, were seen distinctly as if near.

Galileo made a better telescope which magnified about 8 diameters and, a few days later, another of 20 magnification. Through the latter he saw that Jupiter had a round disk while the moon appeared to have a rough, mountainous surface. On January 7, 1610, he observed Jupiter with a fourth and even better telescope of 30 magnification, having ground the lenses himself and spared ‘neither labour nor expense.’

This time he saw three small bright stars near the planet and took them to be fixed stars in the background. On the following night, to his amazement, they had changed their positions both relative to the planet and to one another. By January 13, he had seen a total of four stars near Jupiter, and observations up to March established beyond all doubt that they were not stars at all but four satellites.

The fourth telescope also confirmed his first impression that the moon’s surface was not smooth like a mirror, as many ancient Greeks had thought, but rough with valleys and mountains. He noticed that these features accounted for the irregularities of the terminator and that some of the higher peaks, illumined by the rising sun, shone like stars from the dark side of the disk. He also remarked that the bright spots scattered over the full moon (certain craters and mountain masses), appeared in the darker parts (the maria) as well as in the brighter.

The Keplerian telescope

keplerian telescope1 2

Kepler obtained a telescope in 1610, a gift from Ernest, Archbishop of Cologne, and, in his Dioptrice (1611), Kepler discussed its theory. In this work he enlarged upon his ideas on refraction and wrote about the anatomy of the eye. He described, for the first time, the defect of spherical aberration and stated that it could be over- come by giving optical surfaces hyperboloidal forms.

This notion was suggested to him after he had studied the crystalline lens of the human eye, the anterior refracting surface of which is hyperboloidal. As far as he could judge, the eye was free from spherical aberration and he ascribed this to the form of the crystalline lens.

He showed, also for the first time, that before an object can be seen distinctly, its image must be sharply formed on the retina. This theory of vision, to us obvious enough, was in Kepler’s day in direct contrast to that held by some of his contemporaries, who still believed in the Euclidean idea that light proceeds from the eye to meet the object seen. Leonardo da Vinci had discussed retinal images towards the end of the fifteenth century, but Kepler was the first to establish their existence and the fact that they are inverted.

Hevelius telescope

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Another discovery due to Christian Huygens, and made without any knowledge of Cysat’s nearly forty years previously, was that of the great Orion nebula. Upon hearing of Huygens’ discoveries, Hevelius planned several new telescopes.

He had himself studied the problem of Saturn’s appearance, and the Dutchman’s work convinced him of the superiority of long-focus object glasses. The outcome was that he made telescopes of 60 and 70 feet focus and, finally, one of 150 feet, all of which he described in his Machinae Coelestis. 10 The lenses for the 150-foot were made by a local glassworker expert in all kinds of mechanical as well as optical studies, and occasioned less trouble than the mounting. For this, Hevelius used wood.

A paper tube, although light, would have been too flimsy and fragile, an iron tube too heavy and costly. The tube was sectional, each section consisting of two 40-foot wooden planks fixed at right angles to each other. Three or four of these sections, joined end to end, made a two-sided trough; at the further end was the objective cell, at the other, the eyepiece. This arrangement, braced by wire stays, answered for night use but, during twilight or moonlight, the eyepiece had to be shielded from stray light.

Hevelius, therefore, fixed wooden apertures or ‘stops’ at intervals along the tube. These not only assisted in its re-alignment but added to the rigidity of each section. The entire apparatus was suspended from a mast go feet high and was operated from below by means of ropes and pulleys.

Danzig telescope

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Hevelius spared neither time nor money in the construction of his telescopes. The Machinae Coelestis, with its fine plates and typography, is characteristic of the joy and labour which he derived from and gave to his work. The fame of Sternenburg, his Danzig observatory, spread throughout Europe and, if we judge his illustrations rightly, the erection of his giant 150-foot telescope was a big event in the city. He conceived plans for an ideal’ tower-observatory, suitable for housing giant telescopes and with facilities for their immediate erection.

Huygen's aerial telescope

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Huygens was the first astronomer to appreciate the importance of atmospheric conditions upon telescopic ‘seeing’. He noticed that stars twinkled, and that the edges of the moon and planets trembled in the telescope, even when the atmosphere appeared calm and serene.

One could object to his aerial telescope on the grounds that the string would curl in windy weather, but he points out that a tube would shake far more and that observations made under such conditions would be of little value. So frequent are nights when the seeing is poor that Huygens warns the observer against too hastily blaming his telescope. Newton considered the aerial telescope a great improvement on the long tubes of the period.

The newton’s reflecting telescope

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Curt Richter became aware of the wonderful opportunity offered for the study of effects produced by domestication on this animal, specimens of both the domesticated forms and their wild ancestors being available in large numbers.

This resulted in the discovery of a rat poison-ANTU, a thiourea compound-and also in an extensive study of the physiology of the thioureas in general. Comparison between effect of domestication of the rat and human civilization were discussed in the paper “Rats, Man and the Welfare State.”

Studies of wild rats led also to chance observations on the phenomenon of “sudden death” in animals and humans.

 

In 1663, James Gregory proposed in his Optica Promota a two mirror combination . A large paraboloidal concave collected the light, reflected it to a small ellipsoidal concave which, in turn, formed the final image at the centre of the main speculum. Instead of Gregory’s concave ellipsoidal secondary mirror, Newton used a small flat, inclined at 45° to the axis of the tube, which reflected the convergent beam to a hole in the side of the tube and so into the eyepiece. The first instrument on this plan was made in 1668, but it remained unknown, save to a few of his Cambridge friends, until about three years later. News of his work on colour had then come to the notice of the Royal Society.

Newton’s first telescope was little more than an interesting scientific toy. He writes, The diameter of the Sphere to which the Metal was ground concave was about 25 English Inches, and by consequence the length of the Instrument about six Inches and a quarter. The Eye-glass was Plano-convex, and the diameter of the Sphere to which the convex side was ground was about 1/5 of an Inch, or a little less, and by consequence it magnified between 30 and 40 times. By another way of measuring I found it magnified 35 times.

The concave Metal bore an Aperture of an Inch and a third part, but the Aperture was limited not by an opake Circle, covering the Limb of the Metal round about, but by an opake Circle, placed between the Eyeglass and the Eye, and perforated in the middle with a little round hole for the Rays to pass through to the Eye. For this Circle being placed here, stopp’d much of the erroneous Light, which otherwise would have disturbed the Vision. By comparing it with a pretty good Perspective of four Feet in length, made with a concave Eye-glass, I could read at a greater distance with my own Instrument than with the Glass.

Yet objects appeared much darker in it than in the Glass, and that partly because more light was lost by Reflexion in the Metal, than by Refraction in the Glass, and partly because my instrument was overcharged. Had it magnified but 30 or 25 times, it would have made the Object appear more brisk and pleasant.

In the autumn of 1671, Newton made a second reflecting telescope, news of which so roused Oldenburg’s curiosity that he asked Newton to send the instrument to the Royal Society for inspection by the Fellows. Its appearance at the meeting of January 11, 1672, excited great interest. Among those who examined it were King Charles II, Sir Robert Moray, Robert Hooke, and Christopher Wren. At the same meeting, Newton was elected Fellow of the Society.

Reference

1.King, Henry ,The history of the telescope, London. 2. Sky & Telescope | Astronomy News, Tools & Resources – Sky & Telescope (skyandtelescope.org)

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