PHYSICAL SCIENCE. 



XXI 



gress, he belonged to a noble family in Denmark 

 a class of men entitled, by their rank, to dis- 

 claim the pursuit of knowledge, and extremely 

 jealous of the privilege of ignorance. But his 

 enthusiasm enabled him to break through the 

 trammels of his order. He even acquired the 

 patronage of the king of Denmark, and was able, 

 in consequence, to erect an observatory in the 

 island of Huena, supplied with far better instru- 

 ments than had ever yet been applied to astrono- 

 mical observations. Tycho, by means of them 

 could measure angles to ten seconds, which was 

 sixty times the accuracy of Ptolemy, or of any 

 instruments that had belonged to the school of 

 Alexandria. 



His first object was a catalogue of the stars, 

 which he was anxious to make more accurate 

 than that of Hipparchus and Ptolemy. The great 

 difficulty in executing this task, proceeded from 

 the want of an easy method of determining the 

 distance of one heavenly body east or west of 

 another. The distance north or south was easily 

 determined by the common method of meridian 

 altitudes. For the equator is a plane, which, 

 for any given place on the earth's surface, always 

 retains the same position. But no plane passing 

 through the poles retains a fixed position with 

 respect to an observer. The natural substitute is 

 the measure of time. But this was wanting both 

 to the Greek astronomers and to Tycho. Hip- 

 parchus and Ptolemy determined the longitude 

 of the fixed stars, by referring their places to 

 those of the moon. Thus depending on the 

 most irregular of the heavenly bodies for deter- 

 mining the position of the most fixed. Tycho 

 made use of the planet Venus, instead of the 

 moon ; and his method, though more tedious, 

 was more accurate than that of the Greek astro- 

 nomers. His catalogue contained the place of 

 777 stars. 



The irregularities of the moon's motion were 

 the next subject of inquiry. The ancients had 

 discovered the inequality of this planet, depend- 

 ing on the eccentricity of the centre, and now 

 called the equation of the centre. Ptolemy had 

 added the knowledge of another inequality in 

 the moon's motion, to which the name of evection 

 has been given, causing an increase of the former 

 equation at the quarters, and a diminution of it 

 at the times of new and full moon. Tycho dis- 

 covered another inequality, which is greatest at 

 the octants, and depends upon the difference 

 between the longitude of the moon and of the 

 sun. A fourth* inequality, depending wholly on 

 the sun's place, was known to Tycho, but includ- 

 ed among the sun's equations. Besides, these 

 observations made him acquainted with the 

 changes in the inclination of the plane of the 

 moon's orbit; and, lastly, with the irregular 



motion of the nodes, which, instead of being 

 always retrograde at the same rate, are subject 

 to change that rate, and even to become progres- 

 sive, according to their situation in respect of 

 the sun. These constitute all the irregularities 

 in the moon's motion known before the develope- 

 ment of the theory of gravitation, and all, except 

 the two first, were discovered by Tycho. 



The atmospherical refraction, by which the 

 heavenly bodies are made to appear more elevated 

 above the horizon than they really are, was sus- 

 pected, indeed, before the time of Tycho; but 

 not certainly known to exist. He first became 

 acquainted with it by finding that the latitude of 

 his observatory, as determined by observations at 

 the solstices, and from observations of the great- 

 est and least altitudes of the circumpolar stars, 

 always differed about four minutes. He supposed 

 the effect of refraction to be 34' at the horizon, 

 and to diminish from thence upwards to 45, 

 where it ceases altogether. This last opinion is 

 erroneous; but at 45 it is less than I', a quantity 

 probably not discernible by his instruments in 

 the altitudes measured. He contrived an instru- 

 ment, on purpose to make the refraction dis- 

 tinctly visible. It was an equatorial circle of 

 ten feet diameter, turning on an axis parallel 

 to that of the earth. With the sights of this 

 equatorial, he followed the sun on the day of the 

 summer solstice, and found that, as it descended 

 towards the horizon, it rose above the plane of 

 the instrument. At its setting, the sun was raised 

 above the horizon by more than its own diameter. 



The comet of 1570 was observed by Tycho, 

 and gave rise to a new theory of these bodies. 

 Its parallax was SO*, showing that it was three 

 times farther off than the moon. He considered 

 comets, in consequence, as bodies placed far be- 

 yond the range of our atmosphere, and moving 

 round the sun. His observations on the new star 

 of 1 572, deserve also to be noticed. It appear- 

 ed in Cassiopeia, on the 7th of November, all at 

 once, and surpassed all the stars in splendour, 

 being scarcely inferior to Venus herself. In the 

 month of January, 1573, it was rather less than 

 Jupiter. From this time it was constantly di- 

 minishing in splendour, and disappeared alto- 

 gether in the month of March, 1574. Pliny 

 informs us that it was the appearance of a new 

 star which led Hipparchus to think of making 

 a catalogue of the stars. 



Tycho, notwithstanding his merit as an obser. 

 ver, could not prevail upon himself to adopt the 

 Copernican system of the motion of the earth 

 round the sun. He made the sun move round 

 the earth, while it was at the same time the centre 

 of the planetary motion. 



Kepler followed Tycho. He was born in 1570, 

 and at an early age applied himself to the study 



