BIELAS COMET. 



that the mass assigned to Mercury by Laplace was too great in 

 the proportion of 12 to 7. 



This question is still under examination, and every succeeding 

 perihelion passage of the comet will increase the data by which 

 its more exact solution may be accomplished. 



26. On February 28, 1826, M. Biela, an Austrian officer, 

 observed in Bohemia a comet, which was seen at Marseilles at 

 about the same time by M. Gambart. The path which it pursued, 

 was observed to be similar to that of comets which had appeared 

 in 1772 and 1806. Finally, it was found that this body moved 

 round the sun in an oval orbit, and that the time of its revolution 

 was about 6 years and 8 months. It has since returned at its 

 predicted times, and has been adopted as a member of our system, 

 under the name of Biela's comet. 



Biela's comet moves in an orbit whose plane is inclined at a 

 small angle to those of the planets. It is but slightly oval, the 

 length being to the breadth in the proportion of about 4 to 3. 

 When nearest to the sun, its distance is a little less than that of 

 the earth ; and when most remote from the sun, its distance 

 somewhat exceeds that of Jupiter. Thus it ranges through the 

 solar system, between the orbits of Jupiter and the earth. 



This comet had been observed in 1772 and in 1806 ; but the 

 elliptic form of its orbit, and consequently its periodicity, was 

 not discovered. Its return to perihelion was predicted and 

 observed in 1832, in 1846, and in 1852 ; but that which took 

 place in 1838 escaped observation, owing to its unfavourable 

 position and extreme faintness, 



A Table, showing the elements of this comet during each of its 

 appearances from 1772 to 1846 inclusive, may be seen by refer- 

 ence to my " Hand-Book of Astronomy." 



27. One of the points at which the orbit of Biela's comet 

 intersects the plane of the ecliptic, is at a distance from the earth's 

 orbit less than the sum of the semi-diameters of the earth and the 

 comet. It follows, therefore (2905), that if the comet should 

 arrive at this point at the same moment at which the earth 

 passes through the point of its orbit which is nearest to it, a 

 portion of the globe of the earth must penetrate the comet. 



It was estimated on the occasion of the perihelion passage of 

 this comet in 1832, that the semi-diameter of the comet (that 

 body being nearly globular, and having no perceptible tail) was 

 21000 miles, while the distance of the point at which its centre 

 passed through the plane of the ecliptic, on the 29th October in 

 that year, from the path of the earth was only 18600 miles. If 

 the centre of the earth happened to have been at the point of its 

 orbit nearest to the centre of the comet on that day, the distance 



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