Comets

The following is extracted from Ferguson’s Astronomy, and may not be uninteresting at this time.  (N.Y.E. Post.)

‘The Comets are solid opaque bodies, with long transparent trains of tails, issuing from that side which is turned away from the sun.  They move about the sun in very eccentric ellipses; and are of much greater density than the earth; for some of them are heated to such a degree as would vitrify or dissipate any substance known to us.  Sir Isaac Newton computed the heat of the Comet which appeared in the year 1680, when nearest the sun, to be two thousand times hotter than red hot iron.’

It is believed that there are at least twenty-one comets belonging to our system, moving in all sorts of directions, but the period of three only are known with any degree of certainty.  The first of these Comets appeared in the year 1581, 1607, 1782, 1858, and is expected to appear every 75th year afterwards.   The second of them appeared in 1532 and 1661, and was expected to return in 1789, and every 129th year afterwards.  The third having last appeared in 1680, and its period being no less than 57 years, cannot return till 2225.  This Comet at its greatest distance is about eleven thousand two hundred millions of miles from the sun—and its least distance from the sun’s centre which is 48,000 miles, is within less than a third part of the sun’s semidiameter from his surface. 

In that part of its orbit which is nearest the sun, it flies with the amazing swiftness of eight hundred and eighty thousand miles an hour; and the sun, as seen from it appears a hundred degrees in breadth; consequently forty thousand times as large as he appears to us.  

The astonishing length which this comet runs out into empty space, suggests to our minds an idea of the vast distance between the sun and the nearest fixed stars, of whose attractions all the comets must keep clear, to return periodically and go round the sun.’

None of the Comets have threatened the earth with a nearer appulse than that of 1680; for by calculation Dr. Halley found that November 11th, 1 hour, 9 minutes p.m. that Comet was not above one semi-diameter of the earth to the northward of the way of the earth.  If the earth had been at that time in the part of her orbit nearest to that node of the Comet through which it passed, their mutual gravitation must have caused a change in the plane of the orbit of the earth, and in the length of our year.  Dr. Halley adds that if so large a body, with so rapid a motion as that of this comet near its perihelion, were to strike against our earth, an event by no means impossible, the shock might reduce the beautiful frame to its original chaos.’