Having removed to London, he was admitted (November 6, 1618) a licentiate of the college of physicians, and attracted notice by a publication concerning the comet of 1618.
In Pierre, however, that comet with its long luminous tail aroused no feeling of fear.
Thus a comet may be encountered in the morning dawn or evening twilight, and without such an adjunct the astronomer may lose the whole available opportunity for observation in the vain endeavour to find a suitable comparison-star.
To the right and high up in the sky was the sickle of the waning moon and opposite to it hung that bright comet which was connected in Pierre's heart with his love.
There he observed the transit of Venus of 1882 and photographed the great comet of that year.
It is, however, an open question whether a comet is other than an accumulation of meteoric bodies (see Comet).
Having selected the most suitable one he directs the axis of the finder to the estimated middle point between the comet and the star, turns the finder-micrometer in position angle until the images of comet and star lie symmetrically between the parallel position wires, and then turns the micrometer screw (which moves the distance-wires symmetrically from the centre in opposite directions) till one wire bisects the comet and the other the star.
There he completed his investigation of the comet of 1680, for which the Cotta prize was awarded to him in 1817; he correctly assigned a period of 71 years to the comet of 1812; and discovered the swift circulation of the remarkable comet which bears his name (see Comet).
Cassini, the great comet of 1680 after its perihelion passage; and having returned to England, he married in 1682 Mary, daughter of Mr Tooke, auditor of the exchequer, with whom he lived harmoniously for fifty-five years.
This comet had been observed in 1066, and the accounts which have been preserved represent it as having then appeared to be four times the size of Venus, and to have shone with a light equal to a fourth of that of the moon.