The value of this observation, in correcting the motion of Venus, has already been explained. We must next ascertain how the facts which are deduced from it agree with the calculations of astronomers. This inquiry will doubtless shew the usefulness of the observation to the practical student; especially as it will appear that even the best astronomers have not only disagreed among themselves, but have considerably deviated from the truth.

There are four astronomers from whose tables Ephemerides are at this time chiefly computed, into whose respective merits, as there is some difference of opinion, it may be well carefully to inquire.

1st. Copernicus who compiled the new, or rather the renewed, hypotheses, and the laws of the sidereal motions, in six books of Revolutions, from which Erasmus Reinhold afterwards constructed the Prutenic tables; and from these, Origanus, Maginus, and others derived their Ephemerides which are still extant, and are chiefly used in our prognostics, though now the Prutenic calculation is held in less esteem.

2nd. Longomontanus, the disciple of Tycho Brahé, and as it were the heir of his discoveries, who, in his Danish astronomy, treading faithfully in the footsteps of his master, brought to a conclusion those things which Tycho was by death prevented from finishing.

3rd. The sagacious Kepler, who formerly assisted Tycho in his calculations, was afterwards astronomer to three Emperors, and happily effected the renovation of the science by the publication of the Rudolphian tables, to which his other writings may be considered a prelude.