Philosophical Transactions of the Royal Society - Vol 1 - 1666 / Giving some Accompt of the present Undertakings, Studies, and Labours of the Ingenious in many considerable parts of the World

by Various

9. He represents, that some other Astronomers have also made at Rome several Observations of these Spots of Mars, from March ¹⁴/₂₄. to March ²⁰/₃₀. with Glasses, wrought by Eustachio Divini, of 25. and 45. Palmes; Which Spots he makes little differing from his own, of the first Face; as will by and by appear, by the direction to the Schemes.

10. But he adds, that those other Roman Astronomers, that have observ'd with Divini's Glasses, will have the Conversion of Mars to be performed, not in 24 h, 40 m. (as he maintains it is) but in about 13 h.

11. And to evince, that they are mistaken in these Observations of theirs; he alledges, That they assure that the Spots, which they have seen in this Planet, (by an Eustachian Telescope) the ²⁰/₃₀ of March, were small, very distant from one another, remote from the middle of the Disk, and the Oriental Spot was less, than the Occidental (as is represented by the Fig. O; like that of the first Face of Mars.) whereas, on the contrary,

He (Cassini) pretends to evidence by his Observations, made at the same time at Bononia, that, the same day and hour, those Spots were very large, neer one another, in the midst of the Disk, the Oriental bigger than the Occidental (as appears by Fig. P, which is that of the second Face of Mars.)

12. Besides, he declares, that those Astronomers were too hasty, in determining, after 5 or 6 Observations only, in how much time Mars finish's his Revolution; and denies it to be perform'd in 13 hours: adding, that, though Himself had observ'd for a much longer time, than they; yet he durst not for a great while define, Whether Mars made but one Turn in 24 hours 40 minuts or two; and that all, that he could, for a long time affirm, was onely this, that after 24 h. 40 m. this Planet appear'd in the same manner he did before.

13. But since those first Observations, He affirms to have found cause to determine, that the Period of this Conversion is made in the said space of 24 h. 40 m; and not oftner than once within that time; Alledging for proof;

1. That, whereas Febr. 6. (st.n.) he saw the Spots of the first Face of Mars, moving from eleven of the Clock in the night, until break of day, they appear'd not afterwards in the Evening after the rising of that Planet (witness several intelligent persons, which he names, that were present at the Observations) Whence he infers, that after 12 hours and 20 minuts, the same Spots did not come about; since that the same, which in the morning were seen in the middle, upon the rising of Mars; after 13 or 14 hours, might have appear'd neer the Occidental Limb. But, because he might be imposed upon by Vapors, whilst Mars was yet so neer the Horizon, he gives this other determination, vid.

2. Whereas he saw the first Face of Mars the 6 of February at 11 of the clock of the night following; he did not see the same after 18 daies at the same hour; as he ought to have done, if the Period were absolved in the space of 12 h. 20 m.

3. Again, whereas he saw Febr. 24. in the Evening, the other Face of Mars, he could not see the same, the 13. and 15. day of March, to wit after 17 and 19 days; as he should have done, if the Revolution were made in the newly mention'd time.