The motion of the primary Planets round the Sun (as also of the Satellites round their respective primaries) is called their _Annual Motion_; because they have one Year, or alteration of Seasons compleat, in one of these revolutions. Besides this annual motion, four of the Planets, _viz. Venus_, the _Earth_, _Mars_, and _Jupiter_ revolve about their own _Axis_, from _West_ to _East_; and this is called their _Diurnal Motion_. For by this rotation, each point of their surfaces is carried successively towards or from the Sun, who always illuminates the hemisphere which is next to him, the other remaining obscure; and while any place is in the hemisphere, illuminated by the Sun, it is _Day_, but when it is carried to the obscure hemisphere, it becomes _Night_; and so continues, until by this rotation the said place is again enlightened by the Sun. [Sidenote: Diurnal Motion of the ♁, ♀, ♂ and ♃.] [Sidenote: ☉ and ☽ likewise turn round their Axis.] The _Earth_ performs its revolution round its axis in 23 Hours, 56 Minutes;[2]_Venus_, in 24 Days, 8 Hours; _Mars_, in 24 Hours, and 40 Minutes; and _Jupiter_ moves round his own axis in 9 Hours, and 56 Minutes. The Sun also is found to turn round his axis from West to East, in 27 Days: And the Moon, which is nearest to us of all the Planets, revolves about her axis in a Month, or in the same space of time that she turns round the Earth; so that the _Lunarians_ have but 1 Day throughout the Year. [Sidenote: The Planets are Opaque and Globular.] I. The Planets are all _Opaque_ bodies, having no light but what they borrow from the Sun; for that side of them which is next towards the Sun, has always been observed to be illuminated, in what position soever they be; but the opposite side, which the Solar rays do not reach, remains dark and obscure; whence it is evident that they have no light but what proceeds from the Sun; for if they had, all parts of them would be lucid, without any darkness or shadow. The Planets are likewise proved to be _Globular_; because let what part soever of them be turned towards the Sun, its boundary, or the line separating that part from the opposite, always appears to be circular; which could not happen, if they were not globular. [Sidenote: The Planets turn round the Sun.] II. That the Earth is placed betwixt the Orbs of _Mars_ and _Venus_, and that ☿, ♀, ♂, ♃ and ♄, do all turn round the Sun, is proved from observations as follow: [Sidenote: _Plate 2. Fig. 1. 2._]