Astron o m y.

^ Oppofitions to the Sun, alternate andUc cesfively j which could not be, unless

their

8, '"pi

US als o the apparent Magnitude of Mars , whenan ce is t Y, is to that when his Distance is TY, asHe ar j to that is, as 2523* to 523% or very

as 2 5 to 1. And this we know is true in Fact, byWise 0 L In .& t ^ le Planet in both those Distances. It is like—^ista V '° US to COmmon Sense ; for Mars in his nearesta PP ears so large that he has been often mif-app Ca er Jupiter, whereas in his greatest Distance he*"° tmall as scarcely to be distinguished from aQ Star.

Alo 0n ’j 0 ; 11 what has been said of the Phases of thetiiufl- l tls cas y to understand that Venus and MercuryKn Us . ave nearly the like Appearances. Thus whenturned at Y, all her illumined Hemisphere will beAs a ' re ctly from the Earth, and she will then be Neui.A (Jj 6 P^es from V to A she will appear horned. At£ art ^ ^11 shew just half her enlightened Surface to thet) n 5 and appear bisected, or dichotomised. F rom A to

tv 0u |j tvill appear more and more gibbous ; and at Dhot .1 a Pp e ar a Full enlightned Hemisphere, were ithind ' S then lost in the Sun’s Blaze, or hid be-

nt^ i! S P°dy : All which Phases return again in thele, j\yr ha lf of the Orbit. The fame Thing is obvious^ u PhIr Urh anc * Mars soews Part of those Phases ; butteas 0ri a t>d Saturn appear always with a Full face, by1 0 their very great Distance.sove ra j jT e Appearance of Venus in the Day-time forlag ac _i ^ a ys together, in some certain Years, put thet'T y ® Hr. Halley on resolving the following Problem,1 vben ^ situation of Venus in respect of the Earth,

°f ^ 3e illuminated Part of her Disk is a

Pto

and

a * l »iu7n.

Posed

1 mall here

give

the Solution as he has

also n ^ the Trs-yhK/s-n, N°. 349.;

ij , the Demonstration, which the Doctor omitted.^ en Us i 11 ? r< so s to this, let S be the Sun, V the Planether Dm tae situation required, T the Earth, and T Vut ance sought. Put T S = a, SV=i,TV=r,K 4 and

*35

Pl.LVI.

Fig. 1.