to 90 degrees, so is 6 hours 12 minutes to the degrees of the Arc _DdE_, which measures the Angle _EAL_; from which subtract 90 degrees, and there remains the Angle _OAL_, equal to the Angle _ALC_, under which the Earth’s Semi-diameter _AC_ is seen from the Moon. Now, since all the Angles of a right-lined Triangle are equal to 180 degrees, or to two right Angles, and the sides of a Triangle are always proportional to the Sines of the opposite Angles, say, by the _Rule of Three_, as the Sine of the Angle _ALC_ at the Moon _L_ is to it’s opposite side _AC_ the Earth’s Semi-diameter, which is known to be 3985 miles, so is Radius, _viz._ the Sine of 90 degrees, or of the right Angle _ACL_ to it’s opposite side _AL_, which is the Moon’s distance at _L_ from the observer’s place at _A_ on the Earth’s surface; or, so is the Sine of the Angle _CAL_ to its opposite side _CL_, which is the Moon’s distance from the Earth’s centre, and comes out at a mean rate to be 240,000 miles. The Angle _CAL_ is equal to what _OAL_ wants of 90 degrees. [Sidenote: The Sun’s distance cannot be yet so exactly determined as the Moon’s; How near the truth it may soon be determined.] 191. The Sun’s distance from the Earth is found the same way, but with much greater difficulty; because his horizontal Parallax, or the Angle _OAS_ equal to the Angle _ASC_, is so small as, to be hardly perceptible, being only 10 seconds of a minute, or the 360th part of a degree. But the Moon’s horizontal Parallax, or Angle _OAL_ equal to the Angle _ALC_, is very discernible; being 57ʹ 49ʺ, or 3469ʺ at it’s mean state; which is more than 340 times as great as the Sun’s: and therefore, the distances of the heavenly bodies being inversely as the Tangents of their horizontal Parallaxes, the Sun’s distance from the Earth is at least 340 times as great as the Moon’s; and is rather understated at 81 millions of miles, when the Moon’s distance is certainly known to be 240 thousand. But because, according to some Astronomers, the Sun’s horizontal Parallax is 11 seconds, and according to others only 10, the former Parallax making the Sun’s distance to be about 75,000,000 of miles, and the latter 82,000,000; we may take it for granted, that the Sun’s distance is not less than as deduced from the former, nor more than as shewn by the latter: and every one who is accustomed to make such observations, knows how hard it is, if not impossible, to avoid an error of a second; especially on account of the inconstancy of horizontal Refractions. And here, the error of one second, in so small an Angle, will make an error of 7 millions of miles in so great a distance as that of the Sun’s; and much more in the distances of the superiour Planets. But Dr. HALLEY has shewn us how the Sun’s distance from the Earth, and consequently the distances of all the Planets from the Sun, may be known to within a 500th part of the whole, by a Transit of Venus over the Sun’s Disc, which will happen on the 6th