To find the Moons Horizontal Parallax, or the Angle of the Earth’s semi-diameter as seen from the Moon.
Second Element.
358. Precept. Having found the Moon’s mean Anomaly for the above time, by the first and second Precepts of § [355], enter the XVth Table with the signs and degrees of that Anomaly, and thereby take out the Moon’s Horizontal Parallax: only note, that this is given but to every 6th degree of Anomaly in the Table, because it is very easy to make proper allowance by sight. So the Moon’s Horizontal Parallax April the 1st 1764, at 10 hours 25 minutes in the Forenoon, answering to her mean Anomaly at that time (namely 11s 9° 21ʹ) is 55ʹ 7ʺ; which, diminished by 10ʺ, the Sun’s constant Horizontal Parallax, gives for the semi-diameter of the Earth’s Disc 54ʹ 57ʺ.
To find the Sun’s true Place, and his distance from the nearest Solstice.
Third Element.
359. Precept I. We are to consider, that the beginning of Aries and of Libra, which are the Equinoctial Points, are equidistant from the beginning of Cancer and of Capricorn, which are the Solstitial Points. Hence, when we know in what Sign and Degree the Sun is, we can easily find his distance from the nearest Solstice. Now, to find the Sun’s Place, or Longitude from Aries, April the 1st, 1764, at 10 hours 21 minutes in the Forenoon; being the equated time of New Moon.
Precept II. This being to the time of New Moon, take out the Sun’s mean Place and Anomaly from [Table II.] for that time, and the Equation of his mean Place from [Table XII] by his Anomaly; adding the Equation to his mean Place or subtracting it from the same, as the Table directs, will give his true Place.
EXAMPLE.
| Sun’s Long. from Aries. | Sun’s mean Anomaly. | |||||
|---|---|---|---|---|---|---|
| s | ° | ʹ | s | ° | ʹ | |
| Table I. To the Sun’s mean Place and Anomaly at the mean time of New Moon in March 1764, N. S. | 11 | 17 | 7 | 8 | 2 | 23 |
| Add the same from Tab. VI. for one Lunation, to carry it to April | 0 | 29 | 6 | 0 | 29 | 6 |
| Mean Place and Anomaly at the time of New Moon in April | 0 | 10 | 13 | 9 | 1 | 29 |
| To which place add the Sun’s Equation from Tab. XII. | 1 | 56 | Equal | 1° | 56ʹ | |
| And it gives the Sun’s true place | 0 | 12 | 9 | Additive. | ||
Which is Aries 12° 9ʹ; and this, when taken from three Signs, or the beginning of Cancer, leaves 2 signs 17 deg. 51 min., or 77° 51ʹ for the Sun’s distance from the then nearest Solstice.