370. Make a Scale of any convenient length (six inches at least) as
_AC_, and divide it into as many equal parts as the semi-diameter of the
Earth’s Disc contains minutes, which in this construction of the Eclipse
for _London_ in _April 1764_, is 54 minutes and 57 seconds; but as it
wants only 3ʺ of 55ʹ the Scale may be divided into 55 equal parts, as in
the Figure. Then, with the whole length of the Scale as a Radius,
setting one foot of your Compasses in _C_ as a center, describe the
Semi-circle _AMB_ for the northern Hemisphere or Semi-disc of the Earth,
as seen from the Sun at that time. Had the Place for which the
Construction is made been in South Latitude, this Semi-circle would have
been the Southern Hemisphere of the Earth’s Disc.
[Sidenote: Axis of the Ecliptic.]
371. Upon the center _C_ raise the straight line _CH_ for the Axis of
the Ecliptic, perpendicular to _ACB_.
[Sidenote: North Pole of the Earth.]
372. Make a line of Chords to the Radius _AC_, and taking from thence
the Chord of 23-1/2 Degrees, set it off from _H_ to _g_ and to _h_, on
the periphery of the Semi-disc; and draw the straight line _gNh_, in
which the North Pole of the Disc is always found.
373. While the Sun is in Aries, Taurus, Gemini, Cancer, Leo, and Virgo,
the North Pole of the Disc is illuminated; but while the Sun is in
Libra, Scorpio, Sagittary, Capricorn, and Aquarius, the North Pole is
hid in the obscure part behind the Disc.
374. And, whilst the Sun is in Capricorn, Aquarius, Pisces, Aries,
Taurus, and Gemini, the Earth’s Axis _CP_ lies to the right hand of the
Axis of the Ecliptic _CH_ as seen from the Sun, and to the left hand
while the Sun is in the other six Signs.
[Sidenote: Earth’s Axis.