Astronomy Explained Upon Sir Isaac Newton's Principles: And made easy to those who have not studied mathematicsFerguson, James
Science
Astronomy Explained Upon Sir Isaac Newton's Principles: And made easy to those who have not studied mathematics
Ferguson, James
Astronomy -- Early works to 1800
241. The annexed Table shews the Variation, or Equation of time
depending on the Sun’s Anomaly, and arising from his unequal motion in
the Ecliptic; as the former Table § 229 shews the Variation depending on
the Sun’s place, and resulting from the obliquity of the Ecliptic: this
is to be understood the same way as the other, namely, that when the
Signs are at the head of the Table, the Degrees are at the left hand;
but when the Signs are at the foot of the Table the respective Degrees
are at the right hand; and in both cases the Equation is in the Angle of
meeting. When both the above-mentioned Equations are either faster or
slower, their sum is the absolute Equation of Time; but when the one is
faster, and the other slower, it is their difference. Thus, suppose the
Equation depending on the Sun’s place, be 6 minutes 41 seconds too slow,
and the Equation depending on the Sun’s Anomaly, be 4 minutes 20 seconds
too slow, their Sun is 11 minutes 1 second too slow. But if the one had
been 6 minutes 41 seconds too fast, and the other 4 minutes 20 seconds
too slow, their difference had been 2 minutes 21 seconds too fast,
because the greater quantity is too fast.
242. The obliquity of the Ecliptic to the Equator, which is the first
mentioned cause of the Equation of Time, would make the Sun and Clocks
agree on four days of the year; which are, when the Sun enters Aries,
Cancer, Libra, and Capricorn: but the other cause, now explained, would
make the Sun and Clocks equal only twice in a year; that is, when the
Sun is in his Apogee and Perigee. Consequently, when these two points
fall in the beginnings of Cancer and Capricorn, or of Aries and Libra,
they concur in making the Sun and Clocks equal in these points. But the
Apogee at present is in the 9th degree of Cancer, and the Perigee in the
9th degree of Capricorn; and therefore the Sun and Clocks cannot be
equal about the beginning of these Signs, nor at any time of the year,
except when the swiftness or slowness of Equation resulting from one
cause just balances the slowness or swiftness arising from the other.
Public-domain text, read in full here on John Shaqi.
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