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
the least of all, namely, in Pisces and Aries.
280. The Moon goes round the Ecliptic in 27 days 8 hours; but not from
Change to Change in less than 29 days 12 hours: so that she is in Pisces
and Aries at least once in every Lunation, and in some Lunations twice.
[Sidenote: Why the Moon is always Full in different Signs.
Her periodical and synodical Revolution exemplified.]
281. If the Earth had no annual motion, the Sun would never appear to
shift his place in the Ecliptic. And then every New Moon would fall in
the same Sign and degree of the Ecliptic, and every Full Moon in the
opposite: for the Moon would go precisely round the Ecliptic from Change
to Change. So that if the Moon was once Full in Pisces, or Aries, she
would always be Full when she came round to the same Sign and Degree
again. And as the Full Moon rises at Sun-set (because when any point of
the Ecliptic sets the opposite point rises) she would constantly rise
within two hours of Sun-set during the week in which she were Full. But
in the time that the Moon goes round the Ecliptic from any conjunction
or opposition, the Earth goes almost a Sign forward; and therefore the
Sun will seem to go as far forward in that time, namely 27-1/2 degrees:
so that the Moon must go 27-1/2 degrees more than round; and as much
farther as the Sun advances in that interval, which is 2-1/15 degrees,
before she can be in conjunction with, or opposite to the Sun again.
Hence it is evident, that there can be but one conjunction or opposition
of the Sun and Moon in a year in any particular part of the Ecliptic.
This may be familiarly exemplified by the hour and minute hands of a
watch, which are never in conjunction or opposition in that part of the
dial-plate where they were so last before. And indeed if we compare the
twelve hours on the dial-plate to the twelve Signs of the Ecliptic, the
hour-hand to the Sun and the minute-hand to the Moon, we shall have a
tolerably near resemblance in miniature to the motions of our great
celestial Luminaries. The only difference is, that whilst the Sun goes
once round the Ecliptic the Moon makes 12-1/3 conjunctions with him: but
whilst the hour-hand goes round the dial-plate the minute-hand makes
only 11 conjunctions with it; because the minute hand moves slower in
respect of the hour-hand than the Moon does with regard to the Sun.
[Sidenote: The Harvest and Hunter’s Moon.]
282. As the Moon can never be full but when she is opposite to the Sun,
and the Sun is never in Virgo and Libra but in our autumnal months, ’tis
plain that the Moon is never full in the opposite Signs, Pisces and
Aries, but in these two months. And therefore we can have only two Full
Moons in the year, which rise so near the time of Sun-set for a week
together as above-mentioned. The former of these is called the _Harvest
Moon_, and the latter the _Hunter’s Moon_.
[Sidenote: Why the Moon’s regular rising is never perceived but in
Harvest.]
Public-domain text, read in full here on John Shaqi.
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