Chapter XXIII: Part 23

The nearer an object is to the eye the bigger it appears, and under
the greater Angle is it seen. To illustrate this a little, suppose an
Arrow in the position _IK_, perpendicular to the right line _HA_ drawn
from the eye at _H_ through the middle of the Arrow at _O_. It is
plain that the Arrow is seen under the Angle _IHK_, and that _HO_,
which is it’s distance from the eye, divides into halves both the
Arrow and the Angle under which it is seen: _viz._ the Arrow into
_IO_, _OK_, and the Angle into _IHO_ and _KHO_: and this will be the
case whatever distance the Arrow is placed at. Let now three Arrows,
all of the same length with _IK_, be placed at the distances _HA_,
_HC_, _HE_, still perpendicular to, and bisected by the right line
_HA_; then will _AB_, _CD_, _EF_, be each equal to, and represent
_IO_; and _AB_ (the same as _IO_) will be seen from _H_ under the
Angle _AHB_; but _CD_ (the same as _IO_) will be seen under the Angle
_CHD_ or _AHL_; and _EF_ (the same as _IO_) will be seen under the
Angle _EHF_, or _CHN_, or _AHM_. Also, _EF_ or _IO_ at the distance
_HE_ will appear as long as _CN_ would at the distance _HC_, or as
_AM_ would at the distance _HA_: and _CD_ or _IO_ at the distance _HC_
will appear as long as _AL_ would at the distance _HA_. So that as an
object approaches the eye, both it’s magnitude and the Angle under
which it is seen increase; and as the object recedes, the contrary.

Footnote 45:

The fields which are beyond the gate rise gradually till they are just
seen over it; and the arms, being red, are often mistaken for a house
at a considerable distance in those fields.

I once met with a curious deception in a gentleman’s garden at
_Hackney_, occasioned by a large pane of glass in the garden-wall at
some distance from his house. The glass (through which the fields and
sky were distinctly seen) reflected a very faint image of the house;
but the image seemed to be in the Clouds near the Horizon, and at that
distance looked as if it were a huge castle in the Air. Yet, the Angle
under which the image appeared, was equal to that under which the
house was seen: but the image being mentally referred a much greater
distance than the house, appeared much bigger to the imagination.

Footnote 46:

The Sun and Moon subtend a greater Angle on the Meridian than in the
Horizon, being nearer the Earth in the former case than the latter.

Footnote 47:

The Altitude of any celestial Phenomenon is an arc of the Sky
intercepted between the Horizon and the Phenomenon. In Fig. VI. of
Plate II. let _HOX_ be a horizontal line, supposed to be extended from
the eye at _A_ to _X_, where the Sky and Earth seem to meet at the end
of a long and level plain; and let _S_ be the Sun. The arc _XY_ will
be the Sun’s height above the Horizon at _X_, and is found by the
instrument _EDC_, which is a quadrantal board, or plate of metal,
divided into 90 equal parts or degrees on its limb _DPC_; and has a
couple of little brass plates, as _a_ and _b_, with a small hole in
each of them, called _Sight-Holes_, for looking through, parallel to
the edge of the Quadrant whereon they stand. To the center _E_ is
fixed one end of a thread _F_, called _the Plumb-Line_, which has a
small weight or plummet _P_ fixed to it’s other end. Now, if an
observer holds the Quadrant upright, without inclining it to either
side, and so that the Horizon at _X_ is seen through the sight-holes
_a_ and _b_, the plumb-line will cut or hang over the beginning of the
degrees at _o_, in the edge _EC_; but if he elevates the Quadrant so
as to look through the sight-holes at any part of the Heavens, suppose
to the Sun at _S_; just so many degrees as he elevates the sight-hole
_b_ above the horizontal line _HOX_, so many degrees will the
plumb-line cut in the limb _CP_ of the Quadrant. For, let the
observer’s eye at _A_ be in the center of the celestial arc _XYV_ (and
he may be said to be in the center of the Sun’s apparent diurnal
Orbit, let him be on what part of the Earth he will) in which arc the
Sun is at that time, suppose 25 degrees high, and let the observer
hold the Quadrant so that he may see the Sun through the sight-holes;
the plumb-line freely playing on the quadrant will cut the 25th degree
in the limb _CP_ equal to the number of degrees of the Sun’s Altitude
at the time of observation. _N. B._ Whoever looks at the Sun, must
have a smoaked glass before his eyes to save them from hurt. The
better way is not to look at the Sun through the sight-holes, but to
hold the Quadrant facing the eye, at a little distance, and so that
the Sun shining through one hole, the ray may be seen to fall on the
other.

Footnote 48:

See the Note on § 185.

Footnote 49:

Here proper allowance must be made for the Refraction, which being
about 34 minutes of a degree in the Horizon, will cause the Moon’s
center to appear 34 minutes above the Horizon when her center is
really in it.

Footnote 50:

By this is meant, that if a line be supposed to be drawn parallel to
the Earth’s Axis in any part of it’s Orbit, the Axis keeps parallel to
that line in every other part of it’s Orbit: as in Fig. I. of Plate V;
where _abcdefgh_ represents the Earth’s Orbit in an oblique view, and
_Ns_ the Earth’s Axis keeping always parallel to the line _MN_.

Footnote 51:

SMITH’s Optics, § 1197.

Footnote 52:

All Circles appear ellipses in an oblique view, as is evident by
looking obliquely at the rim of a bason. For the true figure of a
Circle can only be seen when the eye is directly over it’s center. The
more obliquely it is viewed, the more elliptical it appears, until the
eye be in the same plane with it, and then it appears like a straight
line.

Footnote 53:

Here we must suppose the Sun to be no bigger than an ordinary point
(as ·) because he only covers a Circle half a degree in diameter in
the Heavens; whereas in the figure he hides a whole sign at once from
the Earth.

Footnote 54:

Here we must suppose the Earth to be a much smaller point than that in
the preceding note marked for the Sun.

Footnote 55:

If the Earth were cut along the Equator, quite through the center, the
flat surface of this section would be the plane of the Equator; as the
paper contained within any Circle may be justly termed the plane of
that Circle.

Footnote 56:

The two opposite points in which the Ecliptic crosses the Equinoctial,
are called _the Equinoctial Points_: and the two points where the
Ecliptic touches the Tropics (which are likewise opposite, and 90
degrees from the former) are called _the Solstitial Points_.

Footnote 57:

The Equinoctial Circle intersects the Ecliptic in two opposite points,
called _Aries_ and _Libra_, from the Signs which always keep in these
points: They are called the Equinoctial Points, because when the Sun
is in either of them, he is directly over the terrestrial Equator; and
then the days and nights are equal.

Footnote 58:

In this discourse, we may consider the Orbits of all the Satellites as
circular, with respect to their primary Planets; because the
excentricities of their Orbits are too small to affect the Phenomena
here described.

Footnote 59:

If a Globe be cut quite through upon any Circle, the flat surface
where it is so divided, is the plane of that circle.

Footnote 60:

The Figure shews the Globe as if only elevated about 40 degrees, which
was occasioned by an oversight in the drawing: but it is still
sufficient to explain the Phenomena.

Footnote 61:

The Ecliptic, together with the fixed Stars, make 366-1/4 apparent
diurnal revolutions about the Earth in a year; the Sun only 365-1/4.
Therefore the Stars gain 3 minutes 56 seconds upon the Sun every day:
so that a Sidereal day contains only 23 hours 56 minutes of mean Solar
time; and a natural or Solar day 24 hours. Hence 12 Sidereal hours are
1 minute 58 seconds shorter than 12 Solar.

Footnote 62:

The Sun advances almost a degree in the Ecliptic in 24 hours, the same
way that the Moon moves: and therefore, the Moon by advancing 13-1/6
degrees in that time goes little more than 12 degrees farther from the
Sun than she was on the day before.

Footnote 63:

This center is as much nearer the Earth’s center than the Moon’s as
the Earth is heavier, or contains a greater quantity of matter than
the Moon, namely about 40 times. If both bodies were suspended on it
they would hang in _æquilibria_. So that dividing 240,000 miles, the
Moon’s distance from the Earth’s center, by 40 the excess of the
Earth’s weight above the Moon’s, the quotient will be 6000 miles,
which is the distance of the common center of gravity of the Earth and
Moon from the Earth’s center.

Footnote 64:

The Penumbra is a faint kind of shadow all around the perfect shadow
of the Planet or Satellite; and will be more fully explained by and
by.

Footnote 65:

Which is the time that the Eclipse would be at the greatest
obscuration, if the motions of the Sun and Moon were equable, or the
same in all parts of their Orbits.

Footnote 66:

The above period of 18 years 11 days 7 hours 43 minutes, which was
found out by the _Chaldeans_, and by them called _Saros_.

Footnote 67:

A Digit is a twelfth part of the diameter of the Sun or Moon.

Footnote 68:

There are two antient Eclipses of the Moon, recorded by _Ptolemy_ from
_Hipparchus_, which afford an undeniable proof of the Moon’s
acceleration. The first of these was observed at _Babylon_, _December_
the 22d, in the year before CHRIST 383: when the Moon began to be
eclipsed about half an hour before the Sun rose, and the Eclipse was
not over before the Moon set: but by our best Astronomical Tables, the
Moon was set at _Babylon_ half an hour before the Eclipse began; in
which case, there could have been no possibility of observing it. The
second Eclipse was observed at _Alexandria_, _September_ the 22d, the
year before CHRIST 201; where the Moon rose so much eclipsed, that the
Eclipse must have begun about half an hour before she rose: whereas by
our Tables the beginning of this Eclipse was not till about 10 minutes
after the Moon rose at _Alexandria_. Had these Eclipses begun and
ended while the Sun was below the Horizon, we might have imagined,
that as the antients had no certain way of measuring time, they might
have been so far mistaken in the hours, that we could not have laid
any stress on the accounts given by them. But, as in the first Eclipse
the Moon was set, and consequently the Sun risen, before it was over;
and in the second Eclipse the Sun was set, and the Moon not risen,
till some time after it began; these are such circumstances as the
observers could not possibly be mistaken in. Mr. _Struyk_ in the
following Catalogue, notwithstanding the express words of _Ptolemy_,
puts down these two Eclipses as observed at _Athens_; where they might
have been seen as above, without any acceleration of the Moon’s
motion: _Athens_ being 20 degrees West of _Babylon_, and 7 degrees
West of _Alexandria_.

Footnote 69:

Each _Olympiad_ began at the time of Full Moon next after the Summer
Solstice, and lasted four years, which were of unequal lengths because
the time of Full Moon differs 11 days every year: so that they might
sometimes begin on the next day after the Solstice, and at other times
not till four weeks after it. The first _Olympiad_ began in the year
of the Julian Period 3938, which was 776 years before the first year
of CHRIST, or 775 before the year of his birth; and the last
_Olympiad_, which was the 293d, began _A. D._ 393. At the expiration
of each _Olympiad_, the _Olympic Games_ were celebrated in the _Elean_
fields, near the river _Alpheus_ in the _Peloponnesus_ (now _Morea_)
in honour of JUPITER OLYMPUS. See STRAUCHIUS’_s_ _Breviarium
Chronologium_, p. 247-251.

Footnote 70:

The reader may probably find it difficult to understand why Mr. SMITH
should reckon this Eclipse to have been in the 4th year of the 48th
_Olympiad_; as it was only in the end of the third year: and also why
the 28th of _May_, in the 585th year before CHRIST should answer to
the present 10th of that month. But we hope the following explanation
will remove these difficulties.

The month of _May_ (when the Sun was eclipsed) in the 585th year
before the first year of CHRIST, which was a leap-year, fell in the
latter end of the third year of the 48th _Olympiad_; and the fourth
year of that _Olympiad_ began at the Summer Solstice following: but
perhaps Mr. SMITH begins the years of the _Olympiad_ from _January_,
in order to make them correspond more readily with _Julian_ years; and
so reckons the month of _May_, when the Eclipse happened, to be in the
fourth year of that _Olympiad_.

The Place or Longitude of the Sun at that time was ♉ 29° 43ʹ 17ʺ, to
which same place the Sun returned (after 2300 years, _viz._) _A. D._
1716, on _May_, 9^d. 5^h. 6^m. after noon: so that, with respect to
the Sun’s place, the 9th of _May_, 1716 answers to the 28th of _May_
in the 585th year before the first year of CHRIST; that is, the Sun
had the same Longitude on both those days.

Footnote 71:

Before CHRIST 413, _August 27_.

Footnote 72:

Before CHRIST 168, _June 20_.

Footnote 73:

STRUYK’s Eclipses are to the _Old Style_, all the rest to the _New_.

Footnote 74:

This Eclipse happened in the first year of the _Peloponnesian_ war.

Footnote 75:

Although the Sun and Moon are spherical bodies, as seen from the Earth
they appear to be circular planes, and so would the Earth if it were
seen from the Moon. The apparently flat surfaces of the Sun and Moon
are called their _Disks_ by Astronomers.

Footnote 76:

A Digit is a twelfth part of the diameter of the Sun and Moon.

Footnote 77:

This is the same with _the annual Argument of the Moon_.

Footnote 78:

When the _Romans_ divided the Empire, which was about 38 years before
CHRIST, _Spain_ fell to _Augustus_’s share: in memory of which, the
_Spaniards_ dated all their memorable events _ab exordio Regni
Augusti_; as Christians do from the birth of our SAVIOUR. But in
process of time, only the initial letters _AERA_ of these words were
used instead of the words themselves. And thus, according to some,
came the word _ÆRA_, which is made use of to signify a point of time
from whence historians begin to reckon.

Footnote 79:

When the Sun’s Anomaly is 0 signs 0 degrees, or 6 signs 0 degrees,
neither the Sun nor the Moon’s Anomaly have any Equation; which is the
case in this Example.

Footnote 80:

See the Remark, p. 195.

Footnote 81:

_Babylon_ is 42 deg. 46 min. east from the Meridian of _London_, which
is equal to 2 hours 51 min. of time nearly. See § 220.

Footnote 82:

Our SAVIOUR was born in a leap-year, and therefore every fourth year
both before and after is a leap-year in the _Old Stile_: but the
Tables begin with the year _next after_ that of his birth.

Footnote 83:

When only one of the Nodes is mentioned, it is the Ascending Node that
is meant, to which the Descending Node is exactly opposite.

Footnote 84:

When the Moon is North of the Ecliptic and going farther from it, her
Latitude or Declination from the Ecliptic is called _North Ascending_:
when she is North of the Ecliptic and going toward it, her Latitude is
_North Descending_: when she is South of the Ecliptic and going
farther from it, her Latitude is _South Descending_: and lastly, when
she is South of the Ecliptic and going toward it, her Latitude is
_South Ascending_.

Footnote 85:

See Page 193, Example II.

Footnote 86:

M. _Maupertuis_, in his dissertation on the figures of the Celestial
Bodies (p. 61-63) is of opinion that some Stars, by their prodigious
quick rotations on their Axes, may not only assume the figures of
oblate spheroids, but that by the great centrifugal force, arising
from such rotations, they may become of the figures of mill-stones; or
be reduced to flat circular planes, so thin as to be quite invisible
when their edges are turned towards us; as Saturn’s Ring is in such
positions. But when very excentric Planets or Comets go round any flat
Star, in Orbits much inclined to it’s Equator, the attraction of the
Planets or Comets in their perihelions must alter the inclination of
the Star; on which account it will appear more or less large and
luminous as it’s broad side is more or less turned towards us. And
thus he imagines we may account for the apparent changes of magnitude
and lustre in those Stars, and likewise for their appearing and
disappearing.

Footnote 87:

See this word explained in the note at the foot of page 194.

Footnote 88:

See the note on § 323.

Footnote 89:

_Matt._ xxvii. 45. _Mark_ xv. 43. _Luke_ xxiii. 44.

Transcriber’s Note

This book uses inconsistent spelling and hyphenation, which were retained in the ebook version. Some corrections have been made to the text, including correcting the errata and normalizing punctuation. Further corrections are noted below:

Errata: l. 15 from botton -> l. 15 from bottom p. 9: forward in the Eliptic -> forward in the Ecliptic p. 31: is at it were -> is as it were p. 36, Footnote 22 moved from referring to Rutherfurth to Maclaurin, additionally ‘Isacc Newton’ changed to ‘Isaac Newton’. Footnote marker on Rutherfurth removed as there was no footnote associated with it. p. 38: on the the same Axis -> on the same Axis Footnote 32 κοσμὸν -> κόσμον p. 69: who were suprised to find -> who were surprised to find p. 69: than those whch -> than those which p. 72: than tie a thread -> then tie a thread p. 74: is is equal to -> is equal to p. 74: the graduaded limb -> the graduated limb Footnote 49: bove the horizon -> above the horizon p. 78: different lenghts -> different lengths p. 78: from the the Equator -> from the Equator p. 90: is not instantaneons -> is not instantaneous p. 92: Degreees and Parts of the Equtor-> Degrees and Parts of the Equator p. 132: appear supprising -> appear surprising p. 133: When Jupiter at -> When Jupiter is at Sidenote p. 136: The reason of of this -> the reason of this p. 140 the opposite points rises -> the opposite point rises Sidenote p. 141: Harvest aad Hunter’s -> Harvest and Hunter’s p. 154: espeically as to the -> especially as to the Sidenote p. 155: aereal Tides -> aerial Tides p. 158: the the Earth -> the Earth p. 160: goes round him 87 days -> goes round him in 87 days p. 161: Eclipses and revolulution -> Eclipses and revolution p. 167: Jacobus Ptlaumen -> Jacobus Pflaumen p. 168: set set down -> set down p. 172 Table 2, 1st column, 6th row: 1388 -> 1488 p. 174: duplicate entry for 1606 Sept 2. removed p. 177: foretold by Thalls -> foretold by Thales p. 180: the Eclipse is annualar -> the Eclipse is annular p. 193: EAAMPLE II. -> EXAMPLE II p. 202: these two Fquations -> these two Equations p. 203: their Sun will be -> their Sum will be p. 210: the page number was printed as 110 and has been corrected p. 210: Motion and Semi diameter -> Motion and Semi-diameter p. 232: ζωδίακος -> ζωδιακὸς p. 232: ζῶδιον -> ζώδιον p. 238: oblate spheriod -> oblate spheroid p. 261 18 Degres -> 18 Degrees Index Mercury (Quicksiver) -> Mercury (Quicksilver) List of Plates Page number for Plate IV corrected from 15 to 97