The description and use of the globes and the orrery: To which is prefix'd, by way of introduction, a brief account of the solar systemHarris, Joseph
History
The description and use of the globes and the orrery: To which is prefix'd, by way of introduction, a brief account of the solar system
Harris, Joseph
Astronomical models; Globes
Thus, if you turn your face towards the North, you will find the North
Pole of the globe points to the _Pole-Star_; then you may observe two
Stars somewhat less bright than the Pole-Star, almost in a right line
with it, and four more which form a sort of _quadrangle_; these seven
Stars make the constellation called the _Little Bear_; the Pole Star
being in the tip of the tail. In this neighbourhood you will observe
seven bright Stars, which are commonly called _Charles’s Wane_;
these are the bright Stars in the _Great Bear_, and form much such
another figure with those before-mentioned in the _little Bear_: The
two foremost of the square lie almost in a right line with the Pole
Star, and are called the _Pointers_, so that knowing the Pointers,
you may easily find the Pole-Star. Thus the rest of the Stars in this
constellation, and all the Stars in the neighbouring constellations
may be easily found, by observing how the unknown Stars lie either in
_quadrangles_, _triangles_, or strait lines from those that are already
known upon the globe.
After the same manner the globe being rectified, you may distinguish
those Stars that are to the Southward of you, and be soon acquainted
with all the Stars that are visible in our hemisphere.
_SCHOLIUM._
The globe being rectified to the latitude of any place, if you turn it
round its axis, all those Stars that do not go below the horizon during
a whole revolution of the globe, never set in that place; and those
that do not come above the horizon never rise.
PROB. XXXVII. _The latitude of the place being
given; to find the Amplitude, Oblique Ascension and
Descension, Ascensional Difference, Semi-diurnal
Arch, and the time of continuance above the
horizon, of any given point in the heavens._
Having rectified the globe for the latitude, and brought the given
point to the meridian, set the index to the hour of 12; then turn the
globe until the given point be brought to the Eastern side of the
horizon, and that degree of the equinoctial which is cut by the horizon
at that time, will be the _Oblique Ascension_; and where the given
point cuts the horizon, is the _Amplitude Ortive_: If the globe be
turned about until the given point be brought to the Western side of
the horizon, it will there show the _Amplitude Occasive_; and where the
horizon cuts the equinoctial at that time, is the _Oblique Descension_.
The time between the index at either of these two positions, and the
hour of 6; or half the difference between the oblique ascension and
descension is the _Ascensional Difference_.
If the place be in North latitude and the declination of the given
point be (North/South) the ascensional difference reduced into time,
and (added to/subtracted from) 6 o’Clock, gives the _Semi-diurnal
Arch_; the complement whereof to a semicircle, is the _Semi-nocturnal
Arch_. If the place be in South latitude, then the contrary is to be
observed with respect to the declination.
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