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216 #3 The "Sextant"
Dip. The correction for Dip are
Dip subtractive, for when any
of observations are made from the
the deck of a ship, the sea horizon
dips, or is depressed, below
the level of the sensible horizon
horizon and makes the observed altitude
greater than the true apparent
altitude.
Refraction. The correction for
Refraction refraction is also subtractive
Semi Diameter
The Sun, Moon
Semi and important planets, all
have large discs consequently
only the lower limits are observed.
Diameter To obtain the altitude of the
centre, we must add the Semi
Diameter to the altitude of the
lower limit, or subtract it from
the altitude of the Upper. The
Semi Diameter of the above are all
in the Nautical Almanac.
The Astronomer Employs a
The fixed instrument to find the
Latitude, called the Transit
Transit and an Artificial Horizon.
The Navigator uses a Sextant
and the Natural Horizon.
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The Sextant 217
Astronomy
The stars are distinguised into
two Kinds, viz Fixed and Wandering.
The fixed Stars are supposed to
be Suns in the centre of thier
systems, shining with thier own
light, and preserving always the
same situation with respect to Each
other; they are usually divided,
according to thier apparent splendor,
into differant classes called
magnitudes; the brightest being
denominated Stars of the first
Magnitude; the next to them in
' brightness of the second magnitude
and so on to those stars that are
scarcely visible to the naked Eye,
which are termed stars of the
Sixth or Sevent magnitude.
In order to assist
the memory, Astronomers have
divided the heavens into parcels
called "Constellations" these are
a number of fixed stars lying
contiguous, which are supposed to
be curcumscribed, by the outline of
some animal, or other imaginary
figure. Stars which are not
included in the constellations are
called "Unformed Stars". The
Stars in Each Constellation, are
generally distingued [distinguished] by letters of
the Greek Alphabet, and some
of the principal have proper names
thus the Star marked a, in
the comstellation of "Taurus"
is called "Aldebaran"
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