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
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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
_Note_, The best way to find the Sun’s altitude, is by a little
quadrant graduated into degrees, and having sights and a plummet to it:
Thus, hold the quadrant in your hand, so as the rays of the Sun may
pass through both the sights, the plummet then hanging freely by the
side of the instrument, will cut in the limb the altitude required.
These quadrants are to be had at the instrument-makers, with lines
drawn upon them, for finding the hour of the day, and the azimuth; with
several other pretty conclusions, very entertaining for beginners.
_The Latitude and the Day of the Month being given, to
find the hour of the day when the Sun shines._
Having placed the wooden frame upon a level, and the meridian due
North and South, rectify the globe for the latitude, and fix a needle
perpendicularly over the Sun’s place: The Sun’s place being brought to
the meridian, set the hour index at 12 at noon, then turn the globe
about until the needle points exactly to the Sun, and casts no shadow,
and then the index will shew the hour of the day.
PROB. XXVI. _The Latitude, the Sun’s Place,
and his Altitude, being given; to find the hour of
the Day, and the Sun’s Azimuth from the Meridian._
Having rectified the globe for the latitude, the zenith, and the Sun’s
place, turn the globe and the quadrant of altitude, so that the Sun’s
place may cut the given degree of altitude: then the index will show
the hour, and the quadrant will cut the azimuth in the horizon. Thus,
if at _London_, on the 21st of _August_, the Suns altitude, be 36
degrees in the forenoon, the hour of the day will be IX, and the Sun’s
azimuth about 58 degrees from the South part of the meridian.
_The Sun’s Azimuth being given, to place the Meridian
of the Globe due North and South, or to find a
Meridian Line when the Sun shines._
Let the Sun’s azimuth be 30 degrees South-Easterly, set the horizon of
the globe upon a level, and bring the North Pole into the zenith; then
turn the horizon about until the shade of the axis cuts as many hours
as is equivalent to the azimuth (allowing 15 degrees to an hour) in
the North-West part of the hour circle, _viz._ X at night, which being
done, the meridian of the globe stands in the true meridian of the
place. The globe standing in this position, if you hang two plummets
at the North and South points of the wooden horizon, and draw a line
betwixt them, you will have a meridian line; which if it be on a fixed
plane (as a floor or window) it will be a guide for placing the globe
due North and South, at any other time.
PROB. XXVII. _The Latitude, Hour of the Day,
and the Sun’s place being given, to find the Sun’s
Altitude and Azimuth._
Public-domain text, read in full here on John Shaqi.
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