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"Armillary Sphere, an instrument used in astronomy. In its simplest form, consisting of a ring fixed in the plane of the equator, the armilla is one of the most ancient of astronomical instruments. Slightly developed, it was crossed by another ring fixed in the plane of the meridian." — The Encyclopedia Britannica, 1910

Armilla

"Armillary Sphere, an instrument used in astronomy. In its simplest form, consisting of a ring fixed…

"The Earth, whose diameter is 7,912 miles, is represented by the globe, or sphere. The straight line passing through its center, and about which it turns, is called its axis, and the two extremities of the axis are the poles of the Earth, A being the north pole, and B the south pole. The line C D, crossing the axis, passes quite round the Earth, and divides it into two equal parts. This is called the equinoctial line, or the equator. That part of the Earth situated north of this line, is caled the northern hemisphere, and that part south of it, the southern hemisphere. The small circles E F and G H, surrounding or including the poles, are called the polar circles." —Comstock, 1850

Earth Divisions

"The Earth, whose diameter is 7,912 miles, is represented by the globe, or sphere. The straight line…

"Relative positions of the earth and the sun on March 21 (spring equinox) and September 21 (autumn equinox) as seen from the position occupied by the earth on June 21 (summer solstice). Rays of light and heat meet the earth vertically on the Equator, and the days and nights are everywhere of equal length." -Wiswell, 1913

Spring Equinox and Autumn Equinox

"Relative positions of the earth and the sun on March 21 (spring equinox) and September 21 (autumn equinox)…

"Let this figure represent the Earth, N being the north pole, S the south pole, and E W the equator. The lines 10, 20, 30, and so on, are the parallels of latitude, and the lines N a S, N b S, etc., are meridian lines, or those of longitude." —Comstock, 1850

Longitude

"Let this figure represent the Earth, N being the north pole, S the south pole, and E W the equator.…

"Two hoops of thin iron are placed upon an axis which passes through their poles. The two ends of each hoop cross each other at right angles, and are fastened together, and to the axis at the bottom. At the upper end they slide up and down on the axis, which is turned rapidly by wheel-work as represented. These hoops, before the motion begins, have an oval form, but when turned rapidly, the centrifugal force causes them to expand, or swell at the equator, while they are depressed at the poles, the two polar regions becoming no more distant than a and b." —Comstock, 1850

Pole Depression

"Two hoops of thin iron are placed upon an axis which passes through their poles. The two ends of each…

"Relative positions of the earth and the sun on June 21, the summer solstice. Summer and long days north of the Equator; winter and short days south of it." -Wiswell, 1913

Summer Solstice

"Relative positions of the earth and the sun on June 21, the summer solstice. Summer and long days north…

"Relative positions of the earth and the sun on December 21, the winter solstice. Winter and short days north of the Equator; summer and long days south of it. 1, North Frigid Zone; 2, North Temperate Zone; 3, Torrid Zone; 4, South Temperate Zone; 5, South Frigid Zone." -Wiswell, 1913

Winter Solstice

"Relative positions of the earth and the sun on December 21, the winter solstice. Winter and short days…