The astrolabe is a very ancient astronomical computer for solving problems relating to time and the position of the Sun and stars in the sky. Several types of astrolabes have been made. By far the most popular type is the planispheric astrolabe, on which the
celestial sphere is projected onto the plane of the equator. A typical old astrolabe was made of brass and was about 6 inches (15 cm) in diameter, although much larger and smaller ones were made.
A TED TALK ON THE ASTROLABE https://www.youtube.com/watch?v=yioZhHe1i5M
From Wikipedia, the free encyclopedia"Geocentric" redirects here. For orbits around the Earth, see Geocentric orbit.Figure of the heavenly bodies — An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568 (Bibliothèque Nationale, Paris)In astronomy, the geocentric model (also known as geocentrism, or the Ptolemaic system) is a superseded description of the universe with the Earth at the center. Under the geocentric model, the Sun, Moon, stars, and planets all circled Earth. The geocentric model served as the predominant description of the cosmos in many ancient civilizations, such as those of Aristotle and Ptolemy.Two observations supported the idea that the Earth was the center of the Universe. First, the Sun appears to revolve around the Earth once per day. While the Moon and the planets have their own motions, they also appear to revolve around the Earth about once per day. The stars appeared to be on a celestial sphere, rotating once each day along an axis through the north and south geographic poles of the Earth. Second, the Earth does not seem to move from the perspective of an Earth-bound observer; it appears to be solid, stable, and unmoving.Ancient Greek, ancient Roman and medieval philosophers usually combined the geocentric model with a spherical Earth. It is not the same as the older flat Earth model implied in some mythology.[n 1][n 2] The ancient Jewish Babylonian uranography pictured a flat Earth with a dome-shaped rigid canopy named firmament placed over it. (רקיע- rāqîa').[n 3][n 4][n 5][n 6][n 7][n 8] However, the ancient Greeks believed that the motions of the planets were circular and not elliptical, a view that was not challenged in Western culture until the 17th century through the synthesis of theories by Copernicus and Kepler.The astronomical predictions of Ptolemy's geocentric model were used to prepare astrological and astronomical charts for over 1500 years. The geocentric model held sway into the early modern age, but from the late 16th century onward, it was gradually superseded by the Heliocentric model of Copernicus, Galileo and Kepler. There was much resistance to the transition between these two theories. Christian theologians were reluctant to reject a theory that agreed with Bible passages (e.g. "Sun, stand you still upon Gibeon", Joshua 10:12). Others felt a new, unknown theory could not subvert an accepted consensus for geo
video LINK example : https://www.youtube.com/watch?v=eooqcaFFz9o
A horizontal angle can be obtained as a difference between two bearings or two relative bearings. The bearing(1) and relative bearing(2) of a ship are usually measured using a pelorus(3) or radar. A better solution for obtaining a horizontal angle is the use of a sextant (Figure 1). The sextant is a primary instrument for celestial navigation, i.e. instrument designed to measure the altitudes of celestial bodies above the visible sea horizon. However, the sextant also allows us to measure the vertical angle of terrestrial objects (the angle between the top of the object and the visible sea or shore horizon) as well as horizontal angle between two points. Generally speaking, the marine sextant measures the angle between two points by bringing the direct image from one point and a double-re ected image from the other into coincidence, and can usually measure angles up to approximately 120 ̊ (Bowditch, 2002). The main advantage of the sextant compared to other angle measuring instruments is its precision. For example, the pelorus can measure up to a maximum of 1/10 of a degree (in practice the bearing is usually rounded to a whole degree), while the marine sextant normally measures 1/10 of a minute (Figure 2). Figure 3 shows a gyro compass repeater and Figure 2 a sextant with micrometer drum and varnier, that reads a fraction of a minute.
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