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Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point in question above the Earth. [1]
If he were to measure the height of the Sun above the horizon with a sextant, he would find that the altitude of the Sun was 90°. By subtracting this figure from 90°, he would find that the zenith distance of the Sun is 0°, which is the same as his latitude.
Angular distance appears in mathematics (in particular geometry and trigonometry) and all natural sciences (e.g., kinematics, astronomy, and geophysics). In the classical mechanics of rotating objects, it appears alongside angular velocity, angular acceleration, angular momentum, moment of inertia and torque.
A meridian circle enabled the observer to simultaneously determine right ascension and declination, but it does not appear to have been much used for right ascension during the 17th century, the method of equal altitudes by portable quadrants or measures of the angular distance between stars with an astronomical sextant being preferred.
A slow motion of Earth's axis, precession, causes a slow, continuous turning of the coordinate system westward about the poles of the ecliptic, completing one circuit in about 26,000 years. Superimposed on this is a smaller motion of the ecliptic, and a small oscillation of the Earth's axis, nutation. [3] [4]
The frame of a sextant is in the shape of a sector which is approximately 1 ⁄ 6 of a circle (60°), [2] hence its name (sextāns, sextantis is the Latin word for "one sixth"). "). Both smaller and larger instruments are (or were) in use: the octant, quintant (or pentant) and the (doubly reflecting) quadrant [3] span sectors of approximately 1 ⁄ 8 of a circle (45°), 1 ⁄ 5 of a circle (72 ...
The Dark Energy Camera captured a stunning image of “God’s Hand,” a cometary globule 1,300 light-years from Earth in the Puppis constellation. The camera is mounted on the Víctor M. Blanco ...
As the observer moves north (or south), the celestial equator tilts towards the opposite horizon. The celestial equator is defined to be infinitely distant (since it is on the celestial sphere); thus, the ends of the semicircle always intersect the horizon due east and due west, regardless of the observer's position on Earth.