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Angular measure

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Angular Measurements
In astronomy, we often use angular measurements to describe the apparent size of an object in space and the apparent distances between objects. Often these angles are very small. Angles are also used to describe an object's location in space.

[edit] Angular measurement
Using a marine sextant to measure the altitude of the sun above the horizon ...

Angular Measurement
The angle covered or subtended by something (measured in degrees, minutes of arc, seconds of arc), as opposed to linear size of something (measured in units of kilometers or parsecs).
angular momentum ...

A unit of angular measure equal to 60 arc minutes or 3600 arc seconds. One degree is equal to the diameter of two full Moons.
The amount of mass per unit of volume.

The unit of angular measure, defined as the ratio of a length of arc intercepted by two radii to the length of the radius. P radians = 180.
radiant - (n.)
The point in the sky from which all the meteors in a meteor shower appear to be coming.

Spacecraft ~ment
The spacecraft's position in the sky is expressed in the angular quantities Right Ascension and Declination.

A radian is an ~ment.
If we draw two straight lines that cross at the centre of a circle such that the length of the circumference between them (an arc) equals the radius, then the angle between these lines is 1 radian.

As it is unlikely that the ship is sailing across the Pampas, the position in the Atlantic is the correct one. Note that the lines of position in the figure are distorted because of the map's projection; they would be circular if plotted on a globe.
~ment ...

degree A unit of ~. There are 360 degrees in a complete circle.
density A measure of the compactness of the matter within an object, computed by dividing the mass by the volume of the object.

The phase difference of two periodically recurring phenomena of the same frequency, expressed in ~. 2. The angle at a celestial body between the sun and earth. phase constant See propagation constant.

difference of latitude (NASA SP-7, 1965) The shorter arc of any meridian between the parallels of two places, expressed in ~.

Arc Degree A unit of ~ in which there are 360 arc degrees in a full circle.
Arc Second Abbreviated arcsec. A unit of ~ in which there are 60 arc seconds in 1 arc minute and therefore 3600 arc seconds in 1 arc degree.

MORE PRECISELY 1-1Angular Measure
The size and scale of things are often specified by measuring lengths and angles. The concept of length measurement is fairly intuitive.

Recall that the total ~ around a circle is 360 degrees. The angle from (say) the eastern to western horizon (through the point directly overhead, the zenith) is 180 degrees.

(b) A unit of ~ equal to the angle subtended at the centre of a circle by an arc the length of which is equal to the radius. The name is reputed to have been made up by James Thomson in an examination paper in Belfast about 1870 and it was first published in 1873[5].

A unit of ~ consisting of 1/60th of an arc-minute. A dime seen from a distance of 2 miles transects 1 arc second. There are 3,600 arc-seconds in a one-degree angle. It is used to measure the separations of double stars or small deep-sky objects, like planetary nebulas.

A unit of solid (three-dimensional) ~. One steradian is equal to the angle subtended at the centre of a sphere by an area of surface equal to the square of the radius. The name for the unit seems to have come into use about 1880 and was comparatively common by the turn of the century.

Arc Second
abbreviated arcsec. A unit of ~ in which there are 60 arc seconds in 1 arc minute and therefore 3600 arc seconds in 1 arc degree. There are 206,265 arcseconds per radian. One arc second is equal to about 725 km on the Sun.

A unit of ~ equal to one sixtieth of one sixtieth of a degree, or one sixtieth of an arc-minute. As an example, the apparent diameter of Jupiter is about 45 arc-seconds.

So if the universe is "flat," meaning it's ~ments have lines that stay parallel to each other, (assumedly in all 3 dimensions) wouldn't that make our universe more like an expanding box (or quadrilateral maybe), except with invisible edges?

minute of arc: An ~; each degree is divided into 60 minutes of arc.
molecular cloud: An interstellar gas cloud that is dense enough for the formation of molecules; discovered and studied through the radio emissions of such molecules.

RADIAN - A radian is an ~ment. The angular circumference of a circle is 2π radians, or 360; thus, 1 radian = 360/2π degrees = 57.29578 (57 17' 44.8").

Minute of Arc - A unit of ~ment equal to 1/60 of a degree
Mode of Oscillation - A particular pattern of vibration of the Sun
Molecular Cloud - A relatively dense, cool interstellar cloud in which molecules are common ...

f. It is convenient to be able to convert between ~ment and sidereal time measurement. Twenty four (24) hours is equivalent to 360 degrees. From this we can produce the following conversions.
Time measurement Angle measurement ...

radian; rad
The supplementary SI unit of ~, defined as the central angle of a circle whose subtended arc is equal to the radius of the circle.
Energy radiated in the form of waves or particles; photons.

True field - How much sky, in ~, is seen through an eyepiece.
Guide to Amateur Astronomy, Planets and Constellations ...

Declination: Celestial coordinate which is equivalent to the latitude of an object on Earth. This is an ~ of how far above the celestial equator an object lies.
Density: Mass per unit volume; a measure of how much material is in a given space.

The determination of distances and motion along the line of sight is another branch of astronomy, although purely ~ments within the solar system combined with a knowledge of masses of the planets etc can lead to a complete description of their orbits, including distance.

In the 12th century, Sharaf al-Din al-Tusi invented the linear astrolabe, sometimes called the "staff of al-Tusi", which was "a simple wooden rod with graduated markings but without sights. It was furnished with a plumb line and a double chord for making ~ments and bore a perforated ...

The six orbital elements used for comets are usually the following: time of perihelion passage (T) [sometimes taken instead as an ~ called "mean anomaly", M]; perihelion distance (q), usually given in AU; eccentricity (e) of the orbit; ...

See also: See also: What is the meaning of Astro, Earth, Sky, Astronomy, Planet?

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