What is "solid angle"

Solid \Sol"id\ (s[o^]l"[i^]d), a. [L. solidus, probably akin to sollus whole, entire, Gr. ???: cf. F. solide. Cf. Consolidate, Soda, Solder, Soldier, Solemn.]

1. Having the constituent parts so compact, or so firmly adhering, as to resist the impression or penetration of other bodies; having a fixed form; hard; firm; compact; -- opposed to fluid and liquid or to plastic, like clay, or to incompact, like sand.

2. Not hollow; full of matter; as, a solid globe or cone, as distinguished from a hollow one; not spongy; dense; hence, sometimes, heavy.

3. (Arith.) Having all the geometrical dimensions; cubic; as, a solid foot contains 1,728 solid inches.

   Note: In this sense, cubics now generally used.

4. Firm; compact; strong; stable; unyielding; as, a solid pier; a solid pile; a solid wall.

5. Applied to a compound word whose parts are closely united and form an unbroken word; -- opposed to hyphened.

6. Fig.: Worthy of credit, trust, or esteem; substantial, as opposed to frivolous or fallacious; weighty; firm; strong; valid; just; genuine.

   The solid purpose of a sincere and virtuous answer.
   --Milton.

   These, wanting wit, affect gravity, and go by the name of solid men.
   --Dryden.

   The genius of the Italians wrought by solid toil what the myth-making imagination of the Germans had projected in a poem.
   --J. A. Symonds.

7. Sound; not weakly; as, a solid constitution of body.
   --I. Watts.

8. (Bot.) Of a fleshy, uniform, undivided substance, as a bulb or root; not spongy or hollow within, as a stem.

9. (Metaph.) Impenetrable; resisting or excluding any other material particle or atom from any given portion of space; -- applied to the supposed ultimate particles of matter.

10. (Print.) Not having the lines separated by leads; not open.

11. United; without division; unanimous; as, the delegation is solid for a candidate. [Polit. Cant. U.S.]

    Solid angle. (Geom.) See under Angle.

    Solid color, an even color; one not shaded or variegated.

    Solid green. See Emerald green (a), under Green.

    Solid measure (Arith.), a measure for volumes, in which the units are each a cube of fixed linear magnitude, as a cubic foot, yard, or the like; thus, a foot, in solid measure, or a solid foot, contains 1,728 solid inches.

    Solid newel (Arch.), a newel into which the ends of winding stairs are built, in distinction from a hollow newel. See under Hollow, a.

    Solid problem (Geom.), a problem which can be construed geometrically, only by the intersection of a circle and a conic section or of two conic sections.
    --Hutton.

    Solid square (Mil.), a square body or troops in which the ranks and files are equal.

    Syn: Hard; firm; compact; strong; substantial; stable; sound; real; valid; true; just; weighty; profound; grave; important.

    Usage: Solid, Hard. These words both relate to the internal constitution of bodies; but hardnotes a more impenetrable nature or a firmer adherence of the component parts than solid. Hard is opposed to soft, and solid to fluid, liquid, open, or hollow. Wood is usually solid; but some kinds of wood are hard, and others are soft.

    Repose you there; while I [return] to this hard house, More harder than the stones whereof 't is raised.
    --Shak.

    I hear his thundering voice resound, And trampling feet than shake the solid ground.
    --Dryden.

Angle \An"gle\ ([a^][ng]"g'l), n. [F. angle, L. angulus angle, corner; akin to uncus hook, Gr. 'agky`los bent, crooked, angular, 'a`gkos a bend or hollow, AS. angel hook, fish-hook, G. angel, and F. anchor.]

1. The inclosed space near the point where two lines meet; a corner; a nook.

   Into the utmost angle of the world.
   --Spenser.

   To search the tenderest angles of the heart.
   --Milton.

2. (Geom.)

   1. The figure made by. two lines which meet.

   2. The difference of direction of two lines. In the lines meet, the point of meeting is the vertex of the angle.

3. A projecting or sharp corner; an angular fragment.

   Though but an angle reached him of the stone.
   --Dryden.

4. (Astrol.) A name given to four of the twelve astrological ``houses.'' [Obs.]
   --Chaucer.

5. [AS. angel.] A fishhook; tackle for catching fish, consisting of a line, hook, and bait, with or without a rod. Give me mine angle: we 'll to the river there. --Shak. A fisher next his trembling angle bears. --Pope. Acute angle, one less than a right angle, or less than 90[deg]. Adjacent or Contiguous angles, such as have one leg common to both angles. Alternate angles. See Alternate. Angle bar.

   1. (Carp.) An upright bar at the angle where two faces of a polygonal or bay window meet.
      --Knight.

   2. (Mach.) Same as Angle iron.

      Angle bead (Arch.), a bead worked on or fixed to the angle of any architectural work, esp. for protecting an angle of a wall.

      Angle brace, Angle tie (Carp.), a brace across an interior angle of a wooden frame, forming the hypothenuse and securing the two side pieces together.
      --Knight.

      Angle iron (Mach.), a rolled bar or plate of iron having one or more angles, used for forming the corners, or connecting or sustaining the sides of an iron structure to which it is riveted.

      Angle leaf (Arch.), a detail in the form of a leaf, more or less conventionalized, used to decorate and sometimes to strengthen an angle.

      Angle meter, an instrument for measuring angles, esp. for ascertaining the dip of strata.

      Angle shaft (Arch.), an enriched angle bead, often having a capital or base, or both.

      Curvilineal angle, one formed by two curved lines.

      External angles, angles formed by the sides of any right-lined figure, when the sides are produced or lengthened.

      Facial angle. See under Facial.

      Internal angles, those which are within any right-lined figure.

      Mixtilineal angle, one formed by a right line with a curved line.

      Oblique angle, one acute or obtuse, in opposition to a right angle.

      Obtuse angle, one greater than a right angle, or more than 90[deg].

      Optic angle. See under Optic.

      Rectilineal or Right-lined angle, one formed by two right lines.

      Right angle, one formed by a right line falling on another perpendicularly, or an angle of 90[deg] (measured by a quarter circle).

      Solid angle, the figure formed by the meeting of three or more plane angles at one point.

      Spherical angle, one made by the meeting of two arcs of great circles, which mutually cut one another on the surface of a globe or sphere.

      Visual angle, the angle formed by two rays of light, or two straight lines drawn from the extreme points of an object to the center of the eye.

      For Angles of commutation, draught, incidence, reflection, refraction, position, repose, fraction, see Commutation, Draught, Incidence, Reflection, Refraction, etc.

n. (context geometry English) The three-dimensional analog of an angle, equivalent to the area of that portion of the surface a unit sphere that it subtends.

n. an angle formed by three or more planes intersecting at a common point (the vertex)

In geometry, a solid angle (symbol: ) is the two-dimensional angle in three-dimensional space that an object subtends at a point. It is a measure of how large the object appears to an observer looking from that point. In the International System of Units (SI), a solid angle is expressed in a dimensionless unit called a steradian (symbol: sr).

A small object nearby may subtend the same solid angle as a larger object farther away. For example, although the Moon is much smaller than the Sun, it is also much closer to Earth. Indeed, as viewed from any point on Earth, both objects have approximately the same solid angle as well as apparent size. This is evident during a solar eclipse.