Sightline in the context of "Apparent magnitude"

An azimuth (/ˈæzəməθ/ ; from Arabic: اَلسُّمُوت, romanized: as-sumūt, lit. 'the directions') is the horizontal angle from a cardinal direction, most commonly north, in a local or observer-centric spherical coordinate system.

Mathematically, the relative position vector from an observer (origin) to a point of interest is projected perpendicularly onto a reference plane (the horizontal plane); the angle between the projected vector and a reference vector on the reference plane is called the azimuth.

Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to foreshortening, nearby objects show a larger parallax than farther objects, so parallax can be used to determine distances.

To measure large distances, such as the distance of a planet or a star from Earth, astronomers use the principle of parallax. Here, the term parallax is the semi-angle of inclination between two sight-lines to the star, as observed when Earth is on opposite sides of the Sun in its orbit. These distances form the lowest rung of what is called "the cosmic distance ladder", the first in a succession of methods by which astronomers determine the distances to celestial objects, serving as a basis for other distance measurements in astronomy forming the higher rungs of the ladder.

In painting, photography, graphical perspective and descriptive geometry, a picture plane is an image plane located between the "eye point" (or oculus) and the object being viewed. It is usually coextensive to the material surface of the artwork. It is ordinarily a vertical plane perpendicular to the sightline to the object of interest.

Direct fire or line-of-sight fire refers to shooting of a ranged weapon whose projectile/ordnance is launched directly at a target within the line-of-sight of the user. Direct fire weapons typically have a sighting device to ensure accuracy and must have an unobstructed view to the target with no obstacles or friendly units in between, meaning that they can only effectively engage targets that are in enfilade. Conversely, direct fire units are also directly exposed to return fire from the target unless they are shooting from a position of defilade or concealment.

Direct fire is in contrast to indirect fire, which refers to firing a projectile on a curved ballistic trajectory or delivering self-accelerated munitions capable of long range and various degrees of homing abilities to alter the flight path. Indirect fire does not need a direct line-of-sight to the target because the shots are normally directed by a forward observer who provides the coordinates and calibrations needed to hit the target. As such, indirect-fire weapons can shoot over obstacles or friendly units and the weapons can be concealed from counter-battery fire.

The Order of Things: An Archaeology of the Human Sciences (Les Mots et les Choses: Une archéologie des sciences humaines) is a book by French philosopher Michel Foucault. It proposes that every historical period has underlying epistemic assumptions, ways of thinking, which determine what is truth and what is acceptable discourse about a subject, by delineating the origins of biology, economics, and linguistics. The introduction to the origins of the human sciences begins with detailed, forensic analyses and discussion of the complex networks of sightlines, hidden-ness, and representation that exist in the group painting Las Meninas (The Ladies-in-waiting, 1656) by Diego Velázquez. Foucault's application of the analyses shows the structural parallels in the similar developments in perception that occurred in researchers' ways of seeing the subject in the human sciences.

In military usage, a barrage is massed sustained artillery fire (shelling) aimed at a series of points along a line. In addition to attacking any enemy in the kill zone, a barrage intends to suppress enemy movements and deny access across that line of barrage. The impact points along the line may be 20 to 30 yards apart, with the total line length of the barrage zone anything from a few hundred to several thousand yards long. Barrages can consist of multiple such lines, usually about 100 yards apart, with the barrage shifting from one line to the next over time, or several lines may be targeted simultaneously.

A barrage may involve a few or many artillery batteries, or even (rarely) a single gun. Typically each gun in a barrage, using indirect fire, will fire continuously at a steady rate at its assigned point for an assigned time before moving onto the next target, following the barrage's detailed timetable. Barrages typically use high-explosive shells, but may also be shrapnel, smoke, illumination, poison gas (in World War I), or potentially other chemical agents. Barrages are in contrast with concentrated artillery fire, which has a single specific target such as a known enemy position or structure, and in contrast with direct fire which targets enemies within the direct line of sight of the gun.