Semicircle in the context of Region (mathematics)

A circular arc is the arc of a circle between a pair of distinct points. If the two points are not directly opposite each other, one of these arcs, the minor arc, subtends an angle at the center of the circle that is less than π radians (180 degrees); and the other arc, the major arc, subtends an angle greater than π radians. The arc of a circle is defined as the part or segment of the circumference of a circle. A straight line that connects the two ends of the arc is known as a chord of a circle. If the length of an arc is exactly half of the circle, it is known as a semicircular arc.

A hemicycle is a semicircular, or horseshoe-shaped, legislative debating chamber where members sit to discuss and vote on their business. Although originally of Ancient Greek roots, the term and modern design derive from French politics and practice.

In architecture, a semicircular arch is an arch with an intrados (inner surface) shaped like a semicircle. This type of arch was adopted and very widely used by the Romans, thus becoming permanently associated with Roman architecture.

The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. By extension, it is also a plane of reference in the equatorial coordinate system. Due to the Earth's axial tilt, the celestial equator is currently inclined by about 23.44° with respect to the ecliptic (the plane of Earth's orbit), but has varied from about 22.0° to 24.5° over the past 5 million years due to Milankovitch cycles and perturbation from other planets.

An observer standing on the Earth's equator visualizes the celestial equator as a semicircle passing through the zenith, the point directly overhead. 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 the Earth. At the poles, the celestial equator coincides with the astronomical horizon. At all latitudes, the celestial equator is a uniform arc or circle because the observer is only finitely far from the plane of the celestial equator, but infinitely far from the celestial equator itself.

A tiara (from Latin tiara, from Ancient Greek τιάρα (tiára)) is a head ornament adorned with jewels. Its origins date back to ancient Greco-Roman world. In the late 18th century, the tiara came into fashion in Europe as a prestigious piece of jewelry to be worn by women at formal occasions. The basic shape of the modern tiara is a semicircle, usually made of silver, gold, or platinum and richly decorated with precious stones, pearls, or cameos.

Tiaras were extremely popular during the late 19th century and were worn at events where the dress code was white tie. After World War I, wearing a tiara gradually fell out of fashion, except for official occasions at a royal court. Interest in tiaras has increased again since the beginning of the 21st century. The word tiara is often used interchangeably with the word diadem.

An amphitheatre (American English: amphitheater) is an open-air venue used for entertainment, performances, and sports. The term derives from the ancient Greek ἀμφιθέατρον (amphitheatron), from ἀμφί (amphi), meaning "on both sides" or "around" and θέατρον (théātron), meaning "place for viewing".

Ancient Greek theatres were typically built on hillsides and semi-circular in design. The first amphitheatre may have been built at Pompeii around 70 BC. Ancient Roman amphitheatres were oval or circular in plan, with seating tiers that surrounded the central performance area, like a modern open-air stadium. In contrast, both ancient Greek and ancient Roman theatres were built in a semicircle, with tiered seating rising on one side of the performance area.

An ogee (/oʊˈdʒiː/ /ˈoʊdʒiː/) is an object, element, or curve—often seen in architecture and building trades—that has a serpentine- or extended S-shape (sigmoid). Ogees consist of a "double curve", the combination of two semicircular curves or arcs that, as a result of a point of inflection from concave to convex or vice versa, have ends of the overall curve that point in opposite directions (and have tangents that are approximately parallel).

First seen in textiles in the 12th century, the use of ogee elements—in particular, in the design of arches—has been said to characterise various Gothic and Gothic Revival architectural styles. The shape has many such uses in architecture from those periods to the present day, including in the ogee arch in these architectural styles, where two ogees oriented as mirror images compose the sides of the arch, and in decorative molding designs, where single ogees are common profiles (see opening image). The term is also used in marine construction, particularly in shipbuilding, where ogee curves are used in hull design to improve hydrodynamics.The word was sometimes abbreviated as o-g as early as the 18th century, and in millwork trades associated with building construction, ogee is still sometimes written similarly (e.g., as O.G.).

The Gulf of Almería (Spanish: Golfo de Almería) is a bight of the Mediterranean Sea, in the southeastern end of the Iberian Peninsula.

It lies between Punta Sabinar and Cabo de Gata. It is shaped as a semicircle opened to the south with a protrusion in its center featuring the mouth of the Andarax river. Scholars placing ancient Urci in Pechina identify the Sinus Urcitanus cited by Pomponius Mela with the Gulf of Almería.

In astronomy, the meridian is the great circle passing through the celestial poles, as well as the zenith and nadir of an observer's location. Consequently, it contains also the north and south points on the horizon, and it is perpendicular to the celestial equator and horizon. Meridians, celestial and geographical, are determined by the pencil of planes passing through the Earth's rotation axis. For a location not on this axis, there is a unique meridian plane in this axial-pencil through that location. The intersection of this plane with Earth's surface defines two geographical meridians (either one east and one west of the prime meridian, or else the prime meridian itself and its anti-meridian), and the intersection of the plane with the celestial sphere is the celestial meridian for that location and time.

There are several ways to divide the meridian into semicircles. In one approach, the observer's upper meridian extends from a celestial pole and passes through the zenith to contact the opposite pole, while the lower meridian passes through the nadir to contact both poles at the opposite ends. In another approach known as the horizontal coordinate system, the meridian is divided into the local meridian, the semicircle that contains the observer's zenith and the north and south points of their horizon, and the opposite semicircle, which contains the nadir and the north and south points of their horizon.