These time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, the node of its orbit relative to a reference plane, the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit. In human and animal anatomy, three principal planes are used: The sagittal plane or lateral plane is a plane parallel to the sagittal suture. Earth-centered inertial (ECI) coordinate frames have their origins at the center of mass of Earth and do not rotate with respect to the stars. Here Be Dragons: A Creature Identification Quiz. Regular: These moons have relatively small orbits, which are often circular and roughly hover along the parent object's, Closer to Taiwan than Tokyo, and on a similar, The spaceplane was launched into a more highly inclined orbit than previous missions—meaning a larger angle off the, Post the Definition of equatorial plane to Facebook, Share the Definition of equatorial plane on Twitter. A culminating star on the observer's meridian is said to have a zero hour angle (0h). In order to fix the exact primary direction, these motions necessitate the specification of the equinox of a particular date, known as an epoch, when giving a position. In other words, the celestial equator is an abstract projection of the terrestrial equator into outer space. Alternatively to right ascension, hour angle (abbreviated HA or LHA, local hour angle), a left-handed system, measures the angular distance of an object westward along the celestial equator from the observer's meridian to the hour circle passing through the object. In contrast, in the horizontal coordinate system, a star's position differs from observer to observer based on their positions on the Earth's surface, and is continuously changing with the Earth's rotation. 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. [1] Due to 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). ECEF, also known as ECR, is a geographic and Cartesian coordinate system and is sometimes known as a "conventional terrestrial" system. The term "precession" typically refers only to this largest part of the motion; other changes in the alignment of Earth's axis—nutation and polar motion—are much smaller in magnitude. A lunar standstill is the gradually varying range between the northern and the southern limits of the Moon's declination, or the lunistices, over the course of one-half a sidereal month, or 13.66 days. [6] [9] [10]. Although there are two intersections of the ecliptic with the celestial equator, by convention the equinox associated with the sun's ascending node is used as the origin of celestial coordinate systems and referred to simply as the equinox. Superimposed on this is a smaller motion of the ecliptic, and a small oscillation of the Earth's axis, nutation. These are the most globally visible constellations. The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. Equatorial plane definition, the plane passing through the equator of the earth or another celestial body. The position of the Sun in the sky is a function of both the time and the geographic location of observation on Earth's surface. These example sentences are selected automatically from various online news sources to reflect current usage of the word 'equatorial plane.' At the poles, the celestial equator coincides with the astronomical horizon. In astronomy and celestial navigation, the hour angle is one of the coordinates used in the equatorial coordinate system to give the direction of a point on the celestial sphere. Astronomical nutation is a phenomenon which causes the orientation of the axis of rotation of a spinning astronomical object to vary over time. It divides the body into left and right. When paired with declination, these astronomical coordinates specify the location of a point on the celestial sphere in the equatorial coordinate system. In contrast to the common usage of spring and autumn (fall), or vernal and autumnal, equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time. Equatorial plane definition is - the plane perpendicular to the spindle of a dividing cell and midway between the poles. Un système de référence anatomique repose sur un ensemble de plans et d'axes définis par rapport à la position standard de l'organisme décrit. Commonly used reference planes and origins of longitude include: An orbital pole is either point at the ends of an imaginary line segment that runs through the center of an orbit and is perpendicular to the orbital plane. Un système de référence en anatomie, désigne la terminologie utilisée pour se repérer de façon précise dans la structure anatomique d'un organisme, humain ou non. It is convenient to represent the positions and velocities of terrestrial objects in ECEF coordinates or with latitude, longitude, and altitude. The … Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free! In astronomy, there is also a heliocentric rectangular variant of equatorial coordinates, designated x, y, z, which has: This frame is in every way equivalent to the ξ, η, ζ frame, above, except that the origin is removed to the centre of the Sun. [4]. Due to 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). [citation needed], Projection of the Earth's equator out into space,, Articles with unsourced statements from July 2020, Creative Commons Attribution-ShareAlike License, This page was last edited on 10 July 2020, at 15:35. A star's spherical coordinates are often expressed as a pair, right ascension and declination, without a distance coordinate. This plane of reference is coplanar with Earth's orbit around the Sun. Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. One sidereal hour (approximately 0.9973 solar hours) later, Earth's rotation will carry the star to the west of the meridian, and its hour angle will be 1h. All have: The reference frames do not rotate with the Earth (in contrast to Earth-centred, Earth-fixed frames), remaining always directed toward the equinox, and drifting over time with the motions of precession and nutation. The coronal plane or frontal plane divides the body into dorsal and ventral portions. [5]. Although they are caused by the same effect operating over different timescales, astronomers usually make a distinction between precession, which is a steady long-term change in the axis of rotation, and nutation, which is the combined effect of similar shorter-term variations. A common example would be the positions of the centers of a massive body (host) and of an orbiting celestial body at two different times/points of its orbit. [3], Astronomical objects near the celestial equator appear above the horizon from most places on earth, but they culminate (reach the meridian) highest near the equator.

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