Apsis in the context of "Primary body"

An Earth ellipsoid or Earth spheroid is a mathematical figure approximating the Earth's shape and size, used as a reference frame for computations in geodesy, astronomy, and the geosciences. Various different reference ellipsoids have been used as approximations.

It is an oblate spheroid (an ellipsoid of revolution) whose minor axis (polar diameter), connecting the geographical poles, is approximately aligned with the Earth's axis of rotation. The ellipsoid is also defined by the major axis (equatorial axis); the difference between the two axes is slightly more than 21 km or 0.335%.

65489 Ceto, as a binary also (65489) Ceto–Phorcys (provisional designation 2003 FX₁₂₈), is a binary trans-Neptunian object (TNO) discovered on March 22, 2003, by Chad A. Trujillo and Michael Brown at Palomar. It is named after the sea goddess Ceto from Greek mythology. It came to perihelion in 1989.

An orbital spaceflight (or orbital flight) is a spaceflight in which a spacecraft is placed on a trajectory where it could remain in space for at least one orbit. To do this around the Earth, it must be on a free trajectory which has an altitude at perigee (altitude at closest approach) around 80 kilometers (50 mi); this is the boundary of space as defined by NASA, the US Air Force and the FAA. To remain in orbit at this altitude requires an orbital speed of ~7.8 km/s. Orbital speed is slower for higher orbits, but attaining them requires greater delta-v. The Fédération Aéronautique Internationale has established the Kármán line at an altitude of 100 km (62 mi) as a working definition for the boundary between aeronautics and astronautics. This is used because at an altitude of about 100 km (62 mi), as Theodore von Kármán calculated, a vehicle would have to travel faster than orbital velocity to derive sufficient aerodynamic lift from the atmosphere to support itself.

Due to atmospheric drag, the lowest altitude at which an object in a circular orbit can complete at least one full revolution without propulsion is approximately 150 kilometres (93 mi).

In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter (the combined center of mass) or, if one body is much more massive than the other bodies of the system combined, its speed relative to the center of mass of the most massive body.

The term can be used to refer to either the mean orbital speed (i.e. the average speed over an entire orbit) or its instantaneous speed at a particular point in its orbit. The maximum (instantaneous) orbital speed occurs at periapsis (perigee, perihelion, etc.), while the minimum speed for objects in closed orbits occurs at apoapsis (apogee, aphelion, etc.). In ideal two-body systems, objects in open orbits continue to slow down forever as their distance to the barycenter increases.

A near-Earth object (NEO) is any small Solar System body orbiting the Sun whose closest approach to the Sun (perihelion) is less than 1.3 times the Earth–Sun distance (astronomical unit, AU). This definition applies to the object's orbit around the Sun, rather than its current position, thus an object with such an orbit is considered an NEO even at times when it is far from making a close approach of Earth. If an NEO's orbit crosses the Earth's orbit, and the object is larger than 140 meters (460 ft) across, it is considered a potentially hazardous object (PHO). Most known PHOs and NEOs are asteroids, but about a third of a percent are comets.

There are over 37,000 known near-Earth asteroids (NEAs) and over 120 known short-period near-Earth comets (NECs). A number of solar-orbiting meteoroids were large enough to be tracked in space before striking Earth. It is now widely accepted that collisions in the past have had a significant role in shaping the geological and biological history of Earth. Asteroids as small as 20 metres (66 ft) in diameter can cause significant damage to the local environment and human populations. Larger asteroids penetrate the atmosphere to the surface of the Earth, producing craters if they impact a continent or tsunamis if they impact the sea. Interest in NEOs has increased since the 1980s because of greater awareness of this risk. Asteroid impact avoidance by deflection is possible in principle, and methods of mitigation are being researched.

A total solar eclipse occurred at the Moon's ascending node of orbit on Tuesday, August 18, 1868 (also known as "The King of Siam's eclipse"), with a magnitude of 1.0756. A solar eclipse occurs when the Moon passes between Earth and the Sun, thereby totally or partly obscuring the image of the Sun for a viewer on Earth. A total solar eclipse occurs when the Moon's apparent diameter is larger than the Sun's, blocking all direct sunlight, turning day into darkness. Totality occurs in a narrow path across Earth's surface, with the partial solar eclipse visible over a surrounding region thousands of kilometres wide. Occurring about 6.5 days after perigee (on August 17, 1868, at 22:35 UTC), the Moon's apparent diameter was larger.

The path of totality was visible from parts of modern-day Ethiopia, Eritrea, Djibouti, Yemen, India, the Andaman and Nicobar Islands, Myanmar, Thailand, Cambodia, Vietnam, Malaysia, Brunei, Indonesia, and Papua New Guinea. A partial solar eclipse was also visible for parts of East Africa, the Middle East, Central Asia, South Asia, Southeast Asia, Australia, and western Oceania.

A total solar eclipse occurred at the Moon’s ascending node of orbit on Wednesday, August 11, 1999, with a magnitude of 1.0286. A solar eclipse is when the Moon passes between the Earth and the Sun, thereby totally or partly obscuring the light of the sun for a viewer on earth. A total solar eclipse is when the Moon’s apparent diameter is larger than the Sun’s, blocking all direct sunlight, turning day into night. Totality occurs in a narrow path across Earth’s surface, with the partial solar eclipse visible over a surrounding region thousands of kilometres wide. Occurring about 3.5 days after perigee (on August 8, 1999, at 0:30 UTC), the Moon's apparent diameter was larger.

It was the first total eclipse visible from Europe since July 22, 1990, and the first visible in the United Kingdom since June 29, 1927.