Orbit

In physics, an orbit is the gravitationally curved path of an object about a point in space, for example the orbit of a planet about a star. Orbits of planets are typically elliptical, and the central mass being orbited is at a focal point of the ellipse.

Current understanding of the mechanics of orbital motion is based on Albert Einstein's general theory of relativity, which accounts for gravity as due to curvature of space-time, with orbits following geodesics. For ease of calculation, relativity is commonly approximated by the force-based theory of universal gravitation based on Kepler's laws of planetary motion.

History

Historically, the apparent motions of the planets were described geometrically (without regard to gravity) in terms of epicycles, which are the sums of numerous circular motions. Theories of this kind predicted paths of the planets moderately well, until Johannes Kepler was able to show that the motions of planets were elliptical motions.

The celestial spheres model is a geocentric model of the solar system originally used to explain the apparent motion of the planets in the sky. It posited the existence of perfect moving spheres or rings to which the stars and planets were attached. After the planets' motions were more accurately measured, theoretical mechanisms such as deferent and epicycles were added. Although it was capable of accurately predicting the planets' positions in the sky, more and more epicycles were required as the measurements became more accurate, hence the model became more and more unwieldy.

Solar System

The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury.

The Solar System formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with most of the remaining mass contained in Jupiter. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called ices, such as water, ammonia and methane. All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.

Earth mass

Earth mass (M_⊕, where ⊕ is the standard astronomical symbol for planet Earth) is the unit of mass equal to that of Earth. This value includes the atmosphere but excludes the moon. The current best estimate for Earth mass is M_⊕ = 7024597220000000000♠(5.9722±0.0006)×10²⁴ kg Earth mass is a standard unit of mass in astronomy that is used to indicate the masses of other planets, including rocky terrestrial planets and exoplanets.

Value

The mass of Earth is estimated to be:

or:

History of measurement

Modern methods of determining the mass of Earth involve calculating the gravitational coefficient of the Earth and dividing by the Newtonian constant of gravitation

The GM_⊕ product is determined using laser ranging data from Earth-orbiting satellites.

Before the space age (and after 1798), efforts to determine Earth's mass involved measuring G directly as in the Cavendish experiment. Earth's mass could be found by combining two equations; Newton's second law, and Newton's law of universal gravitation.

Major League

Major League may also refer to:

Sport

Major League Baseball

Major League Baseball (MLB) is a professional baseball league. A total of 30 teams now play in two divisions, the American League (AL) and National League (NL), with 15 teams in each. The AL and NL operated as separate legal entities from 1901 and 1876 respectively, until 2000, when they were merged into a single organization known as Major League Baseball. After cooperating but remaining legally separate entities since 1903, in 2000 the leagues merged into a single organization led by the Commissioner of Baseball. The organization also oversees minor league baseball leagues, which comprise about 240 teams affiliated with the major-league clubs. With the World Baseball Softball Confederation, MLB manages the international World Baseball Classic tournament.

Baseball's first professional team was founded in Cincinnati in 1869. The first few decades of professional baseball were characterized by rivalries between leagues and by players who often jumped from one team or league to another. The period before 1920 in baseball was known as the dead-ball era; players rarely hit home runs during this time. Baseball survived a conspiracy to fix the 1919 World Series, which came to be known as the Black Sox Scandal. The sport rose in popularity in the 1920s, and survived potential downturns during the Great Depression and World War II. Shortly after the war, baseball's color barrier was broken by Jackie Robinson.