Hipparcos was a scientific satellite of the European Space Agency, launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial objects on the sky; this permitted the accurate determination of proper motions and parallaxes of stars, allowing a determination of their distance and tangential velocity. When combined with radial velocity measurements from spectroscopy, this pinpointed all six quantities needed to determine the motion of stars; the resulting Hipparcos Catalogue, a high-precision catalogue of more than 118,200 stars, was published in 1997. The lower-precision Tycho Catalogue of more than a million stars was published at the same time, while the enhanced Tycho-2 Catalogue of 2.5 million stars was published in 2000. Hipparcos' follow-up mission, was launched in 2013; the word "Hipparcos" is an acronym for HIgh Precision PARallax COllecting Satellite and a reference to the ancient Greek astronomer Hipparchus of Nicaea, noted for applications of trigonometry to astronomy and his discovery of the precession of the equinoxes.
By the second half of the 20th century, the accurate measurement of star positions from the ground was running into insurmountable barriers to improvements in accuracy for large-angle measurements and systematic terms. Problems were dominated by the effects of the Earth's atmosphere, but were compounded by complex optical terms and gravitational instrument flexures, the absence of all-sky visibility. A formal proposal to make these exacting observations from space was first put forward in 1967. Although proposed to the French space agency CNES, it was considered too complex and expensive for a single national programme, its acceptance within the European Space Agency's scientific programme, in 1980, was the result of a lengthy process of study and lobbying. The underlying scientific motivation was to determine the physical properties of the stars through the measurement of their distances and space motions, thus to place theoretical studies of stellar structure and evolution, studies of galactic structure and kinematics, on a more secure empirical basis.
Observationally, the objective was to provide the positions and annual proper motions for some 100,000 stars with an unprecedented accuracy of 0.002 arcseconds, a target in practice surpassed by a factor of two. The name of the space telescope, "Hipparcos" was an acronym for High Precision Parallax Collecting Satellite, it reflected the name of the ancient Greek astronomer Hipparchus, considered the founder of trigonometry and the discoverer of the precession of the equinoxes; the spacecraft carried a single all-reflective, eccentric Schmidt telescope, with an aperture of 29 cm. A special beam-combining mirror superimposed two fields of view, 58 degrees apart, into the common focal plane; this complex mirror consisted of two mirrors tilted in opposite directions, each occupying half of the rectangular entrance pupil, providing an unvignetted field of view of about 1°×1°. The telescope used a system of grids, at the focal surface, composed of 2688 alternate opaque and transparent bands, with a period of 1.208 arc-sec.
Behind this grid system, an image dissector tube with a sensitive field of view of about 38-arc-sec diameter converted the modulated light into a sequence of photon counts from which the phase of the entire pulse train from a star could be derived. The apparent angle between two stars in the combined fields of view, modulo the grid period, was obtained from the phase difference of the two star pulse trains. Targeting the observation of some 100,000 stars, with an astrometric accuracy of about 0.002 arc-sec, the final Hipparcos Catalogue comprised nearly 120,000 stars with a median accuracy of better than 0.001 arc-sec. An additional photomultiplier system viewed a beam splitter in the optical path and was used as a star mapper, its purpose was to monitor and determine the satellite attitude, in the process, to gather photometric and astrometric data of all stars down to about 11th magnitude. These measurements were made in two broad bands corresponding to B and V in the UBV photometric system.
The positions of these latter stars were to be determined to a precision of 0.03 arc-sec, a factor of 25 less than the main mission stars. Targeting the observation of around 400,000 stars, the resulting Tycho Catalogue comprised just over 1 million stars, with a subsequent analysis extending this to the Tycho-2 Catalogue of about 2.5 million stars. The attitude of the spacecraft about its center of gravity was controlled to scan the celestial sphere in a regular precessional motion maintaining a constant inclination between the spin axis and the direction to the Sun; the spacecraft spun around its Z-axis at the rate of 11.25 revolutions/day at an angle of 43° to the Sun. The Z-axis rotated about the sun-satellite line at 6.4 revolutions/year. The spacecraft consisted of two platforms and six vertical panels, all made of aluminum honeycomb; the solar array consisted of three deployable sections. Two S-band antennas were located on the top and bottom of the spacecraft, providing an omni-directional downlink data rate of 24 kbit/s.
An attitude and orbit-control subsystem ensured correct dynamic attitude control and determination during the operational lifetim
In physics, motion is the change in position of an object with respect to its surroundings in a given interval of time. Motion is mathematically described in terms of displacement, velocity, acceleration and speed. Motion of a body is observed by attaching a frame of reference to an observer and measuring the change in position of the body relative to that frame. If the position of a body is not changing with respect to a given frame of reference, the body is said to be at rest, immobile, stationary, or to have constant position with reference to its surroundings. An object's motion can not change. Momentum is a quantity, used for measuring the motion of an object. An object's momentum is directly related to the object's mass and velocity, the total momentum of all objects in an isolated system does not change with time, as described by the law of conservation of momentum; as there is no absolute frame of reference, absolute motion cannot be determined. Thus, everything in the universe can be considered to be moving.
Motion applies to objects and matter particles, to radiation, radiation fields and radiation particles, to space, its curvature and space-time. One can speak of motion of shapes and boundaries. So, the term motion, in general, signifies a continuous change in the configuration of a physical system. For example, one can talk about motion of a wave or about motion of a quantum particle, where the configuration consists of probabilities of occupying specific positions. In physics, motion is described through two sets of contradictory laws of mechanics. Motions of all large-scale and familiar objects in the universe are described by classical mechanics. Whereas the motion of small atomic and sub-atomic objects is described by quantum mechanics. Classical mechanics is used for describing the motion of macroscopic objects, from projectiles to parts of machinery, as well as astronomical objects, such as spacecraft, planets and galaxies, it produces accurate results within these domains, is one of the oldest and largest in science and technology.
Classical mechanics is fundamentally based on Newton's laws of motion. These laws describe the relationship between the forces acting on a body and the motion of that body, they were first compiled by Sir Isaac Newton in his work Philosophiæ Naturalis Principia Mathematica, first published on July 5, 1687. Newton's three laws are: A body either is at rest or moves with constant velocity and unless an outer force is applied to it. An object will travel in one direction. Whenever one body exerts a force F onto a second body, the second body exerts the force −F on the first body. F and − F are equal in opposite in sense. So, the body which exerts F will go backwards. Newton's three laws of motion were the first to provide a mathematical model for understanding orbiting bodies in outer space; this explanation unified motion of objects on earth. Classical mechanics was further enhanced by Albert Einstein's special relativity and general relativity. Special relativity is concerned with the motion of objects with a high velocity, approaching the speed of light.
Uniform Motion: When an object moves with a constant speed at a particular direction at regular intervals of time it's known as the uniform motion. For example: a bike moving in a straight line with a constant speed. Equations of Uniform Motion: If v = final velocity, u = initial velocity, a = acceleration, t = time, s = displacement, then: v = u + a t v 2 = u 2 + 2 a s s = u t + a t 2 2 Quantum mechanics is a set of principles describing physical reality at the atomic level of matter and the subatomic particles; these descriptions include the simultaneous wave-like and particle-like behavior of both matter and radiation energy as described in the wave–particle duality. In classical mechanics, accurate measurements and predictions of the state of objects can be calculated, such as location and velocity. In the quantum mechanics, due to the Heisenberg uncertainty principle, the complete state of a subatomic particle, such as its location and velocity, cannot be determined. In addition to describing the motion of atomic level phenomena, quantum mechanics is useful in understanding some large-scale phenomenon such as superfluidity, superconductivity, biological systems, including the function of smell receptors and the structures of proteins.
Humans, like all known things in the universe, are in constant motion. Many of these "imperceptible motions" are only perceivable with the help of special tools and careful observation; the larger scales of imperceptible motions are difficult for humans to perceive
In relativity, proper velocity known as celerity, is an alternative to velocity for measuring motion. Whereas velocity relative to an observer is distance per unit time where both distance and time are measured by the observer, proper velocity relative to an observer divides observer-measured distance by the time elapsed on the clocks of the traveling object. Proper velocity equals velocity at low speeds. Proper velocity at high speeds, retains many of the properties that velocity loses in relativity compared with Newtonian theory. For example, proper velocity equals momentum per unit mass at any speed, therefore has no upper limit. At high speeds, as shown in the figure at right, it is proportional to an object's energy as well. Proper velocity w = dx/dτ is the product of two other derivatives in special relativity that describe an object's rate of travel: coordinate velocity v = dx/dt and the Lorentz factor γ = dt/dτ. For unidirectional motion, each of these is simply related to a traveling object's hyperbolic velocity angle or rapidity η by η = sinh − 1 w c = tanh − 1 v c = ± cosh − 1 γ.
In flat spacetime, proper velocity is the ratio between distance traveled relative to a reference map frame and proper time τ elapsed on the clocks of the traveling object. It equals the object's momentum p divided by its rest mass m, is made up of the space-like components of the object's four-vector velocity. William Shurcliff's monograph mentioned its early use in the Brehme text. Fraundorf has explored its pedagogical value while Ungar and Hestenes have examined its relevance from group theory and geometric algebra perspectives. Proper velocity is sometimes referred to as celerity. Unlike the more familiar coordinate velocity v, proper velocity is synchrony-free and is useful for describing both super-relativistic and sub-relativistic motion. Like coordinate velocity and unlike four-vector velocity, it resides in the three-dimensional slice of spacetime defined by the map frame; as shown below and in the example figure at right, proper-velocities add as three vectors with rescaling of the out-of-frame component.
This makes them more useful for map-based applications, less useful for gaining coordinate-free insight. Proper speed divided by lightspeed c is the hyperbolic sine of rapidity η, just as the Lorentz factor γ is rapidity's hyperbolic cosine, coordinate speed v over lightspeed is rapidity's hyperbolic tangent. Imagine an object traveling through a region of spacetime locally described by Hermann Minkowski's flat-space metric equation 2 = 2 − 2. Here a reference map frame of yardsticks and synchronized clocks define map position x and map time t and the d preceding a coordinate means infinitesimal change. A bit of manipulation allows one to show that proper velocity w = dx/dτ = γv where as usual coordinate velocity v = dx/dt, thus finite w ensures. By grouping γ with v in the expression for relativistic momentum p, proper velocity extends the Newtonian form of momentum as mass times velocity to high speeds without a need for relativistic mass; the proper velocity addition formula: u ⊕ v = u + v + u where β w is the beta factor given by β w = 1 1 + | w | 2 c 2.
This formula provides a proper velocity gyrovector space model of hyperbolic geometry that uses a whole space compared to other models of hyperbolic geometry which use discs or half-planes. In physics notation, local proper-velocities w ≡ dx/dτ add as 3-vectors much like coordinate-velocities at low speed, provided that we rescale the magnitude of the "out-of-frame" vector. In other words: w → AC = C + w → BC = γ AC v → AC,where Lorentz-factor γ = 1/β, the magnitude of wAB is rescaled into frame C according to: C ≡
The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five 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 the majority of the remaining mass contained in Jupiter; the four smaller inner planets, Venus and Mars, are terrestrial planets, being composed of rock and metal. The four outer planets are giant planets, being more massive than the terrestrials; the two largest and Saturn, are gas giants, being composed of hydrogen and helium. All eight planets have circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed of ices, beyond them a newly discovered population of sednoids. Within these populations are several dozen to tens of thousands of objects large enough that they have been rounded by their own gravity; such objects are categorized as dwarf planets. Identified dwarf planets include the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets and interplanetary dust clouds travel between regions. Six of the planets, at least four of the dwarf planets, many of the smaller bodies are orbited by natural satellites termed "moons" after the Moon; each of the outer planets is encircled by planetary rings of dust and other small objects.
The solar wind, a stream of charged particles flowing outwards from the Sun, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; the Oort cloud, thought to be the source for long-period comets, may exist at a distance a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy. For most of history, humanity did not understand the concept of the Solar System. Most people up to the Late Middle Ages–Renaissance believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system.
In the 17th century, Galileo discovered that the Sun was marked with sunspots, that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. Edmond Halley realised in 1705 that repeated sightings of a comet were recording the same object, returning once every 75–76 years; this was the first evidence that anything other than the planets orbited the Sun. Around this time, the term "Solar System" first appeared in English. In 1838, Friedrich Bessel measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. Improvements in observational astronomy and the use of unmanned spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun; the principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally.
The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System together comprise less than 0.002% of the Solar System's total mass. Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic; the planets are close to the ecliptic, whereas comets and Kuiper belt objects are at greater angles to it. All the planets, most other objects, orbit the Sun in the same direction that the Sun is rotating. There are exceptions, such as Halley's Comet; the overall structure of the charted regions of the Solar System consists of the Sun, four small inner planets surrounded by a belt of rocky asteroids, four giant planets surrounded by the Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions; the inner Solar System includes the asteroid belt. The outer Solar System is including the four giant planets.
Since the discovery of the Kuiper belt, the outermost parts of the Solar Sys
A red dwarf is a small and cool star on the main sequence, of M spectral type. Red dwarfs range in mass from about 0.075 to about 0.50 solar mass and have a surface temperature of less than 4,000 K. Sometimes K-type main-sequence stars, with masses between 0.50-0.8 solar mass, are included. Red dwarfs are by far the most common type of star in the Milky Way, at least in the neighborhood of the Sun, but because of their low luminosity, individual red dwarfs cannot be observed. From Earth, not one is visible to the naked eye. Proxima Centauri, the nearest star to the Sun, is a red dwarf, as are fifty of the sixty nearest stars. According to some estimates, red dwarfs make up three-quarters of the stars in the Milky Way. Stellar models indicate that red dwarfs less than 0.35 M☉ are convective. Hence the helium produced by the thermonuclear fusion of hydrogen is remixed throughout the star, avoiding helium buildup at the core, thereby prolonging the period of fusion. Red dwarfs therefore develop slowly, maintaining a constant luminosity and spectral type for trillions of years, until their fuel is depleted.
Because of the comparatively short age of the universe, no red dwarfs exist at advanced stages of evolution. The term "red dwarf" when used to refer to a star does not have a strict definition. One of the earliest uses of the term was in 1915, used to contrast "red" dwarf stars from hotter "blue" dwarf stars, it became established use. In terms of which spectral types qualify as red dwarfs, different researchers picked different limits, for example K8–M5 or "later than K5". Dwarf M star, abbreviated dM, was used, but sometimes it included stars of spectral type K. In modern usage, the definition of a red dwarf still varies; when explicitly defined, it includes late K- and early to mid-M-class stars, but in many cases it is restricted just to M-class stars. In some cases all K stars are included as red dwarfs, even earlier stars; the coolest true main-sequence stars are thought to have spectral types around L2 or L3, but many objects cooler than about M6 or M7 are brown dwarfs, insufficiently massive to sustain hydrogen-1 fusion.
Red dwarfs are very-low-mass stars. As a result, they have low pressures, a low fusion rate, hence, a low temperature; the energy generated is the product of nuclear fusion of hydrogen into helium by way of the proton–proton chain mechanism. Hence, these stars emit little light, sometimes as little as 1⁄10,000 that of the Sun; the largest red dwarfs have only about 10% of the Sun's luminosity. In general, red dwarfs less than 0.35 M☉ transport energy from the core to the surface by convection. Convection occurs because of opacity of the interior, which has a high density compared to the temperature; as a result, energy transfer by radiation is decreased, instead convection is the main form of energy transport to the surface of the star. Above this mass, a red dwarf will have a region around its core; because low-mass red dwarfs are convective, helium does not accumulate at the core, compared to larger stars such as the Sun, they can burn a larger proportion of their hydrogen before leaving the main sequence.
As a result, red dwarfs have estimated lifespans far longer than the present age of the universe, stars less than 0.8 M☉ have not had time to leave the main sequence. The lower the mass of a red dwarf, the longer the lifespan, it is believed that the lifespan of these stars exceeds the expected 10-billion-year lifespan of our Sun by the third or fourth power of the ratio of the solar mass to their masses. As the proportion of hydrogen in a red dwarf is consumed, the rate of fusion declines and the core starts to contract; the gravitational energy released by this size reduction is converted into heat, carried throughout the star by convection. According to computer simulations, the minimum mass a red dwarf must have in order to evolve into a red giant is 0.25 M☉. The less massive the star, the longer this evolutionary process takes, it has been calculated that a 0.16 M☉ red dwarf would stay on the main sequence for 2.5 trillion years, followed by five billion years as a blue dwarf, during which the star would have one third of the Sun's luminosity and a surface temperature of 6,500–8,500 kelvins.
The fact that red dwarfs and other low-mass stars still remain on the main sequence when more massive stars have moved off the main sequence allows the age of star clusters to be estimated by finding the mass at which the stars move off the main sequence. This provides a lower limit to the age of the Universe and allows formation timescales to be placed upon the structures within the Milky Way, such as the Galactic halo and Galactic disk. All observed red dwarfs contain "metals", which in astronomy are elements heavier than hydrogen and helium; the Big Bang model predicts that the first generation of stars should have only hydrogen and trace amounts of lithium, hence would be of low metallicity. With their extreme lifespans, any red dwarfs that were a part of that first generation should still exist today. Low metallicity red dwarfs, are rare. There are several explanations for the missing population of metal-poor red dwarfs; the preferred explanation is. Large stars burn out and exp
A constellation is a group of stars that forms an imaginary outline or pattern on the celestial sphere representing an animal, mythological person or creature, a god, or an inanimate object. The origins of the earliest constellations go back to prehistory. People used them to relate stories of their beliefs, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. Adoption of constellations has changed over time. Many have changed in shape; some became popular. Others were limited to single nations; the 48 traditional Western constellations are Greek. They are given in Aratus' work Phenomena and Ptolemy's Almagest, though their origin predates these works by several centuries. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Twelve ancient constellations belong to the zodiac.
The origins of the zodiac remain uncertain. In 1928, the International Astronomical Union formally accepted 88 modern constellations, with contiguous boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations; some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation name. Other star patterns or groups called asterisms are not constellations per se but are used by observers to navigate the night sky. Examples of bright asterisms include the Pleiades and Hyades within the constellation Taurus or Venus' Mirror in the constellation of Orion.. Some asterisms, like the False Cross, are split between two constellations; the word "constellation" comes from the Late Latin term cōnstellātiō, which can be translated as "set of stars".
The Ancient Greek word for constellation is ἄστρον. A more modern astronomical sense of the term "constellation" is as a recognisable pattern of stars whose appearance is associated with mythological characters or creatures, or earthbound animals, or objects, it can specifically denote the recognized 88 named constellations used today. Colloquial usage does not draw a sharp distinction between "constellations" and smaller "asterisms", yet the modern accepted astronomical constellations employ such a distinction. E.g. the Pleiades and the Hyades are both asterisms, each lies within the boundaries of the constellation of Taurus. Another example is the northern asterism known as the Big Dipper or the Plough, composed of the seven brightest stars within the area of the IAU-defined constellation of Ursa Major; the southern False Cross asterism includes portions of the constellations Carina and Vela and the Summer Triangle.. A constellation, viewed from a particular latitude on Earth, that never sets below the horizon is termed circumpolar.
From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic or zodiac ranging between 23½° north, the celestial equator, 23½° south. Although stars in constellations appear near each other in the sky, they lie at a variety of distances away from the Earth. Since stars have their own independent motions, all constellations will change over time. After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring individual stars' common proper motions or cpm by accurate astrometry and their radial velocities by astronomical spectroscopy; the earliest evidence for the humankind's identification of constellations comes from Mesopotamian inscribed stones and clay writing tablets that date back to 3000 BC.
It seems that the bulk of the Mesopotamian constellations were created within a short interval from around 1300 to 1000 BC. Mesopotamian constellations appeared in many of the classical Greek constellations; the oldest Babylonian star catalogues of stars and constellations date back to the beginning in the Middle Bronze Age, most notably the Three Stars Each texts and the MUL. APIN, an expanded and revised version based on more accurate observation from around 1000 BC. However, the numerous Sumerian names in these catalogues suggest that they built on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age; the classical Zodiac is a revision of Neo-Babylonian constellations from the 6th century BC. The Greeks adopted the Babylonian constellations in the 4th century BC. Twenty Ptolemaic constellations are from the Ancient Near East. Another ten have the same stars but different names. Biblical scholar, E. W. Bullinger interpreted some of the creatures mentioned in the books of Ezekiel and Revelation as the middle signs of the four quarters of the Zodiac, with the Lion as Leo, the Bull as Taurus, the Man representing Aquarius and the Eagle standing in for Scorpio.
The biblical Book of Job also
An equinox is regarded as the instant of time when the plane of Earth's equator passes through the center of the Sun. This occurs 23 September. In other words, it is the moment at which the center of the visible Sun is directly above the Equator; the word is derived from aequus and nox. On the day of an equinox and nighttime are of equal duration all over the planet, they are not equal, due to the angular size of the Sun, atmospheric refraction, the changing duration of the length of day that occurs at most latitudes around the equinoxes. Long before conceiving this equality primitive cultures noted the day when the Sun rises due East and sets due West and indeed this happens on the day closest to the astronomically defined event. In the northern hemisphere, the equinox in March is called the Spring Equinox; the dates are variable, dependent as they are on the leap year cycle. Because the Moon cause the motion of the Earth to vary from a perfect ellipse, the equinox is now defined by the Sun's more regular ecliptic longitude rather than by its declination.
The instants of the equinoxes are defined to be when the longitude of the Sun is 0° and 180°. Systematically observing the sunrise, people discovered that it occurs between two extreme locations at the horizon and noted the midpoint between the two, it was realized that this happens on a day when the durations of the day and the night are equal and the word "equinox" comes from Latin Aequus, meaning "equal", Nox, meaning "night". In the northern hemisphere, the vernal equinox conventionally marks the beginning of spring in most cultures and is considered the start of the New Year in the Assyrian calendar and the Persian calendar or Iranian calendars as Nowruz, while the autumnal equinox marks the beginning of autumn; the equinoxes are the only times. As a result, the northern and southern hemispheres are illuminated. In other words, the equinoxes are the only times when the subsolar point is on the equator, meaning that the Sun is overhead at a point on the equatorial line; the subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox.
When Julius Caesar established the Julian calendar in 45 BC, he set 25 March as the date of the spring equinox. Because the Julian year is longer than the tropical year by about 11.3 minutes on average, the calendar "drifted" with respect to the two equinoxes – so that in AD 300 the spring equinox occurred on about 21 March, by AD 1500 it had drifted backwards to 11 March. This drift induced Pope Gregory XIII to create the modern Gregorian calendar; the Pope wanted to continue to conform with the edicts of the Council of Nicaea in AD 325 concerning the date of Easter, which means he wanted to move the vernal equinox to the date on which it fell at that time, to maintain it at around that date in the future, which he achieved by reducing the number of leap years from 100 to 97 every 400 years. However, there remained a small residual variation in the date and time of the vernal equinox of about ±27 hours from its mean position all because the distribution of 24-hour centurial leap days causes large jumps.
This in turn raised the possibility that it could fall on 22 March, thus Easter Day might theoretically commence before the equinox. The astronomers chose the appropriate number of days to omit so that the equinox would swing from 19 to 21 March but never fall on 22 March; the dates of the equinoxes change progressively during the leap-year cycle, because the Gregorian calendar year is not commensurate with the period of the Earth's revolution about the Sun. It is only after a complete Gregorian leap-year cycle of 400 years that the seasons commence at the same time. In the 21st century the earliest March equinox will be 19 March 2096, while the latest was 21 March 2003; the earliest September equinox will be 21 September 2096 while the latest was 23 September 2003. Vernal equinox and autumnal equinox: these classical names are direct derivatives of Latin; these are the universal and still most used terms for the equinoxes, but are confusing because in the southern hemisphere the vernal equinox does not occur in spring and the autumnal equinox does not occur in autumn.
The equivalent common language English terms spring equinox and autumn equinox are more ambiguous. It has become common for people to refer to the September equinox in the southern hemisphere as the Vernal equinox. March equinox and September equinox: names referring to the months of the year in which they occur, with no ambiguity as to which hemisphere is the context, they are still not universal, however, as not all cultures use a solar-based calendar where the equinoxes occur every year in the same month. Although the terms have become common in the 21st century, they were sometimes used at least as long ago as the mid-20th century. Northward equinox and southward equinox: names referring to the appare