Perseus is a constellation in the northern sky, being named after the Greek mythological hero Perseus. It is one of the 48 ancient constellations listed by the 2nd-century astronomer Ptolemy, among the 88 modern constellations defined by the International Astronomical Union, it is located near several other constellations named after ancient Greek legends surrounding Perseus, including Andromeda to the west and Cassiopeia to the north. Perseus is bordered by Aries and Taurus to the south, Auriga to the east, Camelopardalis to the north, Triangulum to the west; some star atlases during the early 19th century depicted Perseus holding the disembodied head of Medusa, whose asterism was named together as Perseus et Caput Medusae, this never came into popular usage. The galactic plane of the Milky Way passes through Perseus, whose brightest star is the yellow-white supergiant Alpha Persei, which shines at magnitude 1.79. It and many of the surrounding stars are members of an open cluster known as the Alpha Persei Cluster.
The best-known star, however, is Algol, linked with ominous legends because of its variability, noticeable to the naked eye. Rather than being an intrinsically variable star, it is an eclipsing binary. Other notable star systems in Perseus include X Persei, a binary system containing a neutron star, GK Persei, a nova that peaked at magnitude 0.2 in 1901. The Double Cluster, comprising two open clusters quite near each other in the sky, was known to the ancient Chinese; the constellation gives its name to the Perseus cluster, a massive galaxy cluster located 250 million light-years from Earth. It hosts the radiant of the annual Perseids meteor shower—one of the most prominent meteor showers in the sky; the constellation of Perseus may be derived from the Babylonian Old Man constellation associated with East in the MUL. APIN—an astronomical compilation dating to around 1000 BCE. In Greek mythology, Perseus was the son of Danaë, sent by King Polydectes to bring the head of Medusa the Gorgon — whose visage caused all who gazed upon her to turn to stone.
Perseus slew Medusa in her sleep, Pegasus and Chrysaor appeared from her body. Perseus continued to the realm of Cepheus whose daughter Andromeda was to be sacrificed to Cetus the sea monster. Perseus rescued Andromeda from the monster by killing it with his diamond sword, he turned Polydectes and his followers to stone with Medusa's head and appointed Dictys the fisherman king. Perseus and Andromeda had six children. In the sky, Perseus lies near the constellations Andromeda, Cassiopeia and Pegasus. Four Chinese constellations are contained in the area of the sky identified with Perseus in the West. Tiānchuán, the Celestial Boat, was the third paranatellon of the third house of the White Tiger of the West, representing the boats that Chinese people were reminded to build in case of a catastrophic flood season. Incorporating stars from the northern part of the constellation, it contained Mu, Psi, Alpha and Eta Persei. Jīshuǐ, the Swollen Waters, was the fourth paranatellon of the aforementioned house, representing the potential of unusually high floods during the end of August and beginning of September at the beginning of the flood season.
Lambda and Mu Persei lay within it. Dàlíng, the Great Trench, was the fifth paranatellon of that house, representing the trenches where criminals executed en masse in August were interred, it was formed by Kappa, Rho, 24, 17 and 15 Persei. The pile of corpses prior to their interment was represented by Jīshī, the sixth paranatellon of the house; the Double Cluster, h and Chi Persei, had special significance in Chinese astronomy. In Polynesia, Perseus was not recognized as a separate constellation. Algol may have been named Matohi by the Māori people, but the evidence for this identification is disputed. Matohi came into conflict with Tangaroa-whakapau over which of them should appear in the sky, the outcome affecting the tides, it matches the Maori description of a blue-white star near Aldebaran but does not disappear as the myth would indicate. Perseus is bordered by Aries and Taurus to the south, Auriga to the east and Cassiopeia to the north, Andromeda and Triangulum to the west. Covering 615 square degrees, it ranks twenty-fourth of the 88 constellations in size.
It appears prominently in the northern sky during the Northern Hemisphere's spring. Its main asterism consists of 19 stars; the constellation's boundaries, as set by Eugène Delporte in 1930, are defined by a 26-sided polygon. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 01h 29.1m and 04h 51.2m, while the declination coordinates are between 30.92° and 59.11°. The International Astronomical Union adopted the three-letter abbreviation "Per" for the constellation in 1922. Algol known by its Bayer designation Beta Persei, is the best-known star in Perseus. Representing the head of the Gorgon Medusa in Greek mythology, it was called Horus in Egyptian mythology and Rosh ha Satan in Hebrew. Located 92.8 light-years from Earth, it varies in apparent magnitude from a minimum of 3.5 to a maximum of 2.3 over a period of 2.867 days. The star system is the prototype of a group of eclipsing binary stars named Algol variables, though it has a third member to make up what is a triple star system.
The brightest compo
The Pleiades known as the Seven Sisters and Messier 45, are an open star cluster containing middle-aged, hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky; the cluster is dominated by hot blue and luminous stars that have formed within the last 100 million years. Reflection nebulae around the brightest stars were once thought to be left over material from the formation of the cluster, but are now considered to be an unrelated dust cloud in the interstellar medium through which the stars are passing. Computer simulations have shown that the Pleiades were formed from a compact configuration that resembled the Orion Nebula. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood; the name of the Pleiades comes from Ancient Greek. It derives from plein because of the cluster's importance in delimiting the sailing season in the Mediterranean Sea: "the season of navigation began with their heliacal rising".
However, in mythology the name was used for the Pleiades, seven divine sisters, the name deriving from that of their mother Pleione and meaning "daughters of Pleione". In reality, the name of the star cluster certainly came first, Pleione was invented to explain it; the Pleiades are a prominent sight in winter in the Northern Hemisphere, are visible out to mid-Southern latitudes. They have been known since antiquity to cultures all around the world, including the Celts, Hawaiians, Māori, Aboriginal Australians, the Persians, the Arabs, the Chinese, the Quechua, the Japanese, the Maya, the Aztec, the Sioux, the Kiowa, the Cherokee. In Hinduism, the Pleiades are associated with the war-god Kartikeya, they are mentioned three times in the Bible. The earliest known depiction of the Pleiades is a Northern German bronze age artifact known as the Nebra sky disk, dated to 1600 BC; the Babylonian star catalogues name the Pleiades MULMUL, meaning "stars", they head the list of stars along the ecliptic, reflecting the fact that they were close to the point of vernal equinox around the 23rd century BC.
The Ancient Egyptians may have used the names "Followers" and "Ennead" in the prognosis texts of the Calendar of Lucky and Unlucky Days of papyrus Cairo 86637. Some Greek astronomers considered them to be a distinct constellation, they are mentioned by Hesiod's Works and Days, Homer's Iliad and Odyssey, the Geoponica; some scholars of Islam suggested that the Pleiades are the "star" mentioned in Sura An-Najm of the Quran. In Japan, the constellation is mentioned under the name Mutsuraboshi in the 8th century Kojiki; the constellation is now known in Japan as Subaru. It was chosen as the brand name of Subaru automobiles to reflect the origins of the firm as the joining of five companies, is depicted in the firm's six-star logo. Galileo Galilei was the first astronomer to view the Pleiades through a telescope, he thereby discovered. He published his observations, including a sketch of the Pleiades showing 36 stars, in his treatise Sidereus Nuncius in March 1610; the Pleiades have long been known to be a physically related group of stars rather than any chance alignment.
John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, so surmised that the Pleiades and many other clusters of stars must be physically related. When studies were first made of the stars' proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related. Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe cluster, Messier's inclusion of the Pleiades has been noted as curious, as most of Messier's objects were much fainter and more confused with comets—something that seems scarcely possible for the Pleiades. One possibility is that Messier wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, so he added some bright, well-known objects to boost his list.
Edme-Sébastien Jeaurat drew in 1782 a map of 64 stars of the Pleiades from his observations in 1779, which he published in 1786. The distance to the Pleiades can be used as an important first step to calibrate the cosmic distance ladder; as the cluster is so close to the Earth, its distance is easy to measure and has been estimated by many methods. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram for the cluster, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods can extend the distance scale from open clusters to galaxies and clusters of galaxies, a cosmic distance ladder can be constructed. Astronomers' understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades, yet some authors argue that the controversy over the distance to the Pleiades discussed below is a red herring, since the cosmic distance ladder can rely on a suite of other nearby clusters where consensus exists regarding the distances as esta
Hyades (star cluster)
The Hyades is the nearest open cluster and one of the best-studied star clusters. Located about 153 light-years away from the Sun, it consists of a spherical group of hundreds of stars sharing the same age, place of origin, chemical characteristics, motion through space. From the perspective of observers on Earth, the Hyades Cluster appears in the constellation Taurus, where its brightest stars form a "V" shape along with the still-brighter Aldebaran. However, Aldebaran is unrelated to the Hyades, as it is located much closer to Earth and happens to lie along the same line of sight; the five brightest member stars of the Hyades have consumed the hydrogen fuel at their cores and are now evolving into giant stars. Four of these stars, with Bayer designations Gamma, Delta 1, Theta Tauri, form an asterism, traditionally identified as the head of Taurus the Bull; the fifth of these stars is a tight naked-eye companion to the brighter Theta2 Tauri. Epsilon Tauri, known as Ain, has a gas giant exoplanet candidate, the first planet to be found in any open cluster.
The age of the Hyades is estimated to be about 625 million years. The core of the cluster, where stars are the most densely packed, has a radius of 8.8 light-years, the cluster's tidal radius – where the stars become more influenced by the gravity of the surrounding Milky Way galaxy – is 33 light-years. However, about one-third of confirmed member stars have been observed well outside the latter boundary, in the cluster's extended halo; the cluster is sufficiently close to the Sun that its distance can be directly measured by observing the amount of parallax shift of the member stars as the Earth orbits the Sun. This measurement has been performed with great accuracy using the Hipparcos satellite and the Hubble Space Telescope. An alternative method of computing the distance is to fit the cluster members to a standardized infrared color–magnitude diagram for stars of their type, use the resulting data to infer their intrinsic brightness. Comparing this data to the brightness of the stars as seen from Earth allows their distances to be estimated.
Both methods have yielded a distance estimate of 153 light-years to the cluster center. The fact that three independent measurements agree makes the Hyades an important rung on the cosmic distance ladder method for estimating the distances of extragalactic objects; the stars of the Hyades are more enriched in heavier elements than our Sun and other ordinary stars in the Solar neighborhood, with the overall cluster metallicity measured at +0.14. The Hyades Cluster is related to other stellar groups in the Sun's vicinity, its age and proper motion coincide with those of the larger and more distant Praesepe Cluster, the trajectories of both clusters can be traced back to the same region of space, indicating a common origin. Another associate is the Hyades Stream, a large collection of scattered stars that share a similar trajectory with the Hyades Cluster. Recent results have found that at least 15% of stars in the Hyades stream share the same chemical fingerprint as the Hyades cluster stars. However, about 85% of stars in the Hyades Stream have been shown to be unrelated to the original cluster on the grounds of dissimilar age and metallicity.
Among the remaining members of the Hyades Stream, the exoplanet host star Iota Horologii has been proposed as an escaped member of the primordial Hyades Cluster. The Hyades are unrelated to two other nearby stellar groups, the Pleiades and the Ursa Major Stream, which are visible to the naked eye under clear dark skies. In Greek mythology, the Hyades were half-sisters to the Pleiades. After the death of their brother, the weeping sisters were transformed into a cluster of stars, afterwards associated with rain; as a naked-eye object, the Hyades cluster has been known since prehistoric times. It is mentioned by numerous Classical authors from Homer to Ovid. In Book 18 of the Iliad the stars of the Hyades appear along with the Pleiades, Ursa Major, Orion on the shield that the god Hephaistos made for Achilles. In England the cluster was known as the "April Rainers" from an association with April showers, as recorded in the folk song "Green Grow the Rushes, O"; the cluster was first catalogued by Giovanni Battista Hodierna in 1654, it subsequently appeared in many star atlases of the 17th and 18th centuries.
However, Charles Messier did not include the Hyades in his 1781 catalog of deep sky objects. It therefore lacks a Messier number, unlike many other, more distant open clusters – e.g. M44, M45, M67. In 1869, the astronomer R. A. Proctor observed that numerous stars at large distances from the Hyades share a similar motion through space. In 1908, Lewis Boss reported 25 years of observations to support this premise, arguing for the existence of a co-moving group of stars that he called the Taurus Stream. Boss published a chart that traced the scattered stars' movements back to a common point of convergence. By the 1920s, the notion that the Hyades shared a common origin with the Praesepe Cluster was widespread, with Rudolf Klein-Wassink noting in 1927 that the two clusters are "probably cosmically related." For much of the twentieth century, scientific study of the Hyades focused on determining its distance, modeling its evolution, confirming or rejecting ca
Kudurru was a type of stone document used as boundary stones and as records of land grants to vassals by the Kassites in ancient Babylonia between the 16th and 12th centuries BCE. The word is Akkadian for "frontier" or "boundary"; the kudurrus are the only surviving artworks for the period of Kassite rule in Babylonia with examples kept in the Louvre, the British Museum, the National Museum of Iraq. The kudurrus recorded the land granted by the king to his vassals as a record of his decision; the original kudurru would be stored in a temple while the person granted the land would be given a clay copy to use as a boundary stone to confirm legal ownership. The kudurrus would contain symbolic images of the deities who were protecting the contract, the contract, the divine curse that would be placed on a person who broke the contract; some kudurrus contained an image of the king who granted the land. As they contained a great deal of images as well as a contract, kudurrus were engraved on large slabs of stone.
Kassite-era kudurrus, in approximate chronological order: Enlil-bānī land grant kudurru Nazimaruttaš kudurru stone Kudurru of Kaštiliašu Tablet of Akaptaḫa, an entitlement narû, or "stele", of a similar character Land grant to Ḫunnubat-Nanaya kudurru Land grant to Marduk-apla-iddina I by Meli-Shipak II Estate of Takil-ana-ilīšu kudurru Land grant to Ḫasardu kudurru Stele of Meli-Šipak Land grant to Marduk-zākir-šumi kudurru Land grant to Munnabittu kudurru Kudurru of GulaPost-Kassite kudurrus: Kudurru for Ritti-Marduk Eanna-shum-iddina kudurru Marduk-nadin-ahhe kudurru Sun God Tablet Marduk-zakir-šumi I kudurru Marduk-apal-iddina II kudurru Kurgan stelae Runestone Encyclopædia Britannica Online article on kudduru BBC Ancient History Online Gallery of Mesopotamian Art US State Department Gallery of Iraqi Cultural Heritage High resolution of the "Michaux Kudurru"
Ancient Greece was a civilization belonging to a period of Greek history from the Greek Dark Ages of the 12th–9th centuries BC to the end of antiquity. Following this period was the beginning of the Early Middle Ages and the Byzantine era. Three centuries after the Late Bronze Age collapse of Mycenaean Greece, Greek urban poleis began to form in the 8th century BC, ushering in the Archaic period and colonization of the Mediterranean Basin; this was followed by the period of Classical Greece, an era that began with the Greco-Persian Wars, lasting from the 5th to 4th centuries BC. Due to the conquests by Alexander the Great of Macedon, Hellenistic civilization flourished from Central Asia to the western end of the Mediterranean Sea; the Hellenistic period came to an end with the conquests and annexations of the eastern Mediterranean world by the Roman Republic, which established the Roman province of Macedonia in Roman Greece, the province of Achaea during the Roman Empire. Classical Greek culture philosophy, had a powerful influence on ancient Rome, which carried a version of it to many parts of the Mediterranean Basin and Europe.
For this reason, Classical Greece is considered to be the seminal culture which provided the foundation of modern Western culture and is considered the cradle of Western civilization. Classical Greek culture gave great importance to knowledge. Science and religion were not separate and getting closer to the truth meant getting closer to the gods. In this context, they understood the importance of mathematics as an instrument for obtaining more reliable knowledge. Greek culture, in a few centuries and with a limited population, managed to explore and make progress in many fields of science, mathematics and knowledge in general. Classical antiquity in the Mediterranean region is considered to have begun in the 8th century BC and ended in the 6th century AD. Classical antiquity in Greece was preceded by the Greek Dark Ages, archaeologically characterised by the protogeometric and geometric styles of designs on pottery. Following the Dark Ages was the Archaic Period, beginning around the 8th century BC.
The Archaic Period saw early developments in Greek culture and society which formed the basis for the Classical Period. After the Archaic Period, the Classical Period in Greece is conventionally considered to have lasted from the Persian invasion of Greece in 480 until the death of Alexander the Great in 323; the period is characterized by a style, considered by observers to be exemplary, i.e. "classical", as shown in the Parthenon, for instance. Politically, the Classical Period was dominated by Athens and the Delian League during the 5th century, but displaced by Spartan hegemony during the early 4th century BC, before power shifted to Thebes and the Boeotian League and to the League of Corinth led by Macedon; this period saw the Greco-Persian Wars and the Rise of Macedon. Following the Classical period was the Hellenistic period, during which Greek culture and power expanded into the Near and Middle East; this period ends with the Roman conquest. Roman Greece is considered to be the period between Roman victory over the Corinthians at the Battle of Corinth in 146 BC and the establishment of Byzantium by Constantine as the capital of the Roman Empire in AD 330.
Late Antiquity refers to the period of Christianization during the 4th to early 6th centuries AD, sometimes taken to be complete with the closure of the Academy of Athens by Justinian I in 529. The historical period of ancient Greece is unique in world history as the first period attested directly in proper historiography, while earlier ancient history or proto-history is known by much more circumstantial evidence, such as annals or king lists, pragmatic epigraphy. Herodotus is known as the "father of history": his Histories are eponymous of the entire field. Written between the 450s and 420s BC, Herodotus' work reaches about a century into the past, discussing 6th century historical figures such as Darius I of Persia, Cambyses II and Psamtik III, alluding to some 8th century ones such as Candaules. Herodotus was succeeded by authors such as Thucydides, Demosthenes and Aristotle. Most of these authors were either Athenian or pro-Athenian, why far more is known about the history and politics of Athens than those of many other cities.
Their scope is further limited by a focus on political and diplomatic history, ignoring economic and social history. In the 8th century BC, Greece began to emerge from the Dark Ages which followed the fall of the Mycenaean civilization. Literacy had been lost and Mycenaean script forgotten, but the Greeks adopted the Phoenician alphabet, modifying it to create the Greek alphabet. Objects with Phoenician writing on them may have been available in Greece from the 9th century BC, but the earliest evidence of Greek writing comes from graffiti on Greek pottery from the mid-8th century. Greece was divided into many small self-governing communities, a pattern dictated by Greek geography: every island and plain is cut off from its neighbors by the sea or mountain ranges; the Lelantine War is the earliest documented war of the ancient Greek period. It was fought between the important poleis of Chalcis and Eretria over the fertile Lelantine plain of Euboea. Both cities seem to have suffered a decline as result of the long war, though Chalcis was the nominal victor.
A mercantile class arose in the first half of the 7th century BC, shown by the introduction of coinage in about 680 BC. This
Triangulum is a small constellation in the northern sky. Its name is Latin for "triangle", derived from its three brightest stars, which form a long and narrow triangle. Known to the ancient Babylonians and Greeks, Triangulum was one of the 48 constellations listed by the 2nd century astronomer Ptolemy; the celestial cartographers Johann Bayer and John Flamsteed catalogued the constellation's stars, giving six of them Bayer designations. The white stars Beta and Gamma Trianguli, of apparent magnitudes 3.00 and 4.00 form the base of the triangle and the yellow-white Alpha Trianguli, of magnitude 3.41, the apex. Iota Trianguli is a notable double star system, there are three star systems with known planets located in Triangulum; the constellation contains several galaxies, the brightest and nearest of, the Triangulum Galaxy or Messier 33—a member of the Local Group. The first quasar observed, 3C 48 lies within the boundaries of Triangulum. In the Babylonian star catalogues, together with Gamma Andromedae, formed the constellation known as MULAPIN "The Plough".
It is notable as the first constellation presented on a pair of tablets containing canonical star lists that were compiled around 1000 BC, the MUL. APIN; the Plough was the first constellation of the "Way of Enlil"—that is, the northernmost quarter of the Sun's path, which corresponds to the 45 days on either side of summer solstice. Its first appearance in the pre-dawn sky in February marked the time to begin spring ploughing in Mesopotamia; the Ancient Greeks called Triangulum Deltoton, as the constellation resembled an upper-case Greek letter delta. It was transliterated by Roman writers later Latinised as Deltotum. Eratosthenes linked it with the Nile Delta, while the Roman writer Hyginus associated it with the triangular island of Sicily known as Trinacria due to its shape, it was called Sicilia, because the Romans believed Ceres, patron goddess of Sicily, begged Jupiter to place the island in the heavens. Greek astronomers such as Hipparchos and Ptolemy called it Trigonon, it was Romanized as Trigonum.
Other names referring to its shape include Triquetrum. Alpha and Beta Trianguli were called Al Mīzān, Arabic for "The Scale Beam". In Chinese astronomy, Gamma Andromedae and neighbouring stars including Beta and Delta Trianguli were called Teen Ta Tseang Keun, representing honour in astrology and a great general in mythology; the 17th-century German celestial cartographer Johann Bayer called the constellation Triplicitas and Orbis terrarum tripertitus, for the three regions Europe and Africa. Triangulus Septentrionalis was a name used to distinguish it from Triangulum Australe, the Southern Triangle. Polish astronomer Johannes Hevelius excised three faint stars—6, 10 and 12 Trianguli—to form the new constellation of Triangulum Minus in his 1690 Firmamentum Sobiescianum, renaming the original as Triangulum Majus; the smaller constellation was not recognised by the International Astronomical Union when the constellations were established in the 1920s. A small constellation, Triangulum is bordered by Andromeda to the north and west, Pisces to the west and south, Aries to the south, Perseus to the east.
The centre of the constellation lies halfway between Alpha Arietis. The three-letter abbreviation for the constellation, as adopted by the IAU in 1922, is'Tri'; the official constellation boundaries, as set by Eugène Delporte in 1930, are defined as a polygon of 14 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 01h 31.3m and 02h 50.4m, while the declination coordinates are between 25.60° and 37.35°. Covering 132 square degrees and 0.320% of the night sky, Triangulum ranks 78th of the 88 constellations in size. Bayer catalogued five stars in the constellation, giving them the Bayer designations Alpha to Epsilon. John Flamsteed added Eta and four Roman letters. Flamsteed gave 16 stars Flamsteed designations, of which numbers 1 and 16 are not used—1's coordinates were in error as there was no star present at the location that corresponds to any star in his Catalogus Britannicus. Baily noted that 16 Trianguli was closer to Aries and included it in the latter constellation.
Three stars make up the long narrow triangle. The brightest member is the white giant star Beta Trianguli of apparent magnitude 3.00, lying 127 light-years distant from Earth. It is a spectroscopic binary system; the secondary is poorly known, but calculated to be a yellow-white F-type main-sequence star around 1.4 solar masses. The two orbit around a common centre of gravity every 31 days, are surrounded by a ring of dust that extends from 50 to 400 AU away from the stars; the second-brightest star, the yellow-white subgiant star Alpha Trianguli with a close dimmer companion, is known as Caput Trianguli or Ras al Muthallath, is at the apex of the triangle. It lies around 7 degrees north-northwest of Alpha Arietis. Making up the triangle is Gamma Trianguli, a white main sequence star of spectral type A1Vnn of apparent magnitude 4.00 about 112 light-years from Earth. It is around double the size around 33 times as luminous as the sun and rotates rapidly. Like Beta, it is surrounded by a dusty de
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