March equinox in the context of First Point of Aries

The Northern Hemisphere is the half of Earth that is north of the equator. For other planets in the Solar System, north is defined as being in the same celestial hemisphere relative to the invariable plane of the Solar System as Earth's North Pole.

Due to Earth's axial tilt of 23.439281°, there is a seasonal variation in the lengths of the day and night. There is also a seasonal variation in temperatures, which lags the variation in day and night. Conventionally, winter in the Northern Hemisphere is taken as the period from the December solstice (typically December 21 UTC) to the March equinox (typically March 20 UTC), while summer is taken as the period from the June solstice through to the September equinox (typically on 23 September UTC). The dates vary each year due to the difference between the calendar year and the astronomical year. Within the Northern Hemisphere, oceanic currents can change the weather patterns that affect many factors within the north coast. Such events include El Niño–Southern Oscillation.

The winter solstice, or hibernal solstice, occurs when either of Earth's poles reaches its maximum tilt away from the Sun. This happens twice yearly, once in each hemisphere (Northern and Southern). For that hemisphere, the winter solstice is the day with the shortest period of daylight and longest night of the year, and when the Sun is at its lowest daily maximum elevation in the sky. Each polar region experiences continuous darkness or twilight around its winter solstice. The opposite event is the summer solstice.

The winter solstice occurs during the hemisphere's winter. In the Northern Hemisphere, this is the December solstice (December 21 or 22) and in the Southern Hemisphere, this is the June solstice (June 20 or 21). Although the winter solstice itself lasts only a moment, the term also refers to the day on which it occurs. Traditionally, in many temperate regions, the winter solstice is seen as the middle of winter; "midwinter" is another name for the winter solstice, although it carries other meanings as well. Other names are the "extreme of winter", or the "shortest day".

The Annunciation (/əˌnʌnsiˈeɪʃən/; from Latin annuntiatio; also referred to as the Annunciation to the Blessed Virgin Mary, the Annunciation of Our Lady, or the Annunciation of the Lord; Ancient Greek: Ο Ευαγγελισμός της Θεοτόκου) is, according to the Gospel of Luke, the announcement made by the archangel Gabriel to Mary that she would conceive and bear a son through a virgin birth and become the mother of Jesus Christ, the Messiah and Son of God, marking the Incarnation.

According to Luke 1:26 the Annunciation occurred in the sixth month of Elizabeth's pregnancy with John the Baptist. Many Christians observe this event with the Feast of the Annunciation on 25 March, an approximation of the northern vernal equinox nine full months before Christmas, the traditional birthday of Jesus.

March is the third month of the year in both the Julian and Gregorian calendars. Its length is 31 days. In the Northern Hemisphere, the meteorological beginning of spring occurs on the first day of March. The March equinox on the 20 or 21 marks the astronomical beginning of spring in the Northern Hemisphere and the beginning of autumn in the Southern Hemisphere, where September is the seasonal equivalent of the Northern Hemisphere's March.

A tropical year or solar year (or tropical period) is the time that the Sun takes to return to the same position in the sky – as viewed from the Earth or another celestial body of the Solar System – thus completing a full cycle of astronomical seasons. For example, it is the time from vernal equinox to the next vernal equinox, or from summer solstice to the next summer solstice. It is the type of year used by tropical solar calendars.

The tropical year is one type of astronomical year and particular orbital period. Another type is the sidereal year (or sidereal orbital period), which is the time it takes Earth to complete one full orbit around the Sun as measured with respect to the fixed stars, resulting in a duration of 20 minutes longer than the tropical year, because of the precession of the equinoxes.

The zodiac is a belt-shaped region of the sky that extends approximately 8° north and south celestial latitude of the ecliptic – the apparent path of the Sun across the celestial sphere over the course of the year. Within this zodiac belt appear the Moon and the brightest planets, along their orbital planes. The zodiac is divided along the ecliptic into 12 equal parts, called "signs", each occupying 30° of celestial longitude. These signs roughly correspond to the astronomical constellations with the following modern names: Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.

The signs have been used to determine the time of the year by identifying each sign with the days of the year the Sun is in the respective sign. In Western astrology, and formerly astronomy, the time of each sign is associated with different attributes. The zodiacal system and its angular measurement in 360 sexagesimal degree (°) originated with Babylonian astronomy during the 1st millennium BC, probably during the Achaemenid Empire. It was communicated into Greek astronomy by the 2nd century BC, as well as into developing the Hindu zodiac. Due to the precession of the equinoxes, the time of year that the Sun is in a given constellation has changed since Babylonian times, and the point of March equinox has moved from Aries into Pisces.

Nowruz (Persian: نوروز, Iranian Persian: [noːˈɾuːz], lit. 'New Day') is the Iranian or Persian New Year. Historically, it has been observed by Iranian peoples, but is now celebrated by many ethnicities worldwide. It is a festival based on the Northern Hemisphere spring equinox, which marks the first day of a new year on the Iranian calendars and the currently used Hijri calendar; it usually coincides with a date between 19 March and 22 March on the Gregorian calendar.

The roots of Nowruz lie in Zoroastrianism, and it has been celebrated by many peoples across West Asia, Central Asia, the Caucasus and the Black Sea Basin, the Balkans, and South Asia for over 3,000 years. In the modern era, while it is observed as a secular holiday by most celebrants, Nowruz remains a holy day for Zoroastrians, Baháʼís, and Ismaʿili Shia Muslims.

The Iranian calendars or Iranian chronologies (Persian: گاه‌شماری ایرانی, Gâh Šomâriye Irâni) are a succession of calendars created and used for over two millennia in Iran, also known as Persia. One of the longest chronological records in human history, the Iranian calendar has been modified many times for administrative purposes. The most influential person in laying the frameworks for the calendar and its precision was the 11th century Persian polymath, Omar Khayyam. The modern Iranian calendar is the Solar Hijri calendar, currently the official civil calendar in Iran.

Nowruz, the Iranian New Year, begins at the midnight nearest to the instant of the northern spring equinox, as determined by astronomic calculations for the meridian of Tehran (52.5°E). Thus the calendar is observation-based, unlike the Gregorian calendar, which is rule-based. This equinox occurs on or about 20 March of the Gregorian calendar. The time zone of Iran is Iran Standard Time, UTC+03:30.

A solar equinox is a moment in time when the Sun appears directly above the equator, rather than to its north or south. On the day of the equinox, the Sun appears to rise directly east and set directly west. This occurs twice each year, around 20 March and 23 September.

An equinox is equivalently defined as the time when the plane of Earth's equator passes through the geometric center of the Sun's disk. This is also the moment when Earth's rotation axis is directly perpendicular to the Sun-Earth line, tilting neither toward nor away from the Sun. In modern times, since the Moon (and to a lesser extent the planets) causes Earth's orbit to vary slightly from a perfect ellipse, the equinox is officially defined by the Sun's more regular ecliptic longitude rather than by its declination. The instants of the equinoxes are currently defined to be when the apparent geocentric longitude of the Sun is 0° and 180°.

The equatorial coordinate system is a celestial coordinate system widely used to specify the positions of celestial objects. It may be implemented in spherical or rectangular coordinates, both defined by an origin at the centre of Earth, a fundamental plane consisting of the projection of Earth's equator onto the celestial sphere (forming the celestial equator), a primary direction towards the March equinox, and a right-handed convention.

The origin at the centre of Earth means the coordinates are geocentric, that is, as seen from the centre of Earth as if it were transparent. The fundamental plane and the primary direction mean that the coordinate system, while aligned with Earth's equator and pole, does not rotate with the Earth, but remains relatively fixed against the background stars. A right-handed convention means that coordinates increase northward from and eastward around the fundamental plane.

As a moveable feast, the date of Easter is determined in each year through a calculation known as computus paschalis (Latin for 'Easter computation') – often simply Computus – or as paschalion particularly in the Eastern Orthodox Church. Easter is celebrated on the first Sunday after the Paschal full moon (a mathematical approximation of the first astronomical full moon, on or after 21 March – itself a fixed approximation of the March equinox). Determining this date in advance requires a correlation between the lunar months and the solar year, while also accounting for the month, date, and weekday of the Julian or Gregorian calendar. The complexity of the algorithm arises because of the desire to associate the date of Easter with the date of the Jewish feast of Passover which, Christians believe, is when Jesus was crucified.

It was originally feasible for the entire Christian Church to receive the date of Easter each year through an annual announcement by the pope. By the early third century, however, communications in the Roman Empire had deteriorated to the point that the church put great value in a system that would allow the clergy to determine the date for themselves, independently yet consistently. Additionally, the church wished to eliminate dependencies on the Hebrew calendar, by deriving the date for Easter directly from the March equinox.

Aries (♈︎; Ancient Greek: Κριός, romanized: Kriós, Latin: Ariēs, lit. 'ram') is the first astrological sign in the zodiac, spanning the first 30 degrees of celestial longitude (0°≤ λ <30°), and originates from the Aries constellation. Under the tropical zodiac, the Sun transits this sign from approximately March 21 to April 20 each year. This time-duration is exactly the first month of the Solar Hijri calendar (Arabic: Hamal; Persian: Farvardin; Pashto: Wray).

According to the tropical system of astrology, the Sun enters the sign of Aries when it reaches the March equinox, typically on March 21. Because the Earth takes approximately 365.24 days to go around the Sun, the precise time of the equinox is not the same each year, and generally will occur about six hours later from one year to the next until reset by a leap year. The leap day February 29 causes that year's March equinox to fall about eighteen hours earlier compared with the previous year.

Pisces is a constellation of the zodiac. Its vast bulk — and main asterism viewed in most European cultures per Greco-Roman antiquity as a distant pair of fishes connected by one cord each that join at an apex — are in the Northern celestial hemisphere. Its traditional astrological symbol is (♓︎). Its name is Latin for "fishes". It is between Aquarius, of similar size, to the southwest and Aries, which is smaller, to the east. The ecliptic and the celestial equator intersect within this constellation and in Virgo. The Sun passes directly overhead of the equator, on average, at approximately this point in the sky, at the March equinox.

The right ascension/declination 00 is located within the boundaries of Pisces.

In astronomy, the ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions, orbits, and pole orientations of Solar System objects. Because most planets (except Mercury) and many small Solar System bodies have orbits with only slight inclinations to the ecliptic, using it as the fundamental plane is convenient. The system's origin can be the center of either the Sun or Earth, its primary direction is towards the March equinox, and it has a right-hand convention. It may be implemented in spherical or rectangular coordinates.