Equinox

An equinox is commonly regarded as the instant of time when the plane (extended indefinitely in all directions) of Earth's equator passes through the center of the Sun.[3] This occurs twice each year: around 20 March and 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 the Latin aequinoctium, from aequus (equal) and nox (genitive noctis) (night). On the day of an equinox, daytime and nighttime are of approximately equal duration all over the planet. They are not exactly equal, however, due to the angular size of the Sun, atmospheric refraction, and the rapidly 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 Vernal or Spring Equinox; the September equinox is called the Autumnal or Fall Equinox. The dates are slightly variable, dependent as they are on the leap year cycle.[4]

Because the Moon (and to a lesser extent the planets) cause the motion of the Earth to vary from a perfect ellipse, the equinox is now 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 longitude of the Sun is 0° and 180°.[5]

Equinozio da Pizzo Vento,tramonto fondachelli fantina, sicilia
The Sun on the equinox as seen from the site of Pizzo Vento, Fondachelli-Fantina, Sicily
UT date and time of
equinoxes and solstices on Earth[1][2]
event equinox solstice equinox solstice
month March June September December
year
day time day time day time day time
2014 20 16:57 21 10:51 23 02:29 21 23:03
2015 20 22:45 21 16:38 23 08:21 22 04:48
2016 20 04:30 20 22:34 22 14:21 21 10:44
2017 20 10:28 21 04:24 22 20:02 21 16:28
2018 20 16:15 21 10:07 23 01:54 21 22:23
2019 20 21:58 21 15:54 23 07:50 22 04:19
2020 20 03:50 20 21:44 22 13:31 21 10:02
2021 20 09:37 21 03:32 22 19:21 21 15:59
2022 20 15:33 21 09:14 23 01:04 21 21:48
2023 20 21:24 21 14:58 23 06:50 22 03:27
2024 20 03:07 20 20:51 22 12:44 21 09:20
Sun at moment of spring equinox 2019
The Sun at the moment of the spring equinox in 2019, when the Sun crossed the celestial equator – the imaginary line in the sky above the Earth's equator – from south to north.

Equinoxes on Earth

General

Systematically observing the sunrise, people discovered that it occurs between two extreme locations at the horizon and eventually noted the midpoint between the two. Later it was realized that this happens on a day when the durations of the day and the night are practically equal and the word "equinox" comes from Latin Aequus, meaning "equal", and Nox, meaning "night".

In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of spring in most cultures and is considered the start of the New Year in the Assyrian calendar, Hindu and the Persian calendar or Iranian calendars as Nowruz (means new day), while the autumnal equinox (September) marks the beginning of autumn.[6]

Earth-lighting-equinox EN

Illumination of Earth by the Sun at the equinox

Ecliptic path

The relation between the Earth, Sun, and stars at the March equinox. From Earth's perspective, the Sun appears to move along the ecliptic (red), which is tilted compared to the celestial equator (white).

North season

Diagram of the Earth's seasons as seen from the north. Far right: December solstice.

South season

Diagram of the Earth's seasons as seen from the south. Far left: June solstice.

The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the equator. As a result, the northern and southern hemispheres are equally illuminated.

In other words, the equinoxes are the only times when the subsolar point is on the equator, meaning that the Sun is exactly 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.

Date

When Julius Caesar established the Julian calendar in 45 BC, he set 25 March as the date of the spring equinox; this was already the starting day of the year in the Persian and Indian calendars. Because the Julian year is longer than the tropical year by about 11.3 minutes on average (or 1 day in 128 years), the calendar "drifted" with respect to the two equinoxes – so that in AD 300 the spring equinox occurred on about 21 March, and 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 (21 March is the day allocated to it in the Easter table of the Julian calendar), and 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, virtually all because the distribution of 24-hour centurial leap days causes large jumps (see Gregorian calendar leap solstice). This in turn raised the possibility that it could fall on 22 March, and 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 (within Europe).

Modern dates

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 approximately 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 (Universal Time).[4]

Names

  • Vernal equinox and autumnal equinox: these classical names are direct derivatives of Latin (ver = spring and autumnus = autumn). These are the historically universal and still most widely used terms for the equinoxes, but are potentially 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 (or fall) equinox are even more ambiguous.[7][8][9] It has become increasingly common for people to refer to the September equinox in the southern hemisphere as the Vernal equinox.[10][11]
  • 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 (as they do not in the Islamic calendar and Hebrew calendar, for example).[12] Although the terms have become very common in the 21st century, they were sometimes used at least as long ago as the mid-20th century.[13]
  • Northward equinox and southward equinox: names referring to the apparent direction of motion of the Sun. The northward equinox occurs in March when the Sun crosses the equator from south to north, and the southward equinox occurs in September when the Sun crosses the equator from north to south. These terms can be used unambiguously for other planets. They are rarely seen, although were first proposed over 100 years ago.[14]
  • First Point of Aries and first point of Libra: names referring to the astrological signs the Sun is entering. Due to the precession of the equinoxes, however, the constellations where the equinoxes are currently located are Pisces and Virgo, respectively.[15]

Length of equinoctial day and night

Hours of daylight vs latitude vs day of year cmglee
Contour plot of the hours of daylight as a function of latitude and day of the year, showing approximately 12 hours of daylight at all latitudes during the equinoxes
GOES East Captures View of Vernal Equinox (47418902822)
Earth at the 2019 vernal equinox

Day is usually defined as the period when sunlight reaches the ground in the absence of local obstacles. On the date of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, so night and day are about the same length. Sunrise and sunset can be defined in several ways, but a widespread definition is the time that the top limb of the Sun is level with the horizon.[16] With this definition, the day is longer than the night at the equinoxes:[3]

  1. From the Earth, the Sun appears as a disc rather than a point of light, so when the centre of the Sun is below the horizon, its upper edge may be visible. Sunrise, which begins daytime, occurs when the top of the Sun's disk appears above the eastern horizon. At that instant, the disk's centre is still below the horizon.
  2. The Earth's atmosphere refracts sunlight. As a result, an observer sees daylight before the top of the Sun's disk appears above the horizon.

In sunrise/sunset tables, the assumed semidiameter (apparent radius) of the Sun is 16 arcminutes and the atmospheric refraction is assumed to be 34 arcminutes. Their combination means that when the upper limb of the Sun is on the visible horizon, its centre is 50 arcminutes below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer.[17]

These effects make the day about 14 minutes longer than the night at the equator and longer still towards the poles. The real equality of day and night only happens in places far enough from the equator to have a seasonal difference in day length of at least 7 minutes,[18] actually occurring a few days towards the winter side of each equinox.

The times of sunset and sunrise vary with the observer's location (longitude and latitude), so the dates when day and night are equal also depend upon the observer's location.

A third correction for the visual observation of a sunrise (or sunset) is the angle between the apparent horizon as seen by an observer and the geometric (or sensible) horizon. This is known as the dip of the horizon and varies from 3 arcminutes for a viewer standing on the sea shore to 160 arcminutes for a mountaineer on Everest.[19] The effect of a larger dip on taller objects (reaching over 2½° of arc on Everest) accounts for the phenomenon of snow on a mountain peak turning gold in the sunlight long before the lower slopes are illuminated.

The date on which the day and night are exactly the same is known as an equilux; the neologism, believed to have been coined in the 1980s, achieved more widespread recognition in the 21st century. (Prior to this, the word "equilux" was more commonly used as a synonym for isophot, and there was no generally accepted term for the phenomenon.)[20][note 1] At the most precise measurements, there is no such thing as an equilux, because the lengths of day and night change more rapidly than any other time of the year around the equinoxes. In the mid-latitudes, daylight increases or decreases by about three minutes per day at the equinoxes, and thus adjacent days and nights only reach within one minute of each other. The date of the closest approximation of the equilux varies slightly by latitude; in the mid-latitudes, it occurs a few days before the spring equinox and after the fall equinox in each respective hemisphere.

Geocentric view of the astronomical seasons

In the half-year centered on the June solstice, the Sun rises north of east and sets north of west, which means longer days with shorter nights for the northern hemisphere and shorter days with longer nights for the southern hemisphere. In the half-year centered on the December solstice, the Sun rises south of east and sets south of west and the durations of day and night are reversed.

Also on the day of an equinox, the Sun rises everywhere on Earth (except at the poles) at about 06:00 and sets at about 18:00 (local solar time). These times are not exact for several reasons:

  • Most places on Earth use a time zone which differs from the local solar time by minutes or even hours. For example, if a location uses a time zone with reference meridian 15° to the east, the Sun will rise around 07:00 on the equinox and set 12 hours later around 19:00.
  • Day length is also affected by the variable orbital speed of the Earth around the Sun. This combined effect is described as the equation of time. Thus even locations which lie on their time zone's reference meridian will not see sunrise and sunset at 6:00 and 18:00. At the March equinox they are 7–8 minutes later, and at the September equinox they are about 7–8 minutes earlier.
  • Sunrise and sunset are commonly defined for the upper limb of the solar disk, rather than its center. The upper limb is already up for at least a minute before the center appears, and the upper limb likewise sets later than the center of the solar disk. Also, when the Sun is near the horizon, atmospheric refraction shifts its apparent position above its true position by a little more than its own diameter. This makes sunrise more than two minutes earlier and sunset an equal amount later. These two effects combine to make the equinox day 12 h 7 min long and the night only 11 h 53 min. Note, however, that these numbers are only true for the tropics. For moderate latitudes, the discrepancy increases (e.g., 12 minutes in London); and closer to the poles it becomes very much larger (in terms of time). Up to about 100 km from either pole, the Sun is up for a full 24 hours on an equinox day.
  • Height of the horizon changes the day's length. For an observer atop a mountain the day is longer, while standing in a valley will shorten the day.
  • The Sun is larger in diameter than the Earth, so more than half of the Earth is in sunlight at any one time (due to unparallel rays creating tangent points beyond an equal-day-night line).

Day arcs of the Sun

Some of the statements above can be made clearer by picturing the day arc (i.e., the path along which the Sun appears to move across the sky). The pictures show this for every hour on equinox day. In addition, some 'ghost' suns are also indicated below the horizon, up to 18° below it; the Sun in such areas still causes twilight. The depictions presented below can be used for both the northern and the southern hemispheres. The observer is understood to be sitting near the tree on the island depicted in the middle of the ocean; the green arrows give cardinal directions.

  • In the northern hemisphere, north is to the left, the Sun rises in the east (far arrow), culminates in the south (right arrow), while moving to the right and setting in the west (near arrow).
  • In the southern hemisphere, south is to the left, the Sun rises in the east (near arrow), culminates in the north (right arrow), while moving to the left and setting in the west (far arrow).

The following special cases are depicted:

Equinox-0

Day arc at 0° latitude (equator)
The arc passes through the zenith, resulting in almost no shadows at high noon.

Equinox-20

Day arc at 20° latitude
The Sun culminates at 70° altitude and its path at sunrise and sunset occurs at a steep 70° angle to the horizon. Twilight still lasts about one hour.

Equinox-50

Day arc at 50° latitude
Twilight lasts almost two hours.

Equinox-70

Day arc at 70° latitude
The Sun culminates at no more than 20° altitude and its daily path at sunrise and sunset is at a shallow 20° angle to the horizon. Twilight lasts for more than four hours.

Equinox-90

Day arc at 90° latitude (pole)
If it were not for atmospheric refraction, the Sun would be on the horizon all the time.

Celestial coordinate systems

The March equinox occurs about when the Sun appears to cross the celestial equator northward. In the Northern Hemisphere, the term vernal point is used for the time of this occurrence and for the precise direction in space where the Sun exists at that time. This point is the origin of some celestial coordinate systems, which are usually rooted to an astronomical epoch since it gradually varies (precesses) over time:

Equinox diagram
Diagram illustrating the difference between the Sun's celestial longitude being zero and the Sun's declination being zero. The Sun's celestial latitude never exceeds 1.2 arcseconds, but is exaggerated in this diagram.

Strictly speaking, at the equinox, the Sun's ecliptic longitude is zero. Its latitude will not be exactly zero, since Earth is not exactly in the plane of the ecliptic. Its declination will not be exactly zero either. The mean ecliptic is defined by the barycenter of Earth and the Moon combined, so the Earth wanders slightly above and below the ecliptic due to the orbital tilt of the Moon.[22] The modern definition of equinox is the instants when the Sun's apparent geocentric longitude is 0° (northward equinox) or 180° (southward equinox).[23][24][25] See the adjacent diagram.

Because of the precession of the Earth's axis, the position of the vernal point on the celestial sphere changes over time, and the equatorial and the ecliptic coordinate systems change accordingly. Thus when specifying celestial coordinates for an object, one has to specify at what time the vernal point and the celestial equator are taken. That reference time is called the equinox of date.[26]

The upper culmination of the vernal point is considered the start of the sidereal day for the observer. The hour angle of the vernal point is, by definition, the observer's sidereal time.

Using the current official IAU constellation boundaries – and taking into account the variable precession speed and the rotation of the celestial equator – the equinoxes shift through the constellations as follows[27] (expressed in astronomical year numbering when the year 0 = 1 BC, −1 = 2 BC, etc.):

  • The March equinox passed from Taurus into Aries in year −1865, passed into Pisces in year −67, will pass into Aquarius in year 2597, and then into Capricornus in year 4312. In 1489 it came within 10 arcminutes of Cetus without crossing the boundary.
  • The September equinox passed from Libra into Virgo in year −729, will pass into Leo in year 2439.

Cultural aspects

The equinoxes are sometimes regarded as the start of spring and autumn. A number of traditional harvest festivals are celebrated on the date of the equinoxes.

Effects on satellites

One effect of equinoctial periods is the temporary disruption of communications satellites. For all geostationary satellites, there are a few days around the equinox when the Sun goes directly behind the satellite relative to Earth (i.e. within the beam-width of the ground-station antenna) for a short period each day. The Sun's immense power and broad radiation spectrum overload the Earth station's reception circuits with noise and, depending on antenna size and other factors, temporarily disrupt or degrade the circuit. The duration of those effects varies but can range from a few minutes to an hour. (For a given frequency band, a larger antenna has a narrower beam-width and hence experiences shorter duration "Sun outage" windows.)[28]

Satellites in geostationary orbit also experience difficulties maintaining power during the equinox, due to the fact that they now have to travel through Earth's shadow and rely only on battery power. Usually, a satellite will travel either above or below the Earth's shadow due to its shifted axis throughout the year; during the equinox, since geostationary satellites are situated above the equator, they will be put into the shadow of the Earth for the longest period of time all year.[29]

Equinoxes on other planets

Saturn, its rings, and a few of its moons
When the planet Saturn is at equinox, its rings reflect little sunlight, as seen in this image by Cassini in 2009.

Equinoxes occur on any planet with a tilted rotational axis. A dramatic example is Saturn, where the equinox places its ring system edge-on facing the Sun. As a result, they are visible only as a thin line when seen from Earth. When seen from above – a view seen during an equinox for the first time from the Cassini space probe in 2009 – they receive very little sunshine, indeed more planetshine than light from the Sun.[30] This phenomenon occurs once every 14.7 years on average, and can last a few weeks before and after the exact equinox. Saturn's most recent equinox was on 11 August 2009, and its next will take place on 6 May 2025.[31]

Mars's most recent equinox was on 22 May 2018 (northern autumn), and the next will be on 23 March 2019 (northern spring).[32]

See also

Notes

  1. ^ This meaning of "equilux" is rather modern (c. 1985 to 1986) and unusual. Technical references since the beginning of the 20th century (c. 1910) use the terms "equilux" and "isophot" to mean "of equal illumination" in the context of curves showing how intensely lighting equipment will illuminate a surface. See for instance John William Tudor Walsh, Textbook of Illuminating Engineering (Intermediate Grade), I. Pitman, 1947. The earliest confirmed use of the modern meaning was in a post on the Usenet group net.astro dated 14 March 1986 net.astro – Spring Equilux Approaches, which refers to "discussion last year exploring the reasons why equilux and equinox are not coincident". Use of this particular pseudo-latin protologism can only be traced to an extremely small (less than six) number of predominently US American people in such online media for the next 20 years until its broader adoption as a neologism (c. 2006), and then its subsequent use by more mainstream organisations (c. 2012) The Equinox and Solstice, UK Meteorological Office.

References

  1. ^ United States Naval Observatory (4 January 2018). "Earth's Seasons and Apsides: Equinoxes, Solstices, Perihelion, and Aphelion". Retrieved 18 September 2018.
  2. ^ Astro Pixels (20 February 2018). "Solstices and Equinoxes: 2001 to 2100". Retrieved 21 December 2018.
  3. ^ a b "Equinoxes". Astronomical Information Center. United States Naval Observatory. Retrieved 26 March 2019.
  4. ^ a b Yallow, B. D.; Hohenkerk, C. Y.; Bell, S. A. (2013). "Astronomical Phenomena". In Urban, S.E.; Seidelmann, P. K. (eds.). Explanatory supplement to the astronomical almanac (3rd ed.). Mill Valley, CA: University Science Books. pp. 506–507. ISBN 978-1-891389-85-6.
  5. ^ "Glossary". Astronomical Almanac 2008. United States Naval Observatory. 2008.
  6. ^ "March Equinox – Equal Day and Night, Nearly". Time and Date. 2017. Retrieved 22 May 2017.
  7. ^ Michelle Skye (2007). Goddess Alive!: Inviting Celtic & Norse Goddesses Into Your Life. Llewellyn Worldwide. pp. 69–. ISBN 978-0-7387-1080-8.
  8. ^ Howard D Curtis (2013). Orbital Mechanics for Engineering Students. Butterworth-Heinemann. pp. 188–. ISBN 978-0-08-097748-5.
  9. ^ Mohinder S. Grewal; Lawrence R. Weill; Angus P. Andrews (2007). Global Positioning Systems, Inertial Navigation, and Integration. John Wiley & Sons. pp. 459–. ISBN 978-0-470-09971-1.
  10. ^ Nathaniel Bowditch; National Imagery and Mapping Agency (2002). The American practical navigator : an epitome of navigation. Paradise Cay Publications. pp. 229–. ISBN 978-0-939837-54-0.
  11. ^ Exploring the Earth. Allied Publishers. pp. 31–. ISBN 978-81-8424-408-3.
  12. ^ Paula LaRocque (2007). On Words: Insights Into How Our Words Work – And Don't. Marion Street Press. pp. 89–. ISBN 978-1-933338-20-0.
  13. ^ Popular Astronomy. 1945.
  14. ^ Notes and Queries. Oxford University Press. 1895.
  15. ^ Spherical Astronomy. Krishna Prakashan Media. pp. 233–. GGKEY:RDRHQ35FBX7.
  16. ^ Forsythe, William C; Rykiel, Edward J; Stahl, Randal S; Wu, Hsin-i; Schoolfield, Robert M (1995). "A model comparison for daylength as a function of latitude and day of year" (PDF). Ecological Modelling. 80: 87–95. doi:10.1016/0304-3800(94)00034-F.
  17. ^ Seidelman, P. Kenneth, ed. (1992). Explanatory Supplement to the Astronomical Almanac. Mill Valley, CA: University Science Books. p. 32. ISBN 0-935702-68-7.
  18. ^ "Sunrise and Sunset". 21 October 2002. Retrieved 22 September 2017.
  19. ^ Biegert, Mark (21 October 2015). "Correcting Sextant Measurements For Dip". Math Encounters (blog). Retrieved 22 September 2017.
  20. ^ Owens, Steve (20 March 2010). "Equinox, Equilux, and Twilight Times". Dark Sky Diary (blog). Retrieved 31 December 2010.
  21. ^ Hilton, James L.; McCarthy, Dennis D. (2013). "Precession, Nutation, Polar Motion, and Earth Rotation". In Urban, S.E.; Seidelmann, P. K. (eds.). Explanatory supplement to the astronomical almanac (3rd ed.). Mill Valley, CA: University Science Books. pp. 205–206. ISBN 978-1-891389-85-6.
  22. ^ "The IAU Working Group on Precession and the Ecliptic...have recommended that the ecliptic be more precisely defined as the plane perpendicular to the mean orbital angular momentum vector of the Earth-Moon barycenter passing through the Sun in the BCRS." [Internal citations omitted].[21]
  23. ^ United States Naval Observatory (2006). Astronomical Almanac 2008. Glossary Chapter.
  24. ^ Meeus, Jean (1997). Mathematical Astronomy Morsels.
  25. ^ Meeus, Jean (1998). Astronomical Algorithms, Second Edition.
  26. ^ Montenbruck, Oliver; Pfleger, Thomas. Astronomy on the Personal Computer. Springer-Verlag. p. 17. ISBN 0-387-57700-9.
  27. ^ J. Meeus; Mathematical Astronomical Morsels; ISBN 0-943396-51-4.
  28. ^ "Satellite Sun Interference". Intelsat. Retrieved 20 March 2019.
  29. ^ Miller, Alex (17 April 2018). "How satellites are affected by the spring and autumn equinoxes". Inside Viasat blog. Retrieved 20 March 2019.
  30. ^ "PIA11667: The Rite of Spring". Jet Propulsion Laboratory, California Institute of Technology. Retrieved 21 March 2014.
  31. ^ Lakdawalla, Emily (7 July 2016). "Oppositions, conjunctions, seasons, and ring plane crossings of the giant planets". The Planetary Society. Retrieved 31 January 2017.
  32. ^ Mars Calendar. The Planetary Society.

External links

Aries (astrology)

Aries (♈) (meaning "ram") is the first astrological sign in the zodiac, spanning the first 30 degrees of celestial longitude (0°≤ λ <30°). Under the tropical zodiac, the Sun transits this sign from approximately March 20 to April 21 each year. This time duration is exactly the first month of the Solar Hijri calendar (Hamal/Farvardin/Wray). The symbol of the ram is based on the Chrysomallus, the flying ram that provided the Golden Fleece.According to the tropical system of astrology, the Sun enters the sign of Aries when it reaches the March equinox, which occurs on average on March 21 (by design). 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. February 29 of a leap year causes that year's vernal equinox to fall about eighteen hours earlier compared with the previous year. From 1800 to 2050 inclusive the vernal equinox date has (or will) range(d) from March 19 at 22:34 UT1 in 2048 to March 21 at 19:15 UT1 in 1903.Under the sidereal zodiac, the sun currently transits Aries from April 15 to 14 May (approximately).

Aries is the first fire sign in the zodiac, the other fire signs being Leo and Sagittarius. Individuals born between these dates, depending on which system of astrology they subscribe to, may be called Arians or Ariens.The equivalent in the Hindu solar calendar is Meṣa.

Bahá'í calendar

The Bahá'í Calendar, also called the Badíʿ Calendar (Badíʿ means wondrous or unique), is a solar calendar with years composed of 19 months of 19 days each (361 days) plus an extra period of "Intercalary Days". Years begin at Naw-Rúz, on the day of the vernal equinox in Tehran, Iran, coinciding with March 20 or 21.

The first year is dated from 21 March 1844 CE, the year during which the Báb proclaimed his religion. Years are annotated with the date notation of BE (Bahá'í Era),

The year 176 BE started on the day of the vernal equinox (in Tehran) in 2019, that is on 21 March 2019.

Chevrolet Equinox

The Chevrolet Equinox is a series of mid-size, later compact crossover SUV from Chevrolet, introduced in 2004 for the 2005 model year.

Earth Day

Earth Day is an annual event celebrated on April 22. Worldwide, various events are held to demonstrate support for environmental protection. First celebrated in 1970, Earth Day now includes events in more than 193 countries, which are coordinated globally by the Earth Day Network.On Earth Day 2016, the landmark Paris Agreement was signed by the United States, China, and some 120 other countries. This signing satisfied a key requirement for the entry into force of the historic draft climate protection treaty adopted by consensus of the 195 nations present at the 2015 United Nations Climate Change Conference in Paris.

In 1969 at a UNESCO Conference in San Francisco, peace activist John McConnell proposed a day to honor the Earth and the concept of peace, to first be celebrated on March 21, 1970, the first day of spring in the northern hemisphere. This day of nature's equipoise was later sanctioned in a proclamation written by McConnell and signed by Secretary General U Thant at the United Nations. A month later a separate Earth Day was founded by United States Senator Gaylord Nelson as an environmental teach-in first held on April 22, 1970. Nelson was later awarded the Presidential Medal of Freedom award in recognition of his work. While this April 22 Earth Day was focused on the United States, an organization launched by Denis Hayes, who was the original national coordinator in 1970, took it international in 1990 and organized events in 141 nations.Numerous communities celebrate Earth Week, an entire week of activities focused on the environmental issues that the world faces. In 2017, the March for Science occurred on Earth Day (April 22, 2017) and was followed by the People's Climate Mobilization (April 29, 2017).

Ecliptic

The ecliptic is the mean plane of the apparent path in the Earth's sky that the Sun follows over the course of one year; it is the basis of the ecliptic coordinate system. This plane of reference is coplanar with Earth's orbit around the Sun (and hence the Sun's apparent path around Earth). The ecliptic is not normally noticeable from Earth's surface because the planet's rotation carries the observer through the daily cycles of sunrise and sunset, which obscure the Sun's apparent motion against the background of stars during the year.

Ecliptic coordinate system

The ecliptic coordinate system is a celestial coordinate system commonly used for representing the apparent positions and orbits of Solar System objects. Because most planets (except Mercury) and many small Solar System bodies have orbits with 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 vernal (northward) equinox, and it has a right-hand convention. It may be implemented in spherical or rectangular coordinates.

Epoch (astronomy)

In astronomy, an epoch is a moment in time used as a reference point for some time-varying astronomical quantity, such as the celestial coordinates or elliptical orbital elements of a celestial body, because these are subject to perturbations and vary with time. These time-varying astronomical quantities might include, for example, the mean longitude or mean anomaly of a body, the node of its orbit relative to a reference plane, the direction of the apogee or aphelion of its orbit, or the size of the major axis of its orbit.

The main use of astronomical quantities specified in this way is to calculate other relevant parameters of motion, in order to predict future positions and velocities. The applied tools of the disciplines of celestial mechanics or its subfield orbital mechanics (for predicting orbital paths and positions for bodies in motion under the gravitational effects of other bodies) can be used to generate an ephemeris, a table of values giving the positions and velocities of astronomical objects in the sky at a given time or times.

Astronomical quantities can be specified in any of several ways, for example, as a polynomial function of the time-interval, with an epoch as a temporal point of origin (this is a common current way of using an epoch). Alternatively, the time-varying astronomical quantity can be expressed as a constant, equal to the measure that it had at the epoch, leaving its variation over time to be specified in some other way—for example, by a table, as was common during the 17th and 18th centuries.

The word epoch was often used in a different way in older astronomical literature, e.g. during the 18th century, in connection with astronomical tables. At that time, it was customary to denote as "epochs", not the standard date and time of origin for time-varying astronomical quantities, but rather the values at that date and time of those time-varying quantities themselves. In accordance with that alternative historical usage, an expression such as 'correcting the epochs' would refer to the adjustment, usually by a small amount, of the values of the tabulated astronomical quantities applicable to a fixed standard date and time of reference (and not, as might be expected from current usage, to a change from one date and time of reference to a different date and time).

Equinox (celestial coordinates)

In astronomy, equinox is either of two places on the celestial sphere at which the ecliptic intersects the celestial equator. Although there are two intersections of the ecliptic with the celestial equator, by convention the equinox associated with the sun's ascending node is used as the origin of celestial coordinate systems and referred to simply as the equinox. In contrast to the common usage of spring and fall, or vernal and autumnal, equinoxes, the celestial coordinate system equinox is a direction in space rather than a moment in time.

The equinox moves because of perturbing forces, therefore in order to define a coordinate system it is necessary to specify the date for which the equinox is chosen. This date should not be confused with the epoch. Astronomical objects show real movements such as orbital and proper motions, and the epoch defines the date for which the position of an object applies. Therefore, a complete specification of the coordinates for an astronomical objects requires both the date of the equinox and of the epoch.The currently used standard equinox and epoch is J2000.0, which is January 1, 2000 at 12:00 TT. The prefix "J" indicates that it is a Julian epoch. The previous standard equinox and epoch was B1950.0, with the prefix "B" indicating it was a Besselian epoch. Before 1984 Besselian equinoxes and epochs were used. Since that time Julian equinoxes and epochs have been used.

Libra (astrology)

Libra (♎) is the seventh astrological sign in the Zodiac.

It spans 180°–210° celestial longitude. Under the tropical zodiac, the Sun transits this area on average between (northern autumnal equinox) September 23 and October 23, and under the sidereal zodiac, the sun currently transits the constellation of Libra from approximately October 31 to November 22. The symbol of the scales is based on the Scales of Justice held by Themis, the Greek personification of divine law and custom. She became the inspiration for modern depictions of Lady Justice. The ruling planet of Libra is Venus. Libra and Aquarius are the only zodiac constellations in the sky represented by inanimate objects. The other eleven signs are represented either as an animal or mythological characters throughout history.Libra is one of the three zodiac air signs, the others being Gemini and Aquarius. The sign of Libra is symbolized by the scales. The moon was said to be in Libra when Rome was founded. Everything was balanced under this righteous sign. The Roman writer Manilius once said that Libra was the sign "in which the seasons are balanced". Both the hours of the day and the hours of the night match each other. Thus why the Romans put so much trust in the "balanced sign".

Going back to ancient Greek times, Libra the constellation between Virgo and Scorpio used to be ruled over by the constellation of Scorpio. They called the area the Latin word "chelae", which translated to "the claws" which can help identify the individual stars that make up the full constellation of Libra, since it was so closely identified with the Scorpion constellation in the sky.According to the tropical system of astrology, the Sun enters the sign of Libra when it reaches the southern vernal equinox, which occurs around September 22.

List of Star Trek Starfleet starships

This is a list of Federation starships from the fictional Star Trek universe. The list is organized first by ship class, then registration number, name, and finally where that vessel was referenced. These vessels appear or are mentioned in the original Star Trek series (TOS), Star Trek: The Next Generation (TNG), Star Trek: Deep Space Nine (DS9), Star Trek: Voyager (VOY), Star Trek: Enterprise (ENT), Star Trek: Discovery (DIS), the Star Trek films, Star Trek games, and Star Trek literature. This list tries to avoid using information found in Star Trek fan fiction. Many of the sources for this list are considered non-canon and the list relies heavily on the non-canon The Star Trek Encyclopedia.

March equinox

The March equinox or Northward equinox is the equinox on the Earth when the subsolar point appears to leave the Southern Hemisphere and cross the celestial equator, heading northward as seen from Earth. The March equinox is known as the vernal equinox in the Northern Hemisphere and as the autumnal equinox in the Southern.On the Gregorian calendar, the Northward equinox can occur as early as 19 March or as late as 21 March at Greenwich. For a common year the computed time slippage is about 5 hours 49 minutes later than the previous year, and for a leap year about 18 hours 11 minutes earlier than the previous year. Balancing the increases of the common years against the losses of the leap years keeps the calendar date of the March equinox from drifting more than one day from 20 March each year.

The March equinox may be taken to mark the beginning of spring and the end of winter in the Northern Hemisphere but marks the beginning of autumn and the end of summer in the Southern Hemisphere.In astronomy, the March equinox is the zero point of sidereal time and, consequently, right ascension. It also serves as a reference for calendars and celebrations in many human cultures and religions.

Northern Hemisphere

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.Owing to the Earth's axial tilt, winter in the Northern Hemisphere lasts from the December solstice (typically December 21 UTC) to the March equinox (typically March 20 UTC), while summer lasts from the June solstice through to the September equinox (typically September 23 UTC). The dates vary each year due to the difference between the calendar year and the astronomical year.

Its surface is 60.7% water, compared with 80.9% water in the case of the Southern Hemisphere, and it contains 67.3% of Earth's land.

Precession

Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In other words, if the axis of rotation of a body is itself rotating about a second axis, that body is said to be precessing about the second axis. A motion in which the second Euler angle changes is called nutation. In physics, there are two types of precession: torque-free and torque-induced.

In astronomy, precession refers to any of several slow changes in an astronomical body's rotational or orbital parameters. An important example is the steady change in the orientation of the axis of rotation of the Earth, known as the precession of the equinoxes.

Right ascension

Right ascension (abbreviated RA; symbol α) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the (hour circle of the) point above the earth in question.

When paired with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.

An old term, right ascension (Latin: ascensio recta) refers to the ascension, or the point on the celestial equator that rises with any celestial object as seen from Earth's equator, where the celestial equator intersects the horizon at a right angle. It contrasts with oblique ascension, the point on the celestial equator that rises with any celestial object as seen from most latitudes on Earth, where the celestial equator intersects the horizon at an oblique angle.

Solstice

A solstice is an event occurring when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial sphere. Two solstices occur annually, around June 21 and December 21. The seasons of the year are determined by reference to both the solstices and the equinoxes.

The term solstice can also be used in a broader sense, as the day when this occurs. The day of a solstice in either hemisphere has either the most sunlight of the year (summer solstice) or the least sunlight of the year (winter solstice) for any place other than the Equator. Alternative terms, with no ambiguity as to which hemisphere is the context, are "June solstice" and "December solstice", referring to the months in which they take place every year. The word solstice is derived from the Latin sol ("sun") and sistere ("to stand still"), because at the solstices, the Sun's declination appears to "stand still"; that is, the seasonal movement of the Sun's daily path (as seen from Earth) stops at a northern or southern limit before reversing direction.

Southern Hemisphere

The Southern Hemisphere is the half of Earth that is south of the Equator. It contains all or parts of five continents (Antarctica, Australia, about 90% of South America, a third of Africa, and several islands off the continental mainland of Asia), four oceans (Indian, South Atlantic, Southern, and South Pacific) and most of the Pacific Islands in Oceania. Its surface is 80.9% water, compared with 60.7% water in the case of the Northern Hemisphere, and it contains 32.7% of Earth's land.Owing to the tilt of Earth's rotation relative to the Sun and the ecliptic plane, summer is from December to March and winter is from June to September. September 22 or 23 is the vernal equinox and March 20 or 21 is the autumnal equinox. The South Pole is in the center of the southern hemispherical region.

Spring (season)

Spring is one of the four temperate seasons, following winter and preceding summer. There are various technical definitions of spring, but local usage of the term varies according to local climate, cultures and customs. When it is spring in the Northern Hemisphere, it is autumn in the Southern Hemisphere and vice versa. At the spring (or vernal) equinox, days and nights are approximately twelve hours long, with day length increasing and night length decreasing as the season progresses.

Spring and "springtime" refer to the season, and also to ideas of rebirth, rejuvenation, renewal, resurrection and regrowth. Subtropical and tropical areas have climates better described in terms of other seasons, e.g. dry or wet, monsoonal or cyclonic. Cultures may have local names for seasons which have little equivalence to the terms originating in Europe.

Tropical year

A tropical year (also known as a solar year) is the time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice. Because of the precession of the equinoxes, the seasonal cycle does not remain exactly synchronized with the position of the Earth in its orbit around the Sun. As a consequence, the tropical year is about 20 minutes shorter than the time it takes Earth to complete one full orbit around the Sun as measured with respect to the fixed stars (the sidereal year).

Since antiquity, astronomers have progressively refined the definition of the tropical year. The entry for "year, tropical" in the Astronomical Almanac Online Glossary (2015) states:

the period of time for the ecliptic longitude of the Sun to increase 360 degrees. Since the Sun's ecliptic longitude is measured with respect to the equinox, the tropical year comprises a complete cycle of seasons, and its length is approximated in the long term by the civil (Gregorian) calendar. The mean tropical year is approximately 365 days, 5 hours, 48 minutes, 45 seconds.

An equivalent, more descriptive, definition is "The natural basis for computing passing tropical years is the mean longitude of the Sun reckoned from the precessionally moving equinox (the dynamical equinox or equinox of date). Whenever the longitude reaches a multiple of 360 degrees the mean Sun crosses the vernal equinox and a new tropical year begins" (Borkowski 1991, p. 122).

The mean tropical year in 2000 was 365.24219 ephemeris days; each ephemeris day lasting 86,400 SI seconds. This is 365.24217 mean solar days (Richards 2013, p. 587).

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