An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

Anomalistic from → anomaly.

Pirâhuri from pirâhur, → perihelion.

The period taken for the right ascension of the mean Sun to increase by 24 hours. The starting point is when the mean Sun's longitude is 280°, corresponding roughly to January 1. It is virtually the same as the tropical year.

→ Besselian; → year.

The time interval between the new year's day in a given calendar system and the day before the following new year's day. In the Gregorian system the calendar year begins on January 1 and ends on December 31. In the Iranian calendar it begins on Farvardin 1, the day closest to the spring equinox and ends on Esfand 29 or 30.

→ calendar; → year.

A calendar year which is not a → leap year. In the → Gregorian calendar, a year with 365 days divided into 12 → months, and only 28 days in February. In the → Iranian calendar, a year consisting of 365 days with 12 months, and only 29 days in the month of Esfand.

→ common; → year.

The interval of time (346.620 03 days) between two successive passages of the Sun through the same node of the Moon's orbit. It takes less than a solar year to complete an eclipse year because the Moon's orbit and the lunar nodes are slowly regressing.

→ eclipse; → year.

In ancient calendars, a year that contains an → embolismic month.

→ embolismic month; → year.

The time taken for the Sun to revolve once around the center of the Milky Way, amounting to about 220 million years.

→ galactic; → year.

A period of 365.25 days adopted in the Julian calendar for the length of the year.

→ Julian calendar; &rarr ;year.

In solar calendars the year that contains 366 days, instead of 365, in order to keep the calendar in pace with the real solar time.

→ leap; → year.

The three criteria that identify → leap years in the → Gregorian calendar: 1) The year must be evenly divisible by 4; 2) If the year can be evenly divided by 100, it is not a leap year, unless; 3) The year is also evenly divisible by 400. This means that in the Gregorian calendar, the years 1600, 2000, and 2400 are leap years, while 1700, 1800, 1900, 2100, 2200, 2300 and 2500 are not leap years.

→ leap; → year; → rule.

→ light-year

→ light; → year.

The distance that light travels in one year at about 300,000 km per second, i.e. 9.5 x 10¹² km. It is equal to about 63,000 → astronomical units. See also → parsec.

→ light; → year.

A year based solely on the Moon's motion, containing 12 synodic months, each of 29.5306 days, that is a year of 354.3672 days. Used by Hebrews, Babylonians, Greeks, and Arabs.

→ lunar; → year.

A → calendar year that contains 365 days and therefore is not a → leap year.

→ ordinary; → year.

The time required for a complete revolution of the Earth's pole on the celestial sphere as the result of → precession. A Platonic year is equal to 25 800 years.

Of or pertaining to Gk. philosopher Plato, from Gk. Platon "broad-shouldered," from platys "broad." → year.

The interval between two successive passages of the Sun, in its apparent → annual motion around the → celestial sphere, through a particular point relative to stars. It is equal to 365.256356 days for the J2000.0 epoch and is 20m 24.5s longer than the → tropical year.

→ sidereal; → year.

The period of time required for the Earth to make one complete revolution around the Sun. Solar year is a general term for: → tropical year, → vernal equinox year, and → autumnal equinox year, which have different lengths.

→ solar; → year.

The Egyptian year of 365 days and 6 hours, as distinguished from them Egyptian vague year, which contained 365 days.

From Fr. sothique, from Gk. Sothis, an Egyptian name of Sirius; → year.

Tištari, of or pertaining to Tištar→ serius; sâl, → year.

The interval during which the Sun's mean longitude, referred to the mean equinox of date, increases by 360 degrees. Its mean length for the epoch J2000.0 is 365.24217879 real solar days (approximately 365.2422 days). This concept of tropical year, adopted by the International Astronomical Union at its General Assembly in Dublin, September 1955, has often been confounded with the → vernal-equinox year. In fact the mean period between two successive true vernal equinoxes is different from the tropical year. This period, which is equal to 365.24236460 solar days (about 365.2424 days), is the real mean length of the year in the Iranian calendar. The difference between the two year lengths is due to the fact that the Earth's orbital velocity around the Sun is not uniform, since the orbit is an ellipse. At the perihelion of its orbit the Earth is closest to the Sun, and therefore moves faster than average, while at aphelion, when it is farthest away from the Sun, it moves slower. Therefore the interval between two successive vernal equinoxes is not the same as the period between two successive summer solstices. In fact the tropical year does not depend on a specific origin for the annual apparent motion of the Sun. For detailed discussion see: A concise review of the Iranian calendar.

→ tropical; → year.

A year of 365 days that overlooks the fraction of less than 0.25 days corresponding to the whole length of the → tropical year. The vague year was used in the → calendars of ancient Egypt, Iran, Mayas, and some other civilizations. Typically the vague year was divided into 12 months of 30 days each plus 5 → epagomenal days.

From annus vagus "wandering year;" → vague→ year.