Date of EasterAs a moveable feast, the date of Easter is determined in each year through a calculation known as computus. Easter is celebrated on the first Sunday after the Paschal full moon, which is the first full moon on or after 21 March (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.
AnalemmaIn astronomy, an analemma (ˌænəˈlɛmə; ) is a diagram showing the position of the Sun in the sky as seen from a fixed location on Earth at the same mean solar time, as that position varies over the course of a year. The diagram will resemble a figure eight. Globes of Earth often display an analemma as a two-dimensional figure of equation of time vs. declination of the Sun. The north–south component of the analemma results from the change in the Sun's declination due to the tilt of Earth's axis of rotation.
Julian calendarThe Julian calendar is a solar calendar of 365 days in every year with an additional leap day every fourth year (without exception). The Julian calendar is still used in parts of the Eastern Orthodox Church and in parts of Oriental Orthodoxy as well as by the Berbers, whereas the Gregorian calendar is used in most parts of the world. This calendar, proposed by Roman consul Julius Caesar in 46 BC, was a reform of the earlier Roman calendar, a largely lunisolar one. It took effect on 1 January 45 BC, by edict.
French Republican calendarThe French Republican calendar (calendrier républicain français), also commonly called the French Revolutionary calendar (calendrier révolutionnaire français), was a calendar created and implemented during the French Revolution, and used by the French government for about 12 years from late 1793 to 1805, and for 18 days by the Paris Commune in 1871, and meant to replace the Gregorian calendar.
Astronomical year numberingAstronomical year numbering is based on AD/CE year numbering, but follows normal decimal integer numbering more strictly. Thus, it has a year 0; the years before that are designated with negative numbers and the years after that are designated with positive numbers. Astronomers use the Julian calendar for years before 1582, including the year 0, and the Gregorian calendar for years after 1582, as exemplified by Jacques Cassini (1740), Simon Newcomb (1898) and Fred Espenak (2007).
Time standardA time standard is a specification for measuring time: either the rate at which time passes or points in time or both. In modern times, several time specifications have been officially recognized as standards, where formerly they were matters of custom and practice. An example of a kind of time standard can be a time scale, specifying a method for measuring divisions of time. A standard for civil time can specify both time intervals and time-of-day.
Position of the SunThe position of the Sun in the sky is a function of both the time and the geographic location of observation on Earth's surface. As Earth orbits the Sun over the course of a year, the Sun appears to move with respect to the fixed stars on the celestial sphere, along a circular path called the ecliptic. Earth's rotation about its axis causes diurnal motion, so that the Sun appears to move across the sky in a Sun path that depends on the observer's geographic latitude.
Astronomical nutationAstronomical nutation is a phenomenon which causes the orientation of the axis of rotation of a spinning astronomical object to vary over time. It is caused by the gravitational forces of other nearby bodies acting upon the spinning object. Although they are caused by the same effect operating over different timescales, astronomers usually make a distinction between precession, which is a steady long-term change in the axis of rotation, and nutation, which is the combined effect of similar shorter-term variations.
Civil timeIn modern usage, civil time refers to statutory time as designated by civilian authorities. Modern civil time is generally national standard time in a time zone at a fixed offset from Coordinated Universal Time (UTC), possibly adjusted by daylight saving time during part of the year. UTC is calculated by reference to atomic clocks and was adopted in 1972. Older systems use telescope observations. In traditional astronomical usage, civil time was mean solar time reckoned from midnight.
Timekeeping on MarsThough no standard exists, numerous calendars and other timekeeping approaches have been proposed for the planet Mars. The most commonly seen in the scientific literature denotes the time of year as the number of degrees on its orbit from the northward equinox, and increasingly there is use of numbering the Martian years beginning at the equinox that occurred April 11, 1955. Mars has an axial tilt and a rotation period similar to those of Earth. Thus, it experiences seasons of spring, summer, autumn and winter much like Earth.
