A synodic day (or synodic rotation period or solar day) is the period for a celestial object to rotate once in relation to the star it is orbiting, and is the basis of solar time.
The synodic day is distinguished from the sidereal day, which is one complete rotation in relation to distant stars and is the basis of sidereal time.
A circadian clock, or circadian oscillator, also known as one's internal alarm clock is a biochemical oscillator that cycles with a stable phase and is synchronized with solar time.
Such a clock's in vivo period is necessarily almost exactly 24 hours (the earth's current solar day). In most living organisms, internally synchronized circadian clocks make it possible for the organism to anticipate daily environmental changes corresponding with the day–night cycle and adjust its biology and behavior accordingly.
Sidereal time ("sidereal" pronounced /saɪˈdɪəriəl, sə-/ sy-DEER-ee-əl, sə-) is a system of timekeeping used especially by astronomers. Using sidereal time and the celestial coordinate system, it is easy to locate the positions of celestial objects in the night sky. Sidereal time is a "time scale that is based on Earth's rate of rotation measured relative to the fixed stars". A sidereal day (also known as the sidereal rotation period) represents the time for one rotation about the planet axis relative to the stars.
Viewed from the same location, a star seen at one position in the sky will be seen at the same position on another night at the same time of day (or night), if the day is defined as a sidereal day. This is similar to how the time kept by a sundial (Solar time) can be used to find the location of the Sun. Just as the Sun and Moon appear to rise in the east and set in the west due to the rotation of Earth, so do the stars. Both solar time and sidereal time make use of the regularity of Earth's rotation about its polar axis: solar time is reckoned according to the position of the Sun in the sky while sidereal time is based approximately on the position of the fixed stars on the theoretical celestial sphere.
1908 (MCMVIII) was a leap year starting on Wednesday of the Gregorian calendar and a leap year starting on Tuesday of the Julian calendar, the 1908th year of the Common Era (CE) and Anno Domini (AD) designations, the 908th year of the 2nd millennium, the 8th year of the 20th century, and the 9th year of the 1900s decade. As of the start of 1908, the Gregorian calendar was 13 days ahead of the Julian calendar, which remained in localized use until 1923.
This is the longest year in either the Julian or Gregorian calendars, having a duration of 31622401.38 seconds of Terrestrial Time (or ephemeris time), measured according to the definition of mean solar time.
The A-train (from Afternoon Train) is a satellite constellation of four Earth observation satellites of varied nationality in Sun-synchronous orbit at an altitude that is slightly variable for each satellite.
The orbit, at an inclination of 98.14°, crosses the equator each day at around 1:30 pm solar time, giving the constellation its name (the "A" stands for "afternoon") and crosses the equator again on the night side of the Earth, at around 1:30 am.
A leap second is a one-second adjustment that is occasionally applied to Coordinated Universal Time (UTC), to accommodate the difference between precise time (International Atomic Time (TAI), as measured by atomic clocks) and imprecise observed solar time (UT1), which varies due to irregularities and long-term slowdown in the Earth's rotation. The UTC time standard, widely used for international timekeeping and as the reference for civil time in most countries, uses TAI and consequently would run ahead of observed solar time unless it is reset to UT1 as needed. The leap second facility exists to provide this adjustment. The leap second was introduced in 1972. Since then, 27 leap seconds have been added to UTC, with the most recent occurring on December 31, 2016. All have so far been positive leap seconds, adding a second to a UTC day; while a negative leap second is theoretically possible, it has not yet occurred.
Because the Earth's rotational speed varies in response to climatic and geological events, UTC leap seconds are irregularly spaced and not precisely predictable. The decision to insert a leap second is made by the International Earth Rotation and Reference Systems Service (IERS), typically about six months in advance, to ensure that the difference between UTC and UT1 does not exceed ±0.9 seconds.
Europe spans seven primary time zones (from UTC−01:00 to UTC+05:00), excluding summer time offsets (five of them can be seen on the map, with one further-western zone containing the Azores, and one further-eastern zone spanning the Ural regions of Russia and European part of Kazakhstan). Most European countries use summer time and harmonise their summer time adjustments; see Summer time in Europe for details.
The time zones actually in use in Europe differ significantly from uniform zoning based purely on longitude, as used for example under the nautical time system. The world could in theory be divided into 24 time zones, each spanning 15 degrees of longitude. However, due to geographical and cultural factors, it is not practical to divide the world so evenly, and actual time zones may differ significantly from those based purely on longitude. In Europe, the widespread use of Central European Time (CET) causes major variations in some areas from solar time. Based on solar time, CET would range from 7.5 to 22.5°E. However, for example Spain (almost entirely in the Western hemisphere) and France (almost entirely west of 7.5°E, as illustrated in the map below) should theoretically use UTC, as they did before the Second World War. The general result is a solar noon which is much later than clock noon, and later sunrises and sunsets than should theoretically happen. The Benelux countries should also theoretically use GMT.