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Astronomical Calendar |
| If you're looking for what's happening in the night sky tonight, this isn't the place. Here you'll find articles to help you understand broad astronomical calendar terms, like what solistice and equinox means and when it occurs. You'll find information on things like Julian days, epochs, Saros cycles and more. Step inside if you're curious.... |
Julian Day
The Julian date (JD) is the interval of time in days and fractions of a day, since 4713 B.C. January 1, Greenwich noon, Julian proleptic calendar.
Astronomical Julian Year
In astronomy, a Julian year is a unit of measurement of time defined as exactly 365.25 days of 86,400 SI seconds each, totalling 31,557,600 seconds.
Names For The Solstice
The two solstices can be distinguished by different pairs of names, depending on which feature one wants to stress.
Saros Series
The Sun and Moon come in conjunction shifts westward by about 0.5° with respect to the Moon's nodes every Saros cycle, and this gives rise to a series of eclipses, called a Saros series.
Metonic Cycle
The Metonic cycle or Enneadecaeteris in astronomy and calendar studies is a particular approximate common multiple of the tropical year and the synodic (lunar) month.
Synodic Month
This is the average period of the Moon's revolution with respect to the sun.
Rotation Period
The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars.
Hipparchic Cycle
The Greek astronomer Hipparchus introduced two cycles that have been named after him in later literature.
Equinox Names
Spring or fall? Vernal or autumnal? It would seem the equinox has more than one name! Let's take a look at them...
Saros Cycle
The Saros cycle is an eclipse cycle with a period of time that can be used to predict eclipses of the Sun and Moon.
Tropical Year
A tropical yearis the length of time that the Sun takes to return to the same position in the cycle of seasons.
Lunar Month
In lunar calendars, a lunar month is the time between two identical syzygies (new moons or full moons).
Equinox
An equinox is the event of the Sun passing over the Earth's equator, in its annual cycle.
Vernal Equinox As A Celestial Coordinate
The vernal point (vernal equinox)—the one the Sun passes in March on its way from south to north—is used as the origin of some celestial coordinate systems:
Sidereal Year
The sidereal year is the time taken for the Sun to return to the same position with respect to the stars of the celestial sphere.
Astronomical Epoch
In astronomy, an epoch is a moment in time used as a reference for the orbital elements of a celestial body.
Equinox and a Heliocentric View of the Seasons
The Earth's seasons are caused by the rotation axis of the Earth not being perpendicular to its orbital plane.
Solstice
Solstices occur twice a year, when the tilt of the Earth's axis is most oriented toward or away from the Sun, causing the Sun to reach its northernmost and southernmost extremes.
Precession of the Equinoxes
In astronomy, precession refers to the movement of the rotational axis of a body, such as a planet, with respect to inertial space.
Gaussian Year
A Gaussian year is defined as 365.2568983 days.
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