In the fields of chronology and periodization, an epoch is an instant in time chosen as the origin of a particular era. The "epoch" then serves as a reference point from which time is measured. Time measurement units are counted from the epoch so that the date and time of events can be specified unambiguously.
A day is defined as one revolution of the earth on its axis, but because the earth is also orbiting the sun, there are actually different types of days.
The period of time measured from local noon to the following local noon is called a solar day; this is what we commonly think of as day.
The period of time it takes the earth to make one entire rotation with respect to the celestial background or a fixed distant star is called a stellar day and is 3 minutes 56 seconds shorter than a solar day.
Cycles of the MoonEditThe Moon makes a complete orbit around the Earth with respect to the fixed stars about once every 27.3 days (its sidereal period).
However, since the Earth is moving in its orbit about the Sun at the same time, it takes slightly longer for the Moon to show the same phase to Earth, which is about 29.5 days (its synodic period); this is what we commonly think of as the moon's cycle.
The tropical year is defined as the period of time for the ecliptic longitude of the Sun to increase by 360°, or more plainly, the time it takes from winter solstice to winter solstice; this is what we most commonly think of as a year. This period averages 365.2421897 mean solar days.
The sidereal year is the time taken for the Earth to complete one revolution of its orbit, as measured against fixed stars and averages 365.256363004 mean solar days.
The anomalistic year is the time taken for the Earth to complete one revolution with respect to its http://en.wikipedia.org/wiki/Apsis apsides], like from one perihelion to the next perihelion; this period averages 365.259635 mean solar days.
Milankovitch Cycles refer to the many long term cycle of the earth. These cycles are affected my many different factors including the sun, the moon, the planets, and the earth's inherent gyrosocpic spin.
Unless an orbit is completely circular (which none are), any elliptical orbit of a planet (like that of the earth) contains two apsides. One is a point where the planet is closest to it's sun, called the perihelion, and the other is a point where the planet is furthest from the sun, called the perihelion. Each anomalistic year (a year with respect to background stars), these apsides move ever so slightly in relation to fixed stars such that over a long period of about 137,000 years, they would have made a complete cycle.
The earth's axis is tilted in relation to it's orbit, which is why we have winter and summer. But over time, the tilt direction changes so that if it is currently titled north, in about 6,500 years, it will be titled west, then south in another 6,500 years and so on and so forth until it starts pointing north again. This 26,000 year cycle ( or a mean 25772 years by some precise estimates) is more commonly known as the precession of the equinoxes. So, on the earth, if a constellation exists at a certain point in the night sky, 2148 years later, that same constellation will appear 30° from it's intial position, 180° after 12886 years, and once again at same position after the full 25772 year axial precession cycle.
Apsidal-Solstice Concurrence CycleEdit
Together, apsidal and axial precession result in a 21,000 year cycle whereby an specific apsis and a specific solstice will occur at the same time once every 21,000 (or more precisely 20935) years. The last occurrence of such a cycle occured in 1246, when the perihelion and the northern winter solstice occurred at the same time on December 22nd. 768 years later, the northern winter solstice still occurs around December 22nd, but the perihelion now occurs around January 4th.
Orbital shape (eccentricity)Edit
Over many years, the shape of the earth's orbit will change from more elliptical to more circular and back again. The earth's eccentricity cycles are very erratic and difficult to measure, though the major component of these variations occurs on a period of 413,000 years. A number of other terms vary between 95,000 and 125,000 years (with a beat period 400,000 years), and loosely combine into a 100,000-year cycle. The Earth's eccentricity varies primarily due to interactions with the gravitational fields of Jupiter and Saturn.
Axial Tilt Cycle (Obliquity)Edit
Currently, the earth's axis is tilted at about 23.44° in relation to it's orbital plane. But over a period of about 41,000 years, this "of the ecliptic" will oscilate between a tilt of 22.1° and 24.5° and back again.
The inclination of Earth's orbit drifts up and down relative to its present orbit with a cycle having a period of about 70,000 years. The inclination of the Earth's orbit has a 100,000-year cycle relative to the invariable plane. This is very similar to the 100,000-year eccentricity period. This 100,000-year cycle closely matches the 100,000-year pattern of ice ages.
Known as the 100,000-year problem, during the past million years, Ice Ages seem to occur about once every 100,000 years.
Cycles of the PlanetsEdit
Another good example of astronomical epochs are the positions of the planets. Tracking the positions of planets independently of one another may be suitable for astronomical epochs. There are calendars that measure these periods of individual planets. http://en.wikipedia.org/wiki/Planet#Planetary_attributes is a good table for comparing the duration periods and other cycles of the planets within our solar system.
Every millenium or so the major planets in our solar systm may sometimes form a somewhat straight line (within about 30° or so) across the solar system, like in 561 BCE. And more often, like in 1983, the planets will be within 96° of eachother. Perfect planetary alignments, however, (basically) never occur.
Just as the earth revolves around the sun, so does the sun revolve around the galaxy. Therefore, the sun has its own cycles.
It takes the solar system about 230 million years to orbit the center of the Milky Way Galaxy one time. This is called a galactic year.
Oscillation Through the Galactic PlaneEdit
Every 62 million years (give or take several million years) the solar system oscillates from 250 light years above to 250 light years below the Milky Way's rotating spiral plane, making the solar system's path around galaxy look like a sine wave around a cylinder.
Pages in category "Astronomical epochs"
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