Year#Astronomical years

The eclipse year is astronomical term as the time interval of successive passages of the same lunar node by the sun and takes about

( 346 days, 14 hours, 53 minutes).

After half eclipse year ( 173.31 d) one of the two lunar nodes is back from the sun. This constellation forms the middle of that seasonal time window of a few weeks in which eclipses or lunar eclipses can be observed from Earth.

For an eclipse year is still occasionally called " Drakonitisches year " as a synonym used. This designation, however, is misleading, because a year is not darkness is composed of Drakonitischen months. In English, this means that eclipse year eclipse / ecliptical year and is in addition also called draconic / draconitic year.

Basics

In the ascending as descending node the Moon's orbit intersects the plane of the ecliptic, and only in the vicinity of these intersections of the Moon can occur so that happened an eclipse. When the moon is doing in conjunction with the Sun, new moon, as it occurs from Earth seen against the sun, causing an eclipse. Is the moon, however, in opposition to the sun, to the full moon, he plunges into the shadow of the Earth and is shaded as a lunar eclipse.

Or their nodal line said connecting line - - For both types of darkness so also the location of one of the lunar nodes in addition to the matching phase of the moon towards the sun necessary condition for the moon in this phase can obscure now or can be darkened.

About every half eclipse year are earth and moon node to the sun in such a position, the different kinds of darkness allowed. When exactly will an eclipse occurs and what type depends on the current moon run. The period from new moon to new moon, ( true ) called lunation, may order more than 13 hours (0.56 d) fluctuate, their calculated mean is the Synodic month and is about 29.53 days. The interval between two passages of the moon through the same path node is on average about 27.21 days and is referred to as Drakonitischer month.

Viewed from the course of the moon eclipses forth are basically only possible within narrow time window are at least close to being met for two different conditions: The moon must in both full or new moon stand as well as go through the on - or descending node. Each of the conditions is now repeated at intervals of integer multiples of half the length of their period, and then take, for example, after 12 half synodic months ( 177.18 days ) or 13 half drakonitischen months ( 176.87 days ) again, perhaps both together. Thus, after about six synodic months, a semester, another eclipse event occur.

An eclipse year is almost 19 days shorter than the solar year to 365.2422 days, because of the precession of the orbiting moon turned its orbital plane and the nodes are therefore shifted annually in the ecliptic plane by about 19 ° decline ( 19.34 ° regression based on a tropical year ), and it takes so shorter than the astronomical lunar year to 354.3671 days, which consists of twelve synodic months with about 346.52 days.

Relationships with darkness cycles

A half eclipse year to about 173.31 days over it obviously takes almost 4 days longer, completed by six lunations of the moon are about 177.18 days, the period for the semester cycle than the shortest of darkness cycles for the multiple repetition of eclipses. Since each changed from event to event, the distance to the node and eventually becomes too great, breaking a semester cycle number from eight, nine or ten repeated eclipse events.

A long darkness cycle contains all of the Canon Eclipses read events. The more individual events are skipped during the summary of a cycle, the longer the cycle series can be. In the known Saros cycle series you will get such with mostly similar to each other darkness 71 events at intervals, take the 223 synodic months ( 6585.32 days ). Compared with 19 years of darkness ( 6585.78 days ), the deviation is less than half a day (0.46 d), even lower by 242 drakonitischen months ( 6585.36 days ). The Saros period is therefore about 18.03 solar years and with this period interrelated eclipses then form a Saros cycle with rows of approximately 1,270 solar years. At about 29.95 solar years a little longer is the Inex period; it lasts half 716 synodic months, differ only very slightly from those 777 drakonitische half months or 61 years half darkness.

Thus, the larger is the period of a read cycle, the more accurate can this as a multiple of half darkness approaching years.

Estimation of the darkness - time window

The half eclipse year is the period between two lunar nodes passages by the sun and thus gives an indication of the date around which darkness can take place; but there are not the actual time between eclipses. As the apparent size of the sun and the moon each approximately amount to half a degree of angle and furthermore the length of the Earth's diameter can make the moon appear under a parallax of up to about 1 °, eclipses can from Earth within a certain period before and be observed after the passage of the moon's nodes. For partial eclipses, this time window is up to plus / minus about 17 days, so just under 5 weeks, for the inconspicuous penumbral lunar eclipses; for total solar eclipses as well as (partial ) umbra - lunar eclipses, it is smaller with plus / minus about 11 days, so good 3 weeks. The actual dates resulting from the entry of the sun and moon in the dark area.

In these for different types of eclipses each different limited areas always occurs an eclipse, if the following three conditions are met each approximate manner:

• ( 1) It is now full moon or new moon ( between passes an average of half a synodic month)
• ( 2) the Moon is in ascending or descending node ( between half a drakonitischer month)
• ( 3) one of the lunar nodes is now seen from Earth from the sun ( between a half eclipse year ).

Starting from an eclipse event is the occurrence of a previous or subsequent darkness can therefore be roughly estimated by the approximate matching of integral multiples of the respective half-cycles, either from synodischem and drakonitischem month (1 and 2) or by synodischem month and eclipse year ( 1 and 3) or drakonitischem month and eclipse year (2 and 3).

The eclipse year is thus the particular circumstances of the intersection of the orbital plane of the Moon with the orbital plane of the earth again as necessary -. Though not sufficient - condition for the event of an eclipse

Precession of the orbital plane of the Moon

The orbital plane of the Moon is inclined by about 5 ° to the ecliptic plane in which the earth attracts its orbit around the sun, so the moon can be seen from Earth, a maximum of this amount higher or lower than the sun. Without this tilt (inclination = 0 °) earth's orbit and lunar orbit would lie in the same plane and each new moon would be connected to a solar eclipse as every full moon with a lunar eclipse.

The South Node as the intersections of the Moon's orbit to the ecliptic plane are in relation to the Sun - in heliocentric reference system - not spatially fixed. During one revolution of the earth, and thus also of the moon around the sun is of course the location of the nodes changed on the ecliptic plane, and they also take different distances with respect to the solar one. Comparing now after a complete orbit of the earth, the position of the two nodes to each other (eg as a nodal line ) with their position to each other a year earlier, it appears beyond a special circumstance: the orbital plane of the Moon is still the same inclination, but it has relative to the ecliptic plane now turned slightly to around 19 ° decline to the circumferential direction. This movement of the lunar orbit plane is understood as a gyroscopic effect and called precession.

Through the annual shift to 19 ° well a full 360 ° rotation is at a period of about 18.6 years completed and reaches the initial position of the nodal line again. The precession of the lunar orbit is also reflected in periodic light variations in the Earth's axis of equal duration; superimposed to the precession of the Earth's axis - the cycle is called by almost 26,000 years, Platonic year - and represent the major proportion of an effect, which is called in astronomy as a nutation of the Earth's axis.

By understanding these relationships, the eclipse year also reflects a measure of the precession of the lunar orbit when it is for instance set to the tropical year in proportion.