Eclipse year
Constellations of the passage of the nodal line through the sun limit eclipse half-years
The eclipse year is the time interval between successive passages of the same lunar node through the sun and lasts around
- (346 days, 14 hours, 53 minutes).
After half an eclipse year (173.31 days) one of the two lunar nodes is again in front of the sun. This constellation forms the middle of a time window of a few weeks in which solar eclipses or lunar eclipses can be observed from Earth. If the moon passes a knot at the same time, the eclipse is centric. If the passage is not too early or too late, it is partial. The determination of the eclipse times is done without using the eclipse year: Based on an event and its location in the time window, those times are determined at which a whole number of sidereal months and a whole number of half draconian months of the Earth's moon have passed.
The term "draconian year" is occasionally used as a synonym for an eclipse year . However, this designation is misleading, because an eclipse year is not made up of draconian months. In English the eclipse year is called eclipse / ecliptical year and is also called draconic / draconitic year .
Basics
(actual inclination only about 5 °)
In the ascending as well as in the descending node, the lunar orbit intersects the plane of the ecliptic and only in the vicinity of these intersection points can the moon appear in such a way that an eclipse occurs. If the moon is in conjunction with the sun, at the new moon , it steps in front of the sun when viewed from the earth and causes a solar eclipse. If, on the other hand, the moon is in opposition to the sun, to a full moon , it dips into the shadow of the earth and is shaded as a lunar eclipse.
For both types of eclipse, in addition to the appropriate phase of the moon, the position of one of the two lunar nodes - or the connecting straight line called the connecting line - in the direction of the sun is a necessary condition so that the moon can now cover or be darkened in this phase.
About every half of the eclipse years, the earth and the lunar nodes to the sun are in such a position that different types of eclipse are possible. When exactly then an eclipse occurs and which type, depends on the current course of the moon. The time span from new moon to new moon, known as the (true) lunation , can fluctuate by more than 13 hours (0.56 d), its calculated mean value is the synodic month and is about 29.53 days. The interval between two passes of the moon through the same orbital node averages around 27.21 days and is known as the Draconite month .
Viewed from the direction of the moon, eclipses are basically only possible within narrow time windows, for which two different conditions are at least almost fulfilled: The moon must be in full or new moon and also go through the ascending or descending node. Each of the conditions is now repeated at intervals of integral multiples of half the length of its period, and they then apply again, for example, after 12 half synodic months (177.18 days) or 13 half draconian months (176.87 days), possibly both together . Thus after about six synodic months, one semester , another eclipse event can occur.
An eclipse year is almost 19 days shorter than the solar year of 365.2422 days, because the precession of the circling moon rotates its orbit plane and the nodes in the ecliptic plane are therefore shifted backwards by around 19 ° annually (19.34 ° regression based on a tropical year), and it is thus shorter with about 346.52 days than the astronomical lunar year of 354.3671 days, which consists of twelve synodic months.
Connections with eclipse cycles
Compared to half an eclipse year of about 173.31 days, it takes almost four days longer until six lunations of the moon are completed in about 177.18 days, the period for the semester cycle as the shortest of the eclipse cycles for the repeated repetition of eclipses. Since the distance to the node changes from event to event and eventually becomes too great, a semester cycle series ends after eight, nine or ten repeated eclipses.
A long cycle of eclipses contains events selected from the canon of all eclipses. The more individual events are skipped when grouping a cycle, the longer the cycle series can become. In the well-known Saros cycle , such series are obtained with mostly 71 similar eclipse events at intervals that last 223 synodic months (6,585.32 days). Compared to 19 eclipse years (6,585.78 days), the deviation is less than half a day (0.46 d), and even less that of 242 draconian months (6585.36 days). The Saros period is thus approximately 18.03 solar years and the eclipses related to this period then form a Saros cycle with series of around 1,270 solar years. The Inex period is a little longer at around 29.95 solar years ; it lasts 716 half synodic months, from which 777 half draconian months or 61 half eclipse years differ only very slightly.
The larger the period of a readout cycle, the more accurately it can be approximated as a multiple of half eclipse years.
Estimation of the eclipse time window
Half an eclipse year is the duration between two lunar node passages through the sun and thus gives an indication of the point in time around which an eclipse can take place; but it does not indicate the actual length of time between eclipses. Since the apparent size of the sun and that of the moon are each about half an angular degree and the length of the earth's diameter can make the moon appear under a parallax of up to about 1 °, eclipses from the earth can occur in a certain period of time before and can be observed after passing the lunar node. For partial solar eclipses this time window is up to plus / minus about 17 days, i.e. just under 5 weeks, also for the inconspicuous penumbral lunar eclipses; for total solar eclipses as well as for (partial) umbra lunar eclipses it is smaller with plus / minus about 11 days, i.e. a good 3 weeks. The actual dates result from the entry of the sun and moon into the respective eclipse area .
In these areas, which are differently limited for different types of eclipses, an eclipse always occurs if the following three conditions are met in each case approximately:
- (1) it is now a full moon or a new moon (an average of half a synodic month passes in between),
- (2) the moon is in the ascending or descending node (half a draconian month in between),
- (3) one of the lunar nodes is now in front of the sun when viewed from earth (half an eclipse year in between).
Based on an eclipse event, the occurrence of a previous or subsequent eclipse can therefore be roughly estimated by the approximate correspondence of integer multiples of the respective half-periods , be it of synodic and draconian months (1 and 2) or of synodic month and eclipse year (1 and 3) or of the draconian month and eclipse year (2 and 3).
The eclipse year thus reflects the special circumstances of the intersection of the plane of the moon with the plane of the earth as a necessary - but 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 moves its orbit around the sun, so that the moon can stand a maximum of this amount higher or lower than the sun when viewed from the earth. Without this inclination (inclination = 0 °) the orbit of the earth and the lunar orbit would lie in the same plane and every new moon would be associated with a solar eclipse like every full moon would be associated with a lunar eclipse.
The lunar nodes as the points of intersection of the lunar orbit with the ecliptic plane are not spatially fixed in relation to the sun - in a heliocentric reference system. During one orbit of the earth and thus also of the moon around the sun, the position of the two nodes on the ecliptical plane naturally changes and they also occupy different distances with respect to the sun. If one now compares the position of both nodes to each other (e.g. as a nodal line) with their position to each other a year before after a complete revolution of the earth, a special circumstance becomes apparent: the orbital plane of the moon is still the same inclination, but it has changed in the meantime somewhat rotated relative to the ecliptic plane, declining by around 19 ° to the direction of rotation. This movement of the lunar orbit plane is understood as a gyroscopic effect and called precession .
Due to the annual shift of a good 19 °, a full 360 ° rotation is completed over a period of around 18.6 years and the starting position of the node line is reached again. The precession motion of the lunar orbit is also reflected in periodic slight fluctuations in the earth's axis of the same duration; these superimpose the precession of the earth's axis - whose cycle of almost 26,000 years is also called the Platonic year - and represent the main part of an effect that is referred to in astronomy as the nutation of the earth's axis.
With an understanding of these relationships, the eclipse year also provides a measure of the precession movement of the lunar orbit, for example when it is related to the tropical year.
Individual evidence
- ↑ Joseph Drecker: Time measurement and star interpretation in historical representation , Berlin: Borntraeger, 1925.
- ↑ Space Globe, astronomical professional
- ↑ For example the Canon of Theodor Oppolzer : Canon der Finsternisse , memoranda of the Imperial Academy of Sciences, mathematical and natural science class, L II.Bd., Vienna 1887.
literature
- Jean Meeus : Astronomical Algorithms , 2nd edition, JA Barth, Leipzig 1994, ISBN 3-335-00400-0