Solar & Lunar Eclips

7/26/2019 Solar & Lunar Eclips

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LUNAR ECLIPS

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MOON


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Lunar & Solar Cycles

From ancient times, lunar and solar eclipses have beenregarded both as signs of awe and fear or of beautyand amazement. It is therefore understandable thatastronomers have continuously searched for methods

of predicting their occurrence and circumstances. Long before the theories of the relative motions of the

Sun and the Moon had reached the stage ofdevelopment that the circumstances of a lunar or asolar eclipse could be solved from first principles,

astronomers noted that eclipses occurred at semi-regular intervals. The earliest of such periods to beemployed would have been the Semester (5 or 6 lunarmonths)


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An overview of the various eclipse cycles

mentioned in the astronomical literature.

Saros (18.0 years), the Metonic Cycle (19.0 years), the Exeligmos (54.1 years) andthe Babylonian Period (441.3 years). New World (Maya) astronomers appear tohave been familiar with the Hepton, the Octon, the Tritos (10.9 years), the Thix(25.6 years) and the Triple Tritos (32.7 years)

Visit: http://www.staff.science.uu.nl/~gent0113/eclipse/eclipsecycles.htm


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SAROS

The Saros arises from a harmonic between three of the Moon's orbital cycles. All three

periods are subject to slow variations over long time scales, but their values as of 2000 CE

are:

Synodic Month (New Moon to New Moon) = 29.530589 days = 29d 12h 44m 03s

Anomalistic Month (perigee to perigee) = 27.554550 days = 27d 13h 18m 33s

Draconic Month (node to node) = 27.212221 days = 27d 05h 05m 36s

One Saros is equal to 223 synodic months, however, 239 anomalistic months and 242

draconic months are also equal (within a few hours) to this same period:

223 Synodic Months = 6585.3223 days = 6585d 07h 43m

239 Anomalistic Months = 6585.5375 days = 6585d 12h 54m

242 Draconic Months = 6585.3575 days = 6585d 08h 35m

With a period of approximately 6,585.32 days (~18 years 11 days 8 hours), the Saros is a

valuable tool in investigating the periodicity and recurrence of eclipses. It was first known to

the Chaldeans as an interval when lunar eclipses repeat, but the Saros is applicable to solar

eclipses as well.


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Any two eclipses separated by one Saros cycle share similar characteristics. They occur at the same node

with the Moon at nearly the same distance from Earth and the same time of year. Because the Saros

period is not equal to a whole number of days, its biggest drawback as an eclipse predictor is that

subsequent eclipses are visible from different parts of the globe. The extra 1/3 day displacement means

that Earth must rotate an additional ~8 hours or ~120 with each cycle. For lunar eclipses, this results in a

shift ~120 west in the visibility zones of each succeeding eclipse. Thus, a Saros series returns to

approximately the same geographic region every three Saros periods (~54 years and 34 days). This triple

Saros cycle is known as the Exeligmos


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About Saros

The periodicity and recurrence of lunar (and solar) eclipses is governed bythe Saros cycle, a period of approximately 6,585.3 days (18 years 11 days8 hours). When two eclipses are separated by a period of one Saros, theyshare a very similar geometry. The two eclipses occur at the same nodewith the Moon at nearly the same distance from Earth and at the sametime of year. Thus, the Saros is useful for organizing eclipses into families

or series. Each series typically lasts 12 to 15 centuries and contains 70 ormore lunar eclipses.

The series began with a penumbral eclipse near the southern edge of thepenumbra on -2570 Mar 14.

Lunar eclipses occurring near the Moon's descending node are given oddsaros series numbers. The first eclipse in such series passes through the

southern edge of the Earth's shadow, and the Moon's path is shiftednorthward each successive saros.

http://en.wikipedia.org/wiki/Lunar_eclipse#Lunar_eclipse_in_culture


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Terms should be know !

The sidereal month is defined as the Moon's orbital period in a non-rotating frame of reference(which on average is equal to its rotation period in the same frame). It is about 27.32166 days (27days, 7 hours, 43 minutes, 11.6 seconds). The exact duration of the orbital period cannot be easilydetermined, because the 'non-rotating frame of reference' cannot be observed directly. However, itis approximately equal to the time it takes the Moon to pass twice a "fixed" star (different stars givedifferent results because all have proper motions and are not really fixed in position).

A synodic month is the most familiar lunar cycle, defined as the time interval between twoconsecutive occurrences of a particular phase (such as new moon or full moon) as seen by an

observer on Earth. The mean length of the synodic month is 29.53059 days (29 days, 12 hours, 44minutes, 2.8 seconds). Due to the eccentric orbit of the lunar orbit around Earth (and to a lesserdegree, the Earths elliptical orbit around the Sun), the length of a synodic month can vary by up toseven hours.

The tropical month is the average time for the Moon to pass twice through the same equinox pointof the sky. It is 27.32158 days, very slightly shorter than the sidereal month (27.32166) days,because of precession of the equinoxes. Unlike the sidereal month, it can be measured precisely.

An anomalistic month is the average time the Moon takes to go from perigee to perigee - the point

in the Moon's orbit when it is closest to Earth. An anomalistic month is about 27.55455 days onaverage.

The draconic month or nodal month is the period in which the Moon returns to the same node ofits orbit; the nodes are the two points where the Moon's orbit crosses the plane of the Earth'sorbit. Its duration is about 27.21222 days on average.


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Terms should be know !

A synodic month is longer than a sidereal month because the Earth-Moonsystem is orbiting the Sun in the same direction as the Moon is orbitingthe Earth. Therefore, the Sun appears to move with respect to the stars,and it takes about 2.2 days longer for the Moon to return to the sameapparent position with respect to the Sun.

An anomalistic month is longer than a sidereal month because theperigee moves in the same direction as the Moon is orbiting the Earth,one revolution in nine years. Therefore, the Moon takes a little longer toreturn to perigee than to return to the same star.

A draconic month is shorter than a sidereal month because the nodesmove in the opposite direction as the Moon is orbiting the Earth, onerevolution in 18 years. Therefore, the Moon returns to the same nodeslightly earlier than it returns to the same star.

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