Encyclopaedia Britannica, 11th Edition, "Echinoderma" to "Edward, prince of Wales"

Chapter IV: Front Matter (4)

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The eclipse of August 30, 1905, offered a duration of 3-1/2 minutes in Spain, the track running from Labrador through Spain to North Africa, and affording excellent opportunities for observers, who flocked to the central line in great numbers. Unfortunately it was cloudy in Labrador, so that the special advantages of the long line of possible stations were lost. Exceptionally good weather conditions were enjoyed in Algeria and Tunisia, and full advantage was taken of them by H.F. Newall, C. Trepied and others at Guelma, by the party from Greenwich and G. Bigourdan at Sfax. That G. Newall's spectroscopic photographs for rotation of the corona again gave no result is a clear indication of the faintness of the corona at 3' from the limb; but F.W. Dyson at Sfax obtained two new lines at 5536 and 5117 in the spectrum of the corona; and a very large number of photographs of the corona (including many in polarized light on several different plans), of its spectrum, and of the spectrum of the chromosphere, were obtained by the various parties, which will afford copious material for discussion. Newall also obtained a polarized spectrum of the corona. Altogether no less than eighty stations were occupied. There were English, American, Russian and German observers in Egypt; English and French in Algeria and Tunisia; English in Majorca; observers of almost all nationalities in Spain; and English and American in Labrador. In Egypt the weather was bright, though the sun was low; in Majorca and Spain there were local clouds. Consequently many observations, in addition to those in Labrador, were lost, notably the special spectroscopic observations undertaken by Evershed on the northern limit of totality, and the observations of radiation undertaken by H.L. Callendar. A search for intra-Mercurial planets was conducted on an elaborate plan, with similar batteries of telescopes, in Egypt, Spain and Labrador, by three parties from the Lick Observatory, but the examination of the plates showed nothing noteworthy. Pending discussion of the greater part of the material, some interesting preliminary results were published in 1906 by the French observers. C.E.H. Bourget and Montangerand conclude that there is a marked division of the chromosphere into two regions or shells, a lower or "reversing-layer," extending only 1" from the limb, and a chromospheric layer extending to 3" or 4"; and that the coronal light contains less blue and violet, but more green and yellow, than sunlight; while Fabry, by visual methods, obtained measures of the total and intrinsic intensity of the light from the corona closely confirming recent photographic observations, finding the total brightness about equal to that of the full moon, and the intrinsic brightness at 5' from the limb about one quarter of that of the full moon. (H. H. T.)

II. _Eclipses of the Moon._

The physical phenomena attending eclipses of the moon are no longer of a high order of interest either to the layman or scientific observer. A brief statement of them and their causes will therefore be sufficient. An observer watching such an eclipse from the moon would see the earth, which has nearly four times the apparent diameter of the sun, impinging on the sun's disk and slowly hiding it. The phenomenon would be quite similar to that of an eclipse of the sun seen from the earth, until the sun was completely covered. During the progress of this partial eclipse the moon would be passing into the earth's penumbra. As the moment of total obscuration approached, a red band of light would rapidly form in the neighbourhood of the disappearing limb of the sun, and gradually extend around the earth. This would arise from the refraction of the sun's light by the earth's atmosphere, and the absorption of its blue rays. When the light of the sun was completely hidden, a reddish ring of great brilliancy would, owing to this cause, surround the entire dark body of the earth during the period of the total eclipse.

The aspect of the moon, as seen from the earth, corresponds to this view from the moon. The fading of the moon's light, due to its entrance into the penumbra, is scarcely noticeable without direct photometric determination until near the beginning of the total phase. Then, as the limb of the moon approaches the earth's shadow, it begins to darken. When only a small portion has entered into the shadow, that portion is completely hidden. But, as the total phase approaches, the part of the moon's disk immersed in the penumbra becomes visible by a reddish coppery light--that of the sun refracted through the lower parts of the earth's atmosphere. The brightness of this illumination is different in different eclipses, a circumstance which may be attributed to the greater or less degree of cloudiness in those regions of the earth's atmosphere through which the light of the sun passes in order to reach the moon. Its colour is due to absorption in passing through the earth's atmosphere.

III. _Laws and Cycles of Recurrences of Eclipses of the Sun and Moon._

It has been known since remote antiquity that eclipses occur in cycles. These cycles are known now to be determined principally by the motion of the moon's node and the relations between the revolutions of the earth round the sun and the moon round the earth.

Eclipse seasons.

Owing to the inclination of the moon's orbit to the plane of the ecliptic, an eclipse of the sun can occur only when the conjunction of the sun and moon takes place within about 16 deg. of one of the nodes of the moon's orbit. The eclipse can be total only within about 11 deg. of the node. An eclipse of the moon can occur only when the line sun-moon-earth makes an angle less than about 11 deg. with the line of nodes; and the eclipse can be total only within about 8 deg. of the node, the average limiting distances varying 1 deg. or 2 deg. according to the circumstances. These conditions being understood, the cycles of recurrence of eclipses of either kind can be worked out geometrically from the mean motions of the sun, moon, node and perigee by the aid of geometric conceptions shown in their simplest form in fig. 4. Here E is the earth, at the centre of a circle representing the mean orbit of the moon around it. MN is the line of nodes which is moving in the retrograde direction from N towards S1, at a rate of about 19.3 deg. in a year, making a complete revolution in 18.6 years. Let the sun at the moment of some new moon be in the line ES1, continued. If the angle NES1 is less than 16 deg. there will probably be an eclipse of the sun, which may be central if the angle is less than 11 deg. Let the next new moon take place in the line ES2 a month later. The mean value of the angle S1ES2 is about 29 deg.; but as the node N has moved towards S1 about 1.4 deg. during the interval, the sum of the angles NES1 and NES2 will be somewhat greater than S1ES2 by about 1.6 deg. The result is that if these two angles are nearly equal there may be two small partial eclipses of the sun, after which no more can occur until, by the annual revolution of the earth, the direction of the sun approaches the opposite line of nodes EM, nearly six months later. The result is that there are in the course of any one year two "eclipse seasons" each of about one month in duration, in which at least one eclipse of the sun, or possibly two small partial eclipses, may occur. One eclipse of the moon will generally, but not always, occur during a season.

Owing to the retrograde motion of the node the direction ES of the sun returns to the node at the end of about 347 days, so that a third eclipse season may commence before the end of a year. In this way there is a possible but very rare maximum of five eclipses of the sun in a year. Owing to the motion of the line of nodes each eclipse season occurs about 19 days earlier in the year than it did the year before. Another conclusion from the greater eclipse limit for the sun than for the moon is that in the long run eclipses of the sun, as regards the earth generally, occur oftener than those of the moon. But as any eclipse of the sun is visible only from a limited region of the earth's surface, while one of the moon may be seen from an entire hemisphere, more eclipses of the moon are visible at any one place than of the sun.

If, starting with a conjunction along some line ES1, we mark by radial lines from E the successive conjunctions year after year, we shall find that at the end of 18 years and about 11 days the 223rd conjunction will fall once more very near the line ES1, the angle NES1 being about 24' greater than before. Successive eclipses will then occur very nearly in the same order as they did 18 years and 11 days before. This period of recurrence has been known from remote antiquity and is called the _Saros_. What is most remarkable in this period is that in addition to the distance from the node being nearly the same as before, the longitude of the sun increases by only 11 deg. and the distance of the moon from its perigee has changed less than 3 deg. The result of this approach to coincidence is that the recurring eclipse will generally be of the same kind--total, annular or partial--through a number of successive periods.

To see the law of recurrence of corresponding eclipses in the successive periods let us suppose the line of conjunction ES1 to be that at which there is a very small eclipse, visible only in high northern or southern latitudes. At the end of 18 years 11 days a second eclipse will occur along a line nearly half a degree nearer EN, the line of nodes. The successive eclipses will occur at the same interval through about ten periods, or 180 years, when the line of conjunction will pass within 11 deg. of EN. Then the eclipse will be central, whether annular or total depending on circumstances: in the first one the central lines will pass only over the polar regions; but in successive eclipses of the series it will pass nearer and nearer to the equator until the conjunction line coincides with the node. The path of centrality will then cross in the equatorial region. During 22 or 23 more recurrences the path will continually approach to the opposite pole and finally leave the earth entirely. The entire number of central eclipses in any one series will generally be about forty-five. Then a series of continually diminishing partial eclipses will go on for about ten periods more. The whole series of eclipses will therefore extend through about sixty-five periods; and interval of time of about twelve hundred years.

Another remarkable eclipse period recurs at the end of 358 lunations. At the end of this period the line of mean conjunction ES1 falls so near its former position relative to the node that we find each central eclipse visible in our time to be one of an unbroken series extending from the earliest historic times to the present, at intervals equal to the length of the period. The recurring eclipses in this period do not, however, have the remarkable similarity of those belonging to the Saros, but may differ to any extent, owing to the different positions of the line of conjunction with respect to the moon's perigee. Moreover, they recur alternately at the ascending and descending node. The length of the period is 10,571.95 days, or 29 Julian years less 20.3 days. Hence 18 periods make 521 years, so that at the end of this time each eclipse recurs on or about the same day of the year. As an example of this series, starting from the eclipse of Nineveh, June 15, 763 B.C., recorded on the Assyrian tablets, we find eclipses on May 27, 734 B.C., May 7, 705 B.C., and so on in an unbroken series to 1843, 1872 and 1901, the last being the 93rd of the series. Those at the ends of the 521-year intervals occurred on June 15, O.S., of each of the years 763, 242 B.C., A.D. 280, 801, 1322 and 1843. As the lunar perigee moves through 242.4 deg. in a period, the eclipses will vary from total to annular, but at the end of 3 periods the perigee is only 7.1 deg. in advance of its original position relative to the node. Hence in a series including every third eclipse the eclipses will be of the same character through a thousand years or more. Thus the eclipses of 1467, 1554, 1640, 1727, 1814, 1901, 1988, &c., are total.

IV. _Chronological Lists of Eclipses of the Sun._

Notable eclipses.

The following is a brief chronological enumeration of those total eclipses of the sun which are of interest, either from their historic celebrity or the nature of the conclusions derived from them. In numbering the years before the Christian era the astronomical nomenclature is used, in which the number of the year is one less than that used by the chronologists. The Chinese eclipses are passed over, owing to the generally doubtful character of the records pertaining to them.

--1069 June 20 and --1062 July 31; total eclipses recorded at Babylon.

--762, June 14; a total eclipse recorded at Nineveh. Computation from
the modern tables shows that the path of totality passed about 100 m.
or more north of Nineveh.

--647, April 6; total eclipse at or near Thasos, mentioned by
Archilochus.

--584, May 28; the celebrated eclipse of Thales. For an account of
this eclipse see THALES.

--556, May 19, the eclipse of Larissa. The modern tables show that the
eclipse was not total at Larissa, and the connexion of the classical
record with the eclipse is doubtful.

--430, August 3; eclipse mentioned by Thucydides, but not total by the
tables.

--399, June 21; eclipse of Ennius. Totality occurred immediately after
sunset at Rome. The identity of this eclipse is doubtful.

--309, August 14; eclipse of Agathocles. This eclipse would be one of
the most valuable for testing the tables of the moon, but for an
uncertainty as to the location of Agathocles, who, at the time of the
occurrence, was at sea on a voyage from Syracuse to Carthage.

F.K. Ginzel (_Spezieller Kanon der Finsternisse_) has collected a
great number of passages from classical authors supposed to refer to
eclipses of the sun or moon, but the difficulty of identifying the
phenomenon is frequently such as to justify great doubt as to the
conclusions. In a few cases no eclipse corresponding to the
description can be found by our modern table to have occurred, and in
others the latitude of interpretation and the uncertainty of the date
are so wide that the eclipse cannot be identified.

Of medieval eclipses we mention only the dates of those visible in
England, referring for details to the works mentioned in the
bibliography. The letter C following a date shows that the eclipse is
mentioned in the Anglo-Saxon Chronicles. The dates in question are:--

Besides these, the tables show that the shadow of the moon passed over
some part of the British Islands on 1424, June 26; 1433, June 17;
1598, March 6; 1652, April 8; 1715, May 2; 1724, May 22. Of these the
eclipse of 1715 is notable for the careful observations made in
England, and published by Halley in the _Philosophical Transactions_.
The next dates are 1927, June 29, when a barely total eclipse will be
seen soon after sunrise in the northern counties near the Scottish
border, and 1999, August 11, when the moon's shadow will graze England
at Land's End.

We give below, in tabular form, a list of the principal total eclipses during the 19th and 20th centuries, omitting a few visible only in the extreme polar regions, and some others of which the duration is very short. The first column gives the civil date of the point on the earth's surface at which the eclipse is central at noon. The next two columns give the position of this point to the nearest degree. The fourth column shows the Greenwich astronomical time of conjunction in longitude. The next column gives the duration of the total phase at the noon-point; this is sometimes 0.1' less than the absolutely greatest duration at any point. Next is given the node near which the eclipse occurs; and then the number in the Saros. Corresponding eclipses at intervals of 18 y. 11 d. have the same number, and occur near the same node of the noon, which is indicated in the next column.

+----------------+--------------+---------------+---------+------+-------+------------------------------------------------------------+
| | Point where |Greenwich M.T. |Duration | | | |
| Date at | Central at |of conjunction | of | | | |
| Noon-Point. | Noon. | in Longitude. |Totality.| Node |Series.| Regions Swept by Shadow. |
| +------+-------+---------------+---------+ | | |
| | Lat. | Long. | d. h. m. | m. | | | |
+----------------+------+-------+---------------+---------+------+-------+------------------------------------------------------------+
| 1803, Feb. 21 | 11 S.| 136 W.| 21 9 20 | 4.2 | Asc. | 1 | Pacific Ocean, Mexico. |
| 1804, Aug. 5 | 38 S.| 66 W.| 5 4 6 | 1.2 | Desc.| 2 | Pacific Ocean, Chile, Argentina. |
| 1806, June 16 | 42 N.| 66 W.| 16 4 22 | 4.6 | Desc.| 3 | New England, Atlantic, Africa. |
| 1807, Nov. 29 | 11 N.| 2 E.| 28 23 48 | 1.4 | Asc. | 4 | Central Africa, Areolia. |
| 1810, April 4 | 12 N.| 154 E.| 3 13 41 | Ann. | Desc.| 5 | Pacific Ocean, Borneo. |
| 1811, Mar, 24 | 39 S.| 26 W.| 24 2 19 | 3.4 | Desc.| 6 | South Atlantic to and across South Africa. |
| 1814, July 17 | 31 N.| 84 E.| 16 18 33 | 6.6 | Asc. | 7 | Africa, Central Asia, China. |
| 1815, July 6 | 88 N.| 175 W.| 6 11 52 | 3.2 | Asc. | 8 | Polar Regions, Western Siberia. |
| 1816, Nov. 19 | 43 N.| 30 E.| 18 22 9 | 1.8 | Desc.| 9 | Eastern Europe, Central Asia. |
| 1817, Nov. 9 | 7 S.| 149 E.| 8 13 53 | 4.7 | Desc.| 10 | Burma, Pacific Ocean. |
| 1821, Mar. 4 | 8 S.| 96 E.| 3 17 50 | 4.3 | Asc. | 1 | Indian and Pacific Oceans. |
| 1822, Aug. 16 | 36 S.| 176 W.| 16 11 22 | 1.4 | Desc.| 2 | Australia, Pacific Ocean. |
| 1824, June 26 | 47 N.| 175 W.| 26 11 43 | 4.4 | Desc.| 3 | Pacific Ocean, Japan, China. |
| 1825, Dec. 9 | 9 N.| 127 W.| 9 8 27 | 1.5 | Asc. | 4 | Pacific Ocean, Mexico. |
| 1828, April 14 | 18 N.| 39 E.| 13 21 18 | 0.3 | Desc.| 5 | Northern Africa, India. |
| 1829, April 3 | 32 S.| 149 W.| 3 10 24 | 4.1 | Desc.| 6 | South Pacific Ocean. |
| 1832, July 27 | 24 N.| 28 W.| 27 2 2 | 6.8 | Asc. | 7 | West Indies and across Central Africa. |
| 1833, July 17 | 78 N.| 76 E.| 16 19 16 | 3.5 | Asc. | 8 | North-eastern Asia and Polar Regions. |
| 1834, Nov. 30 | 40 N.| 101 W.| 30 6 48 | 1.9 | Desc.| 9 | Southern and Western United States. |
| 1835, Nov. 20 | 10 S.| 20 E.| 19 22 31 | 4.6 | Desc.| 10 | Central Africa, Madagascar. |
| 1839, Mar. 15 | 6 S.| 31 W.| 15 2 14 | 4.4 | Asc. | 1 | South America, Africa, Egypt. |
| 1840, Aug. 27 | 34 S.| 72 E.| 26 18 45 | 1.6 | Desc.| 2 | Africa, Madagascar, Indian Ocean. |
| 1842, July 8 | 51 N.| 77 E.| 7 19 2 | 4.1 | Desc.| 3 | Spain, France, Russia to China, and Pacific Ocean. |
| 1843, Dec. 21 | 8 N.| 102 E.| 20 17 10 | 1.6 | Asc. | 4 | Indian and North Pacific Oceans and India. |
| 1846, April 25 | 25 N.| 75 W.| 25 4 49 | 0.9 | Desc.| 5 | Mexico, West Indies, Africa. |
| 1847, April 15 | 24 S.| 90 E.| 14 18 22 | 4.7 | Desc.| 6 | Indian Ocean, Australia. |
| 1850, Aug. 7 | 18 N.| 142 W.| 7 9 34 | 6.8 | Asc. | 7 | Pacific Ocean. |
| 1851, July 28 | 70 N.| 34 W.| 28 2 41 | 3.7 | Asc. | 8 | Scandinavia, Russia and North America. |
| 1852, Dec. 11 | 37 N.| 127 E.| 10 15 32 | 2.0 | Desc.| 9 | China, Pacific Ocean. |
| 1857, Mar. 25 | 4 S.| 155 W.| 25 10 30 | 4.5 | Asc. | 1 | Pacific Ocean, Mexico. |
| 1858, Sept. 7 | 33 S.| 41 W.| 7 2 16 | 1.7 | Desc.| 2 | Peru, South Brazil, Uruguay. |
| 1860, July 18 | 56 N.| 31 W.| 18 2 21 | 3.7 | Desc.| 3 | British America, France, Egypt. |
| 1861, Dec. 31 | 9 N.| 29 W.| 31 1 55 | 1.8 | Asc. | 4 | Caribbean Sea to North Africa. |
| 1864, May 6 | 32 N.| 173 E.| 5 12 14 | 1.4 | Desc.| 5 | Pacific Ocean. |
| 1865, April 25 | 16 S.| 30 W.| 25 2 13 | 5.3 | Desc.| 6 | Brazil to Central Africa. |
| 1868, Aug. 18 | 10 N.| 103 E.| 17 17 12 | 6.8 | Asc. | 7 | India to Pacific Ocean. |
| 1869, Aug. 7 | 61 N.| 145 W.| 7 10 8 | 3.8 | Asc. | 8 | United States and Alaska. |
| 1870, Dec. 22 | 36 N.| 5 W.| 22 0 19 | 2.1 | Desc.| 9 | Gibraltar, Northern Africa, Sicily. |
| 1871, Dec. 12 | 12 S.| 118 E.| 11 16 2 | 4.4 | Desc.| 10 | Southern India, Northern Australia. |
| 1875, April 6 | 2 S.| 83 E.| 5 18 36 | 4.7 | Asc. | 1 | Indian Ocean, Siam, Pacific. |
| 1876, Sept. 17 | 33 S.| 156 W.| 17 9 54 | 1.8 | Desc.| 2 | Pacific Ocean. |
| 1878, July 29 | 60 N.| 139 W.| 29 9 40 | 3.2 | Desc.| 3 | United States and Canada. |
| 1880, Jan. 11 | 10 N.| 160 W.| 11 10 40 | 2.1 | Asc. | 4 | Pacific Ocean, California. |
| 1882, May 17 | 39 N.| 63 E.| 16 19 34 | 1.8 | Desc.| 5 | Egypt, Central Asia, China. |
| 1883, May 6 | 9 S.| 147 W.| 6 9 58 | 6.0 | Desc.| 6 | Pacific Ocean, Caroline Islands. |
| 1886, Aug. 29 | 3 N.| 14 W.| 29 0 54 | 6.6 | Asc. | 7 | South America, Central Africa. |
| 1887, Aug. 19 | 53 N.| 102 E.| 18 17 39 | 3.8 | Asc. | 8 | Northern Europe, Siberia, Japan. |
| 1889, Jan. 1 | 37 N.| 138 W.| 1 9 8 | 2.2 | Desc.| 9 | California, Oregon, British America. |
| 1889, Dec. 22 | 12 S.| 13 W.| 22 0 52 | 4.2 | Desc.| 10 | Central Africa and South America. |
| 1893, April 16 | 1 S.| 37 W.| 16 2 35 | 4.8 | Asc. | 1 | Venezuela to West Africa. |
| 1894, Sept. 29 | 34 S.| 86 E.| 28 17 43 | 1.8 | Desc.| 2 | East Africa, Indian Ocean. |
| 1896, Aug. 9 | 65 N.| 112 E.| 8 17 2 | 2.7 | Desc.| 3 | North Europe, Siberia, Japan. |
| 1898, Jan. 22 | 13 N.| 69 E.| 21 19 24 | 2.3 | Asc. | 4 | East Africa, India, China. |
| 1900, May 28 | 45 N.| 45 W.| 28 2 50 | 2.1 | Desc.| 5 | United States, Spain, North Africa. |
| 1901, May 18 | 2 S.| 97 E.| 17 17 38 | 6.5 | Desc.| 6 | Sumatra, Borneo. |
| 1904, Sept. 9 | 5 S.| 133 W.| 9 8 43 | 6.4 | Asc. | 7 | Pacific Ocean. |
| 1905, Aug. 30 | 45 N.| 12 W.| 30 1 13 | 3.8 | Asc. | 8 | Canada, Spain, North Africa. |
| 1907, Jan. 14 | 39 N.| 89 E.| 13 17 57 | 2.3 | Desc.| 9 | Russia, Central Asia. |
| 1908, Jan. 3 | 12 S.| 145 W.| 3 9 44 | 4.2 | Desc.| 10 | Pacific Ocean. |
| 1911, April 28 | 1 S.| 155 W.| 28 10 26 | 5.0 | Asc. | 1 | Australia, Polynesia. |
| 1912, Oct. 10 | 35 S.| 33 W.| 10 1 41 | 1.8 | Desc.| 2 | Colombia, Ecuador, Brazil. |
| 1914, Aug. 21 | 71 N.| 2 E.| 21 0 27 | 2.1 | Desc.| 3 | Scandinavia, Russia, Asia Minor. |
| 1916, Feb. 3 | 16 N.| 62 W.| 3 4 6 | 2.5 | Asc. | 4 | Pacific Ocean, Venezuela, West Indies. |
| 1918, June 8 | 51 N.| 152 W.| 8 10 3 | 2.4 | Desc.| 5 | British Columbia, United States. |
| 1919, May 29 | 4 N.| 18 W.| 29 1 12 | 6.9 | Desc.| 6 | Peru, Brazil, Central Africa. |
| 1922, Sept. 21 | 12 S.| 106 E.| 20 16 38 | 6.1 | Asc. | 7 | East Africa, Australia. |
| 1923, Sept. 10 | 38 N.| 128 W.| 10 8 53 | 3.6 | Asc. | 8 | California, Mexico, Central America. |
| 1925, Jan. 24 | 42 N.| 44 W.| 24 2 46 | 2.4 | Desc.| 9 | United States. |
| 1926, Jan. 14 | 10 S.| 82 E.| 13 18 35 | 4.2 | Desc.| 10 | East Africa, Sumatra, Philippines. |
| 1927, June 29 | 78 N.| 84 E.| 28 18 32 | 0.7 | Asc. | 11 | England, Scotland, Scandinavia. |
| 1929, May 9 | 1 S.| 89 E.| 8 18 8 | 5.1 | Asc. | 1 | Sumatra, Malacca, Philippines. |
| 1930, Oct. 21 | 36 S.| 155 W.| 21 9 47 | 1.9 | Desc.| 2 | Pacific Ocean, Patagonia. |
| 1932, Aug. 31 | 78 N.| 109 W.| 31 7 55 | 1.5 | Desc.| 3 | Canada. |
| 1934, Feb. 14 | 19 N.| 168 E.| 13 12 44 | 2.7 | Asc. | 4 | Borneo, Celebes. |
| 1936, June 19 | 56 N.| 101 E.| 18 17 15 | 2.5 | Desc.| 5 | Greece to Central Asia and Japan. |
| 1937, June 8 | 10 N.| 131 W.| 8 8 43 | 7.1 | Desc.| 6 | Pacific Ocean, Peru. |
| 1940, Oct. 1 | 19 S.| 16 W.| 1 0 42 | 5.7 | Asc. | 7 | Colombia, Brazil, South Africa. |
| 1941, Sept. 21 | 30 N.| 114 E.| 20 16 39 | 3.3 | Asc. | 8 | Central Asia, China, Pacific Ocean. |
| 1943, Feb. 4 | 47 N.| 176 W.| 4 11 31 | 2.5 | Desc.| 9 | China, Alaska. |
| 1947, May 20 | 2 S.| 25 W.| 20 1 44 | 5.2 | Asc. | 1 | Argentina, Paraguay, Central Africa. |
| 1948, Nov. 1 | 37 S.| 82 E.| 31 18 3 | 1.9 | Desc.| 2 | Central Africa, Congo. |
| 1952, Feb. 25 | 22 N.| 39 E.| 24 21 17 | 3.0 | Asc. | 4 | Nubia, Persia, Siberia. |
| 1954, June 30 | 62 N.| 5 W.| 30 0 27 | 2.5 | Desc.| 5 | Canada, Scandinavia, Russia, Persia. |
| 1955, June 20 | 15 N.| 117 E.| 19 16 12 | 7.2 | Desc.| 6 | Ceylon, Siam, Philippines. |
| 1958, Oct. 12 | 26 S.| 139 W.| 12 8 52 | 5.2 | Asc. | 7 | Chile, Argentina. |
| 1959, Oct. 2 | 23 N.| 6 W.| 2 0 32 | 3.0 | Asc. | 8 | Canaries, Central Africa. |
| 1961, Feb. 15 | 53 N.| 53 E.| 14 20 11 | 2.6 | Desc.| 9 | France, Italy, Austria, Siberia. |
| 1962, Feb. 5 | 4 S.| 179 E.| 4 12 11 | 4.1 | Desc.| 10 | New Guinea. |
| 1963, July 20 | 62 N.| 126 W.| 20 8 43 | 1.5 | Asc. | 11 | Alaska, Hudson's Bay Territory. |
| 1965, May 30 | 4 S.| 137 W.| 30 9 14 | 5.3 | Asc. | 1 | Pacific Ocean. |
| 1966, Nov. 12 | 38 S.| 43 W.| 12 2 27 | 1.9 | Desc.| 2 | Bolivia, Argentina, Brazil. |
| 1970, Mar. 7 | 25 N.| 88 W.| 7 5 43 | 3.3 | Asc. | 4 | Mexico, Georgia, ? Florida. |
| 1972, July 10 | 67 N.| 111 W.| 10 7 40 | 2.7 | Desc.| 5 | North-East Asia, North-East America and Atlantic Ocean. |
| 1973, June 30 | 19 N.| 6 E.| 29 23 39 | 7.2 | Desc.| 6 | South America, Africa and Atlantic Ocean. |
| 1974, June 20 | 32 S.| 107 E.| 19 16 56 | 5.3 | Desc.| 12 | South-West Australia and Indian Ocean. |
| 1976, Oct. 23 | 31 S.| 95 E.| 22 17 10 | 4.9 | Asc. | 7 | Africa, Australia, Indian and Pacific Oceans. |
| 1977, Oct. 12 | 16 N.| 127 W.| 12 8 31 | 2.8 | Asc. | 8 | Venezuela, Pacific Ocean. |
| 1979, Feb. 26 | 61 N.| 77 W.| 26 4 47 | 2.7 | Desc.| 9 | United States, British America, Pacific Ocean, N. Polar Sea|
| 1980, Feb. 16 | 1 N.| 48 E.| 15 20 52 | 4.3 | Desc.| 10 | Africa, Atlantic and Indian Oceans, and India. |
| 1981, July 31 | 54 N.| 127 E.| 30 15 53 | 2.2 | Asc. | 11 | Pacific Ocean, Asia. |
| 1983, June 11 | 7 S.| 111 E.| 10 16 38 | 5.4 | Asc. | 1 | Java, Atlantic Ocean. |
| 1984, Nov. 22 | 39 S.| 170 W.| 22 10 58 | 2.1 | Desc.| 2 | Pacific Ocean, Patagonia. |
| 1987, Mar. 29 | 17 S.| 6 W.| 29 0 45 | 0.3 | Asc. | 13 | Atlantic, Equatorial Africa. |
| 1988, Mar. 18 | 28 N.| 146 E.| 17 14 3 | 4.0 | Asc. | 4 | Indian and Pacific Oceans, Sumatra. |
| 1990, July 22 | 72 N.| 142 E.| 21 14 54 | 2.6 | Desc.| 5 | Finland, North Atlantic. |
| 1991, July 11 | 22 N.| 105 W.| 11 7 6 | 7.1 | Desc.| 6 | Pacific Ocean, Hawaii, Central America. |
| 1992, June 30 | 26 S.| 5 W.| 30 0 19 | 5.4 | Desc.| 12 | South Atlantic. |
| 1994, Nov. 3 | 36 S.| 31 W.| 3 1 36 | 4.6 | Asc. | 7 | Pacific Ocean, South America. |
| 1995, Oct. 24 | 10 N.| 110 E.| 23 16 37 | 2.4 | Asc. | 8 | Pacific and Indian Oceans. |
| 1997, Mar. 9 | 71 N.| 154 E.| 8 13 16 | 2.8 | Desc.| 9 | North-East Asia, Arctic Sea. |
| 1998, Feb. 26 | 6 N.| 81 W.| 26 5 27 | 4.4 | Desc.| 10 | Pacific and Atlantic Oceans, Central America. |
| 1999, Aug. 11 | 46 N.| 18 E.| 10 23 8 | 2.6 | Asc. | 11 | Central and Southern Europe touching England. |
+----------------+------+-------+---------------+---------+------+-------+------------------------------------------------------------+

_Recurrence of Remarkable Eclipses._

From the property of the Saros it follows that eclipses remarkable for their duration, or other circumstances depending on the relative positions of the sun and moon, occur at intervals of one saros (18 y. 11 d.). Of interest in this connexion is the recurrence of total eclipses remarkable for their duration. The absolute maximum duration of a total eclipse is about 7' 30"; but no actual eclipse can be expected to reach this duration. Those which will come nearest to the maximum during the next 500 years belong to the series numbered 4 and 6 and in the list which precedes. These occurring in the years 1937, 1955, &c., will ultimately fall little more than 20" below the maximum. But the series 4, though not now remarkable in this respect, will become so in the future, reaching in the eclipse of June 25, 2150, a duration of about 7' 15" and on July 5, 2168, a duration of 7' 28", the longest in human history. The first of these will pass over the Pacific Ocean; the second over the southern part of the Indian Ocean near Madras.

All the national annual Ephemerides contain elements of the eclipses of the sun occurring during the year. Those of England, America and France also give maps showing the path of the central line, if any, over the earth's surface; the lines of eclipse beginning and ending at sunrise, &c., and the outlines of the shadow from hour to hour. By the aid of the latter the time at which an eclipse begins or ends at any point can be determined by inspection or measurement within a few minutes.

V. _Methods of computing Eclipses of the Sun._

Elements of eclipses.

The complete computation of the circumstances of an eclipse ab initio requires three distinct processes. The geocentric positions of the sun and moon have first to be computed from the tables of the motions of those bodies. The second step is to compute certain elements of the eclipse from these geocentric positions. The third step is from these elements to compute the circumstances of the eclipse for the earth generally or for any given place on its surface. The national Astronomical Ephemerides, or "Nautical Almanacs," give in full the geocentric positions of the sun and moon from at least the early part of the 19th century to an epoch three years in advance of the date of publication. It is therefore unnecessary to undertake the first part of the computation except for dates outside the limits of the published ephemerides, and for many years to come even this computation will be unnecessary, because tables giving the elements of eclipses from the earliest historic periods up to the 22nd century have been published by T. Ritter von Oppolzer and by Simon Newcomb. We shall therefore confine ourselves to a statement of the eclipse problem and of the principles on which such tables rest.

Two systems of eclipse elements are now adopted in the ephemerides and tables; the one, that of F.W. Bessel, is used in the English, American and French ephemerides, the other--P. A. Hansen's--in the German and in the eclipse tables of T. Ritter von Oppolzer. The two have in common certain geometric constructions. The fundamental axis of reference in both systems is the line passing through the centres of the sun and moon; this is the common axis of the shadow cones, which envelop simultaneously the sun and moon as shown in figs. 1, 2, 3. The surface of one of these cones, that of the umbra, is tangent to both bodies externally. This cone comes to a point at a distance from the moon nearly equal to that of the earth. Within it the sun is wholly hidden by the moon. Outside the umbral cone is that of the penumbra, within which the sun is partially hidden by the moon. The geometric condition that the two bodies shall appear in contact, or that the eclipse shall begin or end at a certain moment, is that the surface of one of these cones shall pass through the place of the observer at that moment. Let a plane, which we call the fundamental plane, pass through the centre of the earth perpendicular to the shadow axis. On this plane the centre of the earth is taken as an origin of rectangular co-ordinates. The axis of Z is perpendicular to the plane, and therefore parallel to the shadow axis; that of Y and X lie in the plane. In these fundamental constructions the two methods coincide. They differ in the direction of the axis of Y and X in the fundamental plane. In Bessel's method, which we shall first describe, the intersection of the plane of the earth's equator with the fundamental plane is taken as the axis of X. The axis of Y is perpendicular to it, the positive direction being towards the north. The Besselian elements of an eclipse are then:--x, y, the co-ordinates of the shadow axis on the fundamental plane; d, the declination of that point in which the shadow axis intersects the celestial sphere; [mu], the Greenwich hour angle of this point; l, the radius of the circle, in which the penumbral or outer cone intersects the fundamental plane; and l', the radius of the circle, in which the inner or umbral cone intersects this plane, taken positively when the vertex of the cone does not reach the plane, so that the axis must be produced, and negatively when the vertex is beyond the plane.

Hansen's method differs from that of Bessel in that the ecliptic is taken as the fundamental plane instead of the equator. The axis of X on the fundamental plane is parallel to the plane of the ecliptic; that of Y perpendicular to it. The other elements are nearly the same in the two theories. As to their relative advantages, it may be remarked that Hansen's co-ordinates follow most simply from the data of the tables, and are necessarily used in eclipse tables, but that the subsequent computation is simpler by Bessel's method.

Several problems are involved in the complete computation of an eclipse from the elements. First, from the values of the latter at a given moment to determine the point, if any, at which the shadow-axis intersects the surface of the earth, and the respective outlines of the umbra and penumbra on that surface. Within the umbral curve the eclipse is annular or total; outside of it and within the penumbral curve the eclipse is partial at the given moment. The penumbral line is marked from hour to hour on the maps given annually in the American Ephemeris. Second, a series of positions of the central point through the course of an eclipse gives us the path of the central point along the surface of the earth, and the envelopes of the penumbral and umbral curves just described are boundaries within which a total, annular or partial eclipse will be visible. In particular, we have a certain definite point on the earth's surface on which the edge of the shadow first impinges; this impingement necessarily takes place at sunrise. Then passing from this point, we have a series of points on the surface at which the elements of the shadow-cone are in succession tangent to the earth's surface. At all these points the eclipse begins at sunrise until a certain limit is reached, after which, following the successive elements, it ends at sunrise. At the limiting point the rim of the moon merely grazes that of the sun at sunrise, so that we may say that the eclipse both begins and ends at that time. Of course the points we have described are also found at the ending of the eclipse. There is a certain moment at which the shadow-axis leaves the earth at a certain point, and a series of moments when, the elements of the penumbral cone being tangent to the earth's surface, the eclipse is ending at sunset. Three cases may arise in studying the passage of the outlines of the shadow over the earth. It may be that all the elements of the penumbral cone intersect the earth. In this case we shall have both a northern and a southern limit of partial eclipse. In the second case there will be no limit on the one side except that of the eclipse beginning or ending at sunrise or sunset. Or it may happen, as the third case, that the shadow-axis does not intersect the earth at all; the eclipse will then not be central at any point, but at most only partial.

The third problem is, from the same data, to find the circumstances of an eclipse at a given place--especially the times of beginning and ending, or the relative positions of the sun and moon at a given moment. Reference to the formulae for all these problems will be given in the bibliography of the subject.

AUTHORITIES.--The richest mine of information respecting eclipses of
the sun and moon is T.R. von Oppolzer's "Kanon der Finsternisse,"
published by the Vienna Academy of Sciences in the 52nd volume of its
_Denkschriften_ (Vienna, 1887). It contains elements of all eclipses
both of the sun and moon, from 1207 B.C. to A.D. 2161, a period of
more than thirty centuries. Appended to the tables is a series of
charts showing the paths of all central eclipses visible in the
northern hemisphere during the period covered by the table. The points
of the path at which the eclipse occurs, at sunrise, noon and sunset,
are laid down with precision, but the intermediate points are
frequently in error by several hundred miles, as they were not
calculated, but projected simply by drawing a circle through the three
points just mentioned. For this reason we cannot infer from them that
an eclipse was total at any given place. The correct path can,
however, be readily computed from the tables given in the work. Eduard
Mahler's memoir, "Die centralen Sonnenfinsternisse des 20.
Jahrhunderts" (_Denkschriften_, Vienna Academy, vol. xlix.), gives
more exact paths of the central eclipses of the 20th century, but no
maps. General tables for computing eclipses are Oppolzer's
"Syzygientafeln fur den Mond" (Publications of the _Astronomische
Gesellschaft_, xvi.), and Newcomb's, in _Publications of the American
Ephemeris_, vol. i. part i. Of these, Oppolzer's are constructed with
greater numerical accuracy and detail, while Newcomb's are founded on
more recent astronomical data, and are preferable for computing
ancient eclipses. F.K. Ginzel's _Spezieller Kanon der Sonnen- und
Mondfinsternisse_ (Berlin, 1899) contains, besides the historical
researches already mentioned, maps of the paths of central eclipses
visible in the lands of classical antiquity from 900 B.C. to A.D. 500,
but computed with imperfect astronomical data. Maguire, "Monthly
Notices," _R.A.S._ xlv. and xlvi., has mapped the total solar eclipses
visible in the British Islands from 878 to 1724. General papers of
interest on the same subject have been published by Rev. S.J. Johnson.
A resume of all the observations on the physical phenomena of total
solar eclipses up to 1878, by A.C. Ranyard, is to be found in _Memoirs
of the Royal Astronomical Society_, vol. xli. A very copious
development of the computation of eclipses by Bessel's method is found
in W. Chauvenet's _Spherical and Practical Astronomy_, vol. i. _The
Theory of Eclipses_, by R. Buchanan (Philadelphia, 1904), treats the
subject yet more fully. Hansen's method is developed in the
_Abhandlungen_ of the Leipzig Academy of Sciences, vol. vi.
(Math.-Phys. Classe, vol. iv.). The formulae of computation by this
method are found in the introductions to Oppolzer's two works cited
above. (S. N.)

ECLIPTIC, in astronomy. The plane of the ecliptic is that plane in or near which the centre of gravity of the earth and moon revolves round the sun. The ecliptic itself is the great circle in which this plane meets the celestial sphere. It is also defined, but not with absolute rigour, as the apparent path described by the sun around the celestial sphere as the earth performs its annual revolution. Owing to the action of the moon on the earth, as it performs its monthly revolution in an orbit slightly inclined to the ecliptic, the centre of the earth itself deviates from the plane of the ecliptic in a period equal to that of the nodal revolution of the moon. The deviation is extremely slight, its maximum amount ranging between 0.5' and 0.6". Owing to the action of the planets, especially Venus and Jupiter, on the earth, the centre of gravity of the earth and moon deviates by a yet minuter amount, generally one or two tenths of a second, from the plane of the ecliptic proper. Owing to the action of the planets, the position of the ecliptic is subject to a slow secular variation amounting, during our time, to nearly 47" per century. The rate of this motion is slowly diminishing.

The obliquity of the ecliptic is the angle which its plane makes with that of the equator. Its mean value is now about 23 deg. 27'. The motion of the ecliptic produces a secular variation in the obliquity which is now diminishing by an amount nearly equal to the entire motion of the ecliptic itself. The laws of motion of the ecliptic and equator are stated in the article PRECESSION OF THE EQUINOXES.

Attempts have been made by Laplace and his successors to fix certain limits within which the obliquity of the ecliptic shall always be confined. The results thus derived are, however, based on imperfect formulae. When the problem is considered in a rigorous form, it is found that no absolute limits can be set. It can, however, be shown that the obliquity cannot vary more than two or three degrees within a million of years of our epoch.

The formula for the obliquity of the ecliptic, as derived from the
laws of motion of it and of the equator, may be developed in a series
proceeding according to the ascending powers of the time as follows:
we put T, the time from 1900, reckoned in solar centuries as a unit.
Then,

Obliquity = 23 deg. 27' 31.68" - 46.837" T - 0.0085" T^2 + 0.0017" T^3.

From this expression is derived the value of the obliquity at various
epochs given in the following table. The left-hand portion of this
table gives the values for intervals of 500 years from 2000 B.C. to
A.D. 2500 as computed from modern data. For dates more than three or
four centuries before or after 1850 the result is necessarily
uncertain by one or more tenths of a minute, and is therefore only
given to 0.1'.

B.C. 2000; obl. = 23 deg. 55.5" A.D. 1700; obl. = 23 deg. 28' 41.91"
1500 " = 23 52.3 1750 " = 23 28 18.51
1000 " = 23 48.9 1800 " = 23 27 55.10
500 " = 23 45.4 1850 " = 23 27 31.68
0 " = 23 41.7 1900 " = 23 27 8.26
A.D. 500 " = 23 38.0 1950 " = 23 26 44.84
1000 " = 23 34.1 2000 " = 23 26 21.41
1500 " = 23 30.3 2050 " = 23 25 57.99
2000 " = 23 26.4 2100 " = 23 25 34.56
2500 " = 23 22.5
(S. N.)

ECLOGITE (from Gr. [Greek: ekloge], a selection), in petrology, a typical member of a small group of metamorphic rocks of special interest on account of the variety of minerals they contain and their microscopic structures and geological relationships. Typically they consist of pale green or nearly colourless augite (omphacite), green hornblende and pink garnet. Quartz also is usually present in these rocks, but felspar is rare. The augite is mostly a variety of diopside and is only occasionally idiomorphic. The garnet sometimes forms good dodecahedra, but may occur as rounded grains, and encloses quartz, rutile, kyanite, and other minerals very frequently. The hornblende is usually pale green and feebly dichroic, but, in some eclogites which are allied to garnet-amphibolites, it is of dark brown colour. Among the commoner accessory minerals are kyanite (of blue or greyish-blue tints), rutile, biotite, epidote and zoisite, sphene, iron oxides, and pyrites. The rutile is invariably in small brown prisms; the kyanite forms bladed crystals, with perfect cleavage; felspar, if present, belongs to basic varieties rich in lime. Other minerals which have been found in eclogites are bronzite, olivine and glaucophane. The last mentioned is a bright blue variety of hornblende with striking pleochroism. The eclogites in their chemical composition show close affinities to gabbros; they often exhibit relationships in the field which show that they were primarily intrusive rocks of igneous origin, and occasionally contact alteration can be traced in the adjacent schists. Examples are known in Saxony, Bavaria, Carinthia, Austria, Norway. A few eclogites also occur in the north-west highlands of Scotland. Glaucophane-eclogites have been met with in Italy and the Pennine Alps. Specimens of rock allied to eclogite have been found in the diamantiferous peridotite breccias of South Africa (the so-called "blue ground"), and this has given rise to the theory that these are the parent masses from which the Kimberley diamonds have come. (J. S. F.)

ECLOGUE, a short pastoral dialogue in verse. The word is conjectured to be derived from the Greek verb [Greek: eklegein], to choose. An eclogue, perhaps, in its primary signification was a selected piece. Another more fantastic derivation traces it to [Greek: aix], goat, and [Greek: logos], speech, and makes it a conversation of shepherds. The idea of dialogue, however, is not necessary for an eclogue, which is often not to be distinguished from the idyll. The grammarians, in giving this title to Virgil's pastoral conversations (_Bucolica_), tended to make the term "eclogue" apply exclusively to dialogue, and this has in fact been the result of the success of Virgil's work. Latin eclogues were also written by Calpurnius Siculus and by Nemesianus. In modern literature the term has lost any distinctive character which it may have possessed among the Romans; it is merged in the general notion of pastoral poetry. The French "Eglogues" of J.R. de Segrais (1624-1701) were long famous, and those of the Spanish poet Garcilasso de La Vega (1503-1536) are still admired.

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