CHAPTER X. OCCULTATIONS. How caused.-Table annually given in the "Nautical Almanac."-Occultation by a young Moon.-Effect of the Horizontal Parallax.-Projection of Stars on the Moon's disc.-Occultation of Saturn, May 8, 1859.-Occultation of Jupiter, January 2, 1857.-Historical notices. W THEN any celestial object is concealed by the interposition of another, it is said to be occulted, and the phenomenon is called an occultation. Strictly speaking, an eclipse of the Sun is an occultation of that luminary by the Moon, but usage has given to it the exceptional name of "eclipse." The most important phenomena of this kind are the occultations of the planets and larger stars by the Moon, but the occultation of one planet by another, on account of the rarity of such an occurrence, is exceedingly interesting. Inasmuch as the Moon's apparent diameter is about 40, it follows that all stars and planets situated in a zone extending on each side of her path will necessarily be occulted during her monthly course through the ecliptic. The great brilliancy of the Moon entirely overpowers the smaller stars, but the disappearances of the more conspicuous ones can be observed in a telescope, and a table of them is inserted every year in the Nautical Almanac. It must be remembered that the disappearance always takes place at the limb of the Moon which is presented in the direction of its motion. From the epoch of its New to that of its Full phase, the Moon moves with the dark edge foremost, and from the epoch of its Full to that of its New phase with the illuminated edge foremost during the former interval, therefore, the objects occulted disappear at the dark edge, and reappear at the illuminated edge; and during the latter period they disappear at the illuminated, and reappear at the dark edge. If the occultation be watched when the star disappears on the dark side of the Moon, that is to say during the first half of a lunation, and preferably when the Moon is not more than 2 or 3 days old, the disappearance is extremely striking, inasmuch as the object occulted seems to be suddenly extinguished at a point of the sky where there is nothing apparently to interfere with it. Wargentin relates that on May 18, 1761, he saw an occultation of a star by the Moon during a total eclipse of the latter. He says that the star disappeared more quickly than the twinkling of an eye a." In consequence of the effect of parallax, the Moon, as seen in the northern hemisphere, follows a path different from that which it appears to take as seen in the southern hemisphere; it happens, therefore, that stars which are occulted in certain latitudes are not occulted at all in others, and of those which are occulted the duration of invisibility, and the moment and place of disappearance and reappearance, are different. I must not omit a passing allusion to a circumstance occasionally noticed by the observers of occultations; namely, the apparent projection of the star within the margin of the Moon's disc. Admiral Smyth gives an instance, under the date of October 15, 1829. He says: "I saw Aldebaran approach the bright limb of the Moon very steadily; but from the haze no alteration in the redness of its colour was perceptible. It kept the same steady line to about of a minute inside the lunar disc, where it remained, as precisely as I could estimate, 2 seconds, when it suddenly vanished. In this there could be no mistake, because I clearly saw the bright line of the Moon outside the star, as did also Dr. Lee, who was with me "." Sir T. Maclear saw the same thing happen to the same star on October 23, 1831: "Previous to the contact of the Moon and star nothing particular occurred; but at that moment, and when I might expect the star to immerge, it advanced upon the Moon's limb for about 3 seconds and to rather more than the star's apparent diameter, and then instantly disappeared "." Phil. Trans., vol. li. p. 210. 1761. b Mem. R.A.S., vol. iv. p. 642. Other observers, Maclear included, saw the projection, though F. Baily and others did not see it. e Ibid., vol. v. p. 273. "This phenomenon seems to be owing to the greater proportionate refrangibility of the white lunar light, than that of the red light of the star, elevating her apparent disc at the time and point of contact "." In 1699 La Hire endeavoured to explain the apparition of stars on the Moon's disc by supposing that the true disc is accompanied by a parasitic light, or, as it was formerly termed, a circle of dissipation, which enlarges the star's apparent diameter, and through which it shews itself before passing behind the opaque part of the lunar globe. Arago accepts this theory with the explanation that the observer's eye-piece must be in imperfect focus, and that so the false disc is caused. The fact that some have and some have not seen the phenomenon he considered confirmatory of this explanation. A remarkable occurrence was noticed by Mr. Ralph Copeland, on the occasion of the occultation of κ Cancri on April 26, 1863: "About three-fourths of the light disappeared in the usual instantaneous manner; and after an interval of (as near as I can judge) rather more than half a second, the remaining portion disappeared." Dawes regarded this as a decisive indication of the star being Mr. W. Simms, Sen. thus describes it : "The only remarkable appearance noticed by me during the emersion was the very positive line by which the Moon's limb was marked upon the planet; dark as the mark of a black-lead pencil close to the limb, and gradually softened off as the distance increased ." A representation of this appearance, from a drawing by Lassell, is prefixed [Fig. 80]. An occultation of the planet Saturn by the Moon took place on May 8, 1859. Mr. Dawes thus describes it : "At the disappearance, the dark edge of the Moon was sharply defined on the rings and ball of the planet, without the slightest distortion of their figure. There was no extension of light along the Moon's limb. Even the satellites disappeared without the slightest warning, and precisely at the edge which was faintly visible. "At the reappearance I could not perceive any dark shading contiguous to the Moon's bright edge, such as was seen by myself and several other observers on Jupiter on January 2, 1858 [Qy. 1857]. The dark belt south of the planet's equator was clearly defined up to the very edge; and there was no distortion of any kind, either of the rings or ball. "The very pale greenish hue of Saturn contrasted strikingly with the brilliant yellowish light of the Moon." Mr. W. Simms, Jun. did see a dark shading on the planet contiguous to the Moon's bright edge; but in 1857 he failed to notice it. In an occultation of Saturn on Oct. 30, 1825, Messrs. R. Comfield and J. Wallis plainly saw both one ansa and the ball flattened. The earliest phenomenon of this kind of which we have any mention is an occultation of Mars by the Moon, mentioned by Aristotle. Kepler found that it occurred on the night of April 4, 357 B.c.k Instances are on record of one planet occulting another, but these are of very rare occurrence. Kepler states that he watched an occultation of Jupiter by Mars on January 9, 1591. He also mentions that Mostlin witnessed an occultation. of Mars by Venus on October 3, 1590. Mercury was occulted by Venus on May 17, 17371. As these observations, with the exception of the last, were made before the invention of the telescope, 8 Month. Not. R.A.S., vol. xvii. p. 81. h Ibid., vol. xix. p. 241. Other observations will be found at p. 238 of the same volume. i Mem. R.A.S., vol. ii. p. 457. k Ad. Vitell. Paralipom., p. 307. it is possible that the one planet was not actually in front of the other, but only that they were so close together as to have the appearance of being one object: as was the case with Venus and Jupiter on July 21, 1859. Sometimes stars are occulted by planets. J. D. Cassini mentions the occultation of a star in Aquarius by Mars on October 1, 1672 m. m See a paper on Occultations by A. C. Twining in Amer. Journ. of Science, July, 1858. |