of light, which most distinctly marked the limb of that portion of the planet, and which was doubtless produced by the refraction of sunlight passing through the planet's atmosphere. The effect was very beautifuld."

This illuminated streak, but far less sharply defined than

Prince saw it, was also observed in America by Prof. S. P. Langley, who says:

"It was therefore watched by me, with occasional interruptions, for about 7m. Owing to the boiling of the limb, it was not easy to determine how much of this light lay without, how much within, the planet's contour. When first seen, it suggested for a moment the appearance of Baily's Beads, but the writer's very strong final impression was that it at any rate extended to some degree within the planet, and was brightest on the outside, with a slight gradation toward the planet's centre. Its greatest width was estimated at one-fourth of the planet's radius. Every precaution was taken against instrumental error. The spot was successively examined in different parts of the field, the eye-piece was rotated, and the amount of light

d Month. Not., vol. xliii. p. 64. Dec. 1882.

A a

from the reflectors was varied. It was beyond any question a real, if a most unexpected and unintelligible phenomenon, and it seems to me that it points to a real local cause on the planet. It does not appear to be at all assimilable to the concentric spots which some observers have believed they saw both on Venus and on Mercury in transit, nor to the alleged phosphorescence on the dark side."

This phenomenon, with variations of detail, was seen by Brodie (by whom it was assumed to be a twilight effect resulting from an atmosphere on Venus), by Horners, and probably by others.

Figs. 169-171 were drawn by M. Hatt at Chubut, and represent the phenomena seen at ingress. The observer seems to have been much struck with the appearance presented by the fringe of light which surrounded the planet just before the end of the internal contact.

e Month. Not., vol. xliii. p. 72. Jan. 1883. ↑ Ibid., p. 76.

Ibid., p. 277

CHAPTER XI.

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 Jupiter, January 2, 1857.-Occultation of Saturn, May 8, 1859-Occultation of Saturn, April 9, 1883.-Historical notices.

W

HEN 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 special 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, 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, and parallax will have the effect of further increasing considerably the breadth of the zone of stars subject to occultation. The great brilliancy of the Moon entirely overpowers the smaller stars, but the disappearances of the more conspicuous ones can be observed with 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 is suddenly extinguished at a point of the sky where there seems nothing 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." 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 b."

a Phil. Trans., vol. li. p. 210. 1761. b Mem. R.A.S., vol. iv. p. 642. 1831. Other observers, Maclear included, saw

the projection, though F. Baily and others did not see it.