. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. withoutbeing eclipsed: and therefore there must be at least two solareclipses in a year. The greatest number that can occur is greatest number of lunar eclipses in the year is three, and 144 OCCULTATIONS. there may be none at all. The greatest number of both kindi of eclipses in a year is seven ; the usual number is four. Although the annual number of solar eclipses throughout thewhole earth is the greater, yet at any one place more lunar eclipsesare visible than solar. The reason of this is th
. Elements of theoretical and descriptive astronomy, for the use of colleges and academies. withoutbeing eclipsed: and therefore there must be at least two solareclipses in a year. The greatest number that can occur is greatest number of lunar eclipses in the year is three, and 144 OCCULTATIONS. there may be none at all. The greatest number of both kindi of eclipses in a year is seven ; the usual number is four. Although the annual number of solar eclipses throughout thewhole earth is the greater, yet at any one place more lunar eclipsesare visible than solar. The reason of this is that a lunar eclipse,when it does occur, is visible over an entire hemisphere, whilethe area within which a solar eclipse is visible is very muchmore limited. OCCULTATIONS. 164. An occultation of a planet or a star will occur wheneverthe planet or star is so situated in latitude as to allow the moonto come in between it and the earth. In order to determine thelimit of a planets latitude within which an occultation of theplanet is possible, let us refer to Fig. 61. In this figure, E is the. Fig. 61. centre of the earth, P that of a planet, and if that of the occultation will occur when the moon comes between thetangent lines GB and AH. Let EG be the plane of the is then the geocentric latitude of the planet, and MECthat of the have, PEG = PEG + GED + DEM + MECt and also, GED == EDB — , PEG is the planets semi-diameter, EGD its horizontalparallax, DEM the moons semi-diameter, EDB its horizontalpaiallax, and MEG, as above stated, its latitude. The value OCCTJLTATIONS. 145 of PEC, therefore, can very readily be obtained. If P, insteadof representing a planet, represents a star, the distance PEbecomes so great that AH and BO are sensibly parallel, andthe stars parallax and semi-diameter reduce to zero. In thiscase the greatest value of PEG, within which an occultationcan occur, will be the sum of 5° 20 6, 61 32, and 16 45,which is 6° 38 23. Sinc
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