Occultations of Mars and Venus

Abstract

IF the evening sky is clear on July 17, an interesting observation can be made without a telescope (though, preferably, slight optical aid is desirable) of the occultation of the planet Mars by the moon. The disappearance of Mars, as seen from a station near Greenwich, takes place at the moon's dark limb (the age of the moon being 9·7 days) at position angle 53 ° from the north point of the disk, measured eastwards, at 21h 13m U.T. (22h 13m Summer Time). The reappearance of Mars at the moon's bright limb is at position angle 326 ° at 22h 10m U.T. or 23h 10m Summer Time. The apparent diameter of Mars is about 14" and its stellar magnitude – 0·8. A map in the B.A.A. Handbook for 1937 gives the limits on the earth's surface of the visibility of this occultation. The occultation of Venus by the moon on August 3 takes place in full daylight and cannot, in any event, be seen from southern England. The southern limit of visibility of partial occultation, as given in J. Brit. Ast. Assoc., March, p. 187, runs approximately between Hull, Bradford and a little north of Blackpool ; the southern limit of total occultation runs from just south of Bridlington, to Borough Bridge and just north of Carnforth. At Edinburgh, the disappearance of Venus is due at 8h 26m U.T. at position angle 159 ° from the north point of the moon's disk, and the reappearance at position angle 212 ° at 9h lm (add lh to convert to Summer Time). The phenomenon will require telescopic aid generally, though Venus can on occasions be 'picked up' in daylight by keen-sighted observers. The moon's age is 26·2 days, so that the crescent is a very narrow one. Venus is in its gibbous phase, the ratio of the illuminated area of the disk to that of the entire apparent disk (17"in diameter) being 0·67. Its stellar magnitude is – 3·6. The position of Venus on August 3 at 9h is R.A. 5h 53m·0; Dec. + 21 ° 20'.