9,192,631,771 oscillations of a cesium 

 atom, and the rotation of the earth has 

 ceased to be the standard for time. 



This may seem a bit esoteric if all you 

 want is a three-minute egg, but June's leap 

 second is heady news in the world of com- 

 munications and navigation, where 

 nanosecond accuracy is needed. "In elec- 

 tronic navigation, a time error of a mil- 

 honth of a second can produce a position 

 eiTor of about a quarter of a mile. Get your 

 celestial timing wrong and spacecraft will 

 sail past planets, missiles can fall in the 

 wrong places, and jets can land short of 

 the runway," explains Klepczynski. "Leap 



seconds are the earth's way of keeping us 

 on our toes." 



The Planets in June 



Mercury is visible in the early 

 evenings, low in the southwest at the be- 

 ginning of the month, before slowly fad- 

 ing from view. The planet reaches inferior 

 conjunction (between the earth and the 

 sun) on the 25th. 



Venus bedazzles us this month, shining 

 in the western evening skies at a brilliant 

 -4 magnitude (htore than seventeen times 

 brighter than Sirius, the brightest star in 

 the sky). On the 10th, Venus passes the 



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first-magnitude star Pollux in Gemini, and 

 on the 12th passes by the waxing first- 

 quarter moon. 



Mars rises a couple of hours before the 

 sun in Aries and can be spotted in the 

 southeast just before sunrise. On the 6th, 

 watch as the old crescent moon passes just 

 a few degrees above the ruddy planet. 

 NASA has recently announced that it will 

 continue its exploration of Mars by 

 launching a small orbiter in 1996 to study 

 the surface of the planet. The spacecraft 

 will carry half the science payload that 

 flew on the Mars Observer, which was lost 

 last August. 



Jupiter is nicely placed in Virgo. It is 

 visible as a very bright, silvery-white 

 "star," well up toward the south at sunset, 

 and sets after midnight. On the 18th, the 

 giant planet can be found just north of the 

 waxing third-quarter moon. The refur- 

 bished Hubble telescope's new images of 

 Comet Shoemaker-Levy — due to crash 

 into Jupiter next months — show that the 

 comet is continuing to break up. The 

 largest of its fragments appears to measure 

 2.5 miles in diameter. 



Saturn rises about midnight among the 

 faint stars of Aquarius and is well placed 

 in the southern sky for observing until 

 dawn. On June 1, and again on the 28th, 

 look for Saturn well below the moon — a 

 pretty sight on a warm summer morning. 



Uranus and Neptune are the two blue 

 green worlds that can be seen with binoc- 

 ulars and the help of a detailed sky chart 

 just east of the teapot-shaped constellation 

 Sagittarius, rising one and a half hours 

 after sunset. 



Pluto is not far from Jupiter, just above 

 the star that marks the north "claw" of the 

 scorpion (now part of the constellation 

 Libra), but it is impossible to see without a 

 large telescope. 



The Moon reaches last quarter on the 

 1st at 12:02 a.m., EDT, and again on the 

 30th at 3:31 p.m., EDT. The moon is new 

 on the 9th at 4:26 a.m., EDT, and reaches 

 first quarter at 3:56 p.m., EDT, on 16th. 



The summer solstice occurs at 10:48 

 A.M., EDT, on the 21st, marking the begin- 

 ning of summer in the Northern Hemi- 

 sphere. The sun at noon reaches its highest 

 point in the sky for the year and then be- 

 gins its relentless march south. Although 

 the earth is almost at its farthest distance 

 from the sun, it is tilted so that the North- 

 em Hemisphere most directiy faces the 

 sun. 



Gail S. Cleere lives in Washington, D.C., 

 and writes on popular astronomy. 



88 Natural History 6/94 



