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SKYLOG BY JOE RAO 



n October we have good views of 

 four of the five brightest planets: 

 Venus and Jupiter in the evening sky, 

 Mercury and Saturn in the morning 

 sky. (The fifth, Mars, has disappeared 

 into the evening twilight.) 



The relative brightness of celestial 

 objects from our earthly viewpoint is 

 expressed in terms of "magnitudes," a 

 scale whose roots go back to Hippar- 

 chus ot Nicaea, a second-century B.C. 

 mathematician, philosopher, and as- 

 tronomer. Based upon his naked-eye 

 observations, Hipparchus categorized 

 stars into six levels of brightness, with 

 the brightest as first magnitude and 

 the faintest as sixth magnitude. 



By the nineteenth century, astron- 

 omers could measure a star's appar- 

 ent brightness objectively, but they 

 retained the magnitude system, re- 

 fining and quantifying it. If, for ex- 

 ample, two stars are one magnitude 

 apart, the brighter star is 2.512 times 

 brighter than the dimmer star. The 

 difference between a first-magni- 

 tude and a sixth-magnitude star thus 

 works out to be 2.512 3 , or a hundred 

 times difference in intensity. Today, 

 we also measure brilliant objects with 

 zero or even negative magnitudes. 

 The full Moon is rated -12.6, and 

 the Sun —26.7. At the other extreme, 

 telescopes enable us to see objects 

 much fainter than magnitude 6, and 

 we can detect even fainter ones by 

 using long photographic exposures. 



At its brightest, Venus reaches 

 —4.7; this month it shines at magni- 

 tude —4. Look for the planet thirty 

 or forty minutes after sundown, quite 



OCTOBER NIGHTS OUT 



7 The Moon waxes to first quarter at 

 5:04 a.m. eastern daylight time (EDT). 



14 The Moon becomes full at 4:02 p.m. 

 EDT. Coming after the Harvest Moon, 

 which in the Northern Hemisphere is the 

 full Moon nearest the autumnal equinox, 

 this one is known as the "Hunter's 

 Moon." 



21 The Moon wanes to last quarter at 

 7:55 a.m. EDT, but is still bright enough to 



Hipparchus (ca. 190-120 b c ) maps stars 

 over Alexandria, in a nineteenth-century 

 woodcut. 



low in the southwest. Jupiter stands 

 watch at magnitude —2.2 in the 

 southwest at nightfall. 



Mercury passes inferior conjunc- 

 tion (sweeping between the Sun 

 and Earth in its orbit) on October 6, 

 and soon after becomes a feature of 

 the morning sky. By the 14th, shin- 

 ing at magnitude 1, the planet will 

 rise close to due east, more than an 

 hour before the Sun. On October 

 22, having brightened to magnitude 

 —0.6, Mercury reaches its great- 

 est western elongation, or apparent 

 distance to the west of the Sun, and 

 rises a minute or two before the first 

 glimmer of morning twilight. 



Saturn rises north of due east two 

 hours before sunrise on October 1, 

 and more than four hours before sun- 

 rise by the end of the month. At mag- 

 nitude 1, the planet is faint, mainly 

 because its ring system is turning 

 almost edge-on toward Earth. 



Joe Rao ('hometown.aol.com/skywayincj 

 is a broadcast meteorologist and an associate 

 and lecturer at the Hayden Planetarium in 

 New York City. 



reduce the effect of the Orionid me- 

 teor shower, which peaks this morning 

 before sunrise. The "shooting stars" 

 appear to radiate from a source in the 

 constellation Orion, the Hunter; they are 

 generated as the Earth passes through 

 debris left behind by Halley's Comet. 



22 Mercury reaches its greatest western 

 elongation (see story above). 



28 The Moon is new at 7:14 p.m. EDT. 



63 natural history October 2008 



