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Rare comet – Catch it while you can

Wait around long enough after a spectacular spring sunset, and you're likely to see another wonder in the night sky: comet Ikeya-Zhang, which hangs low in the west as twilight fades.

Ikeya-Zhang last appeared in 1661, making it the first "long period" comet (a round trip of at least 200 years) to be recorded, then spotted on its return, according to Brian Marsden, who runs the International Astronomical Union's Minor Planet Center.

The comet was discovered on Feb. 1 by two amateur astronomers – one in Japan and the other in China. The comet has been visible to the naked eye throughout March, but late April may provide the best viewing for people in the Northern Hemisphere. If you're not a morning person, hunt quickly, because it soon will shift to predawn.

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Once you find Mars, a steady red pinprick of light in the twilight sky, begin to scan to the right of the planet at about the same level above the horizon. The comet appears as a point of light, surrounded by a glowing halo, and sports a faint tail. If you're a night owl, your best viewing is expected to come during the second half of April – around 3 to 4 a.m. Eastern time.

Measuring an earthquake by Hail Marys

Football has its "Hail Mary" pass; seismology may now have its "Hail Mary" earthquake.

Scientists at the University of California at Irvine say they have uncovered geophysical and biological evidence for a temblor that rocked the Los Angeles Basin between 1635 and 1855. The team estimates that the quake, centered on a hidden fault beneath Orange County's coastal San Joaquin Hills, may have reached a magnitude of 7.3 and lifted Orange County's coastline by 3 to 11 feet. This could be the largest quake in recorded history to rock the Los Angeles Basin, they say.

The dates come from examining plants, pollen, and shells in a marsh that lies on an uplifted, ancient shoreline in Upper Newport Bay. The team's favored date, July 31, 1769, comes from the diary of Capt. Gaspar de Portola, who led a 64-member expedition from Baja California to conquer what is now California for Spain. His party camped along the Santa Ana River; three days later a violent quake struck.

Someone had the presence of mind to measure the duration of the quake and its aftershocks by the number of "Hail Marys" expedition members repeated during the shaking. The study was reported in the current issue of the Bulletin of the Seismological Society of America.

Linking humans to global warming not easy

Tree rings are telling tales of medieval warmth in the Northern Hemisphere, suggesting that despite their best efforts, climate scientists still face a difficult task in determining the extent to which humans may be responsible for the current long-term increases in global average temperatures.

According to the researchers, led by Jan Esper of the Swiss Federal Research Institute in Zurich, their study began as an attempt to answer criticisms that tree rings are poor substitutes for thermometers in recreating historical temperature trends. Such ring studies underpinned another study, published in 1999, that has become widely viewed in climate- research circles as a "smoking gun" for at least some human contribution to global warming.

After studying more than 1,200 tree-ring series from 14 sites around the Northern Hemisphere, Dr. Esper and colleagues have concluded that tree rings can, in fact, be effective stand-ins. In the process, however, they also were able to spot in the "Medieval Warm Period" temperature increases over time that look strikingly similar to those recorded during the past century, at least until 1990.

The results highlight the challenge climate researchers face in trying to distinguish between natural and human "forcing" of earth's climate. The study appears in the current issue of the journal Science.

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