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Volume 14, Issue 28 ~ July 13 - July 19, 2006

Sky Watch

by J. Alex Knoll

Lunar Physics

Like a beautifully tuned clock, the moon keeps perfect time

A waning gibbous moon rises late in the evening with week’s end until Sunday, when the last-quarter moon rises with the witching hour of midnight. Month after month, the cycle repeats like a beautifully tuned clock. The full moon rises with sunset and sets with daybreak, moving opposite the sun and reflecting its full light back to earth. At new moon it’s the opposite, with sun and moon rising and setting together, hence the moon’s invisible face. For the two quarter moons, the moon rises juxtaposed at a 90-degree angle between the earth and sun, thus showing but half its face. First-quarter moon rises at midnight, while the last-quarter moon rises at noon.

Sunset, around 8:30 this week, reveals three distinct lights forming a near-straight line above the horizon. Saturn appears just above the west-northwest horizon, but the ringed planet sets within the hour. A few degrees higher yet half as bright is Mars, visible until 10pm. A few degrees above Mars is Regulus, the heart of Leo the lion. Watch over the next week as Mars and the brighter star move closer together until July 22.

Jupiter shines high in the south at sunset and is the brightest light in the evening sky other than the moon until setting at 1:30am. The gas giant hovers between Virgo’s bright star Spica and two dimmer stars in the faint constellation Libra. The stars’ names, Zubeneschamali and Zubenelgenubi, date back a couple millennia to when Libra and Scorpius formed one large constellation; in Arabic they mean the northern claw and the southern claw respectively.

As dawn approaches, Venus rises in the east-northeast. Far brighter than even Jupiter, Venus appears so low against the horizon that its light shifts and pulses as it travels through earth’s atmosphere.

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