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June 7, 1999: This week the planet Mars passes just 1.7 degrees north of
the bright star Spica in the constellation Virgo. It's
a lovely close encounter that star gazers won't want to miss.
Right: Duane Hilton's rendering of Mars and Spica above Cape Royal just after sunset on June 8, 1999. While you're enjoying the view of Mars and Spica in the south, don't forget to look to the west at brilliant Venus which reaches its greatest apparent distance from the sun (48o) later this month. The pair can be seen from both hemispheres as soon as the sky begins to get dark. Simply go outside and look to the south-southwest. Mars appears high in the sky from mid-northern latitudes as a fiery-reddish 1st-magnitude object about 40 degrees above the horizon. Just below Mars is Spica, a 1st magnitude blue-white giant about 220 light years from Earth. It is the 14th brightest star in the sky. Normally, Spica's blueish color is difficult to discern -- it looks white like most other stars. But this week is different. Spica's close proximity to reddish Mars makes its subtle hue stand out. Together, Mars and Spica form a truly vivid red-blue combination.
Although Mars, which shines by relected sunlight, is intrinsically dimmer than a brilliant star like Spica, the planet is much brighter in the night sky. That's simply because it is the closer of the two. The distance from Mars to Earth is a mere 103 million km -- just a hop, skip, and a jump by cosmic standards -- while Spica lies 91,000 times further away (2x1015 km). Mars reached maximum brightness last month when it passed close by Earth and reached magnitude -1.6. As Mars and Earth continue to separate, Mars will grow dimmer and dimmer until it is only +0.4th magnitude by the end of the year. The close approach of Mars to Spica is also a great opportunity to observe a phenomenon that is well-known to avid stargazers: Stars twinkle but planets do not (at least, not much). 
Left: This black and white
video clip of Mars (above)
and Spica (below) doesn't display the pair's dramatic red and blue colors,
but it does show how Spica twinkles but Mars does not.
The images were obtained by Tony Phillips on June 6 using an Astrovid
2000 CCD video camera. The animation consists of 10 video
frames each separated in time by 67
milliseconds.
Neither stars nor planets twinkle as seen from outer space, but the view from Earth's surface is a different matter. Tiny irregularities in the density and temperature of the air above us drift in front of stars and planets as we view them through the atmosphere. When a beam of starlight travels through the atmosphere, these irregularities disturb the beam making it dance back and forth slightly. That's what causes stars to "twinkle." (In scientific terms: tiny inhomogeneities in the atmospheric index of refraction cause point sources like stars to "scintillate.") Stars are so far away that they look like unresolved points of light even in large telescopes. Planets, on the other hand, are relatively nearby. They look like disks even in small telescopes. Planets don't twinkle because the scintillation of light from one side of the disk mixes with scintillation from other parts of the disk, and the disturbances cancel out. The light from planets arrives as a steady stream, not a babbling brook. It's all because planets appear much, much bigger than stars as seen from Earth. |
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This week's sky at a glance -- from Sky & Telescope Mars & Spica in 1998 -- from EarthSky.com Spica -- facts and figures from the University of Wisconsin The Planet Mars - from the SEDS Nine Planets web site Life on Mars - A review of evidence of signs of life in the Allen Hills meteorite Mars Global Surveyor - home page Mars - by Percival Lowell, 1895 Related Stories:
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