Rest in Peace, ISON
It’s official: comet ISON is no more. NASA confirmed Tuesday that after its first-ever venture from the outer limits of the solar system to the sun, the comet did not survive the onslaught of solar radiation. Born 4.5 billion years ago during the formation of the solar system, ISON had resided in the so-called Oort Cloud a full light-year away. Roughly a million years ago, perhaps after a cometary collision, ISON began its journey toward the sun. It was discovered in September of 2012 by Russian astronomers scanning the heavens with the International Scientific Optical Network, hence its name.
Since then, speculation has wavered as to whether this would be the “comet of the century” or if it would fizzle like a candle in the rain. A compact orb of frozen ice and dust, ISON at best hovered at the border of visibility to the unaided eye. But seen through binoculars in pre-dawn skies this past month, the comet hinted at what could be. As ISON neared the sun, astronomers hoped that as it began to warm and melt, the resulting steam would reflect more and more sunlight.
Alas, as ISON looped behind the sun Thanksgiving, coming within 684,000 miles of its surface, the 5,000-degree heat proved too much. What emerged from this close call with the sun was a fragment of its former self — rather fragments, spread in a loose and dim fan shape and fading fast.
However, even though ISON never reached its hoped-for potential for us watching on the ground, space telescopes and observatories have studied the comet’s chemical makeup through spectral analysis, learning a great deal about the earliest days of the solar system.
While ISON has faded away, Venus is at its brightest Friday, blazing at magnitude –4.9 in the early evening sky. Look for it in the southwest a half-dozen degrees from the waxing crescent moon Thursday after sunset and well below the moon Friday night. The evening star sets around 7:30.
Jupiter, brighter than any star but nowhere near as luminous as Venus, rises in the east-northeast around 8pm. Much dimmer Mars rises around 1am and is high in the southeast come dawn. By that time, Mercury and Saturn appear low in the east-southeast, near where Comet ISON was last seen. Saturn is the higher of the two, but Mercury is brighter. The two planets pull farther apart this week as Mercury sinks and Saturn climbs higher.
Saturday marks the earliest sunset of the year, at 4:45pm. The longest night, winter solstice, follows two weeks later on December 21, while the latest sunrise comes another two weeks down the line on January 4.
The cause is the difference between clock time, with its 24 consistent hours, and solar time, measured from high noon one day to the next. Because of earth’s elliptical orbit and its tilted axis, the length of time it takes to complete a full rotation — from noon to noon — this time of year is actually 24 hours and seven minutes.
While the change in sunset is a matter of seconds this week, by Christmas sunset will be four minutes later, and by the end of the year it will be light until nearly 5pm.