Sunday, November 17, 2013

Comet ISON battles moonlight, twilight

Spica and Mercury to the rescue as Comet ISON battles moonlight, twilight

Just when Comet ISON came to life, the moon crept back into the morning sky to rob the comet, at least temporarily, of its splendor. With a full moon on tap for tomorrow, expect the comet to get harder to see.

Pity. Moonlight, while among the most lovely of lights we know, reduces contrast and makes it impossible to see faint stars and wispy things like comet tails. Twilight’s no friend of comets either. Tomorrow morning, Comet ISON stands only about 11 degrees (one fist held at arm’s length) above the southeastern horizon at the start of dawn. In three days that shrinks to just 6 degrees as the comet rapidly approaches perihelion on Nov. 28.

We peer through much more air and haze near the horizon than higher up. Notice the moon’s lower edge is dimmed more than the top for this reason. Credit: Bob King

When you tilt your head to look straight up you’re looking through what’s defined as “one airmass”. An airmass includes the air we breathe plus additional haze and suspended particles called aerosols. Now tilt your head down to look 30 degrees or three fists above the horizon and you’re peering through 2 airmasses.  At 10 degrees, that increases to 5.6 airmasses.  Every additional airmass dims the comet by 0.4 magnitudes. That means Comet ISON appears two magnitudes or about 6 times fainter now than if it were high in the sky. I’m telling you all this so you don’t blame yourself if you’re having difficulty finding the comet. Thick air and moonlight are the culprits.

That doesn’t mean you shouldn’t try for it. You can wait until early twilight when the comet’s higher up. The brightening sky will eventually compromise your view, but fuzzballs as bright as ISON (now around magnitude 5) aren’t too hard to see in early dawn light. The advantage here is that the comet is higher.

Don’t expect to see much of a tail in binoculars at this time unless ISON has another major surge. Recent reports from amateur astronomers using 10×50 binoculars indicate the tail is dim or invisible due to the double whammy of twilight and moonlight.

The moon will be with us until twilight gobbles up ISON, so views of it will be comprised until some days after perihelion when it once again becomes visible in a dark sky. Assuming the comet has survived the sun’s heavy-handed cooking, it should return bright and glorious.

Courtsey - Astro Bob

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