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

Comet ISON this morning Nov. 16 taken by Michael Jaeger of Austria.

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.

Comet ISON battles moonlight now until it’s too low to see at dawn later this month. This map shows the two on opposite sides of the sky tomorrow morning Nov. 17. While the moon’s phase will begin to wane next week, it will move closer to the comet each morning. Stellarium

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.

This map will help you find the comet in binoculars and telescopes in the coming mornings. It shows the sky about 75 minutes before sunrise CST facing southeast. We’re fortunate to have bright Spica and Mercury nearby as guides. The comet will look like a small, dense, fuzzy glow in binoculars. Mercury’s position shown for Nov. 17. Click for large version. Created with Christ Marriott’s SkyMap program

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.

Zoomed-in version of the map above showing both Comet ISON and the more challenging Comet Encke. Time is about 75 minutes before sunrise CST. Stars are shown to magnitude 7. Since Mercury’s on the move just like the comets,  its position is shown on two dates. Click to enlarge.

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.

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About astrobob

My name is Bob King and I work at the Duluth News Tribune in Duluth, Minn. as a photographer and photo editor. I'm also an amateur astronomer and have been keen on the sky since age 11. My modest credentials include membership in the American Association of Variable Star Observers (AAVSO) where I'm a regular contributor, International Meteorite Collectors Assn. and Arrowhead Astronomical Society. I also teach community education astronomy classes at our local planetarium.

8 thoughts on “Spica and Mercury to the rescue as Comet ISON battles moonlight, twilight

  1. Hey Bob just wanted to thank u 4 the GREAT charts u publish on this site and universetoday. Thanks primarily to ur charts (with some assistance from a couple of other ones) i was able to observe both ISON and Lovejoy in binoculars for the 1st time this AM, thus doubling the amount of comets i’m sure i’ve seen in my life. tried for Enke but too low/dim/twilit, and X1 was too dim. anyways, needless to say, AWESOME!!! I am sure i could have seen especially Lovejoy previously but either clouds have interfered or i have been sleeping in anticipation for work. Thankfully i picked a good week to take a vacation! and ISON is kind enough to perihelion on Thanksgiving, if it survives. I’m also wondering if the following day the lunar occultaion of Spica will be visible for me. i think it is an noonish event, and i’m not sure Spica would be visible in my binos at that time (i have no telescope). i’m thinking doubtful.

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