S&T senior editor Alan MacRobert tells you what you need to know to get ready for Comet ISON.

When amateur astronomers discovered Comet ISON in September 2012, its unusual brightness caused some to hail it as the “Comet of the Century,” on a perilous course to graze the Sun in late 2013. However, the most recent observations indicate ISON (C/2012 S1) is on track to produce a somewhat less spectacular display, hopefully (but not definitely) reaching naked-eye visibility in the night sky in December 2013.

To S&T senior editor Alan MacRobert, that doesn’t make the opportunity to see it any less meaningful. MacRobert explains just why it’s so tricky to predict how bright ISON will be, and previews what you’ll need to know to spot it as it approaches the Sun for its close encounter.

Take a look:

Follow MacRobert's updates on ISON. And if you're in a speculative frame of mind, tell us your prediction for how well Comet ISON will perform come December.


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Bob F

September 16, 2013 at 11:19 am

How is ISON on a distinctive elliptical orbit yet considered a New comet? You say its new comet but how can a new comet just manifest itself then orient itself to the gravitational pull of a star (our Sun) that's so far away and not be influenced by another star closer to its origin of creation?
Personally, I feel that your severely underestimating the significance of this body as its coma is 73,000km wide. How can you make these statements? It sounds like your being influenced by something other then the physical body itself! Is your magazine owned by a member of the Cabal?
I can never be part of a membership where this has any possibility of being true case. Regards...

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P Edward Murray

September 16, 2013 at 12:47 pm

Bob F.,

When a comet is discovered other observers are called upon to observe it to and calculate it's position in the sky so that it's orbit can be calculated.

With Comet Ison, it is new because it is traveling from such a long way that it has never been seen before.

While a Comet's Coma may be extremely large it's mass is very small indeed.

I hope this helps you.

Ed Murray

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September 16, 2013 at 12:52 pm

I agree with you Bob F! I don't understand how he speaks with so much assurance when no one knows what is going to happen with this thing. Its never been in the inner solar system before, so does he know what the different planets gravitational pull will do to it. Is there a clear path for its approach to the sun? What if it breaks up (which it appears it already has) into a couple smaller pieces and they going flying wild? I think this video is propaganda. How is it that this video was just created today, and it already comes up at the top of the google search...with only 25 views? I smell a stink here.

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September 16, 2013 at 1:13 pm

Comet ISON path across inner Solar System

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September 16, 2013 at 3:16 pm


Have you noticed that, seemingly every time a new comet is discovered, some group claims that there's a cover-up? That NASA is hiding the truth? That the comet is on a collision course with Earth, or will hit/be deflected by something and end up on a collision course with Earth? That, while we can accurately predict the positions of everything in the Solar System well into the future, that this comet is somehow immune to the same mathematics? How many times have those predictions come true? Is there a logical reason to think things will be different this time around?

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Frank Reed

September 16, 2013 at 4:28 pm

This is an excellent video laying out the real observational issues for Comet ISON and providing a breath of fresh air on reasonable expectations for observing this comet towards the end of this year. As he says, if you're looking for visual spectacle, you won't find it here... Alan MacRobert says, "Astronomy isn't like that. This is a chance to see with your own eyes something that connects you to the wider universe and where we and our world come from. That's the excitement about amateur astronomy: being able to learn your way around the wider universe and to see things that are out there for yourself." Well said! But comets still draw conspiratorial fantasies, like two of the comments above. And it's an amazing historical phenomena: a sort of "historical hysteria". For some reason, comets develop a cult-like following, no matter how they behave. But ignore that nonsense. Don't try to persuade them they're wrong --they won't listen. Instead, get out and observe, and "connect with the wider universe", the REAL universe. And if it turns out that ISON is not an unusually bright comet, which is looking very likely, that just makes it all the more challenging to find and follow. And CHALLENGE is a good thing...

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September 16, 2013 at 6:47 pm

Wow! Talk about "living in an anthill world". Let the naysayers bray, this is a very nicely done video segment that lays out the expectations for this event very realistically. It may or may not be spectacular, no matter what we think we know or hope it will be. All that's left now is to get out there and see for ourselves. I too will relish the challenge.

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Peter W

September 16, 2013 at 7:01 pm

1) The comet will greatly disturb the sun’s corona at closest approach, and NASA’s space-based coronagraphs will provide the biggest wow-factor. 2) Due to the public’s short attention span, NASA will compress the 24 hours of observations made at closest approach into an 8 second time-lapse video. 3) the sun will survive the encounter in one piece.

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September 16, 2013 at 7:17 pm

Thank you, Frank Reed! I share your disgust and praise your restraint.

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Mike Martn

September 16, 2013 at 8:36 pm

Thank-you Alan for giving a Novice Astronomer and others like myself a clear perspective on ISON. I too have gotten caught up in the hype and reading everything I could get my hands on. Quite a few of us who aren't Astro-veterans look to these Blogs for advice and knowledge. Ans sometimes these free-exchange of Ideas may get heated there all done in the advancement of the Science of Astronomy.. In my opinion and it is my opinion only what we need th see is a little more Respect. And Frank talks for saying what a lot of us wanted to say. Thanks All and Clear Skies

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September 16, 2013 at 10:51 pm

I've always followed S&T for my 50+ years of Amateur Astronomy. With all the hype by other press, it is refreshing to see S&T's scientific and disciplined overview.

This video was a 'feet-on-the-ground' review of the current information we have about comet ISON. And with the current press hype, we need that!

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September 19, 2013 at 3:49 pm

I'm amazed at some of these comments! The level of understanding of even elementary science is appalling!

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Dale M

September 20, 2013 at 9:45 pm

I have been an amateur astronomer for only about 6 years or so but this is a VERY big problem in the Astro community, a few of the SO CALLED EXPERTS run their mouths and yell at the top of their lungs "COMET OF THE CENTURY" and many many people like me get so very excited and for what? NOTHING AGAIN...
I am really beginning to believe that these people need to keep their HUGE mouths shut! Now the general public are expecting some WOW comet and WE are going to get a small fuzzy spot (MAYBE) and AGAIN people are going to be VERY DISAPPOINTED in the Astronomy community for speaking when they should have just stayed quite UNTIL they knew for 99% certainty that it would be worth the hype!!

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September 20, 2013 at 11:34 pm


WHEN are these SO-CALLED Experts gonna do the (SMART) Professional thing and keep the hoopla all to themselves UNTIL they REALLY KNOW SOMETHING WORTH REVEILING?

EVERYTIME they think something will be SPECTACULAR it turns into THIS, a bunch of "EXPERTS" BACK-TRACKING! Do you really want to know WHY the general public does not care very much about astronomy? THIS IS WHY, 1 year ago, WE FOUND THE NEXT GREAT COMET "The comet of the CENTURY"
6 months ago: well it will still be GREAT but it may not be as great as we had hoped, 4 months ago: it is not brightening like we had thought it would BUT that don't mean it will not be GREAT!
TODAY: WELL, WE WERE WRONG....AGAIN, sorry public.
I wonder how many people they got to spend money on Binos, or a small scope? WELL PUBLIC, YOU LOSE... but we can watch it in our 3 meter scopes, and we will show you pics!!

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john zemi

September 21, 2013 at 7:56 pm

I really enjoyed watching.

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September 22, 2013 at 12:31 pm

Excuse me. How can any reader of Sky and Telescope not understand the concept and application of mathematics and physics to such astronomical pursuits as comet discovery, orbit calculation and projections? Sheesh! You folks are embarrassing.

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M Carter

September 23, 2013 at 5:08 am

I am very much looking forward to the approach of Comet Ison this November/December.
Having followed a number of predicted Comets over many Years , I know how UN-predictable they really are!
Some have put on a reasonable show while being expected to dazzle, others just flopped.Comet Hale-Bopp came from nowhere to bring a real show, so did Hyukataki.
No one should expect the experts to know what isn't known, it's all part of the FUN!!
Comet Ison may start to show it's real potential only after it crosses MARS orbit, why? only then will it really warm enough to sublimate the water ices it contains.
hey! that's any time soon!
Keep Looking ! MC

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September 23, 2013 at 6:14 am

Just thought I would post some useful ISON and comet URLs

Comet ISON orbit wrt to STEREO spacecraft and planets


Other comet links from the STEREO/SECCHI instrument



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Dean Johnson

September 27, 2013 at 10:03 pm

What a reflief to hear that the Sun will survive its encounter with Comet ISON! Considering that ISON's nucelus is roughly 25 miles in diameter as opposed to the Sun's 864,000 mile girth, I really didn't expect a different result.
What I think will happen is that ISON will be a big a bust for the public as Comet PANSTARRS was this spring. For us astronomers and particularily the die hard observers, it will be fun to watch, whatever happens.
Astronomy publications should be better off not hyping these events. 'Hopeful', not 'Hype' would better serve their purposes, and ultimately the public.

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Steve Albers

October 8, 2013 at 6:19 pm

My comet ephemeris program, developed over a period of decades, has a new addition this year to estimate comet visibility. The calculation for ISON suggests it stays just below naked eye visibility throughout the apparition, at least for the comet's head.

After the govermnent shutdown is over, you can see my ephemerides online at:


Other historical comets are also calculated for comparison.

I'll be updating this ephemeris if conditions change.


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Steve Albers

October 9, 2013 at 12:26 pm


I have been calculating visibility (compared to a naked eye threshold) for ISON with my comet
ephemeris program. My program calculates a modified magnitude based on extinction and
sky brightness and if this effective magnitude is less than 6.0 it is likely visible
naked eye. This is the ephermeris program I've run for decades with various comets, and
updated this year with the visiblity calculations.

I surmise that ISON will stay below naked eye visibility, even if it peaks at magnitude
-6 when close to the sun. The daily (and hourly) output can be seen here, once the
government reopens.



Steve Albers

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October 27, 2013 at 7:25 pm

It is incredible the different stories that are developing over this comet. Watch this video from a Canadian band about the ISON.


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