Posted on March 13, 2013
I’ve been a huge comet geek since I was ten years old (details here), so when I heard about Comet PanSTARRS almost a year ago, I was pretty excited. I became even more excited when I learned that PanSTARRS would be a little more than three degrees left of a new moon on March 12. Checking my calendar, I discovered I’d be on Maui that week. Sweet.
Fast forward to March 12: I’m on Haleakala, the location of the PanSTARRS telescope that discovered the comet, with my Maui workshop group. After a sunset that colored the swirling clouds in all directions, the clouds close in and completely shut down the sky. We stick it out for a while, but when the cold saps the group’s enthusiasm (it’s 35 degrees with 35 mph winds), I reluctantly honor their wishes. Could I really have gotten so close to this event I’ve been anticipating for nearly a year, only to be denied. Descending the mountain with one eye on the sky, I hope for a break. Less than two miles down the road we suddenly pop into the clear and see a thin slice of moon dangling like a Christmas ornament in the blue/orange band separating day and night. While PanSTARRS isn’t bright enough to be visible the twilight, I know I can use its proximity to the moon to guide my lens. I pull the car over and we yank out our cameras and start firing, wider shots at first, then tighter to zero in on the comet and moon. The amount of light necessary to reveal PanSTARRS also revealed exquisite detail in the shadow area.
I’ll write more when I have time but I just had to share. Now, off to Hana….
Posted on January 16, 2013
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PanSTARRS Update, February 6, 2013: Latest word on the street is that PanSTARRS isn’t brightening as fast as hoped. Current predictions put it in the magnitude 2-3 range, about the brightness of the stars in the Big Dipper. This is likely to change, either up or down (welcome to the world of comet watching), as PanSTARRS approaches and passes perihelion (March 10). ISON remains on track for something truly special late this fall. Stay tuned….
PanSTARRS Update, February 23, 2013: After enduring a few weeks of fading hope for PanSTARRS brightness (revised estimates were calling for best-case magnitudes ranging from 2 to 3, and some were in the magnitude 4 range), recent observations have the comet brightening at a faster rate that might put it at magnitude 2 or brighter. I still don’t think we’ll really know what to expect until PanSTARRS reaches perihelion on March 10, but suffice to say it continues to bear watching. Keep your fingers crossed.
PanSTARRS Update, March 4, 2013: Things just keep getting better. PanSTARRS is putting on a great show for the Southern Hemisphere, and it’s about ready to enter Northern Hemisphere skies. Current estimates have PanSTARRS brighter than magnitude 2 right now, and we’re still nearly a week out from perihelion. As it continues to approach the sun, look for PanSTARRS to brighten to magnitude 1 or maybe even brighter. That means it could be visible north of the equator as early as March 7, with improving chances for viewing for at least the next week as it separates from the sun a little each evening.
There’s a lot of excitement in the astronomy community about a pair of comets heading our way in 2013. In late-November and December (and maybe into January), Comet ISON could put on a once-in-a-lifetime celestial display, but before ISON we may be treated to a pretty good warm-up when Comet PanSTARRS graces Northern Hemisphere skies in March. Media hyperbole notwithstanding, the unpredictability of comets is a source for great anxiety among those anticipating these celestial visitors. The safest bet is that PanSTARRS and ISON will either be brighter or fainter than predicted (comet history is rife with examples of both), but it’s just that kind of uncertainty that makes comets so special.
(If all you’re interested in is the photography stuff, feel free to skip to the “PanSTARRS: When and where” section below.)
A comet is a ball of ice and dust a few miles across (more or less) orbiting the sun in an eccentric elliptical orbit: Imagine a circle stretched way out of shape by grabbing one end and pulling–that’s what a comet’s orbit looks like. Looking down on the entire orbit, you’d see the sun tucked just inside one extreme end of the ellipse.
The farther a comet is from the sun the slower it moves, so a comet spends the vast majority of its life in the frozen extremities of our solar system. Some comets take thousands or millions of years to complete a single orbit; others complete their trip in just a few years.
As a comet approaches the sun, stuff starts happening. It accelerates in response to the sun’s increased gravitational pull (though just like the planets, the moon, or the hour hand on a clock, a comet will never move so fast that we’re able to perceive its motion). And more significantly, as the comet approaches the sun, increased heat starts melting the frozen nucleus. Initially this just-released material expands to create a mini-atmosphere surrounding the nucleus; at this point the comet looks like a fuzzy ball when viewed from Earth. As the heat increases, some of the material set free is discarded to form a glowing tail (glowing by reflected sunlight—a comet doesn’t emit its own light) that points away from the sun. The composition and amount of material freed by the sun, combined with the comet’s proximity to Earth, determines the brilliance of the display we see.
With millions of comets in our Solar System, it would be easy to wonder why they’re not a regular part of our night sky. Actually, they are, though most comets are so small, and/or have made so many passes by the sun that their nucleus has been stripped of reflective material, that they just don’t have enough material left to put on much of a show. And many comets don’t get close enough to the sun to be profoundly affected by its heat, or close enough to Earth to stand out.
Most of the “periodic” comets—comets that make regular appearances—are well known to astronomers. These comets have usually lost so much of their material that they’re too faint to be seen without a telescope; a notable exception is Halley’s Comet, perhaps the most famous comet of all. Halley’s Comet returns every 75 years or so and usually puts on a memorable display. Unfortunately, Halley’s last visit, in 1986, was kind of a dud; not because it didn’t perform, but because it passed so far from Earth that we didn’t have a good view of its performance on that pass.
Then there are the “non-periodic” comets, which pivot the sun only once in thousands or millions of years. New non-periodic comets are discovered each year; every once in a while astronomers determine that one of these discoveries is large enough, with a favorable orbit that sends it close enough to the sun to ensure lots of reflective material will be shed, and close enough to Earth that we’ll have a good vantage point, that it just might put on a spectacular display. Enter Comets PanSTARRS and ISON.
Every comet has a different physical make-up, so there’s no way we can tell how it will react during its encounter with the sun. Astronomers also suspect that on its first solar approach an incoming comet may shed a thin, highly reflective outer layer when it’s still a good distance out, giving us a false impression of its intrinsic albedo (reflectivity). Therefore we can’t be certain if a newly discovered comet that appears relatively bright at a great distance (but still much too dim to be seen without a telescope) is going to continue shedding reflective material, or peter out before it arrives.
An even bigger concern is whether the comet will survive its encounter with the sun at all. The closer a comet passes to the sun, the more it is likely to shed the ice and dust a spectacular display requires, but some sun-grazing comets have passed so close to the sun that they completely disintegrated.
In other words, we have no way of knowing whether PanSTARRS and ISON dazzle or fizzle—all we can do is wait. And prepare.
Fortunately, we do have one certainty to work with: the comet’s orbit. We know with great confidence where it will appear (or where it should have appeared had it survived its encounter with the sun) and when it will be there.
PanSTARRS makes its closest approach to the sun, “perihelion,” on March 10. If we’re lucky it will appear as a fuzzy ball low on the western horizon of Northern Hemisphere skies shortly after sunset in the second week of March (it’s done with the Southern Hemisphere). Each evening PanSTARRS will appear above the western horizon shortly after sunset, a little higher and (probably) a little dimmer than the night before. But as it rises each night, it moves farther from the sun into darker sky, so while PanSTARRS may be dimming slightly, the sky surrounding it may darken faster than the comet dims, perhaps and for a week or so (this is anybody’s guess). That would make PanSTARRS more visible as the first week after perihelion progresses. Since PanSTARRS’ tail material may have been stripped by its close encounter with the sun, it will need time to reform and may lengthen with each passing day, another variable that can’t be predicted.
PanSTARRS will eventually rise into the darker part of the sky; by the end of April it will be visible all night in the Northern Hemisphere. But by then it’s very unlikely to be bright enough to be viewed with the unaided eye.
But one of the great thrills of comet watching is the uncertainty. Just as some comets disappoint (Google Comet Kohoutek), others astonish. Hale-Bopp was much heralded before it arrived in late 1996, but nobody expected it to remain visible to the naked eye for eighteen months. And in January 2007 Comet McNaught caught everyone off guard by suddenly brightening to become a spectacular (albeit brief) sight trailing the sun to the horizon in the post sunset twilight.
PanSTARRS approximate location 45 minutes after sunset at 38 degrees north latitude
March 8: Altitude 0º (right on the horizon) Azimuth 260º (due west is 270º)
March 9: Alt. 1º Az. 262º
March 10: Alt. 3º Az. 264º
March 11: Alt. 4º Az. 267º
March 12: Alt. 5º Az. 269º
March 13: Alt. 6º Az. 272º
March 14: Alt. 7º Az. 274º
March 15: Alt. 7º Az. 277º
Since we know when and where PanSTARRS will be visible, there’s no excuse for not preparing now (right?). Preparation starts with knowing where you’re going to shoot PanSTARRS beforehand. Look for unobstructed views to the west with no terrain for PanSTARRS to set behind before the sky is dark enough for it to appear—think beach, hilltop, mountaintop, or flat landscape. The best scenes are worthy of photographing regardless of what’s in the sky, scenes that can use the comet as an accent to take the image to the next level.
I’m guessing that PanSTARRS won’t have a tail anywhere near as dramatic as the tail in the Starry Night illustration above (though I could be, and hope I am, wrong), so unless it brightens far beyond predictions and the tail lengthens more than normal (it’s happened with comets before), you won’t want to go too wide with your composition if the comet is to be your primary subject. On the other hand, even with a relatively short tail, PanSTARRS could make a magnificent accent to an otherwise nice wide scene. I plan to prepare for both tight telephoto and wider landscape shots.
Because PanSTARRS will be in the relatively bright post-sunset sky, and any foreground subject will be in full shade (the sun’s down, but it isn’t completely dark yet), look for striking nearby shapes to silhouette against the sky with PanSTARRS glowing in the distance. For example, near my home in Sacramento the best candidates will be the oak trees dotting hillsides east of town. I’ll need to be on their east side, facing west.
Mountains that stand out against the horizon will work nicely too, though remember that PanSTARRS will already be quite low as the sky darkens, and it will be dropping toward the horizon with each minute (along with everything else in the sky)—if the mountains you choose are too high, or too close, PanSTARRS will disappear below your horizon before the sky is dark enough. For example, Mt. Whitney as viewed from the Alabama Hills might make a great foreground subject for PanSTARRS, but from the Alabama Hills Mt. Whitney juts about 10 degrees above the horizon—on March 15 PanSTARRS will set behind Whitney (drop below 10 degrees) about 30 minutes after sunset. So unless PanSTARRS is extremely bright, it might not be visible at all before it disappears below the mountains.
And before you ask about photographing PanSTARRS in Yosemite, let me strike preemptively and say that pretty much all of the views in Yosemite Valley face east, which means even if you’re lucky enough to glimpse PanSTARRS above Yosemite Valley’s steep walls, while you’re photographing it Half Dome and El Capitan will be behind you. (Wait for ISON, which could be a spectacular pre-sunrise object in the east.)
(I’m assuming you understand the basics of exposure—if not, read this.)
First, it’s important to understand that, unlike ISON in December, photographing PanSTARRS at its brightest will not be night photography, it will be twilight photography. Because it will be in the same area of the sky as a crescent moon, regardless of your focal length choice the rules for photographing PanSTARRS will be similar to those for photographing a crescent moon. (You can read more about that on my Crescent Moon Photo Tips page.)
My general approach to capturing foreground detail in twilight scene like this is to meter on the brightest part of the sky, setting an exposure that’s as bright as possible without overexposing (a graduated neutral density filter helps). After you click, check your histogram to make sure you haven’t blown out the highlights. If at all possible (if your camera shows it and you understand how to read it), I strongly recommend checking each of the channels in the RGB histogram to make sure you haven’t lost any color. On the other hand, if it’s a silhouette I’m going for, I’ll underexpose slightly to hold the color in the sky and/or water. In this case a graduated neutral density filter is unnecessary.
By the time PanSTARRS drops below the horizon, the foreground will be so dark that my exposures will need to be quite long. But since PanSTARRS is in fact moving at the same speed (from our terrestrial perspective) as all the stars and planets, I’ll want to monitor my shutter speed to avoid motion blur (the longer my focal length, the more I’ll need to worry about long exposure motion blur). I’m guessing for my wider shots I’ll be okay at 15 seconds, but I’ll still magnify the image on my LCD to determine whether I need to bump my ISO further to allow an even faster shutter speed.
For wide shots a graduated neutral density filter will help hold down the brightness of the sky enough to enable you to save the twilight color while bringing out some foreground detail. For ocean scenes, where the horizon is flat, I prefer a hard-transition GND; my 3-stop reverse GND will probably get the most use.
You could go super-tight and fill the frame with nothing but comet and sky—I’ll probably try a few of these. But with no foreground, nothing about these compositions will set them apart from the thousands of similar compositions taken from anywhere else in the Northern Hemisphere. Most of my tight shots will include some landscape feature silhouetted against the sky or water. In the telephoto shots that include a foreground subject, the farther from the silhouetted subject I can position myself, the longer the focal length I can use, and the larger the comet will appear in my frame. But don’t forget that the more you magnify with a long focal length, the greater the motion blur you’ll capture—a higher ISO to increase your shutter speed is usually a good idea.
Remember that we’re photographing PanSTARRS after the sun has gone down, but before the sky is completely dark. So for my wide shots I’m going to look for water because water reflects the twilight sky so nicely. I’ll be leading a workshop on Maui during what promises to be PanSTARRS prime-time (the week of March 8-15), so finding water won’t be a problem for me. But no matter where you are, you should be able to find a westward view of a river, lake, or beach.
A particular advantage of photographing PanSTARRS from the beach is the an unobstructed view of the horizon, giving me a very long and clear line of site to PanSTARRS (even better would be shooting downward at the horizon from a mountaintop, like Haleakala).
The night I’m targeting as potentially off-the-charts-special is March 12 (image at the top of the page). That evening PanSTARRS will be aligned with a sliver-thin slice of crescent moon, separated by less than 4 degrees. The next night the moon will be higher, about 10 degrees (the width of a fist held at arm’s length) directly above the moon, with PanSTARRS’ tail pointing directly at the moon. If the tail is long enough, it will appear to pass right through the moon.
On the other hand, I’m fully prepared for disappointment too—I’ve followed comets long enough to understand why astronomer David Levy said, “Comets are like cats: they have tails and do precisely what they want.”
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Read the history of my relationship with comets in my January 11 post.
Posted on January 11, 2013
When I was ten, my best friend Rob and I spent most of our daylight hours preparing for our spy careers—crafting and trading coded messages, surreptitiously monitoring classmates, and identifying “secret passages” that would allow us to navigate our neighborhood without being observed. But after dark our attention turned skyward. That’s when we’d set up my telescope (a castoff generously gifted by an astronomer friend of my dad) on Rob’s front lawn to scan the heavens in the hope that we might discover something—a supernova, comet, black hole, it didn’t really matter.
Our celestial discoveries, while not Earth-changing, were personally significant. Through that telescope we saw Jupiter’s moons, Saturn’s rings, and the changing phases of Venus. We also learned to appreciate the vastness of the universe with the observation that, despite their immense size, stars never appeared larger than a pinpoint no matter how much magnification we threw at them.
To better understand what we saw, Rob and I turned to illustrated astronomy books. Pictures of planets, galaxies, and nebula amazed us, but we were particularly drawn to the comets: Arend-Roland, Ikeya–Seki, and of course the patriarch of comets, Halley’s Comet (which we learned was scheduled to return in 1986, an impossible wait that might as well have been infinity). With their glowing comas and sweeping tails, it was difficult to imagine that anything that beautiful could be real. When it came time to choose a subject for the annual California Science Fair, comets were an easy choice. And while we didn’t set the world on fire with our project presentation, Rob and I were awarded a ribbon of some color (it wasn’t blue), good enough to land us a spot in the San Joaquin County Fair.
The next milestone in my comet obsession occurred a few years later, after my family had moved to Berkeley and baseball had taken over my life. One chilly winter morning my dad woke me and urged me outside to view what I now know was Comet Bennett. Mesmerized, my dormant comet interest flamed instantly, expanding to include all things astronomy. It stayed with me through high school (when I wasn’t playing baseball); I actually entered college with an astronomy major that I stuck with for several semesters, until the (unavoidable) quantification of concepts sapped the joy from me.
While I went on to pursue other things, my affinity for astronomy continued, and comets in particular remained special. Of course with affection comes disappointment: In 1973 Kohoutek broke my heart, a failure that somewhat prepared me for Halley’s anticlimax in 1986. By the time Halley’s arrived, word had come down that it was poorly positioned for its typical display (“the worst viewing conditions in 2,000 years”), that it would be barely visible this time around (but just wait until 2061!). Nevertheless, venturing far from the city lights one moonless January night, I found great pleasure locating (with much effort) Halley’s faint smudge in Aquarius.
After many years with no naked-eye comets of note, 1996 arrived with the promise of two great comets. While cautiously optimistic, Kohoutek’s scars prevented me from getting sucked in by the media frenzy. So imagine my excitement when, in early 1996, Comet Hyakutake briefly approached the brightness of Saturn, with a tail stretching more than twenty degrees (forty times the apparent width of a full moon). But as beautiful as it was, Hyakutake proved to be a mere warm-up for Comet Hale-Bopp, which became visible to the naked eye in mid-1996 and remained visible until December 1997—an unprecedented eighteen months. By spring of 1997 Hale-Bopp had become brighter than Sirius (the brightest star in the sky), its tail approaching 50 degrees. I was in comet heaven.
Things quieted considerably comet-wise after Hale-Bopp. Then, in 2007, Comet McNaught caught everyone off-guard, intensifying unexpectedly to briefly outshine Sirius, trailing a thirty-five degree, fan-shaped tail. But because of its proximity to the sun, Comet McNaught had a very small window of visibility and was easily lost in the bright twilight—it didn’t become anywhere near the media event Hale-Bopp did. I only found out about it by accident on the last day it would be easily visible in the Northern Hemisphere. With little time to prepare, I grabbed my camera and headed to the foothills east of Sacramento, where I managed to capture the image at the top of this post.
Following McNaught I vowed not to be caught off guard by a comet again. After enduring the frustration of seeing others’ images of spectacular Southern Hemisphere-only comets, my heart jumped last year when I came across a website proclaiming the approach of Comet PANSTARRS (a.k.a, C/2011 L4 in less glamorous astro-nerd parlance), discovered not by an individual, but by the Pan-STARRS automated telescope array atop Haleakala on Maui. Researching further, I learned that PANSTARRS could (fingers crossed) hang low in the western sky at magnitudes brighter than Saturn, for about a week beginning around March 10, 2013 (it will rise slowly each night, remaining visible as it fades for a few more weeks). Checking my calendar to see if I had any conflicts that week, I immediately remembered why those dates sounded so familiar—I’ll be on Maui for my workshop then! In fact, my first viewing of PANSTARRS could be almost literally in the shadow of the telescope that discovered it. It’s a sign*.
Since its discovery in June of 2011, astronomers have been monitoring PANSTARRS and updating its orbit and brightness curve—so far everything remains on track (and my crossed fingers are cramping). And as I followed PANSTARRS’ progress, rumbling of another comet could be heard, a comet that may significantly outshine PANSTARRS to achieve historic proportions: In December of this year, Comet ISON (how I long for the days when comets were people and not acronyms) may rival or surpass Hale-Bopp, perhaps even becoming bright enough to be viewed in daylight. (I have to say that you must be careful about such reports—the media seem more interested in generating audience than in actually getting things right.)
Are these comets a sure thing? Of course not. So I make no promises, except that I’ll be checking for updates daily (can you say OCD?) and will keep you posted. Chances are, if they develop as promised (hoped), you won’t have any trouble keeping track on your own (just Google their names for more information than you’ll ever need). And of course if I get any images of PANSTARRS when I’m on Maui, I’ll post them here. Stay tuned….
June 7, 2013
Comet PanSTARRS turned out to be a huge thrill (click image for details):
* Speaking of signs, Rob and I recently reconnected after many years with no contact (sadly, he didn’t go on to become a spy or astronomer either). We’re already talking about going out to see one or both of these comets together.