You’ve probably never heard of 12P/Comet Pons-Brooks. But if you’d been riding on it, you’d now have a grand view of the inner solar system below you as you zoom in toward an April rendezvous with the sun. As you descend sharply at a 74° angle, Jupiter is off to your right at about 2 o’clock, Saturn at about 4. You’ve come a long way since June of 1988 (do you remember what you were doing then?) when you slowly floated high over the frigid reaches of Neptune’s orbit, 33½ long astronomical units from the sun [1]. But now the little circle of inner planets finally appears large, its own busy little solar system embedded within the vastness of the outer planet realm that seemed to take you forever to cross. Pons-Brooks is now roughly Mars’ distance from the sun, and about that distance above it. You’re getting close now. You’re excited, ready to stow your stuff as you would on an airplane approaching the small city below that is your destination.
But despite the slight increases in acceleration and (you are told) increasing speed with each passing day, it still seems like an annoyingly leisurely descent. The pilot finally tells you that you will not swing by the sun until . . .when? Next April 21st, which is the perihelion of this comet (which is the dark shaded dot on the image below, shown below the ecliptic plane), and the beginning of its long ride back. The good news is that you’ll arrive on the solar doorstep in time for the April 8th solar eclipse. So, four long months of travel left to go. (The solar system is so much bigger than we’d imagined.) You’d not be blamed if you decided to deplane on planet earth (the white dot on the third orbit out) and track the comet from there. You’d have a less theatrical view – but you’d avoid all the cometary outgassing and intense solar heat which can be a distinct inconvenience for the inbound traveler.
Being on earth, however, allows you to see the comet at your leisure. Indeed, it will soon be visible in larger binoculars under a reasonably dark sky. My computer math-generated image above shows the relative positions of the earth, sun/moon, and the comet (on the left, shown descending on its orbital path) at the moment of greatest eclipse on April 8, 2024. The comet is now of course higher up than shown on the image, and rapidly picking up speed as it gets ever closer to the sun's immense gravitational embrace. It's now in the constellation Cygnus.
As of the end of December, observers over the world have reported the comet’s brightness to be between 10th and 8th magnitude. Most binoculars can capture seventh magnitude and brighter; sixth magnitude and above means it should be visible to the naked eye in dark, not light-polluted, skies. The comet will brighten much more as it approaches the sun. Hint: get out your binoculars, go to a dark spot out of town and start looking!
You can keep up with it by checking the Heavens Above app https://www.heavens-above.com/ (under tabs “Astronomy” and “Comets.”) You can also see the progress of its brightening and near-term ephemeris in the Comet Observation Database, https://www.cobs.si/. That site incorporates near real-time data from amateur astronomers all over the world.
Discovery
In 1789, 28-year-old Jean Louis Pons (1761-1831) was hired as a doorkeeper for the Observatory at Marseilles, France. His job was to keep watch on the grounds, but Pons spent a fair amount of time looking at the sky too. His apparently home-made telescope, which he called the "le grand chercheur”— “the great seeker” — had an ample three-degree field of view with which he could sweep large areas of the sky during the course of an evening. The skies must have been remarkably dark. And the streets in the summer of 1812 rather devoid of people, at least men, because Napoleon’s Grande Armée of half a million men was off invading Russia. Not among them, fortunately for our story (and for him), was the sharp-eyed Pons. On July 21st Pons noticed a fuzzy, un-starlike object in his field of view. He watched its motion over many nights. He followed its slow drift among the stars till September 28th then lost sight of it. Loss after discovery was not an uncommon experience for a newly discovered comet. W. R. Brooks in New York recovered the comet on its return, seventy-one years later, on September 2nd, 1883. He tracked it until June 2, 1884, but the comet was not seen again till its next apparition in 1954. The comet in our time was spotted in 2020 and will pass its perihelion in 2024.
Pons eventually became one of the most successful comet hunters in history, with at least 26 comets to his name. He seemed to snare about a comet a year. In 1808 his haul was five comets in eight months. With his growing success as a comet hunter, the Marseilles Observatory promoted Pons to assistant astronomer. He went on to become director of two different Italian observatories before his death.
As a relatively short period comet, 12P/Comet Pons-Brooks, as it has been officially named, has swung into our solar neighborhood regularly in recent human history, long before Jean Louis came to be. Indeed, with a period of about 71 years, the comet has clocked many human life spans; who else might have seen it before Pons?
Comets lost, comets found
The great adventure to link comets to other apparitions in history began when Edmond Halley wrote to Isaac Newton in 1695 suggesting that the comets of 1607 and 1682 were one and the same. Later, with a “prodigious deal of calculation,” applying Newton’s methods to 350 years of recorded observations, Halley determined the orbital elements of 24 comets that appeared from 1337 to 1698. His famous 1705 Astronomical Tables showed that the comet of 1531 too was the same as those of 1607 and 1682. He argued that it would return in 1758: “Wherefore if according to what we have already said it should return again about the year 1758, candid posterity will not refuse to acknowledge that this was first discovered by an Englishman.” So it did, and so it ever shall be: What thenceforth became known as Halley’s comet was recovered by a Saxony farmer and amateur astronomer, Johann Georg Palitzsch, on Christmas evening, 1758.
In our day the computer allows us to skip such prodigious deals of calculation that burdened Halley and later astronomers. Now we can turn the clock back (or forward) to any desired year and watch a comet move through history. We can check it against other comets in the historical record and even assess the stability of its orbit.
In the Sherlock Holmes-like spirit pioneered by Halley of investigating the earlier traces of our cometary visitors, Maik Meyer, Takao Kobayashi, Syuichi Nakano, and Daniel W. E. Green recently integrated the orbit of 12P/Pons-Brooks backward until about the year 1000 [2]. Using data from the apparitions of 1883-1884 and 1953-1954 (the latter from the Minor Planet Center's online database), the authors wrote in the International Comet Quarterly that it was “apparent that the orbit for this comet is very stable and does not experience strong planetary gravitational perturbations in the covered period.” (Perturbations are disturbances caused by the gravity of other bodies, mainly planets and asteroids, that can potentially alter a comet’s path.) Moreover, the authors reported the “unambiguous identification of this comet with the historic comets C/1385 U1 and C/1457 A1,” and possibly the even more ancient apparition of a comet seen in AD 245, which may have been the earliest recorded sighting of 12P/Pons-Brooks. Using these and more recent observations, the authors provided linked orbital elements of selected apparitions from the fourteenth century on.
Here is the authors’ chart showing the fascinatingly close match-up of the apparition of 1457, seen and recorded in Italy, with the computed position of Pons-Brooks from the study [3]:
A very stable comet
The ICQ authors characterized the orbit of 12P/Pons-Brooks as stable, at least over the covered interval. What does that actually mean? Out of curiosity, I used mathematical software to plot the comet’s orbit for five of the studied apparitions, 1457, 1812, 1884, 1954, and 2024 (predicted), incorporating the orbital elements of 12P/Pons-Brooks published by Meyer, et al. in their paper. Each color in the image below represents a different apparition:
Nothing wildly unpredictable here. This comet is as adventurous as a nine-to-five commuter riding the same train every day of his working life.
When we talk about the “stability” of a comet, though, it’s not as if it’s a personality trait, inherent more in one comet than another. We are not here referring to the structural integrity of a comet’s body, which can absolutely impact a comet’s lifetime. Biela's comet in 1846 famously separated into two parts and returned as distinct comets in 1852, which later faded; a meteor shower then appeared in November of 1872. The event was a sobering reminder that bodies in our solar system could actually disappear. “Hypotheses, more or less probable, have been offered in explanation, but the significant fact remains - a member of the solar system is lost,” said American astronomer Daniel Kirkwood in 1890. There are many other examples in history. You may recall how in late 2013, Comet ISON completely disintegrated after rounding the sun at frighteningly close quarters.
Here we are considering the stability not so much of the comet itself, but of the comet’s orbit – its permanence over the eons. True, there are certain nongravitational forces, such as outgassing and solar radiation that can accelerate or retard a comet’s mean motion and nudge its track, and Comet Pons-Brooks has been prone to occasional outbursts. But the main forces acting on the comet’s orbit are the comet’s encounters with other solar system bodies big and small, whose gravity-tugs it feels. In other words, it’s about the acquaintances it brushes up against along the way. Fly-by encounters with Jupiter and Saturn, in particular, can sharply disrupt an orbit. Planetary perturbations on his namesake comet were first calculated by Halley. Even Venus has made mischief with Halley’s orbit over time. During the period of the ICQ study, Comet Pons-Brooks passed 3.71 au from Uranus in 1819, and 1.62 au from Saturn in 1957: not close encounters by any means. Comet Pons-Brooks’ closest approaches to our little earth were 0.41 au in 1385, 0.90 au in 1457, and 0.63 au in 1884 – nothing to get worried about. All that non-interaction with the planets over the centuries is why this comet’s orbit has been stable.
Cometary soulmates
You may have noticed some features about this comet that remind you of another far more famous one. Comet 12P/Pons-Brooks’ period is 70+ years. Its aphelion is just past the orbit of Neptune. It has a very high inclination and high eccentricity (.955), indicative of a long, cigar-shaped elliptical orbit. The other comet I have in mind has very similar characteristics. I’m sure you’ve guessed it by now since I’ve already been talking about it.
It’s (of course!) Comet Halley. December 2023 was a special month for Comet Halley. As I noted in my blog two years ago, Comet Halley Remembered, 35 Years Ago, 40 to Go, https://www.douglasmacdougal.com/post/comet-halley-remembered-35-years-ago-40-to-go, December 9th of 2023 marked the aphelion point of Comet Halley, its farthest remove from the sun. Halley is finally back on its way in. Interestingly, Halley and Pons-Brooks are pretty much on opposite sides of their orbits. For the math geeks in the group: can you determine when both Halley and Pons-Brooks will be in the inner solar system (within 1 au) at the same time? (Send me your answers!)
NASA calls Pons-Brooks “Halley-like.” Let’s compare their orbital elements and see why. The elements are the vital characteristics of any orbiting body. I’ve taken this data without rounding directly from NASA/JPL’s massive Small-body Database:
Similar, as you can see, but not identical. They differ greatly in their orbital inclinations but both are steep; Halley’s is past 90 degrees, hence its motion appears retrograde, unlike Pons-Brooks, and travels in reverse direction from Pons-Brooks.
Halley also gets closer to the sun than Pons-Brooks: its q (perihelion distance) is less by about .2 au, which is not insignificant. The smallness of q as it rounds the sun actually makes a great difference in how dramatic the comet will appear. (If you find the quantites in the chart confusing, or want a brief comet refresher course, feel free to check my earlier article, https://www.douglasmacdougal.com/post/an-exceptional-visitor-comes-to-town-comet-c-2020-f3-neowise, which explains orbital elements in the context of several famous comets.) I've talked earlier about the so-called “sungrazers” (of which there is a family known as the Kreutz sungrazers) with extremely small perihelion distances – where the intense solar furnace causes great cometary eruptions that become visible as spectacular tails as they approach and round the sun. Neither Halley nor Pons-Brooks is of that class. Nor are they Oort cloud comets that have multiple thousand- or million-year orbital periods: those random visitors that seem to come into our solar system at all angles from that distant spherical cloud of large ice balls, often arriving and departing in dramatic fashion after skirting close to the sun’s surface. Pons-Brooks and Halley are not of that ilk. They are modest, well-respected senior citizens of the solar system, having settled into stable seven-decade solar system orbits, peacefully drawing their pensions.
The comet and the eclipse
As comets go, Comet 12P/Pons-Brooks this April is not set up for a particularly dramatic show from earth. As the comet descends toward the sun, the earth at the same time is moving away from the comet. The comet will be off on the other side of the sun from us when it passes perihelion, yet is potentially viewable during the April eclipse, even if rather far away. With the sun blotted out, we may, if we’re lucky, catch a glimpse of this elusive visitor from Neptune’s neighborhood hanging to the east of the sun. We’re hoping for a look during the darkness of totality to spot the comet and its tail pointing away from the sun.
I’ve plotted the path of the comet as it nears the sun. The eclipse is on April 8th, and the comet won’t reach perihelion until the 24th. At perihelion the comet will be 1.6 au from the earth, actually .78 au beyond the sun from our perspective. If the comet is visible during the eclipse, we’ll see it above the ecliptic plane. It will thereafter travel below the ecliptic plane before it passes the sun on the 24th, reaching its nearest point to our solar furnace since Queen Elizabeth II was crowned in Westminster Abbey and Edmund Hillary summited Mt. Everest.
One might think that the perihelion would be on or near the ecliptic plane, which would be true of a comet with near zero inclination. But remember that Pons-Brooks’s orbit is highly inclined (tilted) and elongated (stretched out along its “major axis”). The whole configuration of the orbit means, in untechnical terms, that the long part of the orbit tilts way up and the short part tilts down, with the sun at one of the focus points (foci) of the orbit’s ellipse, resting in its orbital plane.
You can see on my computer-generated diagram for the April 8th solar eclipse that brilliant Venus will be westward of the eclipsed sun (a “morning star,” leading it by 15 degrees). Mercury is snug in on the east (trailing it by about 6 degrees, an evening object), with the comet farther out east (by about 25 degrees) [4]. Not shown on the chart but something to look for is Jupiter off to the left (about 30 degrees from the sun). And to the right about 35 degrees from the sun is a sparkling morning conjunction of Saturn and Mars, about 1.5 degrees apart.
All in all, a beautiful line-up! During the total solar eclipse, I’m looking forward to spotting the comet among this little flotilla of sun/moon and planets. If you are lucky enough to be in the path of totality, let me know if you succeed in seeing it!
NOTES
[1] The au, or astronomical unit, is the astronomer’s solar system yardstick. It is the mean distance between the earth and the sun.
[2] Meyer, Maik; Kobayashi, Takao; Nakano, Syuichi; and Green, Daniel W. E. “Comet 12P/Pons-Brooks: Identification with Comets C/1385 U1 and C/1457 A1.” International Comet Quarterly, arXiv:2012.15583v1 [astro-ph.EP] 31 Dec 2020.
[3] I’ve reversed the color in this image from the authors’ dark on white background picture in the article.
[4] The scale sizes of the sun and planets in the diagram are all off, of course, enlarged here for clarity.
Comments