Portion of the Bayeux Tapestry showing Comet Halley
One of the benefits of astronomy is time travel: I enjoy moving around in time, mathematically, exploring what happened or will likely happen – no matter how many years or centuries ahead or behind I spin the clock. In this field, time and distance are both united (in the space-time continuum) and unbounded (except arguably by the Big Bang, 13.7 billion years ago, but what came before?). There is nothing “old” in astronomy: there is intrinsically no greater relevancy to the recent past than to the distant past. I can place my point of view anywhere, and that anywhere becomes the present. Like the geologist, the astronomer sees time as his great workbench spreading out in both directions. It makes no difference whether he or she looks back a thousand or two thousand years, five centuries, or five years, or ahead ten years, a thousand, or a million. The time we pick for our frame of reference can be completely arbitrary, guided by scientific purpose or ordinary curiosity.
In that spirit, and as this summer marks the “35/40 point” of Comet Halley (1P/Halley), whose orbital period is 75 years – near an average human lifespan – I will travel back 35 years to its last apparition. Then I will spin the clock forward 40 years from now to see what’s in store for its return. (It’s never too early to plan!)
1986
It is early 1986. It is sadly the year of the Challenger explosion and the Chernobyl accident. It is also the year of the “Halley Armada” of half a dozen space probes sent to observe the comet at close range, ESA’s Giotto being the first to capture a close-up from about 600 kilometers.
I am living in Hawaii, and the skies then are reasonably dark from my suburban home. I get the comet’s coordinates out of Sky & Telescope magazine and track its progress. Peering through my 6” Newtonian telescope, I let my eyes adjust to the comet’s ethereal glow against the background stars. Here it is at last, history’s most famous comet: described on Babylonian cuneiform tablets as early as 164 BC, depicted on the Bayeux tapestry after the Norman invasion of 1066; painted by Giotto in 1303 in his Adoration of the Magi (after the comet’s 1301 apparition); the great comet whose repetitions through history were first recognized by Edmond Halley in 1705. Here now in my backyard, through my eyepiece, the great comet is just the merest wisp of light. I sketch it and the surrounding stars with graphite pencil in my black sketchbook. I am excited as the comet gradually approaches naked eye visibility. Here are excerpts (in quotes) from my sketchbook; I include two entries from late 1985. The sketch you see here is from the January 5th observation. (Reading my notes over now, I can make no claim to profundity):
November 14, 1985. My first observation. “22:00 LMT. Visibility fair. Seeing fair. Some high haze. Starlike nucleus. No tail seen.”
I estimate the comet’s magnitude at about 7.0. My three pencil drawings show wispy fuzzy round circles with dark centers, depicting clear movement among the stars over time. Halley, earth, and the sun are in a line, with Halley just outside the orbit of Mars. The comet has just emerged from below the ecliptic plane. It is .7 au from earth and closing (the mean distance between earth and sun, roughly 150 million kilometers, being one astronomical unit, or au, the astronomer’s standard yardstick).
[Explanatory note: Halley spends most of its time in its long slender orbit far below the ecliptic plane of the solar system; it only raises its head above that plane when it nears the sun. Like some mythical dragon from the underworld, it comes up for light and air between the orbits of Mars and Jupiter, lingers there briefly to look around and then, having received its solar sustenance, dives back under between the orbits of Venus and Earth to return to its underworld haunt for seven and half more decades.]
December 5, 1985. “22:30. Visibility good; somewhat brightish western sky. RA 0019, DEC -09 47. This [the drawing] gives a sense of what aspects of the tail were visible but hard to discern sharply. The nucleus has grown much larger and brighter. Motion rapid westward. Tail perhaps somewhat fainter in relation to coma than drawing indicates.” Halley is now .65 au from the earth.
January 3, 1986. “Visibility fair – brightish western sky. Windy. Seeing poor (Jupiter not even a disk, stars impossible to clearly focus.) RA 2208, DEC -03 15. Saw it in the western sky just after dusk, in Aquarius, near the Water Jar of that constellation. It was windy and the comet was setting over the house (from where I had the telescope on the patio). As shown above [in the drawing], the tail was more pronounced on the north side of the comet. I tried not to put the extreme contrast in this drawing as it appears in the December 5 drawing – the difference is not attributable to decreased magnitude.”
Halley crossed the earth’s orbit in late December, and at this point our planet is well ahead on its leftward curve around its orbital track while Halley is dashing from right to left behind it, on their way to opposite sides of the sun at perihelion. Halley is about 1.2 au from Earth and widening. At the end of February, though, it starts to close with earth again as earth comes around the rink from the other side to meet it and wish bon voyage the out-bounding comet.
January 5, 1986. “Visibility fair (comet low in the west). Seeing: fair. Ra 22 04 Dec -03 41. Coma somewhat oval, with nucleus in the forward one third, as shown. Was brighter before it got fully dark, when Honolulu lights in the West drowned it out. It was hard to discern any sharp cutoff where coma ended and the tail began. 25 mm view shown. Comet was near α Aquarii. View somewhat enlarged from 1/3/86 drawing, but perhaps closer to actual size.”
Halley is less than .9 au from the Sun and closing rapidly. It intersects Venus’s orbit in about 10 days. (It does not ever intersect Mercury’s orbit.) Halley’s perihelion passage occurs February 5, 1986.
March 20, 1986. Fallbrook, California: “7 x 50 binocs. 3:30 AM, looking southeast, [I saw the comet] just east of Sagittarius, perhaps 30° altitude.” A small faint sketch records the ghostly comet visible above the trees, its distinct nucleus pointing to the horizon and its long faint tail stretching upward at about a 30° from vertical.
March 5, 7th and 11th, 1986. Mariners Ridge, Honolulu, each at about 5:45 AM as it appeared in 7 x 50 binoculars. I note that the comet had a “somewhat fat tail, not as long and graceful as in the best view of March 20, 1986.”
April 16, 1986. A small fan of graphite on the paper: “In the evening South with 7 x 50 binoculars. Globular -like, with discernible direction.”
April 19, 1986. Now a small, “faint fuzzy” in the sky: “Barbers Point [Oahu] – various telescopes. Near Omega Centauri, which resembled it. (High clouds; poor visibility).” This is my last observation of Halley, low on the horizon through many air masses.
Before its February perihelion, Halley is ill-placed for earthly observers on the opposite side of the sun. But as earth motors counterclockwise around its orbit, it somewhat catches up as Halley descends under the Earth’s orbital plane. By April 1986, Halley becomes a respectable half astronomical unit from earth, presenting itself as a pretty photographic object to southern hemisphere observers.
Alas, by summertime Halley says aloha to our solar neighborhood and begins its slow journey outward. In the June 1986 issue of Sky & Telescope, Alan MacRobert writes, “The audience has mostly drifted away and the theater is nearly empty as comet Halley performs its closing act this month. Only a few devotees remain to watch the curtain come down on the 11-month show.” The comet is fading from 7th to 8th magnitude in the constellation of Sextans. The August 1986 issue – 35 years ago – features a Halley Finale, showing many glorious views of the comet photographed mostly from the southern hemisphere.
2021 to 2061
Now we are back home in August 2021, and Comet Halley is a trace over 35 au from the sun, drifting slowly toward the peak of its orbit like a rickety roller coaster creeping toward the top of its run. It passed Neptune’s orbit back in 2009 and by my calculation is a little more than 12 million km away from aphelion, not far by astronomical standards, and at its snail’s pace you can see why it will take over two years to get there. Save the champagne for December 9, 2023, because on that date Halley is at the halfway point: Aphelion occurs at 35.14346 au. At that moment the comet stops moving away from the sun at about one in the morning universal time, then slowly begins its return.
This slow speed actually isn’t too surprising. Think of when you’d shoot a skyrocket vertically upward. You see it gradually slow to a dead stop before it falls back down; there is a moment of zero vertical velocity (though transverse velocity still occurs since the comet’s orbit is an ellipse). For a comet, the sun is “down,” and the little ball of ice and rock simply falls back to the sun. Gravity is always the winner in this game. (At least for comets in bound – elliptical – orbits.) As the comet heads outward, it is relentlessly pulled back, as if the dark voice of gravity were saying “Oh no you don’t – you will come back to me.”
After aphelion, Halley like the Little Engine That Could begins its increasingly rapid return to the sun: it will cross the orbit of Neptune in 2039, the orbit of Uranus in 2053, the orbit of Saturn in 2058, the orbit of Jupiter in 2060 (where it will come particularly close to that planet) and come blazing past all the rest of the orbits for its grand show in 2061.
Why grand? Let’s turn the clock ahead 40 years from now to note Comet Halley’s next perihelion.
2061
Halley’s next perihelion will occur about June 1, 2061. It is a highly favorable apparition for us because the comet and the earth are on the same side of the sun. The sun, Comet Halley, and earth are neatly lined up on June 15, 2051. A few days later, on June 18, 2061, the comet comes closest to the earth and is only .352 astronomical units from us. That is exceedingly, wonderfully close. That’s only a little more than 52 ½ million kilometers.
So mark your calendars! Unlike the 1986 apparition, Halley’s 2061 appearance is at its dramatic best for observers in the northern hemisphere. In June 2061, we can already see the movie: Earth pedals steadily counterclockwise with the rest of the planets but Halley the contrarian – whose inclination exceeds 90° – zips clockwise between the orbits of Venus and Mercury and passes us most closely in mid-June, earth and the comet pass close like cyclists rounding in opposite directions, a fast passing, brilliantly visible in morning skies.
AD 374 to 1986
I want to close by returning to our theme of cosmic perspectives in time. Apropos of that, I attach some images I generated of the paths of Halley’s Comet through history beginning with its apparition in AD 374, and continuing for the years 607, 837, 1066, 1301, 1531, 1759, and 1986. So as not to clutter the image, you might have surmised from the list that I selected only every third appearance, each separated by roughly 230 years. Through its encounters with the other planets in the solar system, particularly Jupiter and Saturn, the orbital elements of the comet (including its period) slightly change with each passage. (If they didn’t, you would see only one fat multicolored line in the picture). This meant that I had to incorporate the appropriate historical elements for each apparition into my program. I used the list of the historical elements for Comet Halley published by Dr. Donald K Yeomans, now retired but formerly with in NASA/JPL and an acknowledged expert on celestial dynamics. Thus each path you see in the picture is the comet path governed by the historical elements for the given date. I had fun creating the program (which is actually quite large) and visiting other moments in history. When I exported the generated plot and inverted the color, I was surprised by how strange it looked!
Another view is shown on my Modeling the Cosmos page. Here is a long distance view with axes added:
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