The “hairy stars” have been absent from the last few Astronomical Calendars – as some readers have lamented! There were several reasons why it had become difficult for me to find information, far enough in advance, about comets likely to be observable.
There is a list of orbital elements for comets, provided by the Minor Planet Center (the International Astronomical Union’s clearing-house for information, at Cambridge, Massachusetts). I thought of using this list and, for each of the comets in it – over a thousand – running calculations for the whole year, to find which of them should become observable. The only criterion I’ve used is brightness (astronomically expressed as “magnitude”), though other factors can affect observability: elongation (angular distance from the Sun) and declination (how far north or south in the map of the sky).
It’s been a longer programming struggle than I anticipated, with pauses and renewed attempts. At last I have it disentangled. It finds these comets reaching peaks of brightness better than magnitude 9.5 in 2024.
dec mag elo 62P Tsuchinshan 2024 Jan 1 13 9.1-111 C/2021 S3 PANSTARRS 2024 Feb 28 -7 7.4 -69 12P Pons-Brooks 2024 Apr 21 10 4.4 23 13P Olbers 2024 Jul 4 42 7.5 31 C/2023 A3 Tsuchinshan-ATLAS 2024 Oct 5 -5 0.4 -15
The columns “dec” and “elo” show approximate declination and elongation, in degrees. Negative elongation means westward – that is, in the morning sky, which is also a consideration for observers.
In the logarithmic magnitude scale, smaller numbers mean brighter. (“1st magnitude” is brighter than “2nd magnitude”.) The naked-eye threshold is about 5 or 6 (depending on sky conditions). So two of these five may reach that higher threshold, though uncomfortably close to the Sun. The others should be findable with binoculars and small telescopes.
The magnitude threshold is arbitrary. Changing it admits more or fewer comets. And there is always the caution that their actual behavior can depart wildly from prediction; they can flare, or shed so much of their cloudy material that they become like asteroids, or can break into several bodies, or disintegrate and disappear. Never bet on a comet! And new comets of the non-periodic kind, which are unpredictable and are liable to be among the brightest, can arrive from deep space at any time.
I checked my result by asking Alan Hale, of Cloudcroft, New Mexico, renowned comet-hunter, discoverer of the greatest of the twentieth century, C/1995 O1 Hale-Bopp. He contributed the “Comets” section to several issues of the Astronomical Calendar, and continued to help me by giving me a list of the comets he expected to be worth featuring in the coming year. Reassuringly, he replied that my list for 2024 “would be identical to the list I would come up with, with that limiting magnitude.”
This space view shows the paths in 2024 of the first two of these comets. The ecliptic plane is indicated by grid lines at intervals of 1 AU (astronomical unit, Sun-Earth distance, about 150 million km or 93 million miles). The thicker line is the vernal equinox direction (zero point for celestial mapping). The viewpoint is 6 AU from the Sun, 35° north of the plane, at ecliptic longitude 35°. Stalks from the comets to the plane are at 1-month intervals. Sightlines from Earth to the comets are drawn at dates of opposition. Earth is exaggerated 500 times in size, the Sun only 5 times. The comets themselves, that is, their solid nuclei. would be vanishingly small. See the end note about enlarging illustrations.
62P Tsuchinsha’s designation means that it was the 62nd comet to be recognized as periodic – with elliptical orbit of under 200 years. January 1 counts as its brightest moment for 2024 because it passed its peak of brightness on December 25. So we described it then, and gave a finder chart with its track across constellation Leo. Reports are that it may be running about one magnitude brighter than predicted. It will be nearest to Earth (0.495 AU) on Jan. 29, but half a magnitude dimmer because farther from the Sun. It will keep climbing higher in the morning sky until we pass it at opposition on March 24, but by then will probably have faded to magnitude 12.
C/2021 S3 PANSTARRSS was one of the discoveries of the automated search program Pan-STARRS (Panoramic Survey Telescope and Rapid Response System, on Mount Haleakala in Hawaii); S3 means it was the 3rd comet discovery or recovery in the second half of September. It has come up from the south, in a vast and steeply inclined orbit (eccentricity given as 1.0002, inclination 58.5°). In contrast with 62P, it is running fainter than the prediction. It will cross the ecliptic plane northward on Feb. 7, pass through perihelion (closest to the Sun, 1.32 AU, not far outside Earth;s orbit) on Feb. 14; should be at its brightest, about magnitude 7, Feb. 28; cross the celestial equator northward on March 4; be nearest to Earth, 1.29 AU, on March 14; will probably fade to mag. 10 by the end of May. In August it is at opposition, far north in the sky, but nearly 2.5 AU away from us and probably as dim as mag. 12.
Here is a chart for this comet as it travels in February almost along the plane of the Milky Way. We draw comets’ tails schematically, to suggest the direction in which they are driven outward by radiation pressure from the Sun; they may or may not be as large as shown.
We’ll have interesting things to say about the others as their times of brightness approach, especially Comet Olbers (whose recovery was another feat of Alan Hale).
But the Minor Planet Center’s list of orbital elements changes, as observations of comets are continually sent in. Magnitude parameters (two numbers on which predictions are based, expressing a body’s size and the rate at which it brightens by shedding cloudy material as it approaches the Sun) may be changed, and new comets will be added. So I shall have to re-load that list and re-run the calculations, and our roster of observable comets may change.
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ILLUSTRATIONS in these posts are made with precision but have to be inserted in another format. You may be able to enlarge them on your monitor. One way: right-click, and choose “”View image”” or “”Open image in new tab”, then enlarge. Or choose “”Copy image””, then put it on your desktop, then open it. On an iPad or phone, use the finger gesture that enlarges (spreading with two fingers, or tapping and dragging with three fingers). Other methods have been suggested, such as dragging the image to the desktop and opening it in other ways.
A comet’s degree of condensation is estimated on a ten-point scale. A comet with zero condensation appears uniformly diffuse. Five is moderately condensed. Nine looks nearly stellar.
A comet with a moderately or very condensed pseudo-nucleus can still have a coma and a tail. It’s easier to see the coma and tail if you can find the pseudo-nucleus and look around it with averted vision.
Thank you very much, Guy. It’s helpful to get a peek “behind the curtain” and to be reminded that all the astronomical information I can so easily take for granted has been painstakingly figured out and vetted by actual human beings!
There’s at least one more thing that greatly influences the visibility of comets, especially for those of us who observe through light pollution: a comet’s degree of condensation. Let’s imagine two comets, each fifth magnitude, bright enough to see with the naked eye in a dark sky, and *possibly* visible through binoculars from the city. The center of one of these comets is highly condensed, it looks almost like a point of light. I’ll be able to see this one through my binoculars from the city. The light of the other comet is evenly spread out across an area as wide as the Moon on the sky. This comet will be invisible through urban light pollution.
Good point. Observers’ reports often mention that the comet’s head is “well condensed” or otherwise. The more condensed, the more like a star, which is point-like. So a well-condensed comet head is almost as easily seen as a star of the same magnitude.
But is there a variable (a number) that specifies condensation, for predicting it, or recording it? I don’t know of one.
And if a comet appears just as one of the stars, and you see no coma or tail, is it really worth seeing, except for recording its position?
It would be nice if you could add comets to your Almanac, l miss it very much..