You may have heard about the “green comet.” Here is my rendering of its trajectory in space.
Grid lines on the ecliptic plane are 1 astronomical unit (AU, Sun-Earth distance, about 150 million km or 93 million miles) apart. Stalks from the comet to the ecliptic plane are 1 month apart. The ram’s-horns symbol marks the vernal equinox direction,
The comet is called C/2022E3 (ZTF). “C” means that it is a non-periodic comet (in an orbit longer than 200 years). “E3” means it was the 3rd comet discovery (or recovery) in the first half of March. “ZTF” is its “name”; it was a by-product of the Zwicky Transient Facility, a program of wide-field imaging of the sky using a camera attached to a telescope in the Palomar Observatory in California. The astronomers who noticed the comet’s faint magnitude-17 image were Bryce Bolin and Frank Masci, but in situations like this it is customary to use the name of the institution.
The orbit is close to being a parabola, with a period perhaps of 50,000 years. It is inclined 109° to the ecliptic plane – more than 90°, meaning that it is almost perpendicular but slightly retrograde – the opposite direction to that of the planets. The perihelion, or innermost point, of the orbit is not far outside Earth’s orbit. A body in such an eccentric orbit whirls swiftly around the perihelion.
Here is how the comet will race across the far north of our sky and then southward in February.
According to my calculations based on orbital elements from Harvard’s Minor Planet Center (which could change slightly as observations continue):
On 2022 March 2, when it was discovered, the comet was almost as distant from the Sun as Jupiter; it was in Aquila, not far north of the celestial equator and low in the morning sky. On July 8 it was at opposition, in Lyra. On July 30 it reached declination about 30° north, in Hercules, before curving back somewhat southward. On Nov. 10 it was at conjunction with the Sun but well north of it, in Serpens just south of Coma Berenices. On 2023 Jan. 12 it was at perihelion, in Coma, 1.11 AU from the Sun. It dawdled in Coma, before seeming to take off northward because Earth came around south of it. On Jan. 28 it will be again at opposition, far north, in Ursa Minor (the Pole Star’s constellation). On Jan. 30 it will be northernmost, only about 9° from the north celestial pole, just above the “head” of Draco.
Then in February it will drop southward only about 0.2 AU outside Earth’s orbit; thus we see it ahead of us and racing vertically south. On Feb. 1 we will be nearest to it, 0.29 AU, and it should be brightest, at magnitude 5. This is comfortably within the naked-eye range. Brightness predictions for comets are notoriously unreliable, because of their “behavior” – releasing dust and gas in irregular amounts and directions to form the cloud-like head and the tail as the ices of the nucleus (the tiny solid body) are heated by the Sun. At this climax, the comet will be 121° from the Sun and very high in the evening sky.
On Feb. 12 it will descend across the ecliptic, in Taurus. On March 2 it will become 1 AU away from us. On March 3, it will descend into the southern celestial hemisphere, its magnitude having sunk to 8. On June 20 it will be again at conjunction with the Sun, well south of it, about 2.6 AU from it and 3.2 AU from Earth.
Green, though in the middle of the visible spectrum, is relatively rare as an impression given to our eyes by astronomical objects. In the comet’s head, it is thought to be due to a reaction of diatomic carbon (C2) molecules.
Here’s the scene. An arrow shows the comet’s movement from 2 days before to 2 days after. The Moon unfortunately is just to the left, in Taurus. Look a few days earlier and later, when the comet should be about as bright. Its tail, pushed outward by the radiation pressure of sunlight, may or may not be spectacular.
The green comet at its nearest and brightest happens to be crossing Camelopardalis, the large but little-known far-northern constellation that tops the cover painting of Astronomical Calendar 2023.
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After weeks of clouds and rain, this morning, January 17, was very clear. Twenty minutes before astronomical dawn I saw this comet through 10×42 image-stabilized binoculars from my back porch in light-polluted San Francisco. It was an easy star hop from Eta Herculis along Hercules’ western leg. The comet was in the same field of view as Hercules’ foot, Chi Her. An obvious fuzzy wedge, I would guess about 5 arcminutes across and about 7th magnitude.
Seeing this comet made my day.
Thank you Guy. That makes sense. The first reference I found was about satellites in Earth orbit, and it used right ascension. Of course for objects orbiting the Sun, ecliptic longitude would be the preferred reference. Anyway, I learned something out of my befuddlement. The next time I see “Omega” in some table of comets, I’ll have a vague notion what it means.
Anthony, I had the same reaction to hearing about “the green comet” ~ many comets look green! Two examples in recent memory for me were Comet Lovejoy in January 2015 and Comet Wirtanen in December 2018:
http://www.starvergnuegen.com/astropix/2015/2015_01_09_lovejoy_index.html
http://www.starvergnuegen.com/astropix/2018/2018_12_11_wirtanen_1.jpg
I haven’t seen ZTF yet but hope to now that the Moon is out of the way.
thanks again for the work that shows clearly the path and locations calendrically! excellent art as usual, as well.
Let’s say it’s the March equinox and the Sun is at the first point of Aries as seen from Earth. A comet is in the north ecliptic hemisphere with an orbital inclination of exactly 90 degrees. It could be traveling from beyond the nighttime side of the Earth toward the Sun, or from the far side of the Sun toward the Earth, or at any angle in between. I poked around the internet and learned that an additional variable, the right ascension of the ascending node (notated as Omega), specifies where the comet crosses the celestial equator.
The inclination of an orbit is the angle, at its ascending node through the ecliptic, between the ecliptic plane and the orbit’s plane.
The position of that ascending node is given by two numbers:
large omega, the longitude of the ascending node, angle from the vernal equinox direction to the ascending node;
small omega, the argument of perihelion, angle from its ascending node to its perihelion.
For instance, for comet C/2022 E3, the longitude of the ascending node is about 303 degrees (way off to the left in my space picture); the argument of perihelion is about 146, bringing it around to the perihelion you see in the picture.
The inclination, back at that ascending node off to the left, is 109.
That’s why you see those stalks falling just beyond the sun.
If the inclination were 90, the comet would pass ecliptically due north of the sun, and drop at a right-angle through the ecliptic plane.
There seems little point in trying to translate all these into the right-ascension system (relative to Earth’s spin), in which it could work out in an infinity of ways depending on the orientation of the orbit (the two omegas).
Diatomic carbon or C2 emits light in the so-called Swan Bands.
https://en.wikipedia.org/wiki/Swan_band#/media/File:Spectrum_of_blue_flame.png
If I’m picturing this right, an object with an inclination of exactly 90 degrees could be going either east to west or west to east. Is that right?
No, neither. It would be moving in the plane perpendicular to the ecliptic, therefpre neither eastward nor westward in relation to the sun.
I had my first view of Comet C/2022 E3 (ZTF) this morning. Here’s my report.
I got up just before 5:00 a.m. EST this morning and was outside about 15 minutes later with a chair and my Canon 15×50 IS binocular. Unfortunately, the “mostly clear” skies were about 40% cloudy, and, of course, clouds covered the area between Hercules and Bootes where Comet C/2022 E3 (ZTF) was located. It was bitterly cold and windy.
Undeterred, I went back inside and returned a bit later. The skies had cleared somewhat so I gave the comet a go. The waning Moon and my red-zone skies weren’t making the effort easy. I saw a couple of satellites pass through the area but was uncertain if I had sighted the comet. I also viewed Alcor-Mizar and Melotte 111.
After another trip inside to warm up and consult various finder charts, I went outside once more. This time I was able to pinpoint its exact location and was able to see C/2022 E3 (ZTF) as a dim, fuzzy spot.
If I have the opportunity, I’ll try again observing the comet again tomorrow morning, this time with a telescope.
Congratulations on a tenacious effort!
Looks like a trip to the cabin up North in Michigan USA is called for.
And comets are extended objects, not points like stars, so a fifth magnitude comet looks fainter than a fifth magnitude star. Binoculars would help.
A few days ago my neighbor asked me if I had seen “the green comet.” I think this epithet is largely a social media artifact. Many comets have greenish comas due to diatomic carbon. C2 molecules fluoresce green light when excited by sunlight. The color can be vivid in photographs. Because of our perspective from Earth, comet ZTF appears to have a short dust tail, so in photographs the coma is more prominent. I suspect you would need a dark sky, a big telescope, and a Swan band filter to visually detect any color in this comet.
Guy, thanks very much for the charts. I hope to catch a glimpse of this comet, if we ever get a break in the clouds and rain.
Please explain more on how it can be greater than 90 degrees from the ecliptic plane. I can’t picture why being retrograde matters.
The inclination of a body’s orbit to the ecliptic is 0 if the body actually orbits in the ecliptic plane, a few degrees for a planet (7 for Mercury, for instance), more for some asteroids and comets. If the angle reaches 90, it means that the orbit is perpendicular to the ecliptic. More than 90, and it is tilted the other way. If it reaches 180, the orbit is flat in the ecliptic plane, but going around the Sun in the opposite direction (clockwise as seen from the north, instead of counterclockwise).
Why green?
As I mentioned briefly, the color is thought to be dure to a reaction involving diatomic carbon (C2). I can’t pretend to understand the full reaction, nor do I know why it is observed in the head of this comet and not of others. More news is probably appearing in the Minor Planet Circulars.
Mag 5 is only comfortably in the naked eye range if you live in a dark area.In my Bortle 6 area mag 5 is about the cut off depending on the night, and Moon,I can sometimes just make out those 2 fainter stars in the box part of Ursa Minor which are around the 5th mag.. In the Bortle 4 area to the south I can make them out no problem.Although my Bortle 6 is a bit better than many people’s as I have the sea to the east which apart from a few ships is dark although even that is filling up with wind turbines with little red lights atop them, better than white I suppose,but not ideal.