You’ve been looking at my description of Comet C/2022 E3 (ZTF) – and may getting a look at the comet itself – and much of the discussion has been about its orbit. That orbit is nearly perpendicular to Earth’s, but slightly retrograde, and what does that mean?
Six numbers, called orbital elements, suffice to describe an orbit; three of them rule its shape, the other three are angles and govern its orientation in space. Those are the ones that have been causing a bit of puzzlement. One of them is inclination (i). The other two are the longitude of the ascending node (small omega) and the argument of perihelion (capital Omega); it’s no wonder they’re a bit intimidating.
So here is a diagram to try to illustrate them. It’s a space view of a fictitious comet, in an orbit much like that of Comet 2P/Emcke, concocted by fiddling with that famous comet’s elements: perihelion date 2000 Jan. 1, perihelion distance 0.34 AU (astronomical units, Sun-Earth distances), eccentricity 0.8, Omega 60#, omega 100°, inclination 12°.
The plane of the comet’s orbit is colored light blue where it’s south of the ecliptic plane, darker blue north of it. Grid lines on the ecliptic plane are 1 AU apart. Stalks from the comet to the plane are one month apart. You see the comet’s movement over 2 years.
The longitude of the ascending node is the angle from the vernal equinox direction to the ascending node, which is the point where the darker blue starts. At that point the orbital plane slants northward through the ecliptic at the inclination angle. Then the angle from that point to the perihelion – which in this case is the start of the comet’s course – is the argument of perihelion.
I hope eventually to use this diagram to illustrate these concepts in a new edition of Albedo to Zodiac, along with another diagram that illustrates the other elements of any elliptical orbit.
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The angular ecliptic elements can also apply to parabolic and hyperbolic orbits, of course, which is relevant for many comets.
Thank you!
Hello Guy,
I’ve always felt “the argument of the perihelion” (or, more generally, “the argument of the periapsis”) is one of the most colorful terms in astronomy.
In the preceding “Green Comet” you mentioned that the orbit of C/2022E3(ZTF) “is close to being a parabola”. That may be true – to the degree that a small sample of that exceptionally elongated orbit (period ~50,000 years), observed near perihelion, may look like a parabola.
But, just as some orbits may look like circles, in the real world all cometary/asteroidal orbits are either ellipses or hyperbolas (Omaurauma) although their eccentricities may be very close to the circular 0 or, as in this case, very close to but not quite the parabolic 1.
Yes, parabola is (like circle) a limiting case; a body could only be in it for an instant before passing into either ellipse or hyperbola.