Orbital holidays
December 12, 2019 1:38 PM Subscribe
I'm trying to think of all the events related to the earth's orbit that might be celebrated on a regular basis. Solstices and equinoxes, obviously. There's also distance to the sun which gives us perihelion and aphelion. I have additional questions about those inside. And what about the inclination of the earth's orbit relative to reference planes of the solar system? Is there a word for the extremes of this and when are they? Which reference plane to use? Are there any other interesting annual events that I'm missing?
Solstice and equinoxes are very easy to find information on, and the most obvious to an earth-based observer.
Perihelion and aphelion took a little longer to find a reference for the exact time, but it is out there. Is there an equinox equivalent for this cycle? The midpoint in time is easy to calculate, but since it's an ellipse the spatial midpoint (average of the two extremes) would be more relevant. When is that, and what is it called? I imagine that's also different than the mean orbital distance - when do we pass through that?
Next, orbital plane. The earth obits in the plane of the ecliptic by definition, but there are other reference planes for the solar system: the invariable plane, which is roughy the average plane of all mass orbiting the sun; There's the plane of the sun's equator. Are there other reference planes?
As the earth orbits, it should have an annual cycle that takes it in and out of these reference planes. When is it at the extremes and when does it pass through the plane? Which reference plane to use? It's not immediately obvious. And what to call it?
That's all the orbital events I can think of, barring long term events like precession that are neat but not something you can make a regular thing out of in a human lifetime, but I'd love to hear about any others.
Solstice and equinoxes are very easy to find information on, and the most obvious to an earth-based observer.
Perihelion and aphelion took a little longer to find a reference for the exact time, but it is out there. Is there an equinox equivalent for this cycle? The midpoint in time is easy to calculate, but since it's an ellipse the spatial midpoint (average of the two extremes) would be more relevant. When is that, and what is it called? I imagine that's also different than the mean orbital distance - when do we pass through that?
Next, orbital plane. The earth obits in the plane of the ecliptic by definition, but there are other reference planes for the solar system: the invariable plane, which is roughy the average plane of all mass orbiting the sun; There's the plane of the sun's equator. Are there other reference planes?
As the earth orbits, it should have an annual cycle that takes it in and out of these reference planes. When is it at the extremes and when does it pass through the plane? Which reference plane to use? It's not immediately obvious. And what to call it?
That's all the orbital events I can think of, barring long term events like precession that are neat but not something you can make a regular thing out of in a human lifetime, but I'd love to hear about any others.
Best answer: There's all the spherical astronomy points: for example, you could celebrate "full Mars" or "full Jupiter" every year (opposition), but it'd be a different day every year.
posted by AzraelBrown at 1:56 PM on December 12, 2019
posted by AzraelBrown at 1:56 PM on December 12, 2019
Best answer: The midpoint in time is easy to calculate, but since it's an ellipse the spatial midpoint (average of the two extremes) would be more relevant. When is that, and what is it called? I imagine that's also different than the mean orbital distance - when do we pass through that?
The spatial midpoint between the perihelion and aphelion of an elliptical orbit is where the ellipse intersects its semi-minor axis. It just so happens that at this point, the distance to the Sun (i.e., one of the foci of the ellipse) is equal to the semi-major axis, which is usually taken to be the "mean orbital distance". So these two points are actually the same.
When we pass through these points I'm not sure, though I'd expect it to be closer to perihelion (January) than aphelion (July) since the Earth is moving faster when it's closer to the Sun.
posted by Johnny Assay at 2:11 PM on December 12, 2019
The spatial midpoint between the perihelion and aphelion of an elliptical orbit is where the ellipse intersects its semi-minor axis. It just so happens that at this point, the distance to the Sun (i.e., one of the foci of the ellipse) is equal to the semi-major axis, which is usually taken to be the "mean orbital distance". So these two points are actually the same.
When we pass through these points I'm not sure, though I'd expect it to be closer to perihelion (January) than aphelion (July) since the Earth is moving faster when it's closer to the Sun.
posted by Johnny Assay at 2:11 PM on December 12, 2019
Best answer: The Earth’s orbit is the ecliptic, which is the usual reference frame for the Solar System.
For any other reference plane, then there are two points in each orbit when the Earth passes through the reference plane, called the ascending node and the descending node, for when the Earth passes “up” (south to north) or “down” through the reference plane, respectively. (Up/down, and north/south, of course being somewhat arbitrarily assigned, but usually when you look from the north, things are rotating and orbiting counterclockwise.) I don’t know of a special name for the points when the Earth is highest or lowest above a reference plane, but they would be 90° (Approx. a quarter year) from the ascending nodes.
posted by BrashTech at 4:20 PM on December 12, 2019
For any other reference plane, then there are two points in each orbit when the Earth passes through the reference plane, called the ascending node and the descending node, for when the Earth passes “up” (south to north) or “down” through the reference plane, respectively. (Up/down, and north/south, of course being somewhat arbitrarily assigned, but usually when you look from the north, things are rotating and orbiting counterclockwise.) I don’t know of a special name for the points when the Earth is highest or lowest above a reference plane, but they would be 90° (Approx. a quarter year) from the ascending nodes.
posted by BrashTech at 4:20 PM on December 12, 2019
Response by poster: Thank you for the orbital node terminology, that's very useful.
Looking around a little more, it seems like the invariable plane is the one being used by astronomers trying to establish a more objective reference for the solar system, which makes sense.
The elliptical nature of the earth's orbit should also affect the timing of it's up and down motion with respect to the invariable plane, right? How the ellipse is oriented relative to the inclination between the two planes would be relevant there, if I'm thinking through that properly.
Which also leads me to the question: is the Earth-Sun barycenter on the invariable plane? I have to imagine that it is (or close enough, anyway), but if it wasn't that would also complicate things. Both that and the elliptical orbit would introduce an asymmetry between the 'above' portion and the 'below' portion of earth's orbit.
posted by vibratory manner of working at 2:54 PM on December 13, 2019
Looking around a little more, it seems like the invariable plane is the one being used by astronomers trying to establish a more objective reference for the solar system, which makes sense.
The elliptical nature of the earth's orbit should also affect the timing of it's up and down motion with respect to the invariable plane, right? How the ellipse is oriented relative to the inclination between the two planes would be relevant there, if I'm thinking through that properly.
Which also leads me to the question: is the Earth-Sun barycenter on the invariable plane? I have to imagine that it is (or close enough, anyway), but if it wasn't that would also complicate things. Both that and the elliptical orbit would introduce an asymmetry between the 'above' portion and the 'below' portion of earth's orbit.
posted by vibratory manner of working at 2:54 PM on December 13, 2019
I'm surprised that nobody's yet mentioned my favorite orbital holidays, the cross-quarter days. You can google "cross-quarter days" for all kinds of great references, but in short, these are the days halfway between the solstices and the equinoxes. They are (or used to be): around Feb 1 or 2, around May 1 or 2, around August 1 or 2, and around November 1 or 2.
What's neat is that most of these are still celebrated today! We call them Groundhog Day/Candlemas, May Day, Lammas (that's an unusual one, I agree), and All Saint's Day/Halloween.
(Unfortunately, the switch from the Julian to the Gregorian calendar, or the precession of the equinox, or something like that, means that these orbital events have drifted a bit from the true solstice-equinox midpoint, which I think now occur on the 6th or 7th, so when we celebrate Groundhog Day on the 2nd we're a bit early for the cross-quarter day. Of course, Christmas was originally on the winter solstice and that's drifted a bit, too, so we shouldn't be too upset.)
posted by fuzzy.little.sock at 9:09 PM on December 13, 2019 [1 favorite]
What's neat is that most of these are still celebrated today! We call them Groundhog Day/Candlemas, May Day, Lammas (that's an unusual one, I agree), and All Saint's Day/Halloween.
(Unfortunately, the switch from the Julian to the Gregorian calendar, or the precession of the equinox, or something like that, means that these orbital events have drifted a bit from the true solstice-equinox midpoint, which I think now occur on the 6th or 7th, so when we celebrate Groundhog Day on the 2nd we're a bit early for the cross-quarter day. Of course, Christmas was originally on the winter solstice and that's drifted a bit, too, so we shouldn't be too upset.)
posted by fuzzy.little.sock at 9:09 PM on December 13, 2019 [1 favorite]
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posted by adamrice at 1:49 PM on December 12, 2019 [1 favorite]