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In our Solar system, all of the planets orbit the Sun along a plane known as the ecliptic.

If we extend the ecliptic plane into a sphere of the same radius centred around our Sun, is there any established name for the "North" or "South" regions in this sphere, where there aren't any planets?

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    $\begingroup$ "where there aren't any planets..." that we know of! Interesting question. I think that northern and southern hemispheres may work, but let's see what others think. $\endgroup$ – uhoh Dec 26 '20 at 23:24
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    $\begingroup$ Actually, only Earth orbits precisely in the ecliptic plane, since Earth's orbit defines that plane. Of the other known planets Mercury's orbit is the most inclined with respect to the ecliptic, at 7.005°. Pluto's is 17.16°. Other, still-undiscovered planet-sized bodies in the far outer solar system could have larger inclinations. $\endgroup$ – Tom Spilker Dec 28 '20 at 21:03
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    $\begingroup$ @TomSpilker the ecliptic is defined by Earth's orbit? Huh, that's interesting. I would've thought it be by the Sun's equator. I wonder how we'll define exosolar ecliptic planes. $\endgroup$ – BMF Dec 29 '20 at 0:14
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    $\begingroup$ @BMF Yeah, my ideal reference plane would be one that goes through the barycenter of the entire system and is perpendicular to the entire system's total angular momentum vector. However, to know this precisely you have to know the position, mass and velocity of every object in the system! We're to the point with the solar system that we could calculate that fairly precisely — as long as there's no 10-Earth-mass "planet 9" out there — but that'd be tough with extrasolar planetary systems. The ecliptic reference plane is well entrenched in the rather change-averse astronomical community. $\endgroup$ – Tom Spilker Dec 29 '20 at 2:05
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    $\begingroup$ @TomSpilker that does sound like a more fundamental reference frame. Though, I wouldn't want to be the guy who has to find the galactic ecliptic $\endgroup$ – BMF Dec 29 '20 at 2:09
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I think the terms would be northern and southern celestial hemispheres respectively, possibly qualified by 'ecliptic'. However those terms depend on the coordinate system you're using. The usual two are both Earth-based, either the equatorial coordinate system, or the ecliptic coordinate system. I think that, unqualified, 'northern celestial hemisphere' would apply to an Earth-based equatorial coordinate system.

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  • $\begingroup$ Thanks @tfb. I wonder if there's another adjective that refers to the Solar System? At least North/South celestial hemispheres can be a good substitute for Earth-based use cases like you've mentioned. $\endgroup$ – SimonT Dec 30 '20 at 18:04

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