Distances in the Honorverse

Gathering Wool

When I think about the Resonance Zone in the MBS and how it affects tactics, I think about the Sol system.

If Earth will be attacked, would Earth being in Perihelion or Aphelion factor into it? What about the arrangement of the planets?

I suppose the alien tech would factor into it as well.

Anyway, in the HV it didn't seem to matter; at the time anyway.

penny wrote:Gathering Wool

When I think about the Resonance Zone in the MBS and how it affects tactics, I think about the Sol system.

If Earth will be attacked, would Earth being in Perihelion or Aphelion factor into it? What about the arrangement of the planets?

Well Sol doesn't have a wormhole -- so it has no RZ.

As for Earth's position.
Because it's a G2 star the Sun should have a 21.12 LM (379,897,000 km) hyper limit.
Earth's perihelion is 147,098,074 km from the Sun (232,798,926 km from that hyper limit)
Earth's aphelion is 152,097,701 km from the Sun (227,799,299 km) from that hyper limit.

Frankly, being an extra ~5 million km in from the limit is basically nothing; it's +/- 2.1% in distance.



Even a glacially slow (150g) ship like a Lenny Det making a zero-zero intercept from the limit would have a transit time difference of just 4 1/2 minutes between Earth at aphelion and perihelion. A 1.07% difference in transit time!

So, the time it'll take something to get to Earth is more affected by hyper emergence inaccuracies than by where in Earth's orbit it is. For comparison, the Peep attack on the Basilisk terminus in EoH missed its emergence point by "twenty-three-point-seven million klicks" -- over 4 times as much as the extra depth Earth gains at perihelion.

Jonathan_S wrote:As for Earth's position.
Because it's a G2 star the Sun should have a 21.12 LM (379,897,000 km) hyper limit.
Earth's perihelion is 147,098,074 km from the Sun (232,798,926 km from that hyper limit)
Earth's aphelion is 152,097,701 km from the Sun (227,799,299 km) from that hyper limit.

Frankly, being an extra ~5 million km in from the limit is basically nothing; it's +/- 2.1% in distance.

And this is even assuming the approximations that RFC has given us and has used throughout the series do apply. For example: is the hyperlimit centred on the star's centre or is it on the system's barycentre? The two are no the same because of everything else in the system, especially Jupiter. The Sun-Jupiter barycentre is about 1.07 Sun radii from the Sun's centre, or 744500 km. That's about 15% of the difference calculated above.

ThinksMarkedly wrote:For example: is the hyperlimit centred on the star's centre or is it on the system's barycentre? The two are no the same because of everything else in the system, especially Jupiter. The Sun-Jupiter barycentre is about 1.07 Sun radii from the Sun's centre, or 744500 km. That's about 15% of the difference calculated above.

One would expect that it is related to the center of mass, which in the case of syzygy (I just wanted to use that word instead of planetary alignment) could be even further out.

But I am replying because of some things that have annoyed me over the years. Back when Stephen Fry hosted QI, the show's elves suggested that the moon does not orbit the Earth. Also recently an astronomy video suggested that the Earth does not orbit the Sun. In a strictly technical sense those statements are true, because the orbits are more closely about the center of mass (plus in the case of the moon, there are perturbations by the other planets). But in a more ordinary sense the Earth never moves outside of the moon's orbit and it is the main object that defines the moon's orbit, so saying the moon orbits the Earth is true, as long as you do not insist that the Earth is at the center of that orbit. I expect the Sun stays with the Earth's orbit, but wonder about Mercury.

ThinksMarkedly wrote:

Jonathan_S wrote:As for Earth's position.
Because it's a G2 star the Sun should have a 21.12 LM (379,897,000 km) hyper limit.
Earth's perihelion is 147,098,074 km from the Sun (232,798,926 km from that hyper limit)
Earth's aphelion is 152,097,701 km from the Sun (227,799,299 km) from that hyper limit.

Frankly, being an extra ~5 million km in from the limit is basically nothing; it's +/- 2.1% in distance.

And this is even assuming the approximations that RFC has given us and has used throughout the series do apply. For example: is the hyperlimit centred on the star's centre or is it on the system's barycentre? The two are no the same because of everything else in the system, especially Jupiter. The Sun-Jupiter barycentre is about 1.07 Sun radii from the Sun's centre, or 744500 km. That's about 15% of the difference calculated above.

Good point.

Just one more reason that (unless you're trying to intercept a comet) the variations in orbital radius just aren't very significant to ships that can pull 100+ gees.


Plus, in combat, you shouldn't be emerging at the closest point on the hyper limit to your target anyway. So since you're coming from further out the variations in orbital radius matter even less.

tlb wrote:One would expect that it is related to the center of mass, which in the case of syzygy (I just wanted to use that word instead of planetary alignment) could be even further out.

But I am replying because of some things that have annoyed me over the years. Back when Stephen Fry hosted QI, the show's elves suggested that the moon does not orbit the Earth. Also recently an astronomy video suggested that the Earth does not orbit the Sun. In a strictly technical sense those statements are true, because the orbits are more closely about the center of mass (plus in the case of the moon, there are perturbations by the other planets). But in a more ordinary sense the Earth never moves outside of the moon's orbit and it is the main object that defines the moon's orbit, so saying the moon orbits the Earth is true, as long as you do not insist that the Earth is at the center of that orbit. I expect the Sun stays with the Earth's orbit, but wonder about Mercury.

The Barycenter of the solar system occasionally ends up slightly outside the sun (up to about 1.4 solar radii from the sun's center -- when all the gas giants line up on one side of the solar system; so up to about 278,536 km above its surface).
That's still way, way, way inside the orbit of Mercury (average 57.9 million km).

Also yeah QI is being pedantic to a level even astronomers normally aren't. As I understand it one object is said to orbit another if their common barycenter (center of mass) is always inside the larger.
The Earth Moon combo is solidly in that category.
OTOH Pluto-Charon is not, they're a dual object system, as both orbit a barycenter that lies between them (though in that case it's much closer to Pluto; about 1,200 km above its surface; and about 13% of the way between the two bodies) -- then the rest of the moons are small enough they mostly orbit that common Pluto-Charon barycenter.

Jonathan_S wrote:Also yeah QI is being pedantic to a level even astronomers normally aren't. As I understand it one object is said to orbit another if their common barycenter (center of mass) is always inside the larger.

The Earth Moon combo is solidly in that category.

OTOH Pluto-Charon is not, they're a dual object system, as both orbit a barycenter that lies between them (though in that case it's much closer to Pluto; about 1,200 km above its surface; and about 13% of the way between the two bodies) -- then the rest of the moons are small enough they mostly orbit that common Pluto-Charon barycenter.

Then what I was saying does not fit the normal definition of something orbiting another. Based on what I was saying, I would have guessed that Charon orbits Pluto (even though the center of that orbit does not lie beneath the surface of Pluto); because the path that Pluto takes is well inside the path that Charon takes. Stating it another way the distance that Pluto travels in one cycle about the center of mass is much less than the distance that Charon travels.

PS: Then in those instances where the center of mass of the Solar System is outside the surface of the Sun, do we say at that time the planets are not orbiting the Sun?

tlb wrote:PS: Then in those instances where the center of mass of the Solar System is outside the surface of the Sun, do we say at that time the planets are not orbiting the Sun?

Probably not. In any case, the only planet that can pull that off is Jupiter: in the tug of war, all other planets in one side and Jupiter on the other, Jupiter wins and pulls the barycentre into the Sun again, past the centre to its side. So the only planet that could legitimately say not to orbit the Sun but only the common barycentre would be Jupiter.

But that's a 0.1% difference, which probably means it's safe to say that it does orbit the Sun.

In the Pluto-Charon case or in the Alpha Centauri A and B (or Manticore A and B for that matter), the barycentre is way over 10% of the distance between the two components.

Where we draw the line may be only of academic curiosity. Literally.

tlb wrote:PS: Then in those instances where the center of mass of the Solar System is outside the surface of the Sun, do we say at that time the planets are not orbiting the Sun?

ThinksMarkedly wrote:Probably not. In any case, the only planet that can pull that off is Jupiter: in the tug of war, all other planets in one side and Jupiter on the other, Jupiter wins and pulls the barycentre into the Sun again, past the centre to its side. So the only planet that could legitimately say not to orbit the Sun but only the common barycentre would be Jupiter.

If all the planets are in a line out from the Sun (a planetary parade), then the CM is as far outside of the Sun as it can get and NO planet is orbiting about a point contained within the Sun. So I do not understand why you limit this to Jupiter. Not that my understanding is of great import.

tlb wrote:If all the planets are in a line out from the Sub (a planetary parade), then the CM is as far outside of the Sun as it can get and NO planet is orbiting about a point contained within the Sun. So I do not understand why you limit this to Jupiter. Not that my understanding is of great import.

If all of them are on conjunction on the same side, sure. But I am saying they can only do that with Jupiter's help.

That is, if you put Jupiter on one side all the others on the other, the others can't pull the barycentre out of the Sun against Jupiter's influence. In fact, it's on Jupiter's side of the Jupiter-Sun-everyone-else's line.

Now, if you were to remove Jupiter altogether, would the other gas giants be able to pull the barycentre from inside of the Sun? I don't know.

And, of course, for a big ball of plasma, where does the Sun end anyway? Why must it be at the photosphere? And why is that defined as optical depth of 2/3 instead of 3/4 or something else? For a gravitational phenomenon, I'd be much more interested in what fraction of the mass of the star / system is inside of a given radius. Even then we'd have an arbitrary number, like 99% or 99.8% - the Sun is 99.86% of the mass of the entire system, but how much of that is within its nominal radius of 695,700 km?

Weren't we celebrating a few months ago that the Parker Solar Probe had touched the Sun? From just over 6 million km (20 light-seconds).