Which Planet Is Closest? | Hacker News Reader

Which Planet Is Closest?

cgpgrey.com

240 pointsvl71 comments

There are some decent arguments that the Moon should be considered to be a planet, which would make the Earth/Moon two planets sharing an orbit rather than a planet and its satellite.

The Moon, if ever recognized as a planet, will of course be the closest one.

Here are some of the ways the Moon is more like a planet than a satellite [1].

I also remember an essay by Isaac Asimov that brought up another one. For all the other satellites in the solar system, if you compute the ratio of gravitation force on them from the planet they orbit to the gravitation force from the Sun on them, that ratio is greater than one.

For the Moon, it is less than one! The Sun exerts more force on the Moon than the Earth does.

Related to this, if you look at the path traced out by all the other satellites in the solar system as their planet makes one orbit around the Sun, that path is sometimes concave and sometimes convex, as the satellite circles the planet. The planet takes a convex path around the Sun, and the satellite makes a little circle around it.

An observer on the Sun would see the planet moving always moving forward, and see the satellite sometimes moving forward and sometimes moving backward.

For the Moon, that path is convex. It's basically a rounded dodecagon, as is the path of Earth. They are out of phase with each other, so that when one is near a vertex of the rounded dodecagon, the other is in the middle of a side, and so can slip past the other.

An observer on the Sun would see the Earth and Moon both always moving in the same direction, with them speeding up and slowing down, so that about 12 times in a year they trade the lead, and the overtaking one always passing on the same side.

[1] https://blogs.nasa.gov/Watch_the_Skies/2009/07/10/post_12472...

Interesting! Thanks for the new perspective. :)

I’ve been thinking about this too, in the context of where the solar system ends. I had been thinking of “the solar system” as a noun for answering multiple choice questions in grade school instead of “objects that are most greatly influenced by sol than any other gravitational field”

I think gravity itself can be taught differently and a lot of concepts would be more intuitive

Luna being the defining example of what a moon is, it would be interesting if its designation would change!

The distinction of "orbiting another body, not a star directly" seems pretty clear to me, but the idea of something being the naming example and then no longer being in the group definitely has happened before.

The word "henge" comes from Stonehenge, grew to encompass similar structures, and then was codified in a way that actually excludes Stonehenge itself:

> The "henge" portion has given its name to a class of monuments known as henges. Archaeologists define henges as earthworks consisting of a circular banked enclosure with an internal ditch. As often happens in archaeological terminology, this is a holdover from antiquarian use. Because its bank is inside its ditch, Stonehenge is not truly a henge site.

https://en.wikipedia.org/wiki/Stonehenge

Yeah, it would be weird if the moon was not a moon.

As I understand one of the reasons astronomers demoted Pluto out of the planet status is they couldn't come up with a definition of planet that kept Pluto a planet that didn't promote the moon to a planet.

I’ve never heard this and find it interesting. Do you have any reference material on this idea?

https://en.wikipedia.org/wiki/Definition_of_planet#Double_pl...

That's an interesting proposal! To give an argument against: the center of mass of the earth-moon system lies 2k km inside earth: https://en.wikipedia.org/wiki/Barycenter#Primary%E2%80%93sec...

Why is the moon vs. planet distinction important?

It would be comically ironic if we decided to categorize it as a planet, and not a moon.... but then continued calling it "The Moon".

> For the Moon, it is less than one! The Sun exerts more force on the Moon than the Earth does.

This is not a very good argument though, because the ratio of Sun influence vs Earth influence only depend on the distance of the orbit from Earth.

You can have a 10 ft size rock in a further orbit and Suns exertion of gravitational force would be even stronger! That doesn't make it a planet.

When we're wandering around on Earth we naturally tend to think of the "closeness" of locations in terms of distance because of the way we travel here. But if you're thinking of traveling to another planet you shouldn't be thinking of linear distance, you should be thinking of delta-v. And by that metric the planets form the same neat line you find in a classroom chart with Venus edging out Mars.

https://www.reddit.com/r/space/comments/29cxi6/i_made_a_delt...

Linear distance isn't entirely irrelevant. It affects light lag for instance. But it is mostly irrelevant.

Delta-V at the time of optimal travel. Otherwise it's probably as all over the place as this video.

Now I want to see porkchop plots for transfers to each planet stacked on top of each other, or otherwise presented together somehow.

Only significant if you try and minimize travel times. Get out of earths orbit and waiting without burning fuel let’s you sync up.

Does anyone know a simulation of the solar system corrected for delta-v?

It would be neat to see visually how the planets compare. Like a 3D rendering, or a screensaver.

You'd need to morph distance such that if a point takes less energy to travel to from earth, it appears either closer or larger.

Bonus points if it accounts for time dilation somehow.

Not exactly a simulation but it is a nice illustration, I think.

https://xkcd.com/681/

Kerbal Space Program?

There is a delta-v map (not to scale) of our solar system that could be used as the basis of this:

https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of...

One of the more interesting consequences of orbital mechanics is it takes far more delta-v to get close to the sun, than it does to leave the solar system entirely.

Think about this in terms of how fast the earth is moving to avoid "falling into" the sun, anything that leaves the earth has that velocity too.

If you do a Hohmann transfer — the most basic sort, used by most Δv calculations — from Earth to the Sun, yes. But if you give yourself solar escape velocity (a savings of about 59% in your hyperbolic Earth escape velocity, which works out to a huge savings of about 18.9 km/s Δv if you do your burn in Low Earth Orbit), then once you get very far from the Sun you can stop yourself virtually for free, and fall straight down. The whole trip would take decades though.

The same basic idea works for reaching any inner object orbiting any body.

In practice you can probably do better by aiming for a planet and slingshotting, but that gets more complicated.

This illustrates why being in a Earth orbit is really “Halfway to anywhere” [1] in the solar system. Well, at least to where you would like to land.

[1] A book by Harry Stein about single stage to orbit attempts. https://en.m.wikipedia.org/wiki/G._Harry_Stine

What's funny is that in terms of delta-V, the surface of Deimos (Mars' second moon) is actually closer to us than the surface of the Moon.

Of course in travel time the distances are very different. Delta-V is not the only factor that matters.

Interesting. How does that work for Deimos specifically? I've read that landing on Mars takes less delta-V than the Moon because you can aerobrake, but that doesn't apply to Deimos. Is that because of its nearly nonexistent gravity, or is there some other factor?

Direct link to the image: https://i.imgur.com/SqdzxzF.png

Came to ask the same question. I would love a nicely animated screensaver that has planets spinning round and highlights the current orbital configuration

It's all about ping time, man!

There are lots of astrology enthusiasts that would care much more about mostest closest than travel path. Also things like “lining up”. You can check with my Aunt Karen.

Delta-V isn't the only consideration. Seeing as how humans need constant supply and environmental control, travel time for a reasonable dV budget is also important; e.g. with aerobraking it takes less dV to get to the Mars surface than to the Lunar surface, but the life support and (even for robotic missions) power/thermal systems need to be much more substantial.

Delta-V to Venus and Mars is only that small when you launch at the correct time. Just as with linear distance, where are plenty of times when delta-V to get to Venus or Mars is going to be a lot larger.

But Delta-V and linear distance are different things. This particular discussion of linear distance to other planets is extremely useful to demonstrate how biased our perception and understanding are. Because of the way we usually look at and discuss planets, everybody blindly assumes that Venus and Mars are closer in linear distance than Mercury, when in reality that's usually not the case. Checking our assumptions and perceptions like that is a useful exercise in itself, even when it's not directly relevant to rocket science.

I'm not sure whether this paper is a great overlooked observation or a trivial result that one can prove with a couple diagrams and no calculation at all. Maybe it's both :)

If you assume all orbits are spherical, then the distance from your planet to the sun is 1 AU.

Now draw a graph, with the sun at the origin, our planet P at 1 AU 'below' the sun, and the other planet P' going around in a circle at distance R around the sun. Now draw a right triangle, with the sun at a near corner, the other planet P' at the far corner, and one of the short sides pointed straight at our planet P.

   __x__P' (t2)
   |   /
   y  /
   | /
   |/
  sun
   |\
  y| \
   |  \
   |_x_\  
   |    P' (t1)
   P

You can see that the y-distance of the planet from the sun exactly averages out to zero - half the time it's on the far side, half the time it's near. Therefore the y-distance from our planet to the other planet always averages out to exactly 1 AU.

However, the x-component is sometimes to the 'left' and sometimes to the 'right' of P. We know from the triangle inequality that |y + x| >= |x|, so the x component always makes the distance between P and P' larger than it would otherwise be, therefore the average must be larger than 1 AU, and this discrepancy grows with the radius of the other planet P'.

That's only if P' is closer to the sun than P. In other words, the average distance between the two planets will be approximately the radius of the outermost planet's orbit.

Somehow my fascination with the result here is my second favorite part of these videos. The curiosity and questioning of knowledge that is generally accepted as understood makes me wonder what else we accept at face value that is only partially correct.

In many ways i find the same joys when building websites/apps. Even using the same technologies it surprises me how my approach to similar-in-theory projects changes year over year. Knowledge that i used to accept as gospel gets overwritten by experience or curiosity, for better or worse.

I had no idea 6 minutes what i assumed were simple questions would invigorate my curiosity like this but color me pleasantly surprised.

Previous (from 7 months before) thread (with a different link): Mercury is closer to Earth, on average, than Venus [0]

[0] https://news.ycombinator.com/item?id=19372323

So which planet is mostest closest to Mercury?

The next largest circle?

Yes, Venus.

EARTH!

Came here for this was not disappointed

Classroom factoids gonna factoid.

I guess the sun is ‘closer’ to earth than any planet, using this ‘metric’.

I think Mercury might still win. When the Sun is closer than Mercury, there is a lot of time when Venus or Mars are closer than the Sun.

But they take time from both Sun and Mercury.

I think Sun will win: let’s say there is a planet A between Mercury and Sun, given math in video, A is better than Mercury. Planet B is between A and Sun. Repeat until you get to the surface of the Sun.

On average the Sun is closer than Mercury, because Mercury spends 2/3 of its time further away from us than the Sun, and only 1/3 closer to us than the Sun. Mercury is closer to us than any other planet entirely because it's the closest to the Sun, which is closer to us than any planet. On average.

The sun isn't a planet. It's a star.

We redefined a planet as a star.

Oh good, we can just use 星 for everything now. Much simpler.

Super Mario 星 makes more sense than Galaxy.

Trivia that will get booed out of existence on HN:

"In astrology, the sun and moon are called luminaries."

(Though Pluto throws a monkey wrench into everything. I like to call it "That heavenly body formerly known as a planet.")

It bothers me that astro-nomy (the laws of the stars) is the regular science and astro-logy (the study of the stars) is the pseudoscience. This is clearly backwards!

The short version is that astrology came first and probably basically began as an attempt to predict the weather for purposes of things like deciding when to plant. Planting before an unseasonably late frost could be a deadly local apocalypse for a village.

Somewhere along the way we began to distinguish scientific study of the stars from attempts at weather prediction and also from attempts at assuring ourselves that the sun will come out tomorrow because the omens are propitious.[3] So we began calling one part of our desire to look up to the heavens and wonder how it impacts our lives Meteorology ("the study of things high in the air" [1]) and another Astronomy and began turning our nose up the stuff we now call astrology even though all three of those used to be basically the same thing.

You can see evidence of this in Farmer's Almanacs which still list things like phases of the moon.[2]

[1] https://en.m.wikipedia.org/wiki/Meteorology

[2] https://www.almanac.com/astronomy/moon/calendar

[3] We remain intensely interested in trying to predict the future and in using that info to try to "bet the right way," we just have more respectable terms for it than signs, portents and omens. These days, we like to use terms like leading indicators, statistical analysis and professional opinion.

If you think about it, it's really the exact same thing, we just have access to better data and tools. When an astrologer was someone kings used as advisors, they were basically relying on proxies. Now, we have satellite imagery of storms on the other side of the planet and we try to predict their path. A hundred years ago, hurricanes just showed up without warning and this lack of warning directly resulted in lost lives.

But we scoff at astrology like "only a fool would take that seriously," completely forgetting that we have always been able to look up and infer relationships between things happening in the heavens and events here on the ground, but we didn't always have satellites and the like.

We make fun of astrologers for being silly now not because the heavens aren't connected to the earth, but because astrologers never bothered checking to see whether or not the specific connections they believed in were true.

I think part of the problem boils down to "90% of everything is crap."

What most people know of astrology is "popular" or Sun Sign astrology which posits that all 7 billion people on the planet can be broken up into twelve sub categories depending on the (astrological) month they were born in and we can then predict that roughly one twelfth of the human race "will fall in love today!" or whatever (even though some of those people are infants).

Predictive astrology caters to the omnipresent human need to reassure ourselves that things will be okay. It's the stuff that shows up in newspapers and magazines and it has little to do with what serious astrologers do.

But it helps pay the bills for some people. So it's probably here to stay, though most of it is amazingly eye-roll worthy.

(Said as someone who actually believes in astrology, yet hates "popular" astrology with something of a burning passion.)

What's the difference between not believing in astrology (or any other theory) and not believing that it can predict anything?

A very large part of astrology is about understanding yourself and your relationships to other people. Natal or birth charts are about wondering "Who the hell am I?" and relationship astrology -- also called Synastry -- is about understanding how you interact with people around you.

It's not terribly different from things like Meyers-Briggs Personality Types, which also have both avid followers and derisive critics.

I will note that both Natal charts and Synastry map out the positions of the sun, moon, planets and other points of interest and additionally examine their mathematical relationships. They don't rely on "birth month"/sun sign as anything remotely resembling sufficient information.

But what you're describing is predictive. If it were true, you could use it to predict the future personality and behavior of a newborn baby. If an astrologer was clever they would have gone around checking in on babies to make sure their natal charts were accurate, keeping a tally of the ones that went as fate predicted and the ones that didn't, but that was before science so the idea was not as obvious as it is now.

Astrologers actually do the kind of research you talk about. It's an ongoing thing and astrologers with large followings sometimes use social media to ask for ongoing feedback concerning such things.

There is a long-standing and ongoing interest in trying to find ways to categorize human personality traits and figure out how X interacts with Y. There are many systems for that. Astrology and Meyers-Briggs are just two of countless such attempts.

Here's a recent article called The Church of Interruption that actually has nothing to do with religion. It's about communication styles:

https://news.ycombinator.com/item?id=21044009

There are people who believe in astrology as a kind of religious or spiritual thing and that belief system is basically that souls reincarnate and time and place of birth is chosen to correspond to soul states and life goals.

It's not necessary to believe in that to find astrology useful as a kind of "choose your own adventure" tool for thinking about yourself and how you relate to people.

You could think of it as the oldest and most fleshed out attempts to make personality categories based on long human observation that certain traits seem to very often go together.

Astrology uses a lot of provisional language that makes it easy for people to pick and choose what parts of the personality descriptors apply to them. You get a list of traits and you go "Why, yes, I am chatty and intellectual, but I'm not shallow. So most of these traits fit me and I find this makes sense."

It almost doesn't matter if there is any validity to planetary positions. You get a menu of things that kind of go together and I find that useful and haven't found anything else that provides a similarly useful menu to choose from.

So if it's a problem with a Saturn influence, Saturn rules both limitations, such as poverty, plus time and effort. You can accept your limitations or put in the time and effort to overcome them.

Maybe it's absolute bullshit and you can certainly come up with insights like "I can accept my limitations or work to overcome them" without astrology, but I find it useful.

Sociologists can find logical explanations for the value of religion in fostering a sense of community, etc, without accepting the tenets of any particular religion. Christians will be personally offended if you tell them God isn't real and they just go to church for a sense of community, but it's not actually necessary to draw any firm conclusions about whether or not God actually exists to see a valid role for religion in the lives of many people.

Same sort of thing is true for astrology.

From what I gather, astrology is a very popular subject in Japan, even though most Japanese aren't serious believers in astrology. In that case, it appears to be a socially acceptable means to indicate birth month in a culture that needs to know relative ages to determine appropriate honorifics to use in conversation when asking people their age is unforgivably rude.

So if you all go to school together, you know you are all born within about a year. Naming your sun sign is de facto naming your birth month, which helps sort who is older and who is younger without ever directly asking that question.

For many people outside of Japan, astrology serves a similar purpose of being a non threatening conversation starter. Not believing in it too earnestly actually makes it a better social tool because it makes it no more important than social exercises like asking "What's your favorite color?" and making up personality traits that go with that.

“The observation of the world began from the noblest spectacle that was ever placed before the human senses and that our understanding can bear to follow in its vast expanse, and it ended in — astrology.”

Immanuel Kant, Critique of Practical Reason

The comment you're responding to didn't claim that; rather, it claimed that the sun is closer than any planet by this metric, which is true.

I'm aware of that. But the piece in question is literally titled Which Planet is Closest?

So while it's possible that the sun actually is the winner, it's not an answer to the question being investigated.

If they can pedant, so can I.

That's not the issue here. The video asks which planet is the closest, and presents its measurements as common sense.

Then it gets controversial results. Imo the base setup is wrong and Sun and Mercury always being the closest is a demonstration of that.

It should either measure the minimum or maximum distance between planets/stars, not the average. Or measure distance between two orbits.

While this metric is technically the most accurate, it's (to me) uninteresting and not useful because it is essentially a constant function (the closest planet is always the one with the smallest orbit).

Wait til we redefine the moon as a binary planet.

After reading tzs' comment, I'm convinced.

This is strangely apropos given its namesake.

Earth

I wonder if this is more due to Mercury's orbit being so frequent that it is more often able to occupy that "closest window" while other planets remain further away for longer. Would this mean the second closest for any planet is Venus being the planet with the second highest orbital period?

Yes, that's what the video is saying and showing.

My guess is that as long as the orbital periods of the involved planets are sufficiently different (not being rational multiples of each other for example) , making the system ergodic . Then Mercury will always be the closest by the metric.

Somewhat related:

The three inner Galilean moons of Jupiter are in a 1:2:4 resonance https://en.wikipedia.org/wiki/Galilean_moons#Orbit_animation https://xkcd.com/1300/

Also, the ex-planet Pluto is in a 2:3 resonance with Neptune. https://en.wikipedia.org/wiki/Resonant_trans-Neptunian_objec...

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