
Which Planet Is Closest? - vl
http://www.cgpgrey.com/blog/which-planet-is-closest
======
tzs
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...](https://blogs.nasa.gov/Watch_the_Skies/2009/07/10/post_1247240047519/)

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

~~~
Latty
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](https://en.wikipedia.org/wiki/Stonehenge)

------
Symmetry
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...](https://www.reddit.com/r/space/comments/29cxi6/i_made_a_deltav_subway_map_of_the_solar_system/)

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

~~~
sillysaurusx
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.

~~~
fyfy18
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...](https://www.reddit.com/r/space/comments/1ktjfi/deltav_map_of_the_solar_system/)

~~~
throwaway2048
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.

~~~
DavidSJ
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.

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smallnamespace
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'.

~~~
zuminator
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.

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liquidise
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.

------
dyukqu
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](https://news.ycombinator.com/item?id=19372323)

------
ShteiLoups
So which planet is mostest closest to Mercury?

The next largest circle?

~~~
kolla
Yes, Venus.

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slimscsi
EARTH!

~~~
sideshowb
Came here for this was not disappointed

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campfireveteran
Classroom factoids gonna factoid.

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

~~~
testplzignore
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.

~~~
vl
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.

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slg
This is strangely apropos given its namesake.

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Iwan-Zotow
Earth

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twright
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?

~~~
ravar
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.

~~~
gus_massa
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://en.wikipedia.org/wiki/Galilean_moons#Orbit_animation)
[https://xkcd.com/1300/](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...](https://en.wikipedia.org/wiki/Resonant_trans-
Neptunian_object)

