[PLUG-TALK] Perhelion Re: Happy Consensus New Year

[Keith Lofstrom]

On Mon, 31 Dec 2018, Russell Senior wrote:
>less arbitrary marker that everyone could agree on is also coming
...
>"Perihelion in Portland, Oregon, USA is on Wednesday, January 2,
>2019 at 9:19 pm PST"

On Mon, Dec 31, 2018 at 12:56:05PM -0800, Paul Heinlein wrote:
> I'm not sure that something being measurable makes it less arbitrary
> (although, in this case, timezones are measurable). Being measurable
> makes something easier to repeat, but its applicability to anything
> else isn't a given.

Time zones over oceans and on continents with plate tectonics
are mildly arbitrary for fussy measurement geeks.  Portland
moves east a centimeter per year, and may suddenly move a few
meters west any year now.  That happened on Friday January 26,
1700 at 21:00 pm PST, and is one reason why the Pacific
Northwest was depopulated and ready for European settlement.
Adjudicating property lines after the Big Rip tumbles the west
hills on top of downtown will only add to our misery :-(

---

The speed of light, the electronic charge, etc., are not
arbitrary, though measuring the universe with those constants
as yardsticks is an arbitrary (but practical) choice.  Better
than the King's foot, which shrivels after a beheading.

Perihelion TIME involves an arbitrary choice, the point of
view.  Earth's perihelion tonight will be 0.9833012
astronomical units ( == 149,597,871 km ), 1681 km farther
from the Sun than the long term average.  On the night side
of the Earth, rotating towards midnight, we will be 4500 km
farther away.  Portland's local "perihelion" will be closer
to noontime today (Weds).

Measured from the Earth,  perihelion might be 2019 Jan 2 at
9:19 pm PST (and 2019 Jan 3 0519 UTC), but due to the speed
of light delay it won't appear as perihelion until 490 seconds
later viewed from the surface of the Sun ("hmm, that tiny ball
of rock seems close, and I am ionized vapor ...").   

In our reference frame, perihelion and aphelion are special,
because at those extremes our radial velocity from the Sun
is zero.  While measuring distance and Doppler shift from
the center of an incandescent ball of gas seems arbitrary,
measuring the distance to all the large planets is critical
when aiming an interplanetary probe for an aero-braking
maneuver, years later. 

Perihelion is slightly more optimal for launching such missions,
because Earth's "tangential" (round-and-round) velocity is
highest, and the propellant your rocket launches backwards
has the maximum kinetic energy effect.  

Understanding the mission value of periapsis (closest orbital
distance) should help you understand the ignorance of 
politicians yammering about Moon orbit as a staging area
for interplanetary missions.   Lifting propellant to the
Moon costs energy, slowing it to orbit the Moon costs energy,
pushing it back out of Moon orbit costs energy.  Unless you
have magic machines on the Moon that can turn icy south pole
rock into liquid hydrogen and oxygen, and launch them to your
orbiting spacecraft, the Moon is a pointless distraction.

No, you want to launch interplanetary missions from a very
low perigee ( == periapsis, resembling perihelion) of a very
high apogee EARTH orbit (highly eccentric, as the mechanics
who build orbits say).  

It is sad that most politicians who pay for this stuff do
not understand the subject matter; accommodating their 
ignorance can add billions to mission expense.  At least
some know which experts to consult - Kennedy did - and
that is why Apollo landed men on the Moon.  And why he
did NOT call for a "space force"; the aftermath of a
space battle is an impenetrable ring of debris.

>From the 2019 perspective, all this seems abstract.  From
the 3019 perspective, it will be as important as charting
barrier reefs was during the second age of exploration,
the age that delivered most of our ancestors to America.

Thanks, Russell, for reminding us of where we are.

Keith

-- 
Keith Lofstrom          keithl at keithl.com