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View Full Version : The nearest star to earth is Proxima Centauri!!!



The_Radiation_Specialist
2005-Dec-22, 12:47 PM
Does anybody here have the book "Escape from Earth" by Peter Ackroyd?
If like me you are unfortunate enough to have bought this book, grab the book and jump to page 140. Read the fact sheet on the stars and look at the first paragraph:
"The nearest star to earth is Proxima Centauri, a red dwarf star, which lies 4.2 light years away"
WOW! If our only energy source comes from 4.2 light years away, man! I wonder how we are still alive!

Doodler
2005-Dec-22, 03:08 PM
Its a commonly understood that the phrase except the Sun is unspoken there. Kinda like a silent letter, ya know?

Relmuis
2005-Dec-22, 04:30 PM
One might get funny replies, too, if one would ask someone to name the nearest planet. Especially if one added some clause like "last october".

Charly
2005-Dec-22, 11:07 PM
Shouldnt it be called Alpha Centauri C anyway?

The Bad Astronomer
2005-Dec-22, 11:25 PM
This is a funny coincidence. Do you read my blog? Just a few days ago I posted on this topic (http://www.badastronomy.com/bablog/2005/12/17/the-sun-of-all-fears/)!

DukePaul
2005-Dec-23, 12:36 AM
If space can be folded artificially or if wormholes can be created and traversed then the question is moot.

PhantomWolf
2005-Dec-23, 04:59 AM
One might get funny replies, too, if one would ask someone to name the nearest planet. Especially if one added some clause like "last october".

Doesn't matter. The nearest planet to Earth at all times is the Moon. ;)

The_Radiation_Specialist
2005-Dec-23, 07:23 AM
This is a funny coincidence. Do you read my blog? Just a few days ago I posted on this topic (http://www.badastronomy.com/bablog/2005/12/17/the-sun-of-all-fears/)!
Of course! read it every day! I liked your drawing of the evil cloud taking over the planet!:evil:

montebianco
2005-Dec-23, 07:37 AM
Well, if the universe were governed by special relativity, then we could perform the following exercise.

a) Choose any star at all, subject only to the constraint that the path from the earth to the other star does not pass through any other star

b) Break the distance from earth to every star into two components, one parallel to the vector from earth to the chosen star, and one perpendicular. For every star except the chosen one (and possibly others on the back side of the chosen star), the perpendicular vector will be non-zero; for the chosen star, it will be zero.

c) Move extremely rapidly exactly in the direction of the chosen star, so the parallel vectors are dilated, and the perpendicular ones are not.

With sufficient velocity in the right direction, the chosen star is the closest to earth.

So, every star in the universe is the closest to earth, for an appropriate observer (as long as there isn't another right in between).

I don't know if it still works in general relativity. (I'm hoping I didn't do anything silly under special relativity here...)

N

Chuck
2005-Dec-24, 03:09 AM
I think that if you're going fast enough then objects that are to the side of you in their reference frame are ahead of you in yours. The faster you go, the closer they are to directly ahead.

If you don't count the sun then we don't know which star is closest. There could be a dim star closer than Proxima Centauri that we haven't noticed yet.

Lord Jubjub
2005-Dec-25, 10:07 PM
When will Mercury be the closest planet to Earth?

PhantomWolf
2005-Dec-26, 07:35 AM
When will Mercury be the closest planet to Earth?

Ummmm, never?

eburacum45
2005-Dec-26, 08:35 AM
By November 2006 Mercury will be the closest planet to Earth by a wide margin.

PhantomWolf
2005-Dec-26, 10:12 AM
By November 2006 Mercury will be the closest planet to Earth by a wide margin.

Nah, the Moon still beats it. :P

eburacum45
2005-Dec-27, 02:12 PM
Doh!

If we allow that the Earth-Moon system is a double planet system you are right. I obviouly should have qualified the statement somewhat...

Doodler
2005-Dec-27, 02:17 PM
When the IAU releases a statement that the Moon is a planet, it'll be a planet, until then, its a satellite.

Kinda like Pluto being a planet till they say otherwise. If you wanna go that route, then Xena's in contention for 12th place, behind the Moon and Charon.

PhantomWolf
2005-Dec-28, 02:53 AM
When the IAU releases a statement that the Moon is a planet, it'll be a planet, until then, its a satellite.

Kinda like Pluto being a planet till they say otherwise. If you wanna go that route, then Xena's in contention for 12th place, behind the Moon and Charon.
Well the trouble is that the IAU goes not by physics, but by tradition. As far as the physics goes, the moon orbits the sun, not us. The Earth merely perturbs its orbit so that it swings backand forth across the Earth's orbit, and length of the path to the next crossing determining which passes the crossing point first. ie. when it's outside Earth's orbit it has to travel further to the next crossing point so Earth pulls ahead and when it's inside Earth's orbit it has to travel a shorter distance so it beats us. Because of that frm an Earth referance frame the Moon appears to orbit us. From the Sun's referance frame, it doesn't, both the Earth and moon orbit it within the same space. This is not true for all the other moons (barring prehaps Charon) where they all loop back in their orbits as their primary orbits the sun. Also the Moon is twice as attracted to the sun than to Earth so the sun is the moon's primary, not Earth.

HenrikOlsen
2005-Dec-28, 05:06 AM
The problem is that all the reasons you cite is caused by orbital mechanics, not because the moon is a planet.
Move the earth-moon system to twice the distance from the sun and the earth will pull more than the sun without changing anything about the moon-earth system.
No satellite at all moves backwards relative to the sun.

Just did a quick calc comparing the Moon with Deimos.

Mass of Sun=1.98892*1030kg
Mass of Earth=5.9742*1024kg
Mass of Mars=6.42*1023kg
Mass of Moon=7.36*1022kg
Mass of Deimos=1.8*1015kg
Mean distance Earth to Moon=3.844*108m
Mean distance Earth to Sun=1.50*1011m is also mean distance Moon to Sun
Mean distance Deimos-Mars 2.3460*108m
Mean distance Sun-Mars 2.28*1011m

Fg=-G(Mm)/r2 where
G=6.673*10-11Nm2kg-2

FSun-Moon=-6.673*10-11Nm2kg-2*1.98892*1030kg*7.36*1022kg/(1.50*1011m)2=-4.341*1020N
FEarth-Moon=-6.673*10-11Nm2kg-2*5.9742*1024kg*7.36*1022kg/(3.844*108m)2=-1.986*1020N
Yes, the sun pulls more on the moon than the earth does

FSun-Deimos=-6.673*10-11Nm2kg-2*1.98892*1030kg*1.8*1015kg/(1.50*1011m)2=-1.06*1013N
FMars-Deimos=-6.673*10-11Nm2kg-2*6.42*1023kg*1.8*1015kg/(9.378*106m)2=-1.40*1012N
But it also pulls more on Deimos than Mars

Tangential speed is given by v=sqrt(GM/r) (not quite right, but close enough)
Moons tangential speed relative to earth vMoon-Earth=sqrt(-6.673*10-11Nm2kg-2*5.9742*1024kg/3.844*108m)=1.0183*103ms-1
Earths tangential speed relative to sun
vSun-Earth=sqrt(-6.673*10-11Nm2kg-2*1.98892*1030kg/1.50*1011m)=2.9745*104ms-1
Yes, the moon does not move backwards relativ to the sun

Deimos tangential speed relative to Mars vDeimos-Mars=sqrt(-6.673*10-11Nm2kg-2*5.9742*1024kg/2.3460*108m)=4.27*102ms-1
Mars tangential speed relative to sun
vSun-Mars=sqrt(-6.673*10-11Nm2kg-2*1.98892*1030kg/2.28*1011m)=2.41*104ms-1
but neither does Deimos, incidentally neither does Phobos, though I haven't included the calculations.

So by your definition Deimos is a planet.

Yes the Moon orbits the Sun, but so does every other satellite.

PhantomWolf
2005-Dec-28, 09:37 AM
Yes, it's been pointed out that number of "moons" do, though I'd point out that these moons are for the most part captured asteriods and if they hadn't been so captured would be considered minor planets such as Ceres, Juno, Apollo and Icarus. The moon isn't a captured asteriod and is also a lot larger compared to the Earth then those asteriod moons of Mars, Jupiter and Saturn.

HenrikOlsen
2005-Dec-28, 11:26 AM
From the Sun's referance frame, it doesn't, both the Earth and moon orbit it within the same space. This is not true for all the other moons (barring prehaps Charon) where they all loop back in their orbits as their primary orbits the sun.
This is true of all moons, none of them loop back.


Yes, it's been pointed out that number of "moons" do, though I'd point out that these moons are for the most part captured asteriods and if they hadn't been so captured would be considered minor planets such as Ceres, Juno, Apollo and Icarus. The moon isn't a captured asteriod and is also a lot larger compared to the Earth then those asteriod moons of Mars, Jupiter and Saturn.
I'm sorry to say this, but this sounds like armwaving to save a trasured theory in the face of evidence. Something I'd expect from the woowoo crowd but not from you.

mickal555
2005-Dec-28, 02:21 PM
Doesn't matter. The nearest planet to Earth at all times is the Moon. ;)

Maybe...

but the question was nearest planet...

that'd be the earth.

mickal555
2005-Dec-28, 02:24 PM
Ummmm, never?

What if mercury was at inferior conjunction, venus superior conjunction and mars at conjunction?

HenrikOlsen
2005-Dec-28, 02:41 PM
He was arguing from the position that the moon is a planet, so that's always nearest.