What is exactly an AU ?

[DanSteph]

1 AU or astronomical unit is equal to 149'598'000'000 meters (149.6 million kilometers) it's the medium distance from the sun to the earth.

what I love when programing is that you learn a lot of things on various subject.
I knew what was an AU but not it's exact value.
I was even more surprised while playing with the new computer antenna's display
to see various target distance.


display not finished yet

if jupiter is less far then I was thinking (4 AU "only") saturn and uranus
are really more far (10UA and ...29UA 8o ) (depend of course where they are on Orbit
but this give a good idea)

Dan

[ChristopherT]

 Now you see why I've got the DG3 docked inside the USS Constitution (NCC-1700)?  The Connie
has all the speed, and the DG3 all the great data collecting tools.

    Christopher

[MattNW]

That makes the distance from the Sun to the Earth real easy to remember. My Astronomy
professor had that one on the final.

Question: How far is Earth from the Sun.
Answer: 1AU.

That was the only answer he would accept. He said it was amazing how many people would try to
give it in miles, kilometers and every other unit of measurement ever invented by man forgetting
the simple answer.

[Arkalius]

The first person who can describe what a parsec is and what it equates to in light years gets a
cookie.

[MattNW]

In Lightyears a Parsec is 3.26 ly. The measurement however is defined by the distance that 1 AU
equals one second of arc.   

Edit. Little change there. My dyslexia is showing.



Post Edited (02-09-04 03:31)

[Spets]

hehe, I had the "distance from earth to sun" question on my math test.. my teacher was expecting
the opposite, to give the miles value. hah, I just gave her the AU.

she didn't specify units!

[Urwumpe]

Well, an AU is astronomically speaking just the length of the major semiaxis of earth.  or the average
distance of earth to the sun.  I don't know if the physical definition of it depends on the changing
orbital parameters of earth or on the rather fixed speed of light .

as long as nobody provided a time, one AU is the correct answer  :D

[acehunter]

To add to MattNW's description:

A 'parsec' is an abbreviation for 'parallax second'.  It is measured by taking a sighting on a star at
two points in the Earth's orbit around the sun where the line between the two points (ideally) is
perpendicular to the line through the star itself and our sun.  Simple geometry then allows a
distance calculation.  The same measurement can be performed using simultaneous observations
on opposite sides of the Earth, but that only provides a 12,750km baseline for measurements,
whereas the solar orbit method provides a 300,000,000km baseline (which produces a much more
accurate measurement).  A parsec is equal (as MattNW said) to about 3.26LY, or about
30,820,870,000,000km (206,000AU)

[sofias sotiris]

AU is the mean distance of the earth from the sun.It is 149.6 million Km

[McBrain]

huh?

[freespace2dotcom]

Allrighty.

This ancient thread should not have been revived.

but since it has been, I'll mention that the speed of light is 186,282.4 miles per second in traditional units. Seeing as
that hadn't been mentioned yet, I think it only fair to do so.


"no, everyone, NO!! Put those guns away! No! Anywhere but there! Aaaaiiiiiyyaaaaaa!!!!"


....

[Simonpro]

Tuttuttut, accuracy is everything! 186282.397051mps!
*Runs off and hides.

[freespace2dotcom]

well, personally, I think that accuracy's all fine and dandy, but I think that there's a line to be drawn somewhere.

generally, I consider two decimal place accuracy more than enough for anything, although I'm sure you can see why I
rounded.

much like how people convienently round the spped of light as 300,000,000 meters per second, when in all honestly,
we both know that's a wrong inaccurate hideous grevious error.

Although to me I really don't care how extremely accurate we measure the speed of light. for small distances it's
going to be virtually unnoticable.

And for long distances, say from here to The Crab Nebula. Who cares if it's in the range of 5995-6005 LY away?

It's FAR AWAY!

eh.. now this post is getting too long for an antiquated thread.  Dammit, simon!

Doc, seal the thread or something!

[Simonpro]

Heh, you got me thinking about that.
For a satellite in a sun synchronous earth orbit (alt ~800km) that is measuring the height of the sea a radar signal
will take approximately 0.00533333 seconds to reflect and return if we take c=300000000m/s. If, however, we take
c=299792458m/s then we get 0.005337 seconds.
Doesn't sound much different, does it?

But these radars are used to measure the height of waves on the sea - say 2m high. The difference in reflection time
from the top and bottom of the wave is thus 4/c = 1.3373e-8 seconds - smaller than the error produced with our two
values of c!

So, in some cases the actual value of c is very very important.
Lol, not sure if that is interesting, but i thought i'd share it with y'all anyway :p

[DocHoliday]

You forgot to vector in the relativistic effects. Satellite being where it is, should according to my grasp of things,
measure time at a slower pace... And the gravitational effect of Moon.. And then there is the lateral movement of the
satellite while it measures that needs to be accounted for.. And what of the thin molecules that keep changing the
orbital velocity of the sat.. Hm.. surely the subatomic effects of solar wind upon the measuring eq on the sat.. And..
and .. and Jupter being in opposition with Venus.. and Mars in conjuction with Pluto. See, you can never know.. gotta
be VERY accurate here mates

I'm sure if we try like hell, we can pump up the difference to say 0.005s. Heh, at that rate it could mean a difference
between a Nobel prize and another crashed probe on Mars.

[Simonpro]

Well, if we're being pedantic the difference cause dby variation in c values used in the calculation is much smaller than
the difference caused by atmospheric effects.
And yes, you do need to factor all the other things into consideration too - so even finding out how high up
something on the earth is involves an incredibly complez set of mathematics

[freespace2dotcom]

Yes, yes.

I'm just waiting for scientists to redefine the KG from a nice simple weight to the amount of force given from
987654321 Carbon-13 atoms traveling at a speed of 434 meters per second acting upon a surface 789
picometers wide.


All in the name of accuracy.



Post Edited ( 05-17-05 16:40 )

[DocHoliday]

simon, tell free, how 1 meter is defined. I'm sure he'll get a kick out of it

[freespace2dotcom]

I already know. It's defined by the the lenght light trvels in something of a second.

Which is funny, because light spped is based on the meter.

Isn't that a paradox or what?


(I do know the relationship between them, so don't answer that.)

[Simonpro]

The numerical value for lightspeed is actually given by c=1/sqrt(u0*e0)

[freespace2dotcom]

I prefer simply saying  c= sqrt (e/m)

or c= sqrt (e/delta T)        if you're talking about spacetime and not matter.

*nyuk, nyuk*





Post Edited ( 05-17-05 19:18 )

[Simonpro]

I wouldn't use the first of those - it tends to imply various things that the original equation was not meant for.
Comparing a velocity to an energy in this way leads to confusion

[freespace2dotcom]

oh, pooh.

And here I was trying to sound smart by rearranging a formula that everyone knows..



Post Edited ( 05-17-05 21:23 )

[Simonpro]

Lol, it took me a good few minutes to figure out what it was - i got confused by e/m which is the charge to mass ratio
for an electron, was wondering what it had to do with energy.
Who says physics is straightforward, eh? The same damn symbols are used for everything :p

[freespace2dotcom]

Some will likely try to change that in the future by coming up with a whole new system to learn, but as people are
used to what they know, it will likely take a long time to see results, and it's possible the new system may never
replace the old one.

'cuz those traditional symbol users are stubborn, you know



Post Edited ( 05-18-05 01:35 )