Mail Archive

The Sun...

• Subject: The Sun...
• From: Ray
• Date: Wed Jun 21 18:37:17 1995
• Message-Id: id527

Dear Ray,

I hope everything is going well with your work.

Well the scientists seem to agree that you can't really tell whether it is the
universe or the earth rotating! That is nice...

>
>The growth of what is on the web is indeed fantastic.  I don't
>think that anyone can say where this will all lead to.  In effect
>it is a new life form and we are all just cells in that greater
>being.  This was of course already true of towns and countries
>and the world (Gaia), but now the form is a mind not just a body.

I think it's the expression of many minds... rather than a mind...

>
>> I've been wondering about the sun... I suppose the base of the
>> parallax triangle is 93,000,000 * 2.
>

>That's right, but parallax are quoted based on just 93,000,000.
>
>> How do we know for certain
>> that the sun is 93,000,000 miles away? Could it be closer?
>
>The distances of the planets are now known extremely accurately.
>The relative distances were known accurately a long time ago,
>but the absolute scale was uncertain by about 0.05% until it
>became possible to bounce radio signals off other planets.
>This, and then space spaceships which send radio signals have allowed
>the astronomical unit (earth-sun distance) to be determined to
>within 0.02km. It is 149,597,870.66 km.

So they have never actually measured the distance to the sun? They depend on the
relationship between the distances of the planets to determine the distance to the
sun?

[BTW I noticed someone posted the question in sci.astro today, "What is the
simplest way of determining the distance to the sun?" - I will be watching with
interest.]

>
>Because distance is now defined in terms of the speed of light
>(1 meter = distance light travels in 1/299,792,458 second) and
>time can be measured very accurately from atomic clocks (which
>is also the definition of time) then we can say for sure that
>the distances of the planets are known to this accuracy.

The radio waves travel at the speed of light? You send them to a planet and they
bounce back and then you time it, divide it by 2 and multiply by .299792458?

[BTW once upon a time I lived in Canberra, which is close to the Mt. Stromolo
Observatory. There are three places in the world from which they fire a lazar beam
through a telescope aimed at a prism which is meant to be on the moon, put there
by one of the Apolo flights. Then they try to catch the lazar beam on its way back
to measure the time and thus calculate the distance to the moon... I spoke to the
scientist in charge of the experiment and he told me that although, in Australia
we are theoretically in the best place to do the experiment they had never been
able to get conclusive results. He said out of so many photons fired to the moon
so few would even make it all the way to the moon, through the prism and back into
the telescope and it was very difficult to tell if they were actually the photons
from the lazar or some stray light from somewhere else. He said the two other
places, the US and France were able to get reliable results. In France they said
they could even do it in broad daylight!... Why do they bother with this if they
can just bounce radio waves off it?]

>
>Even if time and distance are defined some other way (which
>will be less accurate) then it is still true that the sun is
>at that distance, but to a little less accuracy.

But we can't bounce anything off the sun to check it out? Or can we?

>
>Incidentally, time can now be measured to about 14 digits precision
>or a millionth of a second per year.
>
>Distances start to become unsure for other than the nearby stars.
>The accepted accuracies are say 10% for stars in the galaxy and
>perhaps 30% for nearby galaxies and 100% for distant galaxies.
>By 100% I mean the true value could be half to double the estimate.
>There are however some slightly weak links in the chain, and it
>is conceivable that a larger error than this could exist.
>In the past several times adjustments by a factor of 2 have been made
>and it could happen again.  Some new methods of galaxy measurements
>could improve the accuracies considerably, but there is a lot of
>theory involved in large distances, and if the theory is wrong then
>it could all collapse.  I would say the chance of this was quite
>small.


What is the theory it is based on?

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