FAQ for uk.sci.astronomy

Dear subscribers to uk.sci.astronomy,

The FAQ for this newsgroup may be found at:
http://www.astronomycentre.org.uk/UKSA/faq.htm
http://www.astunit.com/faq/uksciastrofaq.htm


Contents:
~~~~~~~~
A. Introduction
B. Astronomical Societies
C. Astronomical Publications
D. Astronomical Software
E. Astronomical Equipment – Choosing and Purchasing
F. Astronomical Equipment – Maintenance
G. Astronomical Equipment – Mounts
H. Astronomical Equipment – Eyepieces
I. Astronomical Equipment – Accessories
J. Astronomical Equipment – Telescope Making
K. Astrophotography – Conventional Film
L. Astrophotography – Digital Cameras (DigiCams)
M. Astrophotography – CCD Cameras
N. Astrophotography – Other Electronic Cameras
O. Visual Observation
P. Light Pollution
Q. Famous UK Astronomers
W. Miscellaneous
X. Other Useful Internet Resources
Y. Contributors
Z. Requests for Other Sections

As you will note from the above (and also when reading the FAQ) there
are bits that still need to be written. If there are any offers towards
this, please email me.


Best,
Stephen

Remove footfrommouth to reply

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JRS: In article , dated Fri, 17 Dec 2004
18:21:06, seen in news:uk.sci.astronomy, Stephen Tonkin news05footfromm
posted :

As you will note from the above (and also when reading the FAQ) there
are bits that still need to be written. If there are any offers towards
this, please email me.

Could you not post a selected such topic, let us write here for a while,
and choose the good bits that appear? Then repeat with another ...
Nothing encourages a response more than reading a wrong one.

There are topics in Z that I could not write a FAQ part on, but in
respect of which I could contribute; there must be others similarly
placed.

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

Dr John Stockton wrote:
JRS: In article , dated Fri, 17 Dec 2004
18:21:06, seen in news:uk.sci.astronomy, Stephen Tonkin news05footfromm
posted :

As you will note from the above (and also when reading the FAQ) there
are bits that still need to be written. If there are any offers towards
this, please email me.

Could you not post a selected such topic, let us write here for a while,
and choose the good bits that appear? Then repeat with another ...

I second that motion.

Perhaps prefixing the subject line with [FAQ discussion] or similar
would be useful, so those not interested would easily be able to spot
and ignore the relevant thread(s).

Of course we are assuming Stephen (or someone) can be bothered to
trawl through the resulting arguments, opinions, off-topic babble and
general noise to extract the useful information and present it in a
legible form.


Tim
--
This is not my signature.

As you will note from the above (and also when reading the FAQ) there
are bits that still need to be written. If there are any offers towards
this, please email me.

Stephen, I have the section on Energy nearly done along with the other
newsgroups bit. I shall forward them in a week or so.

Graeme


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Dr John Stockton wrote:
Could you not post a selected such topic, let us write here for a
while, and choose the good bits that appear?

In principle, that is a good idea. However, at the moment I do not have
time to rehash a usenet discussion into "FAQable" material (which is why
I've recently been less of a windbag than usual on this NG).

What would be better, IMO, is if someone who does have time to do this
chooses a "missing" topic of interest and initiates the thread here. If,
as you say, John:
There are topics in Z that I could not write a FAQ part on, but in
respect of which I could contribute;
why not get the ball(s) rolling with threads on these topics and
subsequently fashion them into a bit for the FAQ?

This would also mean that I would not need to assume the role of "Mystic
Steve" in order to find out which topics other people have sufficient
interest and time to do this for.

Best,
Stephen

Remove footfrommouth to reply

--
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +
+ Stephen Tonkin | ATM Resources; Astro-Tutorials; Astro Books +
+ (N51.162 E0.995) | http://astunit.com +
+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

JRS: In article , dated Sat, 18 Dec 2004
07:43:05, seen in news:uk.sci.astronomy, Stephen Tonkin news05footfromm
posted :
Dr John Stockton wrote:
Could you not post a selected such topic, let us write here for a
while, and choose the good bits that appear?

In principle, that is a good idea. However, at the moment I do not have
time to rehash a usenet discussion into "FAQable" material (which is why
I've recently been less of a windbag than usual on this NG).

What would be better, IMO, is if someone who does have time to do this
chooses a "missing" topic of interest and initiates the thread here. If,
as you say, John:
There are topics in Z that I could not write a FAQ part on, but in
respect of which I could contribute;
why not get the ball(s) rolling with threads on these topics and
subsequently fashion them into a bit for the FAQ?

I suspect that most of Section Z was contributed by me on the basis that
I did not know as much about it as I'd like to see in the FAQ!

Noting Graeme's article (Energy), could you post here an update to
Section Z which marks what's already been dealt with and includes any
topics that are already intended to go into Z? To save everyone looking
for Z, the version that I believe to be current (2004 July 23) has (my
numbering; suggest change to ol type=a

Z. Requests for Other Sections
There have been requests for sections or subsections, or
expansions on the following topics

1 Gravity
2 Energy
3 Universe
4 Solar System
5 Other FAQs
6 Radio-astronomy
7 Astronomy-from-space
8 UK Astronomical Web Sites

add 9 Galaxy


Now 2 is in hand, by Graeme.

For 5, I'd suggest cribbing from my URL:http://www.merlyn.demon.co.uk/
astro.htm#FAQs and #Linx 2 below -
liFAQs :-ul
lia href="http://www.faqs.org/faqs/astronomy/faq/"sci.astro/a
lia href="http://www.saaweb.org/"sci.astro.amateur/a;
lia href=
"http://www.geocities.com/CapeCanaveral/4411/sss-faq.htm"FAQ/a
for news:sci.space.shuttle, with links to others for news:sci.space.*
/ul
that is if the URLs still work.

In fact, a fair proportion of my site could be mined; it has a small
number of sites for 8, but 8 would be better done by a broadband user.

Some of those who observe should know something about 4 5 9 - that's
what they look at.


For 1, there's too much, and not everything, on my site (via URL:http:/
/www.merlyn.demon.co.uk/gravity0.htm); I could pick out bits of Newton,
G, Kepler and of Lagrange & Roche, for a start. Shall we start there?


Those who definitely intend not to participate - please don't look at my
site on Dec 20-24, for bandwidth reasons.


P.S. On reading another article in the group :-

10 Orbital Elements

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

JRS: In article , dated Sat, 18
Dec 2004 23:49:52, seen in news:uk.sci.astronomy, Dr John Stockton
posted :

For 1, there's too much, and not everything, on my site (via URL:http:/
/www.merlyn.demon.co.uk/gravity0.htm); I could pick out bits of Newton,
G, Kepler and of Lagrange & Roche, for a start. Shall we start there?

Postulating a Yes, I've merged and severely pruned and edited my gravity
pages 0..3; the present stage should be visible at
URL:http://www.merlyn.demon.co.uk/grav-faq.htm.

At one stage, I forgot that the newsgroup FAQ existed in HTML; if I've
removed too much mark-up, it can be restored. I've not yet adapted it
so that it would match style if dropped into the FAQ.

Comment?

--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

JRS: In article , dated Sun, 19
Dec 2004 15:30:27, seen in news:uk.sci.astronomy, Dr John Stockton
posted :
JRS: In article , dated Sat, 18
Dec 2004 23:49:52, seen in news:uk.sci.astronomy, Dr John Stockton
posted :

For 1, there's too much, and not everything, on my site (via URL:http:/
/www.merlyn.demon.co.uk/gravity0.htm); I could pick out bits of Newton,
G, Kepler and of Lagrange & Roche, for a start. Shall we start there?

Postulating a Yes, I've merged and severely pruned and edited my gravity
pages 0..3; the present stage should be visible at
URL:http://www.merlyn.demon.co.uk/grav-faq.htm.

At one stage, I forgot that the newsgroup FAQ existed in HTML; if I've
removed too much mark-up, it can be restored. I've not yet adapted it
so that it would match style if dropped into the FAQ.

Comment?

Deathly hush. I've adjusted it a bit, and a text rendition follows.
The part not extracted from my site has been added to my site, except of
course for the last bit. For the more readable version, see
URL:http://www.merlyn.demon.co.uk/grav-faq.htm

Ö is square root sign.

=========


-----------------------------------------------------------------------

= 2004-12-19, tweaked 2004-12-20, -22.

For more on these topics, see gravity0.htm ff., astro.htm ff., and other
sources.

Links to other sites can be taken from those pages.

-----------------------------------------------------------------------

Basics of Newtonian Gravity

In Newtonian gravity, every element of mass attracts every other element
with a force proportional to the product of their masses divided by the
square of the distance between them.
F = G m1 m2 / d2

G is about 6.675×10-11 m3 kg-1 s-2. The local field, g, is about 9.81 m
s-2 at Earth's surface.

A uniform spherical shell causes no field inside itself. Outside a
symmetrical sphere, its field is as if all its mass were at the centre.

Circular orbital speed balances "centrifugal force" and gravity, v =
Ögr. Escape speed is the speed needed to reach infinity, Ö2gr. Surface
escape speed is what would be needed to attain a height of one radius
against the surface field.

The period of a circular orbit about a spherically symmetric mass
distribution depends only and inversely on the square root of the
average density D within the circumscribing sphere; it is Ö ( 3 p / (G
D) ). That relates immediately to Kepler's Third Law.

------------------------------------------------------------------------

Kepler's Laws, etc.

Kepler's Laws, derived from observation :-

1. The orbits are ellipses with the centre of mass at one focus.
2. If two bodies revolve about each other under the influence of a
central force, the line joining them sweeps out equal areas in equal
times.
3. The square of the period of the orbit is proportional to the cube of
its semi-major axis.

After Newton, they apply in the case of two bodies interacting by any
attracting force along the line of their centres, proportional to the
product of their masses and inversely proportional to the square of
their separation. The Third Law applies among the secondaries of a given
primary.

-----------------------------------------------------------------------

Orbital Paths

The path of the Moon is everywhere curved towards the Sun, because the
Sun's pull exceeds the Earth's.

Seen from a celestial pole, the paths of Europa and Dione cross
themselves, because they orbit at higher speeds than their planets.

The paths of Uranian satellites look peculiar, as the system is tilted
so.

The true paths of our other major natural satellites are wavy lines,
like the path of Earth around the Galactic centre and the paths of
artificial satellites.

------------------------------------------------------------------------

Lagrange Points

If a lesser mass is in a circular orbit around a much greater, there are
co-rotating locations (the Lagrange Points, L1..L5) where a particle may
reside, stably or unstably.

For the Earth-Moon system, L3, Earth, L1, Moon, L2 are in that order in
a straight line; L4 & L5 are in the Moon's orbit, 60° away; L4 leads, L5
trails.

(However, Henry Spencer has said that astronomers prefer the order L1,
Earth, L2, Moon, L3; again L4 leads, L5 trails.) - IS HE RIGHT?

Points L1, L2, L3 are not stable. Halo orbits exist around them and are
used. The SOHO solar observatory orbits the Sun-Earth L1 point. The
future James Webb Space Telescope may go to L2.

If the primary:secondary mass ratio exceeds about 25, there are stable
regions including the "Trojan Points" L4 & L5. The original Trojan
asteroids, named after heroes in Homer's Iliad, are in the L5 position
of the Sun-Jupiter system.

QUERY : different authorities assert 25 & 26. Can 25 be confirmed?

-----------------------------------------------------------------------

The Hill Limit

I don't know much about this - please advise. It concerns the greatest
radius from a planet at which a satellite will be bound and not escape
onto Solar orbit.

-----------------------------------------------------------------------

Tides

Sea-tides are mainly caused by the lunar gravity gradient (with a
component from the solar).

"Spring" tides occur when Earth, Sun, and Moon are in line, and the
gradients reinforce; "Neap" tides occur when Sun, Earth, and Moon form a
right angle. Eclipses only occur at Spring Tides.

Tides tend to cause the orbits of planets and moons to circularise and
synchronise. The tidal effect of a body is proportional to its density &
the cube of its apparent angular diameter, only.

-----------------------------------------------------------------------

The Roche Limit

Roche's Limit for a non-cohesive non-rotating satellite of a primary
body is that the satellite will be broken up tidally if the centre-
centre distance is less than about 2.423 primary radii times the cube
root of the density ratio.

-----------------------------------------------------------------------

Rings

The outer limits of the icy particulate ring systems of the four major
planets of the Solar System are near, but within, the Roche limit.

-----------------------------------------------------------------------

ADDED MATERIAL, = 2004-12-19 18:00 Z
This section has been copied into gravity0.htm and edited there.

Note - predominantly off the top of my head - needs checking. ISTR that
there was a third confirmation of Einstein.

Relativistic

Cite relativity FAQs. Web astro3.htm calculates relativistic travel.

Perihelion of Mercury

The advance of the perihelion of Mercury is largely explained by
Newtonian gravity. But there is a residual advance of 43" of arc per
century, explained by relativitistic gravitation.

Deviation of Light

Light grazing the Sun is deviated by gravity. Eddington's 1919 trip
confirmed Einstein's relativitistic prediction of 1.74" of arc, twice
the Newtonian value.

Gravitational Lensing

In that fashion, the Sun focuses light from infinity at a distance of
550 AU (McAndrew, A M, et al.). Galaxies can focus more distant ones.
Focusing is not lens-like, but it can magnify and intensify.

Blue Shift

Light reaching a massive body is gravitationally blue-shifted, and vice
versa. This is not the Hubble-related shift; it is [shown by] the
Mossbauer effect.

Black Holes

Even classically, light cannot escape from a sufficiently large body.
General Relativity is needed to attempt a full explanation of Black
Holes. One can enter a sufficiently large Black Hole unharmed, but
cannot leave nor survive long. A Black Hole must contain a singularity.
Black Holes evaporate, slowly. When a large enough star runs out of
fuel, it must collapse to a Black Hole?

GRAVITY IN OTHER TOPICS

In Universe - Big Bang, expansion/crunch?, flatness?.

In Galaxy - BH at centre ??

-----------------------------------------------------------------------

Existing UK.S.A. FAQ
Re "Who are the UK astronomers of note?"
It was not 1643 when Newton was born - 1642-12-25 02:?? to 1727-03-20
(Julian, of course). British dates should be British civil dates unless
otherwise specified. Anyway, 1643 actually started on March 25th.

Possible additions, if UK and noteworthy and astronomers : Freeman J
Dyson (the Sphere, not the A.R. nor the vacuum cleaner) ?? Charles
Sheffield ??

-----------------------------------------------------------------------


--
© John Stockton, Surrey, UK. Turnpike v4.00 MIME. ©
Web URL:http://www.merlyn.demon.co.uk/ - FAQqish topics, acronyms & links;
some Astro stuff via astro.htm, gravity0.htm; quotes.htm; pascal.htm; &c, &c.
No Encoding. Quotes before replies. Snip well. Write clearly. Don't Mail News.

(However, Henry Spencer has said that astronomers prefer the order L1,
Earth, L2, Moon, L3; again L4 leads, L5 trails.) - IS HE RIGHT?
Points L1, L2, L3 are not stable. Halo orbits exist around them and are
used. The SOHO solar observatory orbits the Sun-Earth L1 point. The
future James Webb Space Telescope may go to L2.
If the primary:secondary mass ratio exceeds about 25, there are stable
regions including the "Trojan Points" L4 & L5. The original Trojan
asteroids, named after heroes in Homer's Iliad, are in the L5 position
of the Sun-Jupiter system.
QUERY : different authorities assert 25 & 26. Can 25 be confirmed?

I know of someone that knows a good deal about orbital mechanics, I can ask
if you like (I don't think they visit this group)

The Hill Limit
I don't know much about this - please advise. It concerns the greatest
radius from a planet at which a satellite will be bound and not escape
onto Solar orbit.

I understood the explanation on your website no problem, just wonder if it
should be put differently (not sure how though!) Don't know if this helps
http://snipurl.com/bjx4 as its a bit maths heavy.

In Galaxy - BH at centre ??

I don't think anyone has come down with a definitive answer on this one
thats convincing yet.

Re "Who are the UK astronomers of note?"
It was not 1643 when Newton was born - 1642-12-25 02:?? to 1727-03-20
(Julian, of course). British dates should be British civil dates unless
otherwise specified. Anyway, 1643 actually started on March 25th.

http://www-groups.dcs.st-and.ac.uk/~...ns/Newton.html

http://www.maths.tcd.ie/pub/HistMath...RB_Newton.html


Possible additions, if UK and noteworthy and astronomers : Freeman J
Dyson (the Sphere, not the A.R. nor the vacuum cleaner) ?? Charles
Sheffield ??

I can add these to the list no problem.

Graeme



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Dr John Stockton wrote:

If the primary:secondary mass ratio exceeds about 25, there are stable
regions including the "Trojan Points" L4 & L5. The original Trojan
asteroids, named after heroes in Homer's Iliad, are in the L5 position
of the Sun-Jupiter system.

QUERY : different authorities assert 25 & 26. Can 25 be confirmed?

The value comes from the Routh stability criterion (derivation is in A E
Roy's book Orbital Motion ~ p138).

In terms of the reciprocal mass ratio mu = 1/2 - sqrt(23/108)

So in your notation 25.959936... pretty close to 26

-----------------------------------------------------------------------

The Hill Limit

I don't know much about this - please advise. It concerns the greatest
radius from a planet at which a satellite will be bound and not escape
onto Solar orbit.

It may come from the equation for Hill's limiting surface. ie the
requirement that kinetic energy = 0. Or the the other one who did
numerical integration of the solar system (GW vs JG or voice versa).

I haven't heard of it before.

In Galaxy - BH at centre ??

Thought to be. Cite evidence from strong acceleration of stars observed
close to galactic centre. Something compact with a lot of pulling power
is there.

Regards,
Martin Brown