Bye, bye Moon

Thanks to a comment by Chuck Wood, it was brought to my attention that
my octet image Moons had different diameters. Chuck was interested as
to whether I had adjusted the image size for the component Moons.

As it happened, I hadn't but I promised him I'd have a look into why
there was variation. He'd asked whether it was due to the Moon moving
to perigee.

Well this graphic

http://www.digital-astronomy.co.uk/t...strips_500.jpg

shows that it is due to this fact. Amazing. Although I knew the Moon
changed apparent diameter, I had no idea that it was so dramatic an
effect.

Thanks go to Chuck for pointing this out.

--
Pete Lawrence
http://www.pbl33.co.uk
Most recent images http://www.pbl33.fast24.co.uk/recent_images.html

Pete Lawrence wrote:
Thanks to a comment by Chuck Wood, it was brought to my attention that
my octet image Moons had different diameters. Chuck was interested as
to whether I had adjusted the image size for the component Moons.

As it happened, I hadn't but I promised him I'd have a look into why
there was variation. He'd asked whether it was due to the Moon moving
to perigee.

Well this graphic

http://www.digital-astronomy.co.uk/t...strips_500.jpg

shows that it is due to this fact. Amazing. Although I knew the Moon
changed apparent diameter, I had no idea that it was so dramatic an
effect.

Thanks go to Chuck for pointing this out.

I've just performed a couple of quick calculations with just a few
assumptions.
1) Small angle approximations can be used, so theta = x/D, where theta is
the angle subtended, x is the moon's radius & D the distance to the moon.
This means we can use the linear size of the strips to represent the angle.
2) Ignoring the moon's and earth's radii. 3) We know the moon's perigee is
359000km

I've measured the lengths of your strips to be 359 & 331 pixels for the
longest and shortest respectively.

From this, the apogee should be 359 * 359000 / 331 = 389000km, which is
actually a bit on the low side.


You can find similar stuff at
http://www.fourmilab.ch/earthview/moon_ap_per.html


It's nice to know that amateurs can do interesting things. Now has anyone
tried similar measurements using a reticle?


DaveL

On Wed, 8 Sep 2004 11:33:08 +0100, "Dave"
wrote:

I've just performed a couple of quick calculations with just a few
assumptions.
1) Small angle approximations can be used, so theta = x/D, where theta is
the angle subtended, x is the moon's radius & D the distance to the moon.
This means we can use the linear size of the strips to represent the angle.
2) Ignoring the moon's and earth's radii. 3) We know the moon's perigee is
359000km

I've measured the lengths of your strips to be 359 & 331 pixels for the
longest and shortest respectively.

From this, the apogee should be 359 * 359000 / 331 = 389000km, which is
actually a bit on the low side.


You can find similar stuff at
http://www.fourmilab.ch/earthview/moon_ap_per.html


It's nice to know that amateurs can do interesting things. Now has anyone
tried similar measurements using a reticle?

Thanks for that Dave. However, there are a couple of points that need
to be made he

1) The strips don't necessarily start from apogee (or for that fact
finish at perigee). They are, as far as I'm aware, in the middle of a
cycle (I've not checked this though).

2) I've not included any azimuth figures for the strips yet. Azimuth
will affect the distance from the observer to the Moon. When the
Moon's in the East, the distance is greater than when it's directly
overhead. This effect will be small though, I'm guessing.

3) The strips were put together very quickly to check to see what was
happening. To measure them correctly, it will be necessary to compare
like with like - i.e. make sure that the Moon is orientated exactly
the same before taking the sample. I didn't do this in the example
presented (although the strips are approximately similar - they are
not exact).

My intention is to add the two (and hopefully more) additional images
that I currently have into the mix. I'll post the final, formal
version when I've created it.


--
Pete Lawrence
http://www.pbl33.co.uk
Most recent images http://www.pbl33.fast24.co.uk/recent_images.html

Pete Lawrence wrote:


Thanks for that Dave. However, there are a couple of points that need
to be made he

1) The strips don't necessarily start from apogee (or for that fact
finish at perigee). They are, as far as I'm aware, in the middle of a
cycle (I've not checked this though).

2) I've not included any azimuth figures for the strips yet. Azimuth
will affect the distance from the observer to the Moon. When the
Moon's in the East, the distance is greater than when it's directly
overhead. This effect will be small though, I'm guessing.

3) The strips were put together very quickly to check to see what was
happening. To measure them correctly, it will be necessary to compare
like with like - i.e. make sure that the Moon is orientated exactly
the same before taking the sample. I didn't do this in the example
presented (although the strips are approximately similar - they are
not exact).

Here's the same effect on the Sun - two photos taken with the same
setup and superimposed:

http://www.hadastro.org.uk/sunsize.htm
My intention is to add the two (and hopefully more) additional images
that I currently have into the mix. I'll post the final, formal
version when I've created it.

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47

On Wed, 08 Sep 2004 15:45:00 +0100, Martin Frey
wrote:

Here's the same effect on the Sun - two photos taken with the same
setup and superimposed:

http://www.hadastro.org.uk/sunsize.htm

Great stuff Martin. While not Earth shattering in it's own right,
it's nice to see it in action. Dave's link (now I've had time to view
it) has a good Apogee/Perigee generator in it. It shows that the Moon
was at Perigee on August 27th at 05h38m. My first image on the 29th
at 22h18m is a way off that, so the secion of variation that I'm
showing starts part way through the cycle. However, I do have the
Moon imaged on the next apogee date (September 8th 02h43m). My image
will be a couple of hours adrift of that time, but close enough.

If the weather holds, I should just about have enough images to show
the size beginning to increase again. Now I've just got to think of
some snazzy way to present it and I can then get on with something a
bit more serious ;-)

--
Pete Lawrence
http://www.pbl33.co.uk
Most recent images http://www.pbl33.fast24.co.uk/recent_images.html

The difference in time of night will also need to be considered. We're
closer to to the moon by one earth radius at midnight (on a full moon) than
at sunset.

Also of interest is the different colouration on each of the strips.
Something that Pete had raised on his original posting of the moon.

Regards


Chris



"Pete Lawrence" wrote in message
...
On Wed, 08 Sep 2004 15:45:00 +0100, Martin Frey
wrote:

Here's the same effect on the Sun - two photos taken with the same
setup and superimposed:

http://www.hadastro.org.uk/sunsize.htm

Great stuff Martin. While not Earth shattering in it's own right,
it's nice to see it in action. Dave's link (now I've had time to view
it) has a good Apogee/Perigee generator in it. It shows that the Moon
was at Perigee on August 27th at 05h38m. My first image on the 29th
at 22h18m is a way off that, so the secion of variation that I'm
showing starts part way through the cycle. However, I do have the
Moon imaged on the next apogee date (September 8th 02h43m). My image
will be a couple of hours adrift of that time, but close enough.

If the weather holds, I should just about have enough images to show
the size beginning to increase again. Now I've just got to think of
some snazzy way to present it and I can then get on with something a
bit more serious ;-)

--
Pete Lawrence
http://www.pbl33.co.uk
Most recent images http://www.pbl33.fast24.co.uk/recent_images.html

Pete Lawrence wrote:
On Wed, 8 Sep 2004 11:33:08 +0100, "Dave"
wrote:

I've just performed a couple of quick calculations with just a few
assumptions.
1) Small angle approximations can be used, so theta = x/D, where
theta is the angle subtended, x is the moon's radius & D the
distance to the moon. This means we can use the linear size of the
strips to represent the angle. 2) Ignoring the moon's and earth's
radii. 3) We know the moon's perigee is 359000km

I've measured the lengths of your strips to be 359 & 331 pixels for
the longest and shortest respectively.

From this, the apogee should be 359 * 359000 / 331 = 389000km, which
is actually a bit on the low side.


You can find similar stuff at
http://www.fourmilab.ch/earthview/moon_ap_per.html


It's nice to know that amateurs can do interesting things. Now has
anyone tried similar measurements using a reticle?

Thanks for that Dave. However, there are a couple of points that need
to be made he

1) The strips don't necessarily start from apogee (or for that fact
finish at perigee). They are, as far as I'm aware, in the middle of a
cycle (I've not checked this though).

2) I've not included any azimuth figures for the strips yet. Azimuth
will affect the distance from the observer to the Moon. When the
Moon's in the East, the distance is greater than when it's directly
overhead. This effect will be small though, I'm guessing.

I hadn't thought about the dates, was just trying to do what before we had
computers would have been a quick back of the postage stamp calculation. I
was specifically ignoring the moon's altitude by ignoring the earth's radius
in the calculation. Hey, I'm a physicist, if you was precision, ask an
engineer


3) The strips were put together very quickly to check to see what was
happening. To measure them correctly, it will be necessary to compare
like with like - i.e. make sure that the Moon is orientated exactly
the same before taking the sample. I didn't do this in the example
presented (although the strips are approximately similar - they are
not exact).

My intention is to add the two (and hopefully more) additional images
that I currently have into the mix. I'll post the final, formal
version when I've created it.


I look forward to it, it's nice to see such things are easily demonstrated
with amateur equipment. I liked your idea of showing strips of the moon
instead of the whole discs, it makes the effect much more obvious.

I'd still be interested to know if any of the more experience observers has
ever noticed this with a reticle.


DaveL

Pete Lawrence wrote:

Great stuff Martin.

But sadly not mine - but all praise accepted willingly of course

If the weather holds, I should just about have enough images to show
the size beginning to increase again. Now I've just got to think of
some snazzy way to present it and I can then get on with something a
bit more serious ;-)

I suppose one accuracy limitation with any method like this is focus.
While not for a moment suggesting that your pics are not "in focus" it
can be a subjective judgement. The first thing that varies with focus
is image size - so maybe a whisker away from perfect focus can produce
a much larger variation in size than in the blurriness on which we
tend to judge focus.

--
Martin Frey
http://www.hadastro.org.uk
N 51 02 E 0 47

On Thu, 09 Sep 2004 14:12:40 +0100, Martin Frey
wrote:

Pete Lawrence wrote:

Great stuff Martin.

But sadly not mine - but all praise accepted willingly of course

If the weather holds, I should just about have enough images to show
the size beginning to increase again. Now I've just got to think of
some snazzy way to present it and I can then get on with something a
bit more serious ;-)

I suppose one accuracy limitation with any method like this is focus.
While not for a moment suggesting that your pics are not "in focus" it
can be a subjective judgement.

This is a hypothetical thought experiment then? Ok I'll try to
imagine what "out of focus" means. I read about this once ;-)

The first thing that varies with focus
is image size - so maybe a whisker away from perfect focus can produce
a much larger variation in size than in the blurriness on which we
tend to judge focus.

Agreed. However, I think that the variations in size due to focus
problems are going to be pretty small. Even a focussed image can
appear blurred due to poor seeing conditions.

--
Pete Lawrence
http://www.pbl33.co.uk
Most recent images http://www.pbl33.fast24.co.uk/recent_images.html

"Dave" wrote in message
...
I hadn't thought about the dates, was just trying to do what before we had
computers would have been a quick back of the postage stamp calculation. I
was specifically ignoring the moon's altitude by ignoring the earth's
radius in the calculation. Hey, I'm a physicist, if you was precision, ask
an engineer
Ask an engineer what? They often work with tolerances of 100%. I've a number
of amusing anecdotes regarding engineers, and a couple of tragic ones. To
take one; British Aerospace (formerly the British Aircraft Corporation) used
to manufacture plane parts to quite a wide tolerance (I believe it was
something like 0.005"). After test flying one of their prototype planes (or
not, in this instance) they cut it down to 0.001" and made other changes in
their work scheduling. The reason? One of their machinists always worked to
the upper limit, and he happened to be making most of the parts for one side
of the plane while his colleague always worked to the lower limit, and he
was working on the parts for the other side (often mirror image parts). When
the test pilot tried to take off, he could only get the landing gear on one
side of the plane off the ground.
Grim