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Bart

Belgium
76 Posts

Posted - 29 Nov 2011 :  03:57:25  Show Profile  Reply with Quote
The Moon (O) and observer (A) move with largely the same velocity and orientation (up to 30km/s) relative to the star (S).
As a consequence, stellar aberration as observed on the mooon and the Earth are largely the same (up to 20.5 arcsec).

The moon rotates in 27.3 days around the Earth which equals a displacement of 0.55 arcsec/second.
Light takes 1.3 seconds to travel from the Moon to the Earth.
So light-time correction accounts for (up to) 0.7 arcsec which is way below the value for stellar (or planeteray) aberration.

So Light-time correction and stellar/planetary aberration are different effects ...
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Michiel

Netherlands
85 Posts

Posted - 29 Nov 2011 :  09:49:06  Show Profile  Reply with Quote
Yes Larry, but if the speed of light is the same for all observers (and in all directions), how can aberration be a function of the speed of the incoming particle or wave? In absence of some kind of local or absolute reference frame, there seems to be a catch 22.

Bart quoted from the wiki:

"[aberration] depends solely upon the transverse component of the velocity of the observer, with respect to the vector of the incoming beam of light (i.e., the line actually taken by the light on its path to the observer)."

Doesn't the word "actually" imply a pre-defined reference frame?
Maybe my confusion comes from the fact that we wouldn't able to recognize absolute aberration, we can only observe changes in aberration.
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Jim

1862 Posts

Posted - 29 Nov 2011 :  14:14:29  Show Profile  Reply with Quote
What happens in the case of the sun setting/rising 5 minutes before we see the effect? An observer on Earth is spinning and you have not factored that motion in have you?
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Jim

1862 Posts

Posted - 29 Nov 2011 :  14:38:00  Show Profile  Reply with Quote
Does this effect generate a red shift in the spectrum of stars??
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Bart

Belgium
76 Posts

Posted - 29 Nov 2011 :  15:04:13  Show Profile  Reply with Quote
Where it relates to the sun, one can look at what happens during Solar Eclipses.

What Tom Van Flandern wrote on this site:
"We find that maximum eclipse occurs roughly 381.9 seconds of time, on average, before the time of gravity maximum."
(the time of gravity maximum = when Sun, Moon and Earth are exactly aligned)

This implies that light arriving from the sun is reaching us with an aberration of 20.5 arcsec.

This is often explained to as: "we observe the sun where it was 8 minutes ago and that's why we observe the sun with this exact aberration".
I would challenge this as follows: Suppose the Earth would not be rotating and not have an inclination; then an observer on the Equator at noon time would always observe the sun in the exact same perpendicular position at any moment during the day.

So my take on this is that the light arriving from the sun must be reaching us through a curved path.
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Bart

Belgium
76 Posts

Posted - 29 Nov 2011 :  15:46:39  Show Profile  Reply with Quote
Where it relates to the motion of the observer on Earth: this is the "Diurnal aberration", which has a maximum of 0.32 arcsec (on the Equator).
For the observer in Florida, the Moon/Jupiter were relatively low in the sky so I would expect just a minor effect.

I would not expect this to create a red shift / doppler effect.
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Jim

1862 Posts

Posted - 29 Nov 2011 :  23:17:39  Show Profile  Reply with Quote
It seems to me this is mostly geometry of objects moving in 3D space. Is that correct? And if so, would a curved path be needed as the only means to explain the effects? Would the effect be different without curved paths? And what force is involved to cause the curving path? If the path of light curves when will it curve 360 degrees and return to where it started?
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Bart

Belgium
76 Posts

Posted - 30 Nov 2011 :  04:59:57  Show Profile  Reply with Quote
A curved path is indeed required to explain why a star or planet can become visible when we know that there is an other object in between the obserer and the point where the light originated from (considering a straight line).

The effect without a curved path:
Relative to the object that is occulting:
- at one end: star/planet shows overlayed with the object
- at the other side: star/planet disappears before it has reached the border of the object

There is no 'force' involved causing the curved path in the same way there is no 'force' involved with stellar aberration occuring locally or with the bending of light due to gravitation. The cause of the curving must be closely related to the very nature of the propagation of light.

For someone travelling at the speed of light (tangential to the star being observed), the observed aberration would be 45 degrees (independent if the medium surrounding this observer is travelling at the same speed or not). Travelling beyond the speed of light would still pose a limit of 90 degrees. So a 360 degrees turn doesn't look like an option...
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Bart

Belgium
76 Posts

Posted - 30 Nov 2011 :  14:54:54  Show Profile  Reply with Quote
On the topic brought up by Michiel: "speed of light is the same for all observers (and in all directions)".

This statement goes back to the interpretation of the Michelson-Morley experiment:
http://en.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment

The picture on this Wikipedia page: "A depiction of the concept of the "aether wind"" is fundamentally different from the model used to explain the planetary aberration which starts from a depiction whereby the light carrying medium rotates around the Solar System and whereby the planets move along with this light carrying medium with exactly the same speed (so without the planets dragging the medium or vica-versa).

Using this 'alternative depiction', it is logical that speed of light is the same for all observers (and in all directions).

At the same time, it is logical that it is possible to measure the Earth rotation through the Michelson Gale experiment:
http://en.wikipedia.org/wiki/Michelson%E2%80%93Gale%E2%80%93Pearson_experiment
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Jim

1862 Posts

Posted - 30 Nov 2011 :  16:28:15  Show Profile  Reply with Quote
Bart, Is the bending of light in the Einstein gravity model adding to your curved light beam model? If you added your curve to the data we have from bent light from stars how much of the total would be caused by the curve? I'm assuming both effects are operating.
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Larry Burford

USA
2235 Posts

Posted - 30 Nov 2011 :  22:27:18  Show Profile  Reply with Quote
[Bart] "The Moon (O) and observer (A) move with largely the same ..."

I constructed my example so that observer A is stationary with respect to the incoming beam. It is observer B (on Earth, or in the rocket moving like Earth moves) that is moving almost as fast as Luna (or the rocket O). Observer C is moving just as fast as observer B, but in a retrograde orbit. This is why B and C see S out in front of where it actually is, with the same magnitude of angular displacement, but the opposite sign (from our third party perspective - from their first party perspectives, B anc C just see S "out in front" of where it ought to be).

And why A sees S with zero angular displacement. Right where it ought to be. Right where it actually is.

[Michiel] "[aberration] depends solely upon the transverse component of the velocity of the observer ..."

They are being a little lazy with their explanation, and as a result their claim is misleading. If it were literally true, aberration would be expressed in meters per second rather than in degrees or radians. Aberration is a function of two velocities, not one.

I suppose they phrase it as they do because the speed of light (the speed of the incoming wave) is the same for all observers.


                          transverse speed_observer
angle_aberration = ------------------------------------------------
                    incoming speed_particle or wave being observed

This approximate formula works for just about anything, as long as the calculated angle is less than about 4 or 5 degrees. Bullets, arrows, water waves, protons, light waves, and so on.

[Jim] "Does this effect generate a red shift in the spectrum of stars??"

If you look in the direction of your motion, the stars will have a slight blue shift. If you then turn 180, those stars behind you will have a slight red shift. But if you look perpendicular to your line of travel (where we have been looking to measure angular displacement of the star S and other stars near-by) you will see no doppler effect.

BTW, when you do see the doppler shift of stars in front of you or behind you, you will see no angular displacement of those stars.

One or the other, but not both.


[Bart] "Suppose the Earth would not be rotating ..."

To keep Sol directly overhead all the time, Earth must rotate once per year. From Luna, Earth appears to always be in the same place in the sky, because Luna rotates once per month.
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Bart

Belgium
76 Posts

Posted - 01 Dec 2011 :  15:15:13  Show Profile  Reply with Quote

(S)

| |
| |
| | Medium static to S
.................................................................................
S_b | Medium static to B or A
\ S_a O
\ S_a O
\ | OOO S_c
\ |OOOOO /
\<---OOOOOOO/
\ OOOOO/
\ OOO/
\ O/
<-- B /
A /
C -->

FIG 2 The left edge of O has just crossed the straight line between S and A

Assuming that B (simulating Earth) is static relative to the light carrying medium:
At the instant when the leading edge of O just crossed the straight line between S and A/B/C:
- Light for all three observers follows the path S_b -> A/B/C and is therefore visible to all three observers
- B observes light coming from S_b and B observes the left edge of O from the direction of S_a
- A observes light as if coming from S_a and observes the left edge of O as coming from S_c
- C observes light as if coming from S_c and observes the left edge of O as coming from a position to the right of S_c

Assuming that A is static relative to the light carrying medium.
At the instant when the leading edge of O just crossed the straight line between S and A/B/C:
- Light hits O as a result of which none of the observers can see star S
- A observes the edge of O coming from S_a (behind which S is obscured)
- B observes the edge of O as coming from S_b (behind which S is obscured)
- C observes the edge of O as coming from S_c (behind which S is obscured)

So, independent of the direction of the light carrying medium:
- all observers A/B/C will either see S or not see S
- every of the observers A/B/C see light S coming from a direction that depends on their own motion

In other words: it does not help to move faster in an attempt to observe more stars behind the edge of moon ...
The reason being that the edge of the moon will be subject to the same amount of additional aberration as for the stars behind it.
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Joe Keller

USA
958 Posts

Posted - 05 Dec 2011 :  01:01:38  Show Profile  Reply with Quote
"Reverse Occultation" of Venus

Previously I cited observations of Lunar occultation anomalies of Jupiter, the Galilean moons, and Uranus. These included what I called "reverse occultation", where Jupiter's disk showed in front of Luna. This is not the "oil drop effect". The overlap is typically about half the planet's disk: many arcseconds, much more than, say, two or three arcsecond "seeing" due to atmospheric scatter.

Rechecking the Astronomical Journal tonight (the indexes of all the issues on the shelf at Iowa State Univ., from the first issue, in 1849, through 1890) I find three reports of "reverse occultation" of Venus, involving two different observatories and two different occultations. The first is by J. Ferguson at the U. S. Naval Observatory, for the April 18, 1855, Lunar occultation of Venus, observed with the "large equatorial" (AJ 4:95, 1855). Ferguson reports: "8h39m10s (M. T. Washington) The planet was seen half its diameter within the limb of the moon, exhibiting no diminution of light or of magnitude, but showing as if it were on this, and not on the other side of the moon. 8h39m38s The first diminution of magnitude was apparent; the inside or cut edge being straight and well defined; the planet still showing as if projected on the surface of the moon."

Again for the April 24, 1860 Lunar occultation of Venus, Ferguson (AJ 6:107, 1860) says, "The planet was distinctly seen projected on the moon's dark surface, and, when it began to be eclipsed, the diminution of light was made by a circular line, inside the moon's limb and parallel to it. I had witnessed the same appearance at an occultation of Venus, 1855...".

A similar report for this 1860 occultation comes from Gloucester, Massachusetts by Tuttle (AJ 6:119): "At the immersion the planet appeared to advance nearly its whole diameter upon the dark limb of the moon." (Tuttle was using a famous telescope that formerly had been used by Hind in England for asteroid discovery at Bishop's Observatory, reported in the Monthly Notices of the Royal Astronomical Society, 1851.)

Addendum Dec. 5: For the 1855 occultation, Venus was 1.38 AU distant; the dark part of Luna was covering the lit part of Venus. For the 1860 occultation, the configuration was roughly the same, but Venus was only 0.83 AU distant.

Edited by - Joe Keller on 05 Dec 2011 17:34:59
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Larry Burford

USA
2235 Posts

Posted - 05 Dec 2011 :  12:01:56  Show Profile  Reply with Quote
If it was naked-eye visible in the mid 1800s, a CCD ought to give even better results. Recordable results, too.

What percentage of astronomical observations are routinely recorded these days?

How many reports like this have you seen? Is there a pattern of any sort that would allow us to predict when an occultation will turn "reverse"?

It might be an illusion, caused by the mind's ability to fill in (create) missing detail. Especially in low light conditions. Any evidence for or against this idea?

Have stars ever been observed 'behaving' like this?

===

Hmm. I wonder if there is a way to create a laboratory version of this?

LB
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Larry Burford

USA
2235 Posts

Posted - 05 Dec 2011 :  14:21:39  Show Profile  Reply with Quote
Bart,

I believe I have figured out part of our misunderstanding.

What an astronomer calls "stellar aberation", a physicist calls "angular displacement". As a physicist rather than an astronomer, I think in terms of angular displacement. And I am aware of four different phenomena that can cause an object to exhibit angular displacement relative to where I think it ought to be. (There may be others.)

  • aberration angle (caused by observer motion perpendicular to the particle or wave being observed)

  • lead angle (caused by target motion perpendicular to the observer)

    • lead angle is actually the same thing as aberration angle, but the observer is firing a particle or wave at the target instead of observing a particle or wave comming from the target

  • media drag (caused by particle or wave motion through a surrounding medium)

  • media drift (caused by motion of the surrounding medium, through which the particle or wave moves, relative to the observer)

    • if that drift is perpendicular to the path of the particle or wave, it is seen by the observer as similar to aberration

    • if that drift is parallel, it is seen as similar to drag

Lead angle is not an issue for astronomers. But it is a problem for those who plan space missions. And duck hunters, and snipers. And navigators.) Main stream astronomers further assume that no medium is involved so drift and drag are also not an issue for them. I presume that you are not a main stream astronomer, however, so you (and I) do not have the luxury of assuming that drift and drag can not contribute to the observed angular displacement of a star or planet.

===

The total (or observed) angular displacement (stellar aberration) is actually the sum of angular displacements caused by aberration, media drift and media drag. Any of these terms, or all of them, can be zero under the right circumstances.

If there is an aether (a light carrying medium) and if it contributes to the observed angular displacement of a particular star, that contribution should be listed separately and called drift. Drag seems to be always zero, whether light is assumed to propagate as wave or particle.

===

When drift is present, it is separate from the aberration contribution caused by the observer's motion. But both cause the object to be displaced from where it "ought" to be. Drift is a possible cause for some of the unexpected angular position observations being discussed here.

Regards,
LB
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Bart

Belgium
76 Posts

Posted - 05 Dec 2011 :  16:31:04  Show Profile  Reply with Quote
Examples of "Reverse Occultation" ?
http://assabfn.blogspot.com/2010/09/more-photos-venus-and-moon-occultation.html
http://www.icstars.com/HTML/JupiterMoon/MoonJupiter.html
When enlarging the picture of the Occultation of Jupiter by the Moon on 7 Dec 2004 9:14 UT:
http://spaceweather.com/occultations/07dec04/parker1_huge.jpg there is also a small overlap that can be observed.

Next steps: try to simulate the path followed by the light coming from these planets at the moments indicated.

Therebe consider the 'lead angle' as light will not arrive at the observer on Earth without a 'lead angle' (relative to the straight line between the planet and the Earth). My hypothesis is that the lead angle continues to play on the full length of the path, thereby creating a difference between the light reflected by the moon and the light from the observed planet razing by that some point on the moon.
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Larry Burford

USA
2235 Posts

Posted - 06 Dec 2011 :  16:32:37  Show Profile  Reply with Quote
[Bart] "Therebe consider the 'lead angle' as light will not arrive at the observer on Earth without a 'lead angle' (relative to the straight line between the planet and the Earth)."

Since we are observing light comming from a star or planet rather than sending light to a star, the name of the phenomenon involved is 'drift', not 'lead'.

Drift (what used to be called the 'aether wind' in the old days) does in fact act on the light beam from the moment it leaves the source to the moment it is detected. Assuming of course that there actually is a light carrying medium.

That drift can cause the star being observed to appear displaced from its actual position. Suppose that the drift between there and here is always parallel to Sol's polar axis, and in the direction from north to south. The light of a beam propagating from there to here will be carried 'downward' for the entire trip (if we define Solar north as up), causing it to appear to be 'south' of where it actually is.

The last few hours of travel for that beam will be within the entrained bubble rotating with our solar system, so there will also be a slight 'westward' angular displacement of the beam caused by this last minute change in the 'wind' direction.

-edit1-
The first few hours of travel for that beam will be within the entrained bubble rotating with the source star, so there will also be a slight '???ward' angular displacement of the beam caused by this inial 'wind' of unknown speed and direction.

Because it happens so far away, it is probably not detectable. But I mention it (belatedly) to make my discussion of the overall process more complete.
-edit1-

Finally, at the moment of observation, aberration will be added to the total observed angular displacement. So you see that relying only on an aberration calculation to know where a star is 'supposed to be' might not always be accurate.

The difference between the expected location and the observed location might be evidence for the light carrying medium. This would be especially so if there is a non-zero observed angular displacement when we expect (calculate) "stellar aberration" to be zero.

-edit2- Summary
lead - occurs at the instant a particle or wave leaves the source
drift / drag - occur continuously while a particle or wave is traveling through a medium between source and target
aberration - occurs at the instant a particle or wave is detected at the target

All of these, separately or in various combinations, can cause the observed particle or wave to arrive from a direction other than the stratight line between source and target.
-edit2-


LB
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Joe Keller

USA
958 Posts

Posted - 06 Dec 2011 :  23:09:42  Show Profile  Reply with Quote
Searching Iowa State University's holdings of the Astronomical Journal, forward through 1960, I find some more planetary occultations by Luna that received one or more reports. Occultation of Mars by Luna, on three different dates in 1948, was reported by four different amateurs who gave little information other than contact times; the instruments used were 9.5 inch, 2 inch and 82mm refractors, and "a 12 power German infantry range finder" (AJ 54:200, 1949 & AJ 53:216, 1948).

(Also, prior to 1900, there is in the AJ a report of an occultation of Saturn by Luna, from Pulkovo Observatory. No anomalies were reported; contact times are in Pulkovo sidereal time.)

The Jan. 12-13 (GMT) 1923 occultation of Venus by Luna, was reported by three major U.S. observatories using large telescopes: the U. of Virginia 26 inch & 5 inch (AJ 35:8, 1923), the U. of Illinois 12, 4 & 2.25 inch (AJ 35:102-103), and the Cincinnati Observatory 16 & 4 inch (AJ 35:107). None reported anomalies. This occultation differed from those of 1855 and 1860, discussed in my previous post: the former occultations were of the lit side of Venus by the dark side of Luna; the 1923 occultation was of the dark side of Venus by the lit side of Luna. Emersion in 1923, would have been of the lit side of Venus from the dark side of Luna, but emersion phenomena are more difficult to observe with the eye because the planet is hidden and there is little warning that emersion is about to occur.

Edited by - Joe Keller on 06 Dec 2011 23:15:40
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Joe Keller

USA
958 Posts

Posted - 07 Dec 2011 :  17:16:00  Show Profile  Reply with Quote
There will be an occultation of Venus by Luna on Aug. 13, 2012, with ingress at about 19h UT or thereabouts (I gather, somewhat later than that, in the U.S., due to lunar parallax) and egress at about 20:30 UT. (This is not to be confused with the transit of Venus across the Sun, in June 2012.)

This would have to be seen in daylight from the U.S., but, I gather, will be far enough above the horizon on the west coast, or better yet Alaska or Hawaii. It will still be night in northeast Asia and that would be the best place to see it. As in 1923, it will be the lit part of Luna covering the dark part of Venus.
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Joe Keller

USA
958 Posts

Posted - 17 Dec 2011 :  21:56:55  Show Profile  Reply with Quote
Noises at sundown

At sundown, usually on a sunny winter day when the temperature has been near zero Celsius, I have on about four recent evenings heard one or two loud "crack" sounds resembling either distant gunfire or a breaking branch. One, was more like a cannon or a large, loud, distant fireworks explosion. I've heard these sounds from two different farmsteads, but am unsure whether they come from the direction of trees or the direction of farm buildings. All the times I've heard them, they have been from the north or northwest, even though many trees and farm buildings lay in other directions.

I thought it might be freezing water in trees causing branches or trunks to crack. A neighboring farmer suggested that when the winter sun suddenly stops shining on the vertical metal wall of a farm building, sudden contraction of the metal might cause the cracking sound.

Today I timed it. I found that the two cracks I heard, were five minutes and six minutes after sundown (sun center at zero elevation).
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