# The Naked Scientists Forum

### Author Topic: Why can we see stars that are actually behind the Moon?  (Read 5927 times)

#### BartLeplae

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##### Why can we see stars that are actually behind the Moon?
« on: 20/10/2013 18:25:24 »
Due to the effect of annual aberration (which is one aspect of stellar aberration), we observe stars up to 20.5 arcsec displaced from their long-term average position. On the contrary, light emitted by the Moon is not subject to the effect of stellar aberration. As a consequence, the actual position (or better: the long-term average position) of a star can be up to 20 arcsecs behind the Moon at the moment when the apparent position is still in a visible location next to the Moon.

This implies that the effect of annual aberration must take place before light arrives at the distance of the Moon.

Illustrations of the "Aberration of light" such as in Wikipedia show an effect that takes place near the observer on Earth. For the situation described above: if we draw a straight line in the direction of the actual direction of the star, than we hit the surface of the Moon (even if we take into account that the Moon proceeds with around 0.5 arcsec/sec). This provides evidence that these illustrations, although allong to derive the right formula, are not reflecting the actual mechanism.

The effect of annual aberration can be explained through relativistic aberration: light curves when it passes through frames with different transversal velocities.  So instead of applying the formula for relativisitic aberration at the location of the obverser, it needs to be applied as an incremental effect while light passes through the Solar System. These incremental changes add up to the exact same amount of aberration as if the annual aberration would occur near the observer. Because the Earth and the Moon find themselves in the same frame relative to the Solar System, there is no incremental aberration when light continues its travel from the Moon to the Earth.

The "frames with different transversal velocities" referred to above can be defined as follows: at any point in the plane of the Solar System, the frame is rotating around the Solar System with the same speed as a planet in a circular orbit would do. The frame is rotating faster near the Sun and rotates slower towards the boundary of the Solar System.

Remark: The "actual position" of a star is different from its "long term average position" because the latter includes the secular aberration term which is due to the motion of the Solar System relative to the Milky Way. As a consequence, the actual position of a star can be significantly more behind the Moon than the 20 arcsec mentioned above.

"The curvature of light due to relativistic aberration"

I once received the following response to the question "Why can we see stars that are actually behind the Moon?":
"Between the moment light has passed the Moon and arrives with the observer, the observer on Earth has moved forward with 30km/s. If the observer would not have moved, then the star would not have been visible as it would have remained hidden behind the Moon; but since the observer moved forward, the star has become visable to the observer.

My own consideration with respect to this reponse:
It takes 1.3 seconds for light from the Moon to reach the Earth. The question is not why the observer wasn't able to observe the star 1.3 seconds ago but why, at the exact moment of the observation, the observer is able to observe the star while the actual direction of the star is behind the Moon.

The astromer Tom Van Flandern describes the absence of the aberration for the Moon in publications such as "What the Global Positioning System Tells Us about Relativity". newbielink:http://www.metaresearch.org/cosmology/gps-relativity.asp [nonactive]
Tom Van Flandern describes aberration as an effect that takes place when light enters the Earth's gravity field (so before light reaches the Moon).
My own consideration towards this perspective is that the Earth's gravity field doesn't have clear boundary. If the Earth's gravity field would play a role, than the Sun's gravity field would need to play an ever bigger role.

When Michelson published the findings of the famous MMX experiment "On the Relative Motion of the Earth and the Luminiferous Ether" he started to describe Stellar Aberration as a reason why to expect the measurement of the 30km/s speed of the Earth.
My own consideration is that this experiment was executed with a local light source that is not subject to stellar aberration.
« Last Edit: 26/10/2013 19:43:57 by BartLeplae »

#### yor_on

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##### Re: Why can we see stars that are actually behind the Moon?
« Reply #1 on: 11/12/2013 09:29:33 »
"The effect of annual aberration can be explained through relativistic aberration: light curves when it passes through frames with different transversal velocities.  So instead of applying the formula for relativisitic aberration at the location of the obverser, it needs to be applied as an incremental effect while light passes through the Solar System. These incremental changes add up to the exact same amount of aberration as if the annual aberration would occur near the observer. Because the Earth and the Moon find themselves in the same frame relative to the Solar System, there is no incremental aberration when light continues its travel from the Moon to the Earth."

Are you saying that you imagine it as a result of 'gravity' from all sorts of objects, not only referable to Earths gravity? But to the whole of the solar system, creating the (curved to a Earthly observer) geodesic? I'm sort of losing myself here, but it is interesting. As for Earth's motion, that one may play a role, but very small, so I totally agree with you at that.
=

It's this about "So instead of applying the formula for relativisitic aberration at the location of the observer, it needs to be applied as an incremental effect while light passes through the Solar System." that gets me confused. Are you considering it to be different frames that change the 'curvation of light'? The point you're aiming at might be what a observer dependency should be thought of as? as related to the observer (limited to this particular case of questioning, if I now are following your thoughts?), or related to a defined  observer independent 'cosmos', having gravity dynamically acting on particles (waves/photons)??
« Last Edit: 11/12/2013 09:47:43 by yor_on »

#### BartLeplae

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##### Re: Why can we see stars that are actually behind the Moon?
« Reply #2 on: 11/12/2013 20:11:36 »
I don't see this as an effect related to gravity because the direction and magnitude of stellar aberration isn't related to gravity.
On the other hand, relativistic aberration describes annual aberration from the perspective of the inertial frame of the Earth that moves relative to the overall frame of the Solar System.

A rule in astronomy is that transits (such as Venus passing in front of the Sun) and occultations (such as the Moon passing in front of a planet or a star) occur at the moment when the apparent (and not the true) directions of the objects align with each other.

This is a very intuitive behavior: we observe Jupiter advancing very slowly through the sky and when when it overlaps with a background star, the star disappears for a while. We know for sure that the apparent position of the background star is different from the position where light departed due to the effect of secular aberration (which is due to the motion of the Solar System). We are also sure that the apparent position of Jupiter is not influenced by secular aberration. If this would be the case, than the apparent position of Jupiter would be highly influenced by this effect (which we know isn't the case).

So although the true direction of Jupiter and the true direction of background star are significantly different from each other, the occultation occurs at the moment when the apparent positions overlap.  This implies that light from both Jupiter and the background star must be following the exact same path between Jupiter and the Earth. As a consequence, the secular aberration for light originating from the background star must occur prior to reaching Jupiter.

The same reasoning is true from light from Jupiter that is occulted by the Moon (which we know is not subject to aberration).
So the aberration of light originating from Jupiter must be occuring prior to reaching the Moon.

My reasoning is that space must be rotating around the Solar System and rotating around the Milky Way on a larger scale.
Light becomes subject to stellar aberration when it moves through space(s) that have different speeds.
The aberration angle is hereby the exact same amount as descibed through the formulas of relativistic aberration.

So secular aberration must be occuring when light from a star enters the rotating space of the Solar System.
Annual aberration is a gradual effect when light travels through the Solar System (where space rotates faster when closer to the Sun).

The reason why, despite the circumstantial evidence, stellar aberration is assumed to occur near the observer on Earth:
- the aberration direction and magnitude match exactly with the velocity of the Earth
- we assume that light always follows a straight path in free space (with the exception of a small effect related to gravity)
« Last Edit: 13/12/2013 20:35:03 by BartLeplae »

#### BartLeplae

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##### Re: Why can we see stars that are actually behind the Moon?
« Reply #3 on: 02/01/2014 19:28:04 »
I visualized the effect of stellar aberration through the following "Prezi" presentation:

#### BartLeplae

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##### Re: Why can we see stars that are actually behind the Moon?
« Reply #4 on: 24/02/2014 21:08:37 »
I posted a commented version of my presentation on YouTube:

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##### Re: Why can we see stars that are actually behind the Moon?
« Reply #4 on: 24/02/2014 21:08:37 »