APOD: Mercury Spotting (2012 May 27)

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APOD: Mercury Spotting (2012 May 27)

Postby APOD Robot » Sun May 27, 2012 4:05 am

Image Mercury Spotting

Explanation: Can you spot the planet? The diminutive disk of Mercury, the solar system's innermost planet, spent about five hours crossing in front of the enormous solar disk in 2003, as viewed from the general vicinity of planet Earth. The Sun was above the horizon during the entire transit for observers in Europe, Africa, Asia, or Australia, and the horizon was certainly no problem for the sun-staring SOHO spacecraft. Seen as a dark spot, Mercury progresses from left to right (top panel to bottom) in these four images from SOHO's extreme ultraviolet camera. The panels' false-colors correspond to different wavelengths in the extreme ultraviolet which highlight regions above the Sun's visible surface. This was the first of 14 transits of Mercury which will occur during the 21st century. Next week, however, an event much more rare but easier to spot will occur -- a transit of Venus across the Sun. Need help spotting Mercury? Just click on the picture.

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Re: APOD: Mercury Spotting (2012 May 27)

Postby Boomer12k » Sun May 27, 2012 4:11 am

I have only seen Mercury twice with my 10" Meade. Both times had to be in Winter when no leaves are on the trees and was seen in between the branches. Wish I'd been able to take a picture at the time. It was bright fiery red in the early dawn, and I was soooooo excited...it was the first time I had really seen it. :D

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Re: APOD: Mercury Spotting (2012 May 27)

Postby Chris Peterson » Sun May 27, 2012 1:53 pm

Boomer12k wrote:I have only seen Mercury twice with my 10" Meade. Both times had to be in Winter when no leaves are on the trees and was seen in between the branches. Wish I'd been able to take a picture at the time. It was bright fiery red in the early dawn, and I was soooooo excited...it was the first time I had really seen it. :D

Mercury is pretty uninteresting telescopically. It's really more fun to pick it up visually, which you can do fairly often if you have a low horizon to the west or east. Of course, transits are best seen with a telescope, given how tiny Mercury really is. (I'm looking forward to trying to catch the Venus transit next week, weather gods permitting.)

Chris

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Re: APOD: Mercury Spotting (2012 May 27)

Postby deathfleer » Sun May 27, 2012 2:46 pm

between solar w?ind and the sun's gravity: which is stronger.
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Re: APOD: Mercury Spotting (2012 May 27)

Postby Chris Peterson » Sun May 27, 2012 2:53 pm

deathfleer wrote:between solar w?ind and the sun's gravity: which is stronger.

You need to frame that question differently. What do you mean by "stronger"? What objects are you considering to be influenced by these effects? For instance, the position of the Earth is unaffected by the solar wind, while many of the particles that make up that wind are moving away from the Sun at greater than the solar escape velocity. So you might say that in the case of a massive body like a planet, gravity is the "stronger" of the two, while for a tiny particle, the solar wind might be "stronger".
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Re: APOD: Mercury Spotting (2012 May 27)

Postby Guest » Sun May 27, 2012 3:40 pm

Chris Peterson wrote:
deathfleer wrote:between solar w?ind and the sun's gravity: which is stronger.

You need to frame that question differently. What do you mean by "stronger"? What objects are you considering to be influenced by these effects? For instance, the position of the Earth is unaffected by the solar wind, while many of the particles that make up that wind are moving away from the Sun at greater than the solar escape velocity. So you might say that in the case of a massive body like a planet, gravity is the "stronger" of the two, while for a tiny particle, the solar wind might be "stronger".
.

thanks for the answer.
In the case of Mercury which today's photos make it seems to be very near to the sun. Is it the solar wind that pushes mercury away from the sun from succumbing to the sun's gravity to fall into the fire. (I was imagining a floating leaf above a bonfire)
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Re: APOD: Mercury Spotting (2012 May 27)

Postby geckzilla » Sun May 27, 2012 3:58 pm

Guest wrote:thanks for the answer.
In the case of Mercury which today's photos make it seems to be very near to the sun. Is it the solar wind that pushes mercury away from the sun from succumbing to the sun's gravity to fall into the fire. (I was imagining a floating leaf above a bonfire)


Mercury is not as close to the sun as it appears. Example: http://legault.perso.sfr.fr/iss_atlanti ... _2010.html
You know the ISS is orbiting the Earth and not close to the sun. However, when photographed in front of the sun, it has the appearance of being very near it.
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Re: APOD: Mercury Spotting (2012 May 27)

Postby Chris Peterson » Sun May 27, 2012 3:59 pm

Guest wrote:thanks for the answer.
In the case of Mercury which today's photos make it seems to be very near to the sun. Is it the solar wind that pushes mercury away from the sun from succumbing to the sun's gravity to fall into the fire. (I was imagining a floating leaf above a bonfire)

Mercury doesn't fall into the Sun because it's in orbit. Bodies in orbit never fall into the object they are orbiting, unless something else takes energy away from them. In fact, the solar wind does just that, although from a practical viewpoint the effect is insignificant. But all the planets are moving through the solar wind, which creates a miniscule amount of drag, and therefore results in a (very, very) slow spiraling in towards the Sun. If the Solar System had a much longer lifetime, and the Sun remained in its current state, the planets would eventually end up hitting the Sun.

The "pushing" effect of the solar wind is insignificant.
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Re: APOD: Mercury Spotting (2012 May 27)

Postby DavidLeodis » Mon May 28, 2012 10:38 am

It's interesting (but initially a bit confusing) how the Universal Time (UT) times of the transit differ a lot from the view on Earth to that from the SOHO. From a link in the explanation it stated of the transit on May 7 2003 as seen from Earth "roughly between 5:11 UT and 10:32 UT, a Mercury transit happened". The times in UT in the SOHO images are noticeably later, presumably due to its different viewing position. That there would be such a large difference did however surprise me :!:
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Re: APOD: Mercury Spotting (2012 May 27)

Postby flash » Tue May 29, 2012 6:17 pm

Chris Peterson wrote:Mercury doesn't fall into the Sun because it's in orbit. Bodies in orbit never fall into the object they are orbiting, unless something else takes energy away from them. In fact, the solar wind does just that, although from a practical viewpoint the effect is insignificant. But all the planets are moving through the solar wind, which creates a miniscule amount of drag, and therefore results in a (very, very) slow spiraling in towards the Sun. If the Solar System had a much longer lifetime, and the Sun remained in its current state, the planets would eventually end up hitting the Sun.

The "pushing" effect of the solar wind is insignificant.


If the body is in a nearly circular orbit around the Sun, and the solar wind is flowing radially outward from the Sun, then the force imparted by the solar wind on the body should also be directed radially outward and in fact should add to the energy of the orbit, not subtract from it. This would cause the orbit to spiral outwards, not inwards.

Now if the solar wind were not flowing outward with such great speed (relative to the tangential orbital velocity of the body in orbit) there would be a drag effect which would remove energy from the orbit and cause eventual orbital decay and inward spiraling.

Having said that I see that there must be a drag effect regardless of the speed difference between the solar wind and the orbital velocities. I can't say which would dominate with any certainty, but Chris apparently believes that the radial "pushing" is "insignificant", and also that the "drag" is "miniscule". I wonder which would win in the long run?

The reason that the solar wind causes orbital decay for bodies orbiting a planet is that the solar wind is not radially aligned, and so the perturbance is half the time net outward, and half the time net inwards causing the orbit to become increasingly eccentric until it intersects either the atmosphere or the planet itself.
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Re: APOD: Mercury Spotting (2012 May 27)

Postby neufer » Tue May 29, 2012 7:04 pm

flash wrote:
If the body is in a nearly circular orbit around the Sun, and the solar wind is flowing radially outward from the Sun, then the force imparted by the solar wind on the body should also be directed radially outward and in fact should add to the energy of the orbit, not subtract from it. This would cause the orbit to spiral outwards, not inwards.

Now if the solar wind were not flowing outward with such great speed (relative to the tangential orbital velocity of the body in orbit) there would be a drag effect which would remove energy from the orbit and cause eventual orbital decay and inward spiraling.

Having said that I see that there must be a drag effect regardless of the speed difference between the solar wind and the orbital velocities. I can't say which would dominate with any certainty, but Chris apparently believes that the radial "pushing" is "insignificant", and also that the "drag" is "miniscule". I wonder which would win in the long run?

The reason that the solar wind causes orbital decay for bodies orbiting a planet is that the solar wind is not radially aligned, and so the perturbance is half the time net outward, and half the time net inwards causing the orbit to become increasingly eccentric until it intersects either the atmosphere or the planet itself.

http://en.wikipedia.org/wiki/Solar_sail wrote:
<<Radiation pressure, solar wind and gravity are the main forces caused by the sun.

Radiation pressure is much stronger than the solar wind, by about three orders of magnitude.

Maxwell's equations (1861) show that photons have a momentum p=E/c; therefore, each light photon absorbed by or reflecting from a surface exerts a small amount of radiation pressure. This results in forces of about 4.57x10−6 N/m2 for absorbing surfaces perpendicular to the radiation in Earth orbit, and a little less than twice as much if the radiation is reflected.

Charged particles from the solar wind can cause geomagnetic storms that knock out power grids on Earth, and point the tails of comets away from the Sun. The solar wind averages 6.7 billion tons per hour at 520 km/s with "slow" low energy coronal ejections reaching 400 km/s and "fast," higher energy ejections averaging 750 km/s. At the distance of the Earth, the average solar wind pressure is 3.4×10−9 N/m2, three orders of magnitude less than radiation pressure. The solar wind dominates many phenomena (only) because its interaction cross section with gases and charged particles is about 109 times larger than that of the photons. Both the radiation pressure and solar wind are small forces that decrease in an inverse square law relationship to the distance from the Sun.>>

Note that the power imparted upon an object is: Image
such that forces in the direction of a orbiting body's motion are the most important:
http://en.wikipedia.org/wiki/Poynting%E ... son_effect wrote:
<<The Poynting–Robertson effect, also known as Poynting–Robertson drag, is a process by which solar radiation causes a dust grain in the Solar System to slowly spiral into the sun. The drag is essentially a component of radiation pressure tangential to the grain's motion.>>
http://en.wikipedia.org/wiki/Yarkovsky_effect wrote:
<<The Yarkovsky effect is a force acting on a rotating body in space caused by the anisotropic emission of thermal photons, which carry momentum. It is usually considered in relation to meteoroids or small asteroids (about 10 cm to 10 km in diameter), as its influence is most significant for these bodies.>>
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Re: APOD: Mercury Spotting (2012 May 27)

Postby flash » Tue May 29, 2012 7:43 pm

I must say that in my previous note I was not distinguishing between solar wind and solar radiation since they are both acting in the same direction.

And I erred in sayng that "the perturbance is half the time net outward, and half the time net inwards": It is always radially outward from the Sun. But half the time the body in orbit around the planet is moving with a radial component of velocity inward toward the Sun, and half the time it is moving outward from the sun. This has the effect of altering the body's orbit making it ever more eccentric. The effect (and lots of other orbital mechanics effects) are quite easily explained (without calculus) in a paper I recall from school, and which I located online. It is a good, easy read for anyone interested in understanding orbits better. It contains what may be for many an introduction to the useful concept of "velocity space". Its title is "Velocity Space and the Geometry of Planetary Orbits" by Abelson, diSessa, and Rudolf.

http://dspace.mit.edu/bitstream/handle/1721.1/5788/AIM-320.pdf?sequence=2
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Re: APOD: Mercury Spotting (2012 May 27)

Postby neufer » Tue May 29, 2012 10:25 pm

flash wrote:
I must say that in my previous note I was not distinguishing between solar wind and solar radiation since they are both acting in the same direction.

Solar wind is irrelevant for un-ionized bodies

flash wrote:
And I erred in sayng that "the perturbance is half the time net outward, and half the time net inwards": It is always radially outward from the Sun. But half the time the body in orbit around the planet is moving with a radial component of velocity inward toward the Sun, and half the time it is moving outward from the sun. This has the effect of altering the body's orbit making it ever more eccentric.

http://en.wikipedia.org/wiki/Poynting%E ... son_effect wrote:
<<Since the Sun's gravity varies as Image whereas the Poynting–Robertson force varies as Image, the Poynting–Robertson effect gets relatively stronger as the object approaches the Sun, which tends to reduce the eccentricity of the object's orbit in addition to dragging it in. Rocky dust particles sized a few micrometers need a few thousand years to get from 1 AU distance to distances where they evaporate.

For particles much smaller than [a few micrometers], radiation pressure, which makes them spiral outwards from the Sun, is stronger than the Poynting–Robertson effect that makes them spiral inward. For rocky particles about half a µm in diameter, the radiation pressure equals gravity, and they will be always blown out of the Solar System even though the Poynting–Robertson effect still affects them. Particles of intermediate size will either spiral inwards or outwards depending on their size and their initial velocity vector.
>>
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Re: APOD: Mercury Spotting (2012 May 27)

Postby flash » Wed May 30, 2012 3:55 pm

neufer wrote:Solar wind is irrelevant for un-ionized bodies

Why is that? If the reason is that wind is composed of charged particles, and so the orbiting body cannot "absorb" them without accumulating a subsequent repulsive force, why does the Sun not become charged as a result of expelling the charged particles, thereby eventually preventing the wind? The answer I think must be that the wind contains both positive and negatively charged particles which then they can be absorbed by the orbiting body without it accumulating charge. The small momentum of the "absorbed" solar wind particles is transfered to the orbiting body, resulting in a radial outward force. Yes?
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Re: APOD: Mercury Spotting (2012 May 27)

Postby UniversalHandle » Fri Jun 08, 2012 4:15 pm

I'm curious why Mercury's orbit appears so high relative to our view of the sun. I know that parallax could cause this, but it also seems like the distances involved would make any parallax much smaller than this.
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Re: APOD: Mercury Spotting (2012 May 27)

Postby Chris Peterson » Fri Jun 08, 2012 4:31 pm

UniversalHandle wrote:I'm curious why Mercury's orbit appears so high relative to our view of the sun. I know that parallax could cause this, but it also seems like the distances involved would make any parallax much smaller than this.

I'm not sure I understand the question. Are you asking why Mercury doesn't seem to be transiting closer to the Sun's equator?

Mercury and the Earth do not orbit on the same plane- they are tilted with respect to each other by 7°. Normally when Mercury is at inferior conjunction (between Earth and the Sun) it is well above or below the Sun from our perspective. That's why transits are fairly rare, occurring just once every 7 or 8 years on average, despite the fact that inferior conjunctions occur about 3 times every year.
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