star - Standard conditions for a heliacal rising

What I do is compute the limiting magnitude for the position of the object I am interested in, and subtract it from the magnitude of the object (I call this the magnitude contrast). If negative, it should be visible with the naked eye. I use the function limmag_jd() in the Fortran library libTheSky for this.

In order to find the best moment in a given night, I compute the contrast for say every 15 minutes, and find the minimum. You can do this for a number of nights, until the best contrast found is negative, which would be the heliacal rising.

I'm not sure whether there is an easy way using a simple equation - the limiting magnitude will depend on things like the positions of Sun, Moon and the object in question, the phase of the Moon, the magnitude of the object, etc.

gravity - How can a galaxy in-between our view and the galaxy behind it have a 'lensing effect'?

It's called gravitational lensing. Here's a link to the wikipedia article on the subject: http://en.wikipedia.org/wiki/Gravitational_lens.

Gravity affects everything, including light. A massive object such as a star, a galaxy, or in this case, a cluster of galaxies, bends the path of photons that pass very close to the massive object. Bending light is the basis of the lenses in an optical telescope.

The behavior of a gravitational lens is different from that of an optical lens., but the end result is the same: Gravitational lenses enable astronomers to see more distant objects than they would without them. Gravitational lenses create multiple images of the remote objects behind the lens. Oftentimes these multiple images are in the form of streaks. See the below image of gravitational cluster Abell 1689, which is the cluster that let those astronomers see A1689-zD1.

The cluster magnified the remote galaxy A1689-zD1 by a factor of 9.3. Without that magnification, that remote galaxy would be invisible to current technology.

zoology - List of species recently found of economic value

I can only answer for plants. The short answer is no, there is no central source of information of that kind.

You are basically asking about the sub-classifications economic botany and ethnobotany, which cover the economy and human uses of plants respectively. Neither field is particularly fast-paced, so you could simply keep up to date with the latest issues of journals like Economic Botany or Ethnobotany.

One key area of development for new valuable crops is in crop wild relatives, but there is no particular publication you should watch for that. Keep an eye on the Crop Trust as they will shortly be releasing, in collaboration with us (the Millennium Seed Bank) some reports and data about this.

If I wanted to keep up to date with all those things over time, I would set up some saved search alerts on Google Scholar for key search terms like 'emerging crop', 'new crop', 'crop wild relative', etc. Just perform a search on Scholar and then click the envelope button to the right of the search field to create a recurring search alert.

One other potential source of information would be to check FAOSTAT each year. You can examine global and local data about area under cultivation, yield, investment, commodity value, etc. of most crops. If something new were to emerge as a big player, it would appear on there.

Relationship between the Casimir force and dark energy

There are two problems that arise here. One obvious problem is that the Casimir effect is attractive¹, while dark energy is repulsive. The other problem is one of scale.

Casimir (1948) shows that, between two particles (instead of the oft-cited case of two plates)
$$delta E propto R^{-7}$$
That's an enormous drop-off. On large scales, this should be negligible. Dark energy, however, is clearly non-negligible on these scales, while its effects are unobservable on small scales.

The Casimir effect is certainly one manifestation of vacuum energy, which may also be the cause of dark energy (in some theories), but that doesn't mean that dark energy is an example of the Casimir effect, or vice versa.

---

^{1 There are exceptions, as pointed out by Stan Liou, which arise under certain conditions, but they wouldn't play a part here.}

solar system - What Causes the Large Radiation Fields Around Jupiter?

As you stated, we have not been able to simulate the pressure and temperatures required to generate those that are believed to exist in Jupiter's interior other than in short-lived shockwave experiments, according to the NASA webpage A Freaky Fluid inside Jupiter?, observing that

"Liquid metallic hydrogen has low viscosity, like water, and it's a good electrical and thermal conductor," says Caltech's David Stevenson, an expert in planet formation, evolution, and structure. "Like a mirror, it reflects light, so if you were immersed in it [here's hoping you never are], you wouldn't be able to see anything."

Going further, according to the article Jumpin' Jupiter! Metallic Hydrogen (Lawrence Livermore National Laboratory), discuss the shock wave results, finding the level at which hydrogen metallises as being

from 0.9 to 1.4 Mbar, resistivity in the shocked fluid decreases almost four orders of magnitude (i.e., conductivity increases); from 1.4 to 1.8 Mbar, resistivity is essentially constant at a value typical of that of liquid metals. Our data indicate a continuous transition from a semiconducting to metallic diatomic fluid at 1.4 Mbar, nine-fold compression of initial liquid density, and 3,000 K.

The findings from the researchers above are summarised in the diagram below

The source is the Jumping Jupiter link above.

the sun - Do sungrazing comets leave a field of meteoroids near the Sun?

The SOHO image you attached, provides some evidence, that most of the debris continues the trajectory of the original body.
Most of the debris hence won't replenish the Vulcanoid population.
But due to the disruptive event you get an additional delta-v, which may change the orbit a bit, however insufficient to slow down to a Vulcanoid orbit; compare the velocity of the comet near perihelion with the speed of sound as an order of magnitude estimate of the upper limit of the delta-v for debris.
Dust may either be slowed down in this or in future orbits in the solar corona to end up in the Sun, or blown away by solar radiation pressure, again no Vulcanoids.

biochemistry - What happens to colloidal particles in a liquid medium? And how to stabilize it?

"Colloidal particles: What are colloidal systems, give an example,--"

Milk, bear foam, paste, blood, ---

"what happens to colloidal particles in a liquid medium."

I understand the q as solid in liquid so a gel. Examples about gel are "agar, gelatin, jelly and opal" according to my lecture slides. The electrostatic forces are in some sort of middle phase: not enough to repulse into liquid form but not strong enough to form liquid either.

"How to stabilize colloidal systems in a liquid medium."

Wikipedia about stability (here)

"The stability of a colloidal system is the capability of the system
to remain as it is. Stability is hindered by aggregation and by
sedimentation phenomena, that determine phase separation."

Now different things to consider are:

• Electrostatic stabilization

"In a stable colloid, mass of a dispersed phase is so low that its buoyancy or kinetic energy is too weak to overcome the electrostatic
repulsion between charged layers of the dispersing phase."

• steric stabilization

"particles in polymers which prevents the particle to get close in the range of attractive forces."

The big bang and our expanding universe

There are a couple of misunderstandings here. Let's take one at a time:

The Hubble telescope can see the light from the big bang.

The Hubble Space Telescope (HST) doesn't actually see light from the Big Bang. HST has several instruments on board, both for imaging and spectroscopy, but they all operate in the infrared, optical (i.e. "visible to humans"), and ultraviolet wavelength range. When you mention "light from the Bing Bang", I suppose you're thinking of the cosmic microwave background radiation (CMB), which is light emitted 380,000 after Big Bang. This is "the closest" we can get to Big Bang (yet). HST is unable to detect microwaves; instead we have other telescopes for that, the most recently-launched being Planck.

My question is if our universe is expanding that would make everything
that's in front of our solar system be in the future?

The expansion of the Universe is one of the most difficult concepts to get one's head around. But firstly, Big Bang wasn't an explosion, hurling matter outwards from a central point in an otherwise empty Universe. Galaxies, on average, don't travel much through space. Rather, they lie relatively still in space, but space itself expands. The distances between galaxies increase all the time, and previously, before galaxies were formed, the distance between atoms and other particles increased. An often-used analogy — which you shouldn't take too far — is a balloon with dots painted on. The dots are fixed on its surface, and when you blow it up, the distances between the dots increase despite the dots still being fixed.

Secondly, your question seems to confuse space and time. Even if the expansion were an explosion, we would travel through space, and yes, you could say that we travel through time as well, but saying that the future is "everything in front of our Solar system" doesn't really make sense (to me, at least). What's in front of you when you travel through space is just more space; you can look at it before you get there, but that doesn't mean you look into the future (in fact you look at the past, since the light you detect with your eyes has spent some time traveling toward you).

And also can Hubble see ahead of our solar system

It depends what you mean by "ahead". It can definitely see things outside the Solar system. The Solar system consists of one star and it satellites (planets, comets, asteroids, etc.) out of the few hundred billions that make up the galaxy we call the Milky Way. Our Galaxy is itself only one out of at least some hundred billion — and quite possibly infinitely many — galaxies that float around in our Universe. And yes, HST can see many of these. But if by "ahead" you mean "into the future", then the answer is no.

and does the bang big light ends behind our solar system?

As mentioned above, the Big Bang light — or the CMB — was emitted shortly after Big Bang. That means that it has been traveling for 13.8 billion years through space. Light travels one lightyear per year, but since the Universe is expanding, the distance that the CMB has traveled is more than 13.8 billion lightyears; in fact it's some 46 billion lighyears. This is the most distant "thing" we can see, way, way beyond the Solar system which for comparion is of the order of one lightyear large.

evolution - Why do some plant species have lobed leaves, while similar species in the same habitat don't?

This is a question for which, I think at the moment, we don't have a clear answer.

It is important to bear in mind that the leaf plays a number of important roles in the plant (photosynthesis, thermoregulation etc.) so leaf shapes probably evolved through a process of successive trade-offs. This may make it difficult to identify the exact selection processes operating on any one species. In contrast, something like the eye has a well-defined single function, which in principle at least, makes it easier to understand the link between form and function.

From Niklas (1988):

Life history and optimisation theory suggest that the number of
phenotypic solutions that allow for different equally successful trait
combinations increases as the number of trade-offs increases – a
conclusion that applies to traits within the leaf (e.g. for shape) as
well as to leaf–branch relationships.

However, there are a number of ideas to explain leaf shape diversity which include:

• Thermoregulation

It has been shown that by adding lobes to leaves, the rate of heat transfer across a leaf is greater than that of an unlobed leaf of the same area (e.g. Gurevitch and Schuepp 1990). So, lobed leaves may be selected for under certain environmental conditions.

• hydromechanical constraints

Lobed leaves may have greater hydraulic efficiency. For smaller veins, hydraulic pressure increases as they present an increased resistance to water flow. This places stress on the deliate outer leaf tissues. If lobed leaves have relatively less mesophyll tissue than large, highly conductive veins, they may have reduced hydraulic resistance compared unlobed leaves (Sack and Tyree 2005).

• optimization for photosynthesis




• adaptations against herbivorous animals




• random (non-selected) variation. This is a possibility, although probably unlikely due to the importance of the leaf in plants.

So, some or all of the above possibilities (which are not necessarily mutually exclusive) may shape the leaves of species which may appear to have undergone very similar selective pressures.

This is a good read by Nicotra et al. (2011) which summarizes the state of play.

Gurevitch J, Schuepp PH (1990) Boundary layer properties of highly dissected leaves – an investigation using an electrochemical fluid tunnel. Plant, Cell & Environment 13, 783–792. doi:10.1111/j.1365- 3040.1990.tb01094.x

Nicotra et al., The evolution and functional significance of leaf shape
in the angiosperms, Functional Plant Biology, 2011, 38, 535–552

Niklas KJ (1988) The role of phyllotactic pattern as a developmental constraint on the interception of light by leaf surfaces. Evolution 42, 1–16. doi:10.2307/2409111

Sack L, Tyree MT (2005) Leaf hydraulics and its implications in plant structure and function. In ‘Vascular transport in plants.’ (Eds NM Holbrook, MA Zwieniecki) pp. 93–114. (Elsevier Academic Press: Burlington, MA, USA)

evolution - Why would stablising selection ever happen?

Evolution has no aim, you seem to be repeating a caricature of what the theory of evolution really says. Evolution works by a mechanism where pressures on a population select for the traits that are best suited for that environment. If there are no pressures on a population, then the selection of specific traits will be slowed down, as all members of the population have an equal chance at reproducing.

Now to wax a bit philosophical on the issue.

As John Rennie said in Scientific American:

"Survival of the fittest" is a conversational way to describe natural selection, but a more technical description speaks of differential rates of survival and reproduction. That is, rather than labeling species as more or less fit, one can describe how many offspring they are likely to leave under given circumstances. Drop a fast-breeding pair of small-beaked finches and a slower-breeding pair of large-beaked finches onto an island full of food seeds. Within a few generations the fast breeders may control more of the food resources. Yet if large beaks more easily crush seeds, the advantage may tip to the slow breeders. In a pioneering study of finches on the Gal pagos Islands, Peter R. Grant of Princeton University observed these kinds of population shifts in the wild [see his article "Natural Selection and Darwin's Finches"; Scientific American, October 1991].

The key is that adaptive fitness can be defined without reference to survival: large beaks are better adapted for crushing seeds, irrespective of whether that trait has survival value under the circumstances.

Michael Shermer states:

Living fossils (organisms that have not changed for millions of years) simply means that they evolved an adequate structure for a relatively static and unchanging environment, good enough to maintain a niche.

Or as a particularly brilliant writer that goes by the name of Calilasseia said:

The static species fallacy.

This is a particularly stupid canard, which the above discourse on inheritance, and variation brought about by meiosis, should flush down the toilet at a stroke. But, in order to reinforce how stupid this canard is, it is necessary to cover rigorously what a species is.

A species is a population entity, and as a corollary thereof, a dynamic entity. A species is defined in rigorous biological work, as a population of living organisms, whose members can produce viable offspring with each other, but whose members can not produce viable offspring with a separate, distinct population. Actually, this is only one extant definition, but it is the one that matters with respect to evolution, because once again, it points to the central role of inheritance.

Of course, part of the problem arises because of taxonomy. Because scientists need a reference point from which to launch further investigation, they have alighted, courtesy of our old friend Linnaeus, upon the process of cataloguing organisms and providing them with a unique, unambiguous identity. This, of course, has been most helpful in furthering our understanding of the biosphere, and indeed, Linnaeus himself, on the basis of comparative anatomy alone, alighted upon the idea that organisms were related to each other a hundred years before Darwin, which is why he constructed his taxonomic scheme in the manner he did. Yes, that's right, a creationist (though he was only a creationist because no other option existed in 1758) alighted upon the idea of biological interrelatedness, as a result of paying attention to reality. But the very same taxonomic practices that have been useful to science, have also led to a popular misconception. This is because taxonomists base their classification upon individually sampled organisms, one of which is chosen as a 'type specimen' that is henceforth declared to be the reference standard against which all others are compared. Other specimens are maintained in order to provide a record of likely variation in characteristics from that reference standard. The trouble is, of course, basing the entire classification system upon such reference standards promotes the illusion that those standards remain in place for all time. Scientists, of course, recognise that this is not the case, but it takes diligent intellectual effort to recognise that the taxonomic standards are merely particular snapshots of the state of the species at a given point in its history, which scientists then choose as their reference benchmark for current work. The species itself, however, courtesy of all that dissemination of variation across generations, does not stay still. It is NOT static.

I cannot reinforce this strongly enough. A taxonomic classification is merely a historical snapshot of the state of a species, used as a reference point for further work, and does NOT constitute "the species" itself. The species itself, is the sum total of all the living organisms comprising that interfertile population, and with each new generation, that population undergoes change, because in the new generation, each of the organisms comprising that population are genetically different from those in the previous generation.

So, if anyone wishes to erect the ridiculous idea that a species is a static entity, the simple retort is this. Look at your family album. Are you identical to either of your parents? No? There's your evidence for the dynamic nature of a species. Now replicate that evidence across millions of humans, and picture what happens with each new generation, remembering that across generations, inheritance is a dynamic process. There goes the static species fallacy.

I hope that clears it up for you.

gravity - Can the theory of multiuniverse explain dark matter?

This is a bit of a speculative question, but I can answer it.

Dark matter has been observed in galaxies, and the distributions of dark matter in galaxies have also been measured. It seems clear that the matter is firmly in our universe - we just can't detect it with electromagnetic radiation.
*However, there are ideas (extremely speculative) that dark matter is from a different spatial dimension. It sort of explains why gravity is so weak (it "leaks off" through the other dimension(s)). Note, though, that this highly, highly speculative.

There are ideas like this in string theory, too. Some say that we are trapped on a brane, a section of space-time with three spatial dimensions. However, there are one or more additional dimensions. All of these dimensions are known as the bulk. Gravity can "leak off" through these extra dimensions. Some theories take this to the extreme, and say that there exist many universes (of a certain number of dimensions each, but with less dimensions that those of the bulk) that are branes, just like ours. These have been considered the source of phenomena such as dark energy - in which case your scenario is within the realm of plausibility.

All of these ideas, however, are speculative and have no evidence in their favor.

hydrogen - Why are there no green stars?

Human color vision is based on three types of "cones" in the eye that respond differently to different wavelengths of light. Thus, not counting overall brightness, the human color space has two degrees of freedom. In contrast, the spectra of stars are very close to a black body, which depends only on effective temperature. As one varies the temperature, the color of a star should make a one-dimensional curve in this color space. Thus, unless some perverse shenanigans are going on, it is intuitive that we necessarily miss most of colors, i.e. there will be no stars of those colors.

Our Sun actually has a peak at about $500,mathrm{nm}$, which is a green. However, that's just the peak: since the Sun also radiates lots of light with shorter wavelength (bluer) and also longer wavelengths (redder), the resulting mixture doesn't look green to human eyes.

An image from wikipedia on the color of a blackbody of a given temperature:

Note that the colors at near edges of this color space aren't accurate, because the sRGB standard used in computers only covers a fairly small triangle portion of it. Still, that's a complication that's not very important here.

enzymes - What conditions are necessary for HPL (human pancreatic lipase) to activate?

The protein referred to in the question is encoded by gene PNLIP, pancreatic lipase. From this annotation of the protein, I see that there is a signal peptide from amino acids 1 to 16. Thus, this signal peptide must be cleaved before the protein can be active in its digestion of emulsified triacylglyerides.

A paper describes the structural changes induced in human pancreatic lipase by lowering the pH. The secondary structure of the enzyme is stable within a pH range of 3.0 to 6.5. At this pH, a reversible opening of the lid controlling the access to the active site was observed. So, there is another aspect of activation - pH and the ability to open the enzyme lid so that the fat molecule enters the active site.

terminology - Is it possible for the year to have 11 full moons (i.e. the opposite of a blue moon)

Since a full moon happens once every 29 days (and that's not exact, but close enough), the longest period of time where you can have 11 full moons but not 12 is 11 x 29 + 28, or 347 days, so in a normal year, it's not possible. There are slight variations in the time between full moons but not nearly enough to make up the extra 18 days needed.

So you'd need a special circumstance. Great Brittan's short year in 1752 wasn't sufficiently short, it had 11 days removed.

There is a phenomenon where not everywhere on Earth gets a full moon every 29 days. In the North and South poles for example, where during summer they get 24 hours of sun and winter, no sun, a somewhat similar effect happens with the moon. The Moon orbits the Earth at 5 degrees off the ecliptic or basically, the plane between the Earth and the Sun and that makes the Moon's apparent movement across the sky a little more complicated than the Sun's, but because full moons only happen when the moon is roughly opposite the sun, it's not hard to see that when the sun is in the sky for 24 hours during the Polar summer, they don't get a full moon for several months because the full moon is below the horizon when the sun is in the sky 24 hours. You might only see 6-9 full moons a year at the poles (if I was to guess), and there's probably a range of latitudes close to the poles where 11 full moons might be about the yearly average.

If you want to split hairs even further, the Moon only takes an instant to pass the 180 degree point in relation to the Earth and Sun, so in a sense, a perfect 180 degree full moon is only visible to half the earth and by the time the other half of the earth turns to make that moon visible, the moon has already gone past 180 degrees and from a certain perspective, it's no longer full. But, a full moon is generally regarded as a roughly 24 hour period, not an instantaneous moment, so I don't like that argument personally.

What effect does a natural satellite have on a planet's rotation and revolution?

A body and its satellite in truth form a system and both rotate around their common centre of gravity. Hence both the Earth and Moon rotate around this centre - however because the Earth is significantly more massive than the Moon this can be approximated as the Moon orbiting the Earth.

The Moon does have a measurable impact on the Earth - tides are the obvious example, and tidal friction is also slowing down the Earth's rotation by a very small amount (around 2.3ms per century).

In the Moon's case, though, the Earth's gravitational pull has already effectively locked its rotation period with its orbital period around the Earth - so we essentially only ever see the same face of the Moon on Earth.

The Earth and Moon are much closer in mass than most other planet/satellite systems in the Solar System - think of the huge mass of Jupiter - and so it is difficult to show how these planets' satellites have a similar impact on the planet - but the same effects are there, albeit in much smaller scale.

What kind of things I could "see" with an amateur radio telescope?

From my simplistic analysis, it's not good for much.

For comparison, the first radio telescope was 9 meters.

One of the favorite parts of the spectrum for radio telescopes is the water hole - 21 cm.

From my quick mental arithmetic, this dish would be able to resolve sources of 21 cm signals of they were about 5 degrees apart.

I'll update with links and more when my computer is working; this is written on my phone :-)

star - brown dwarfs and planets

Stanley, there really isn't a very clear definition and this is still a keenly argued point.

Definitions include:

Browns dwarfs burn deuterium. In models this happens if they are more massive than about 13 times Jupiter. The weakness of this that we think isolated brown dwarfs could condense from a gas cloud that are less massive than this; and young brown dwarfs won't have got around to fusing deuterium.

Planets must form from the disk around a star. This is ok, but: brown dwarfs may also form from the disk and it is also possible for planets to be tidally stripped from their stars and be found alone in space.

Planets must have a rocky core. This used to be thought definite, but now we think maybe sometimes planets can collapse from a gas instability in the disk in some circumstances, without the need for a rocky/icy core. It is true that brown dwarfs should not have a rocky core. However as an observational definition this is fairly hopeless since we can't even tell yet if Jupiter has a rocky core.

A flavour of the controversy can be gleaned from reading between the lines of the IAU statement on the definition of planets vs brown dwarfs.

the sun - Does the Moon capture radiation pressure from the sun causing momentum from photon propulsion?

If we assume the 15 CM per year is accurate (which I'm not 100% sure it is), then it's possible to answer this. In a loose sense, yes, the pressure should move the moon away from the sun faster, but more accurately and the real answer is that doesn't happen because there are 3 bodies in play and when you have 3 body gravitation, things get a lot more complicated.

First, just for fun, a source for the 15 CM.

The moon away each year by photon momentum.The moon is the perfect
color,size and The moon has a mass of 7.35 x 10²² kilograms. It is
only about 60 percent as dense as Earth so it should be effected by
the photons from the sun causing movement.And that movement should be
combined with earths 15cm giving a new total distance a year right

If we only look at equal force per surface area, which is what you're describing, it's not density, it's the ratio surface area to mass. The Moon's surface area is about 7.4% of Earth's (about 1/13.5) and it's mass about 1/81st of Earths. Source. 81/13.5 = about 6. That means given equal pressure, the moon should accelerate 6 times as much which corresponds over small distances to 6 times the distance or 90 CM per year - BUT, that's if you ONLY take into account the pressure.

The effect of the sun losing mass has equal effect on both the Earth and the smaller Moon.

And a 3rd factor to consider is the tidal bulge on the sun caused by the Earth-Moon system, which is very small but all these effects are small. The Earth-Moon system creates a tiny bulge on the surface of the Sun and because the Sun rotates ahead of the Earth-Moon, that tiny tidal bulge has a tiny pull on the Earth-Moon system that slowly accelerates them and pushes them slowly away from the sun. That effect is equal for both the Moon and Earth too.

All 3 of those are factors in the 15 CM per year estimate and only one of them has 6 times the effect on the Moon than the Earth. The planets Venus and Jupiter might also be factors in that 15 CM per year estimate too, but lets leave that alone for now.

If we consider the 3 body problem it gets very mathy, but I'll just talk about how it applies to solar pressure. Lets start with a picture.

When the Moon is waning its moving towards the sun and any pressure from the sun slows the moon down, (a tiny bit) and that slow-down moves the Moon closer to the Earth which speeds it up even more - funny how that works, pushing to slow something down in orbit and it goes faster - but that's how it works, cause potential energy converts to kinetic as the orbit drops, kinda like how falling makes things go faster.

When the Moon is waxing, it's moving away from the sun and any pressure from the sun speeds it up which moves it away from the earth and that in turn, slows it down. So that's your answer in a nutshell. The solar pressure doesn't push the moon away from the sun so much as it pushes the moon into a lower and then higher orbit around the Earth depending on where the Moon is in it's orbit around the Earth and the various positions in both the Moon's and Earth's elliptical orbits. The overall effect is very difficult to calculate and I suspect it could go either way depending in part on the timing of large coronal mass ejections hitting the moon and in part on any possible resonance between the eccentricity of the Moon around the Earth and the Earth around the sun, er, I think.

What is safe to say is that the Earth-Moon system obeys the same surface area to mass ratio the Earth alone because that's simple momentum which needs to be conserved. The Earth-Moon system has a combined surface area of 1.074 Earths and a combined mass of 1.012 Earths, so the Moon being in the Earth-Moon system makes the Earth move away due to solar pressure about 6% faster. 15 CM to 15.9 CM. Not a very big change, and it's possible that the 15 CM estimate is based on the Earth-Moon barycenter not the Earth itself.

So, the effect is more of a wobble in the Moon's orbit around the earth than an actual force that pushes the Moon away from the sun faster than it pushes the Earth away.

All this, is, of-course, unimportant compared to the much bigger solar tidal force on the Earth-Moon system. As the Moon gets closer to the sun (new moon in the picture above) The sun effectively pulls the Moon a little bit away from the Earth and when it's further from the sun (full moon), The Moon is effectively pulled towards the earth. The net effect of this solar tide is a measurable wobble in the Moon's orbit around the earth. The effect on the Moon's orbit around the earth due to photon pressure and coronal mass ejection pressure - basically insignificant.

Hope that's clear.

biochemistry - Human perception of time depending on age

This is not really a biological answer, but a psychological one:

One important fact to consider is that the perception of time is essentially a recollection of past experience, rather than perception of the present.

Researchers who study autobiographical memory have suggested that part of this effect may be explained by the number of recallable memories during a particular time period. During one's adolescence, one typically has a large number of salient memories, due to the distinctness of events. People often make new friends, move frequently, attend different schools, and have several jobs. As each of these memories is unique, recollection of these (many) memories gives the impression that the time span was large.

In contrast, older adults have fewer unique experiences. They tend to work a single job, and live in a single place, and have set routines which they may follow for years. For this reason, memories are less distinct, and are often blurred together or consolidated. Upon recollection, it seems like time went by quickly because we can't remember what actually happened.

In other words, it can be considered a special case of the availability heuristic: people judge a time span to be longer in which there are more salient/unique events.

Incidentally, (and to at least mention biology), episodic memory has been shown to be neurally distinct from semantic memory in the brain. In particular, a double dissociation has been shown for amnesics who suffer from semantic or episodic memory, but not both.

My apologies for the lack of citations, but a good bit about autobiographical memories can be found in:

Eysenck, M.W., & Keane, M.T. (2010). Cognitive Psychology: A
Student's Handbook.

You may also be interested in some responses or references to a related question on the Cognitive Science StackExchange:

Perception of time as a function of age

natural satellites - Is it possible that Mercury was originally the moon of Venus after a giant impact?

This was originally going to be a comment, but it ran too long, so I'm making it an answer.

Some models argue that the scenario of a satellite of Venus escaping like this is unlikely. Alemi & Stevenson (2006) have explored the possibility of a prior Venusian moon, starting from the assumption that Venus would not have been able to avoid a giant impact. Here's their sequence of events:

1. A large body collides with Venus in a similar manner to the proposed Earth-Theia collision.


2. Debris from the impact moves outwards into a disk surrounding Venus,


3. A moon coalesces from the disk, and begins to slowly recede because of tidal acceleration.


4. Another large body hits Venus. It reduces Venus's angular momentum, reversing its rotation.


5. The moon spirals into Venus as it undergoes tidal deceleration, finally colliding with it again.

One of the tricky things about testing this model is that the authors say that there would not necessarily have been drastic composition changes, meaning that it would be hard to analyze the planet's surface and see if there is evidence supporting the double impact hypothesis. So far, there have not been tests.

It is certainly true that Venus could have suffered other impacts - the model does not preclude that. There are a couple problems with Mercury arising from such a collision:

• Other impacts could have ended up with the same result as the original moon.


• The chances of many more impacts aren't too high.


• Solar tides would likely have destabilized the orbit of any moon larger than a few kilometers in diameter (see Sheppard & Trujillo (2009)).


• MESSENGER determined that Mercury has a high potassium/thorium ratio on its surface, which would seem to disprove any events involving extremely high temperatures, including any giant impact variant.

Of course, if we accept that Venus could have captured a moon, only the third objection remains - still a strong point against the survival of a satellite, even by itself.

observation - Parabolic or hyperbolic trajectories

Yes, and it is not uncommon for an orbit have an eccentricity close to one. The wikipedia site, linked in a comment above, notes C/1980 E1, which entered the inner solar system with an eccentricity close to one, but had a close encounter with jupiter and was accelerated. It left the inner solar system with a eccentricity of 1.05, and so is on a hyperbolic trajectory, and will escape from the sun's gravity

Orbits that are highly hyperbolic are very unlikely. Comets formed as part of the solar system.

They are not really harder to spot than any other comet. A comet takes many months to make its passage through the inner solar system. There is plenty of time for them to be spotted, especially if you have probes like SOHO or NEAT

How does gravity really work

First of all: "How gravity really works" is a deep question, and any serious scientist would quickly concede that all we have is an incomplete working model. You certainly have heard about General Relativity; the first image on the page is your trampoline.

Our working model, General Relativity, is working because it explains a lot of observations very nicely. (Careful, here is another deep question lingering: "Explains" means that we can predict some observations from other observations with the model of gravity we have in our mind. It does not necessarily mean that we understand the "real nature" of the underlying issues.) But we are very confident that the model is working over a wide range of observations. One of the last "first-time" observations which followed the predictions and thus gave us more confidence in the model was the two black holes colliding lately. Lately? Well, billions of years ago. We just learned about it lately. Here is a link to a New York Times article with an impressive video. (I think one can still read a limited number of Times articles for free, so try it out.)

Our model of gravity is incomplete because it doesn't connect well to the model of nature we have for other things (elementary particles, quantum physics). For a while (like 70 years or so) it didn't connect at all; Einstein himself completely failed to connect the dots, which was probably not encouraging since he had received the Nobel Price for laying one of the foundations of quantum physics and was the obvious authority about gravity. If he couldn't do it, who could?

If I'm not mistaken, the physicists today are making progress, slowly. This connection between quantum physics and gravity is one of the main unresolved problems in modern physics.

Last, let me address your concern about the planets spiraling into the sun. This idea probably comes from actual balls on an actual trampoline spiraling in, I suppose. You probably know that the balls lose speed due to friction, much the same way you slow down on your bike when you stop pedaling. Some of the kinetic energy is transformed into heat.

And you know what? You are right. Given enough time, the planets would eventually fall into the sun. Low-flying satellites fall back to earth after a few years, because there are still traces of atmosphere slowing them down out there. The reason is that there is "friction" in the wider sense involved in all large-scale processes in the universe. That is actually one of the fundamental physical principles making up the world we know. It's just that the near-vacuum between the planets doesn't provide that much friction, and the planets are fairly massive bodies with an enormous mass and kinetic energy. It will take a long long time for them to lose enough energy that they'll be so close as to touch the sun. (Perhaps too long to happen at all.) In fact, over human life times the planets, moons and stuff are almost perfect examples for movement without friction. But in the astronomical time scale -- billions of years --, there certainly is friction. For example, the moon is showing us always the same side because friction slowed its rotation so that the rotation is now "locked" with its orbit.

Bottom line: The idea that gravity bends space and time "explains" all large-scale observations so far; the "trampoline" is a good model for a 2-dimensional "space", i.e. a surface, if you ignore friction.

asteroids - How does large body gravity affect planet formation?

why didn't the sun's gravity keep the inner planets from forming?

The short answer is, the asteroid belt orbits the sun, it doesn't orbit Jupiter, and you shouldn't expect the same effect with 2 different relations.

The sun is the gravitational object that the Asteroid belt orbits. Jupiter is not and it has a stabilizing effect on the material that orbits around it. Jupiter has a (mostly) destabilizing effect on nearby material that orbits around the sun, though there is some stability with orbital resonance and trojan points. The reason why that is gets a little bit mathy, but I'll cover the basics.

When a solar system forms, probably out of a combination of strong solar wind from a supernova and a dense enough gas cloud, most of the solar-system material forms the young sun and maybe 2%-4% remains in orbit and over time, spirals and flattens out into a disk. (see cool video)

The sun's gravity stabilizes that disk. There's no reason for it to disburse it, though the young and usually strong solar wind can blast it and clear out the closer regions of smaller particles and any ices.

Jupiter is entirely different. Jupiter and other planets which form inteh disk, tends to eat everything in their path. That's one of the definitions of a planet. It clears out it's orbital area. These planets on their own would remain in orbit around the sun, but they can interact over time with each other. Pretty much anything in Jupiter's path, once Jupiter gets large enough, either gets eaten or gets thrown into a very different orbit, either further our or closer in.

The asteroid belt is (as Suhrid Mulay points out) a relatively low density region of space that doesn't have enough mass to coalesce into it's own planet, but it might not have always been that low density and that's probably only part of the reason. The other part of the reason is the proximity of Jupiter which tends to disrupt anything that orbits too close to it and Jupiter may have at one point, moved in quite a bit closer to Mars before moving back out. Tossing much of the material in that region far away.

Planets don't do well forming too close to other planets because they will gravitationally disturb each other. A planet the size of Jupiter had pretty significant reach when it comes to disturbing other planets and planet formation.

neuroscience - Why does regular exercise increase brain volume?

Well, Erickson et al (2011) attribute the increase in brain volume in the aerobic exercise group to brain-derived neurotrophic factor (BDNF).

Specifically (p. 3020):

In fact, we found here that changes in serum BDNF levels were
associated with changes in anterior hippocampal volume; an important
link because the hippocampus is rich in BDNF, and BDNF levels increase
with exercise treatments in both rodents and humans. BDNF is a
putative mediator of neurogenesis and contributes to dendritic
expansion and is also critical in memory formation. Our results suggest that
cell proliferation or increased dendritic branching might explain increased
hippocampal volume and improvements in memory after exercise

with the caveat:

however, increased vascularization (15, 16, 33) and dendritic
complexity (34) may also be contributing to increased volume

big bang theory - Calculating the age of the universe

The simplest assumption about the global properties of the Universe is that it looks the same outside the part that is observable to us, as it does inside. That is, we see a finite part of a Universe that is (probably) infinite in extend. If so, then the calculated age — which is finite — applies to all of the Universe, not just the observable part.

The age is calculated on the basis of the observed expansion rate, and the observed densities of the constituents of the Universe. It is possible to imagine a universe with the right mixture of constituents that has existed forever$^{dagger}$, but for our particular Universe, this just doesn't seem to be the case; it is ruled out by observations.

As a first-order approximation, you can simply take the age $t_mathrm{Uni}$ to be the reciprocal of the expansion rate $H_0 = 70,mathrm{km},mathrm{s}^{-1},mathrm{Mpc}^{-1} = 2times10^{-18}$ s. That is,

$$t_mathrm{Uni} sim frac{1}{H_0} = 14,mathrm{billion,years}.$$

However, this assumes that the Universe has been expanding at the same rate throughout its entire history, which is hasn't. More generally, the age is calculated from integrating (numerically except for simplified approximations) the Friedmann equation, yielding 13.819 billion years.

I should say that the calculated age is the time from the Big Bang till now. I guess the safest thing to say is that we don't know what happened the first tiny fraction of a second or so after creation, and in principle it could have existed before this instant, collapsed, and then re-expanded. But no observations I know of suggest this.

$^dagger$An example of a temporally infinite universe is one containing energy only in the form of a cosmological constant. In this case, the Friedmann equation reduces to $da/dt=aH_0$, with $a$ the scale factor ("size") of the universe, the solution of which is an exponential function with zero size only at $t = -infty$.

cosmology - Seeing a galaxy (quasar) greater than 46.6 billion light years away

The edge of the observable universe is actually 46.6 billion light years away, despite the Big Bang being only 13.8 Billion years ago. This is because the light which we are now receiving as the furthest visible stuff had to travel through ever expanding space in between, being redshifted down into what we call the Cosmic Microwave Background Radiation (CMBR). There is a little bit further than that which we are technically receiving, but it has been redshifted infinitely.

To see anything further away than 46.6 Bly, it would have had to existed literally before time itself, or travelled faster than the speed of light. Two highly improbable things