the sun - What is the Neupert Effect?

Following the previous answer stating that the HXR emission occurs during rise phase of the flare as evidenced by the full-Sun SXR flux observed i.e. by GOES in 1-8 Angstrom, I would add that the HXR maximum is typically observed during the strongest increase of the SXR flux, i.e., during d(flux_SXR)/dt = max.

data analysis - How to determine period of pulsar?

I've got a file which contains data about photons coming from some pulsar. For each photon I know: a) time, when it was registered, b) a probability that this photon came from pulsar but not from galactic background, c) energy of a photon. My task is to find a period of pulsar.
I suppose I have to use FFT to find a period but there are two issues. First of all, there are lots of photons with low probability (background photons), so I have to somehow select relevant photons. Second, I've got an unequally spaced data, so there is a problem with using FFT.
I am a novice in data processing. Could you please give me links to some lectures or books where this topic is discussed clearly?

How long was the hyper inflation phase?

Googling "hyperinflation" mostly returns articles about Zimbabwe and the Weimar Republic, so I'm going to assume that you are referring to what is usually just called (cosmological) inflation.

We are still pretty uncertain about when and for how long inflation occured, but we do however have some sort of constraints. In general, inflation is thought to be caused by some sort of scalar field, usually written as $phi(mathbf{r},t)$, and its associated potential energy $V(phi)$. Various forms of how this field changes as a function of time exist, but a popular one is, or has been at least, the slow-roll field, which makes it behave like a cosmological constant, enabling it to drive an exponential inflation.

Inflation can be initiated by a symmetry break when the unified forces of electromagnetism and the weak and strong forces are separated in the strong and the electroweak force (the "GUT" era, for Grand Unified Theory), at temperatures of $Tsim10^{28}$ K. It will continue until the field has "rolled" from its metastable false vacuum state to a true vacuum. During this time, the Universe expands something like $e^{100}$ times, and its temperature drops by the same factor, i.e. to a freezing temperature of $Tsim10^{-15}$ K. This doesn't last long, though, because the energy of the field is converted to photons and other relativistic particles. This is called reheating.

The factor by which the Universe expands during inflation is essentially set by the minimum factor needed to explain in particular three cosmological "problems", viz. the flatness of the Universe, the causality of the horizon, and the missing monopoles, and the maximum factor allowed in order not to blow stuff too far apart that no structure would have had the time to form before gravitational attraction was too small, so that we wouldn't be here to ask the question. This requires at least 60 e-foldings (i.e. expanding by a factor of $e^{60}$), but could be 100 or maybe more. The factor is discussed e.g. in Galaxy Formation And Evolution by Mo, Bosch & White, Sec. 3.6.2.

Since the size evolution of the Universe during inflation is exponential, the scale factor can be written as
$$a(t) propto e^{Ht},$$
where $H$ is the Hubble constant during inflation, so for an expansion of, say, $e^{100}$ we have
$$e^{100} = frac{a(t_{mathrm{end}})}{a(t_{mathrm{begin}})}
= frac{e^{H t_{mathrm{end}}}}{e^{H t_{mathrm{begin}}}}
= e^{H(t_{mathrm{end}} - t_{mathrm{begin}})},$$
or
$$Delta t equiv t_{mathrm{end}} - t_{mathrm{begin}} = 100/H,$$
where $t_{mathrm{begin}}$ and $t_{mathrm{end}}$ marks the time of beginning and end of inflation, respectively.

During inflation, the Hubble constant should not evolve, but have a constant value of $H=(Lambda/3)^{1/2}$, where $Lambda$ is a cosmological constant. If inflation started when the GUT era ended, which was around $t_{mathrm{GUT}} sim 10^{-36}$ s, the Hubble constant was of the order $H sim t_{mathrm{GUT}}^{-1} sim 10^{36}$ s. Then a hundred e-foldings will take you to $t_{mathrm{end}} sim 10^{-34}$ s. Wikipedia states that inflation should last somewhat longer, something like $10^{-33}$ to $10^{-32}$ s. This might be if you do a more careful analysis, or maybe a different model. For instance, H is not exactly constant during inflation, but does evolve a little.

Various observations can constrain the physical parameters that enter the equations, e.g. cosmic microwave background and baryon acoustic oscillations. The latest results from the Planck satellite can be found here. Wikipedia also has a summary of the observational status of inflation.

So, I guess the answer to your question is, "We don't, really, but we have several theories that together with several observational constraints tell us that the duration of inflation should be of this order of magnitude. If it were much shorter, it wouldn't solve the cosmological problems that gave rise to the idea of inflation, and if it were much longer, the Universe would be blown apart".

the sun - How can the sun burn without oxygen?

As you are suspecting, the sun burns in a different sense, not by chemical reaction with oxygen.

Atoms consist of a tiny, heavy nucleus, surrounded by an almost empty space, populated by electrons. Burning by chemical reaction with oxygen doesn't change the nucleus of atoms, but takes place in the hull of atoms: Atoms may assemble to form molecules; electrons change their orbitals (the way they surround the nucleus), and release some energy as heat.

Atomic nuclei are (positively) electrically charged, and repell each other.
But if small nuclei, like those of hydrogen atoms, come close together, they can fuse and form a larger nucleus. This nuclear fusion of hydrogen to helium (in this case) releases much energy, more even than fission of uranium in a nuclear power plant. The notion "burning" is used sometimes for reactions of atomic nuclei, too, if they release energy as heat.

To overcome the electrostatic repulsion of hydrogen nuclei, high pressure and temperature are needed. These conditions occur in the core of the Sun.

exoplanet - Is the surface of TrES-2b actually dark?

I don't know, how it works there, but perhaps it is easier to imagine, how it could look.

TrES-2b is a gas giant, so it doesn't have a well defined surface. When you are somewhere in the atmosphere then:

-When looking up, you will see the light, as deep sea creatures do. The amount of light and its color will depend on how deep you are.

-When looking down, it should be dark (black), comparing to how it looks up. Otherwise, the atmosphere would be reflecting light.

-When looking to the sides, it should also be dark, for if the light were scattered at some angles, it would also be scattered upwards.

So, one simplest possible model is: like in deep sea.

Note, that at some other wavelengths, particularly in infrared, it all can look very differently.

cosmology - Hubble law, cosmological redshift and distance

I think that the wikipedia page on Hubble's law is reasonably clear. The distance in Hubble's law is the proper distance. This is the separation between two objects measured by observers at the same cosmic time. That is, if you imagine (instantaneously) stretching a load of metre rulers end-to-end from us to a distance galaxy, it is how many metre rules you would need. The velocity in Hubble's law is the rate of change of proper distance with cosmic time.

On large enough scales we find that that the ratio of velocity to proper distance is the Hubble parameter, which itself changes with cosmic time.

$$ v(t) = H(t) D(t)$$

In practice we cannot measure the velocity or proper distance in Hubble's law - we can generally only estimate these quantities for a galaxy as we observe it some time in the past, when it emitted the light we detect. This is why Hubble's law in terms of measured redshifts and estimated distances is only applicable over relatively small (cosmologically speaking) distances and for recession velocities much less than the speed of light.

For more distant galaxies the distance cannot be calculated from a redshift without a cosmological model for how the Hubble parameter changes with cosmic time. This in turn depends on the adopted cosmological parameters. See Evolution of the Hubble parameter

Proper distance is also discussed here.

How can I set the live video feed from the ISS as my desktop background?

In VLC, you can choose to align the video to the left, centre or the right. In addition, set the ratio of the display you want the feed to be placed on in the Aspect Ratio or Crop settings. I have it playing perfectly in full screen on my left monitor of a dual head setup.

Also, you can open the playlist file mentioned above and edit it to only have the 480 feed within it. This can be done in Wordpad or a competent text editor. Notepad will corrupt it, since the linefeeds are not completely windows-compatible.

Your playlist.m3u8 file should look as follows:

#EXTM3U
#EXT-X-STREAM-INF:PROGRAM-ID=1,BANDWIDTH=1194092,CODECS="avc1.77.31",RESOLUTION=854x480
http://iphone-streaming.ustream.tv/watch/playlist.m3u8?cid=17074538&stream=live_6&appType=103&appVersion=3&conn=wifi&group=iphone

To make the video display only on one screen, go to VLC settings, then go to the Show Settings box at the bottom and choose All.

Next, go to the Video section in the left tree, near the bottom, and click on it. In the right pane that appears, scroll down to the bottom of the Video settings and, in the Video alignment section, select Left, Center or Right, depending on your preference. Left will align to your left screen. Center will display the video across both displays with black bars on either side, and Right will align to your right display.

Finally, switch back to Simple settings, then under Video make sure your Output drop-down in the Display section is set to Automatic.

Close the settings for now. Next, in the main VLC window, go to the Crop settings and choose the correct ratio for the display you are using. In my case, the ratio is 16:10.

This will both align and crop the video to a single display only. Perfect! The crop is important so the VLC viewport doesn't spill onto the other display. Try experimenting with it until you get the desired effect.

Update: Maybe I was incorrect about the display switch. I can't get it to show on my right display! Any ideas? It still sits perfectly on my left screen though. Aspect ratio seems to be a better idea so it doesn't chop off the video, even though it smooshes it a little bit as the feed is wider.

If anyone has suggestions on how to create a desktop shortcut for this, I'd greatly appreciate it!

orbit - Does the orbital variation in planetary gravity affect the Sun's corona

Dimitris (see below) argues that the syzygies of the Earth and Venus and those of Mercury, Earth and Jupiter distort the Sun's corona, which in some way affects climate on the scale of hundreds of years. Is there any evidence to support the idea that the planets do have such an effect on the sun, other than the correlation noted by Dimitris?

Planetary orbits’ effect to the Northern Hemisphere climate, from solar corona
formation to the Earth climate.

Poulos Dimitris

Abstract
The four planets that influence the most the solar surface through tidal forcing seem to affect the Earth climate. A simple two cosine model with periods 251 years, of the seasonality of the Earth – Venus syzygies, and 265.4 years, of the combined syzygies of Jupiter and Mercury with Earth when Earth is in synod with Venus, fits well the Northern Hemisphere temperatures of the last 1000 years as reconstructed by Jones et al (1998). Later reconstructions that give too much emphasis on multy-centenial variation are due to increased error. The physical mechanism proposed is that planetary gravitational forces derange the Solar Corona that in turn deranges the planetary geomagnetic field causing
temperature variations.

http://www.itia.ntua.gr/getfile/1486/1/documents/PoulosPaper.pdf

cosmology - Going from the second moments of an object to its ellipticities and half-light-radius

You simply cannot obtain any information on the half-light radius from the quadrupole moment only of a galaxy, without knowing the galaxy profile. Consider for example two (extreme) light profiles:

1. A galaxy consisting of a uniform disk of radius $R$ – that is, its surface brightness would be something like $I(r) propto H(R_1 - r)$, where $H$ is the Heaviside function.




2. A galaxy consisting of a point source + a uniform disk – that is $I(r) propto delta(r) + H(R_2 - r)$.

You can easily find a combination of radii $R_1$ and $R_2$ such that the quadrupole moments of the two galaxies are identical. However, clearly their half-light radii cannot be (for the second galaxy, in particular, the half-light radius vanishes).

From what I can see, the relation you wrote holds for a Gaussian profile, and even then it is not correct: your $r$ is not the half-light radius but the standard deviation of the Gaussian profile. For different profiles there is no guarantee that your relation will work (in general it will not): in general, it will only give you something proportional to the square of the half-light radius, but the proportionality constant will depend on the specific light profile.

exoplanet - Are astronomers continuously monitoring exoplanetary systems?

I'm reading many of the Wikipedia pages about exoplanets and the different methods they are using to detect them. But I wonder, it seems that the emphasis is on detecting and finding new exoplanets, but are astronomers continuously monitoring the exoplanets that have already been discovered? For instance, if there is a collision between a detected exoplanet and a previously unknown planet, this could cause a change in some of the orbital parameters, and could be potentially detected. If this happens, would we learn of this change, or will it have to wait until astronomers again take another look at the system?

Estimating the angle covered by the star trails and deducing how long the exposure lasted

The answer is given here. One minute of time corresponds to 15 arcminutes (written as 15'). This is because in 24 h the Earth revolves 360º, so
$$textrm{angle per time} = frac{360º}{24 textrm{ h}} =frac{21,600'}{1440 textrm{ min}} = 15'/textrm{min}.$$
If you turn this fraction upside down, you see that 1' corresponds to 1/15 min, or 4 seconds.

That is, you measure the angle (let's call it $theta$) of any of the star traces, as seen from the center (notice that the Northern Star is not exactly at the center, so that it itself traces a tiny arc instead of a dot). From the picture below, I get roughly $theta = 135º$. The exposure time is thus:
$$t_mathrm{exp} = frac{theta}{textrm{angle per time}} = frac{135º}{360º/24 textrm{ h}} sim 9textrm{ h}.$$

By the way, if you mark the position of the ends of the trails, you can recover the stellar sky. I found Ursa Major, marked by the yellow dots.

the moon - Lunar Soil: what are those "bumps" and how were they formed?

It's a bit difficult to understand what exactly you're asking about, but I will presume you meant the topological features in images like this one, taken from the video of which you provided a link to:

         

For the most part, they are meteorite and micrometeorite impact craters, with some regolith boulders and impact ejecta scattered along the edge of the larger impact crater at the bottom right of the image.

These impact craters appear strange to an untrained eye when they're the only visible feature on images, because there is not atmospheric scattering of light on the Moon that would somewhat illuminate the shadow side too and reduce contrast to what we're used to seeing on Earth, so the high contrast can appear as bumps or dents interchangingly and at will, similar to how you can see either either a vase / candlestick in the middle (yellow) or two faces facing each other (black) on this image:

                             

A trained eye will have less problems recognising them for what they are, though. One nifty trick is to search the image for different looking topological features, such as already mentioned larger crater ejecta along its edge at the bottom right of the image, or perhaps prominent features within the features themselves, like some larger micrometeorite impact craters show shockwave formed uplifted centers, and so on. It also helps to understand how these features were likely made, but once you orient yourself, it will prove difficult to convince yourself into seeing what you saw initially, if you saw indentations where there are perturbations, or vice versa.

magnetic field - Can Magnetars destroy planets?

Completely disregarding magnetic fields for now, an extremely dense object like a magnetar (which is a kind of pulsar, which itself is a kind of neutron star) could destroy planets just with gravity (see Roche Limit). Now, if the planet is far away from the star, things might not be necessarily that ominous. The problem with electromagnetic forces is that their range is not as far-reaching as gravity. In fact, the first exoplanets ever detected were found around a pulsar (PSR B1257+12), and these objects already have pretty damn strong magnetic fields.

Here's the thing: atoms nuclei are held up by outrageously strong nuclear forces, but they are even more short-ranged than electromagnetic forces. I haven't run any numbers on this, but I think that these forces are so strong, that in order to break apart the atoms of the rocky planet just by using the magnetic fields, the planet would have to be so close to the magnetar, that its gravity would already have destroyed the planet.

However, the way gravity and electromagnetism precisely work on such extraordinary environments like the regions around a magnetar are still mysteries to be solved. Magnetism is counter-intuitive and the calculations are hard (been there). Even so, I think gravity would still win.

probe - How do we know that Voyager's data is correct?

This may answer some of your questions, but not all. Additional information may exist in the book.

Also take into account that both Voyager spacecrafts could perform the same measurements before they diverged. The results could be compared.

Following excerpts are from the book Deep Space Craft: An Overview of Interplanetary Flight, Dave Doody, Springer Science & Business Media, 2010, parts of which are available in the free preview (use chapter links to the source of the two excerpts):

5.6.2 Extensible Booms

[...] A potential source of error in the magnetometer instruments’
measurements is the amount of twist in the long fiberglass
magnetometer boom established after deployment. To compensate, a
magnetic coil, which can be energized on command to create a magnetic
field of known strength and orientation to calibrate the instruments,
is mounted on the spacecraft.

6.4.8 Calibrations and Ground Truth

Scientific measurements are made by using instruments that have
quantifiable error. Calibrations are carried out by instruments on a
spacecraft to acquire baseline data for comparison with an actual
observation, thus allowing instrument errors to be quantified.

Prior to carrying out an infrared spectral measurement of a target,
the IR spectrometer will be aimed toward a spot of deep space free of
any bright objects in its field of view. An absolute reference value
is obtained, and any defects in the instrument’s sensors can be
recorded and later included in data analysis.

For the same reason, radio science experiments always begin and end
with a measurement of the spacecraft’s unobstructed, unmodulated radio
tones lasting tens of minutes before and after encountering the
target.

Many imaging instruments may be aimed toward a special calibration
target mounted on the spacecraft bus. Voyager’s calibration target was
a rectangular plate mounted below the bus (see Appendix A, page 294)
coated with a material of known grayscale and albedo values. The
spacecraft’s scan platform could aim the cameras so that the target
plate would fill the field of view for calibration.

The same target plate on Voyager serves as the thermal radiator for
the spacecraft’s electrical system regulator, so Voyager’s infrared
instrument, the infrared radiometer-spectrometer (IRIS) could be
calibrated.

The operation of every science instrument and experiment includes some
sort of procedure or other means for calibrating its measurements.

star formation - How is the Lithium Depletion Boundary used to determine the age of a stellar cluster?

When low mass stars are very young, they are termed pre main sequence (PMS) stars. These PMS stars have larger radii than main sequence stars of the same mass, and energy transport in their interiors occurs primarily through convection. The convection ensures that the star is thoroughly mixed and chemically uniform.

As the PMS star radiates away its gravitational potential energy, it contracts. The virial theorem tells us that as it does so, its interior becomes hotter. Roughly speaking, the core temperature is proportional to $M/R$, where $M$ is the mass and $R$ the radius.

Nuclear fusion of hydrogen will not commence until the core temperature reaches more than 10 million K, however there are other fusion reactions that become possible at lower temperatures - namely deuterium burning at around 1 million K and then lithium burning at around 3 million K. The latter reaction is not energetically important in the star's life because there is not much lithium in the star to begin with (about 1 part in a billion), however this lithium can be observed in the photosphere of the PMS star (via the 670.8 nm Li I resonant absorption line) and the convective mixing means whatever we see at the photosphere also represents the abundance at the core.

The Li burning reaction is extremely temperature dependent (like $T^{20}$ or thereabouts), so it turns on like a switch once the core reaches the appropriate temperature (e.g. see Bildsten et al. 1997). The time it takes for a PMS star to reach this core temperature basically depends on its mass. More massive, and hence more luminous stars contract faster and reach the Li burning temperature quicker. Once they do so then the Li in the star is rapidly and thoroughly consumed by fusion. The relationship between the age at Li destruction and the luminosity of the PMS star at that time is the age-LDB luminosity relationship that you refer to.

The result is that if you look at a bunch of stars in a cluster (assuming they all have the same age), then the more massive, more luminous PMS stars will have destroyed their Li, whilst the lower mass, lower luminosity stars will still contain their original Li content. The luminosity at the reasonably sharp transition between these two regimes is known as the LDB.

LDB ages are arguably the most accurate way to find the ages of stars in clusters. All age determinations are to some extent dependent on what physical ingredients are in stellar evolution models, but the sensitivity of the age-LDB luminosity relationship to various uncertainties is quite weak (e.g. Burke et al. 2004) - we basically understand the physics of a contracting, fully convective ball of gas quite well. The LDB ages can also be precise, because the very sharp turn-on of Li burning and its rapidity should lead to a sharp transition between stars with low luminosities that have Li and stars with only slightly higher luminosities that don't.

Interestingly, in the last couple of years it has become apparent to us (and others) that there are some model ingredients that are not completely understood, namely the effects of dynamo generated magnetic fields and dark starspots. Both of these may lead to the suppression of heat transport, either throughout the star or just at the surface, slowing the PMS contraction so that at a given age, the PMS star has a cooler core temperature. This might delay the onset of Li burning and mean that the currently determined LDB ages are underestimates by 10-20% or so (e.g. Jackson & Jeffries 2014; Somers & Pinsonneault 2015 ).

planet - Are there large underground caverns on Mars?

As evidence of caverns detected on Mars, consider the following recent image taken from HiRISE instrument on the Martian Reconnaissance Orbiter :

Image source: NASA

Scientists believe that

The hole appears to be an opening to an underground cavern, partly illuminated on the image right. Analysis of this and follow-up images revealed the opening to be about 35 meters across, while the interior shadow angle indicates that the underlying cavern is roughly 20 meters deep.

Which accompany larger openings, reported in the National Geographic article Mars Has Cave Networks, New Photos Suggest (Norris, 2007) of several larger pits detected by Odyssey, and they offer a possible origin:

The surface of Mars is strewn with craters from meteor impacts and depressions formed by the collapse of underground chambers formed by flowing lava, the experts said.

There are many examples of lave tubes (and indeed, in other places in the Solar System), caverns etc reported in the article Mars Cave-Exploration Mission Entices Scientists (Wall, 2012), who also state, in regards to detection and exploration of these caverns that robotic means are being explored.

However, according to this NASA article, a method to find and characterise caves on Mars (or indeed Titan, and maybe other places, including the Moon) is to detect them via thermal differences caves cause in comparison to the surrounding surface. This method is used on Earth, and

Techniques developed
through this research will ultimately be applied to
locating subterranean cavities on the Martian surface.
We anticipate one of the best techniques for systematically
finding caves on Mars will be via remotely
sensed thermal imagery.

cosmology - Two species of dark matter?

Hot dark matter would be made from very light, fast moving particles. Such particles could not possibly be gravitationally bound to any structure, but rather would be dispersed all across the universe.

But dark matter is always "found" (or "inferred") either gravitationally bound to some visible structure (e.g. weak lensing detection of dark matter associated with colliding galaxy clusters / flat rotation curves of spiral galaxies / abnormal velocity dispersion in galaxy clusters) or not associated to anything visible but nevertheless forming clumps (weak lensing detection of galaxy clusters previously unseen). That is why dark matter is thought to be cold.

Additionally, there is a clear distinction between both types: there is not such thing as dark matter that is "not too cold but not too hot either" (see footnote as well). Dark matter is either made of particles with less than ~10 eV (hot dark matter, made of light particles, mostly dispersed everywhere) or particles with more than ~2 GeV (heavier, slower particles gravitationally bound to some structure). Both limits are found when imposing the maximum amount in which the candidate particles (neutrinos or something more exotic) can possibly contribute to the actual value of the density parameter due to matter in our expaning Universe.

Thus, either DM appears gravitationally bound (cold DM) or dispersed (hot DM), and both types are clearly distinct (10 ev vs 2 Gev). Observations favour the first case. However, Cold Dark Matter is not the ultimate solution, and still faces some problems.

Regarding the possibility of mixed solutions, many of them have been already ruled out. Microlensing has ruled out the possibility of unseen compact objects (brown dwarfs, stars, stellar black holes) in galactic halos, in our galactic neighbourhood as well as in the extragalactic domain. Ordinary matter (stones, bricks, dust) cannot possibly be, otherwise they would become hot and re-radiate. Any exotic mix of known particles doesn't work.

All we think we know is that DM must be made of some heavy particles yet to be discovered. In order to introduce a more complex model (e.g. different types of particles depending on the structure they appear attached to) one needs a justification (i.e. some predictions that better agree with reality) and nobody has been able to do that yet.

---

Remark Note that Dark Matter particles, either from the hot or the cold type, cannot possibly "slow down" and clump too much (e.g. forming planets) because they don't interact electromagnetically like ordinary matter, that is why DM is said to be collisionless. Wherever infalling ordinary matter forms any structure (e.g. protostars or accretion disks), a very important part of the process is thermalisation, i.e. the redistribution of energy of the infalling particles by means of numerous collisions. This cannot happen with Dark Matter.

During night on the Moon is there Earth light and Earth phases?

Yes, there are Earth phases, viewing from the Moon. Full earths, half earths, quarter earths, waning and waxing earths. The easiest way to visualize this is, imagine the earth is still, one half of the Earth facing the sun, the other half away from the sun, so you have half the Earth is light, half is dark, now, imagine you're on the moon orbiting the Earth every 28 days. When you're over the sunny half of the Earth (night on your part of the Moon) the Earth is full. When you're over the dark side of the Earth (day on your part of the Moon), the Earth is new. As the moon takes 28 days to orbit the earth, like the moon in our sky, every 28 days would complete one cycle.

What's different is the Earth wouldn't move in the night sky. It would actually go back and forth a bit, but it would stay in the same general area, every day, every year, every century, because the Moon is tidally locked to the Earth, but apart from not moving, it would be similar to the Lunar cycles.

Another difference is that you could observe the Earth's rotation. Here's a pretty good video on what it would look like. 28 days squeezed into about 1 minute.

https://www.youtube.com/watch?v=-HgHEO0DUig

I would imagine the Earth looks quite bright from the point of view of the Moon, and I'd guess the pictures don't really do it justice, but that's just a guess. I've never seen it for myself. I'm also not sure it would be pitch black and not visible as a "New Earth" either. I remember reading that you can see stars from the moon even during the day, that's because there's no atmosphere to diffract the light so you could probably see the Earth even at new earth too. We can see the new moon from Earth sometimes, so I would think a "new earth" would be visible but dark.

Here's a discussion on being able to see the new moon. I would think, seeing a new earth from the moon would be even easier.

Short discussion: http://scienceline.ucsb.edu/getkey.php?key=26

Long discussion: http://physics.stackexchange.com/questions/1907/why-can-we-see-the-new-moon-at-night

Does the Perseid peak cause enough light pollution to be a problem?

Tought to answer, because the amount of airborne dust due to meteor showers and its stability in the upper atmosphere will vary significantly enough. We had many meteor showers, Perseids included, in the past that were a bit of a letdown and didn't produce as many shooting stars as initially predicted, and also the other way around of course.

But here's the thing tho, that discounting for any additional atmospheric glow during a few crucial hours of dusk and dawn, this airborne meteor dust might even improve the average atmospheric refraction with mostly extremely fine powder silicate dust settling onto the lower atmosphere and dispersing water molecules and binding to other solids (read: decrease humidity and pollutants), by being highly electromagnetically charged from the atmospheric reentry and electrostatically binding to larger atmospheric or pollution molecules, or simply for being hygroscopic and binding really good to water molecules, making them heavier enough to fall in a light spray back on the ground and remove the pollutants from the lower atmosphere in the process, too.

There might be other physical phenomena going on that I'm not aware of, but what I wanted to illustrate is, that there might even be positive side-effects in terms of light pollution vs increase in meteors, which is how we call meteoroids entering Earth's atmosphere.

amateur observing - How does a Bahtinov mask work?

The idea is to create a specific diffraction pattern so that you know if you image is in focus. The pattern is so that you end up with 3 lines crossing the source.

The mask consists of three separate grids, positioned in such a way that the grids produce three angled diffraction spikes at the focal plane of the instrument for each bright image element (star). As the instrument's focus is changed the central spike appears to move from one side of the star to the other. In reality, all three spikes move but the central spike moves in the opposite direction to the two spikes forming the 'X'. Optimum focus is achieved when the middle spike is centered on the star and symmetrically positioned between the other two spikes. Small deviations from optimal focus are easily visible.

https://en.wikipedia.org/wiki/Bahtinov_mask

You can construct a mask with some materials:

http://www.deepskywatch.com/Articles/make-bahtinov-mask.html

There is also a Hartmann Mask that splits the object into multiple when not in focus. This seems a little easier to construct.

http://www.cloudynights.com/item.php?item_id=518

Free neutrons and stellar nucleosynthesis

I have yet to find good information regarding the s-process, so I'll just talk about the r-process here.

The key difference between the two is the conditions under which they take place. The r-process takes place during supernovae, specifically those due to gravitational collapse. In a collapsing star, many of the protons and electrons are squeezed tightly together - so tightly, in fact, that a process called neutronization begins. The protons and electrons actually combine to form neutrons (and neutrinos). This creates a large neutron flux, and so there are now lots of neutrons available for the r-process.

None of this lasts incredibly long, though, because the "explosion" doesn't last long. It soon propels much of the outer material of the star out into space, and the remains become either a neutron star or a black hole. That said, an interesting follow-up might address the relationship between the r-process, neutronization and the formation of neutrons stars in more detail.

Sources:

https://en.wikipedia.org/wiki/R-process

http://www.ucolick.org/~bolte/AY4_00/week8/SNeII.html

Find the height of the shooting star from the Earth's ground

This is basically a trigonometry question. I wasn't sure from the question if the apparent location from your friend's position was six degrees higher or lower. I chose higher for the example, but you could change it.

You have two triangles with the same altitude, but different bases and angles.

$$text{altitude} = x tan(81^{circ}) = (x + 20km) tan (75^{circ})$$ Do you see how to calculate it from there?

planet - Estimation of average rock and asteroid mass associated with different stars

The composition of the gas from which stars and their planetary systems form is reasonably well known. About 1-2% of this gas is in the form of chemical elements heavier than Helium (the so-called metallicity of the gas).

A fraction of these "metals" - the iron, silicon, oxygen etc. is capable of forming dust and then accumulating to form "rocky" material.

So to answer your last question first, given the available reservoir it seems most unlikely that even 1% of the baryonic (normal) matter in the universe could be rocks, even if none of it were gathered into luminous stars and galaxies.

Forming stars are surrounded by circumstellar material from which planets and other rocks form. Observations of young stars suggest these discs can be as massive as 10% of the stellar mass, but more usually 1% or less. This means that as a fraction of the whole formed star system the rocks will be at most 1% of 10% (ie 0.1%) of the stellar mass. For a Sun-like star that means there's less than 330 Earth masses of rocky material around it.

the moon - A curious relationship between lunar periods and the solar year

The synodic month is the "average period of the Moon's revolution with respect to the line joining the Sun and Earth". However, the Earth also moves in its orbit around the Sun during this month. From our vantage point, the Sun has appeared to move in the sky with respect to the background stars, in the same direction as the Moon moves in the sky with respect to the background stars.

Your calculation deals with the sidereal month, so your result is the calculation of the sidereal year.

When the Sun returns to the same spot in the sky, that is the sidereal year, whose length is 365.256363004 days, very close to your calculation. So why is that off from the tropical year by 20 minutes? Because the tropical year (the year that keeps the seasons in place throughout the calendar year) is slightly shorter than the sidereal year.

The tropical year is about 20 minutes shorter than the time it takes Earth to complete one full orbit around the Sun as measured with respect to the fixed stars (the sidereal year).

You just found the difference between the sidereal year and the tropical year, and that was no coincidence.

Addition

The number of sidereal months in a sidereal year is one more than the number of synodic months in a sidereal year. That is because the Earth goes around the Sun once of year (of course), leading to one less synodic month than sidereal month.

Here, $P_{syn}$ is the synodic period and $P_{sid}$ is the sidereal period, and $Y_{sid}$ is the sidereal year, all in days.

$$frac{Y_{sid}}{P_{sid}} = frac{Y_{sid}}{P_{syn}} + 1$$

Dividing both sides by $Y_{sid}$ yields:

$$frac{1}{P_{sid}} = frac{1}{P_{syn}} + frac{1}{Y_{sid}}$$

Solving for $Y_{sid}$...

$$frac{1}{P_{sid}} - frac{1}{P_{syn}} = frac{1}{Y_{sid}}$$

or

$$P_{sid}^{-1} - P_{syn}^{-1} = frac{1}{Y_{sid}}$$

Multiplying both sides by $Y_{sid}$ and dividing both sides by $P_{sid}^{-1} - P_{syn}^{-1}$ yields

$$Y_{sid} = frac{1}{P_{sid}^{-1} - P_{syn}^{-1}}$$

astrophysics - Where can I learn these Astrophysical techniques? [Read below]

If you are going to do data analysis, you need to understand how the fitting procedure works. This means a lot of statistics, and new terminology and so on, which is hard and takes much time.

If you want to start to fit, e.g., a spectrum, you should definitely read the XSPEC guide. You can find a pdf online as well.

In short, fitting is to take a model and measure how well this model fits to your data. To quantify this "how well", usually the chi-squared distribution is considered:

$chi^2 = sumlimits_{i=1}^n(frac{X_i - mu_i}{sigma_i})^2$

where $X$ is your data (observed value), $mu$ is your expected value (which, in this case, corresponds with the prediction of the model), and $sigma$ is the variance on your data point (the error).

This is the most general information that you need.
You can read more here, and especially on the Numerical Recipes (don't be scared by the huge format of the last document, the pages you need are only few around Chapter 15).

For fun, you can just try to play with XSPEC, to take confidence, and see at least how things change when you change your model or your data (especially if you know which model is the best-fit for your data). To understand everything will take years, but if you never start, you'll never arrive ;)

How much heat is generated from waxing and waning of reflected radiation from the Sun?

The article doesn't go into specifics and appears, not to be written by Dr. Evans at all, but I'll pull some quotes.

When it is completed his work will be published as two scientific
papers. Both papers are undergoing peer review

and

He has been summarising his results in a series of blog posts on his
wife Jo Nova’s blog for climate sceptics.

He is about half way through his series, with blog post 8, “Applying
the Stefan-Boltzmann Law to Earth”, published on Friday.

(footnote, I'm guessing in Australia, summarizing and skeptics are spelled differently than in the US, cause that's a copy-paste)

I've only read one of his summaries, and in Mathematics, you have to look at the details, which he's not provided, so, he's basically saying little more than "this is true, here's generally why, I've done the math, please take my word for it, I'll publish the numbers later". Now Michael Mann is also presenting non peer reviewed work to Paris, so, what's good for the goose is good for the gander I suppose.

Now, I'm just a guy who likes science, but my understanding is that ideas have been presented before they are finished or peer reviewed fairly often. Einstein did this in fact regarding general relativity and another scientist actually beat him to publishing the theory (though, the other scientist was gracious about it and gave Einstein full credit), so, I don't think it's necessarily bad to present a summary prior to peer review.

I think it is, however, unusual to post summaries of an idea on a blog saying "I figured it out, the majority of research on this subject is wrong". Dr. Evans is saying he has proof, but he's acting like a junk science blogger.

Dr. Evans "trust me I did the math" claim kind of requires that we look
at his track record, and his track record isn't very strong, though he will say, he's being attacked by the establishment.

According to this site, he hasn't published anything peer-reviewed since the 1980s and he's on a their Climate Denier List. He's also on Skeptical Science's Climate Misinformers List.

And here's a list of debunked claims he's made (and if this list is accurate, he's not a scientist at all, just a guy on a fishing expedition. No good scientist would agree with pretty much every counter argument against climate change cause that's not how the scientific method works. You can disagree with something, that's fine, but to agree with every counter argument - that's silly. Here's another more detailed explanation of what he's gotten wrong, from 2011. Evans seems to be in this debate to disagree with it much more than he's in it to do scientific research.

It's pretty much impossible to make a true scientific argument against Evans "proof" without his specifics, and, that kind of proof/disproof can get a little long and complicated, but for now, disproving him is impossible. But should we listen to him?

It says above he's published 8 blogs related to his recent research. I'm not going to dig up all 8, but here's the most recent one. Applying the Stefan-Boltzmann Law to Earth. Now, I'm just a layman, but even I can see problems with his argument here (and he should too given that he's an engineer with a PhD). The Stefan-Boltzmann law is an approximation. It's a physical model to calculate radiation into space.

The problem with his approach is, the best way to measure how much heat/energy leaves the earth by radiation is to measure it directly, by satellite. The amount of energy that radiates from the Earth into space varies with temperature, snow cover, cloud cover, even humidity, and probably 1 or 2 other things I'm overlooking. If you try to calculate this energy leaving the earth into space by playing with with the Stefan-Boltzmann Law instead of relying on direct measurements, you're allowing yourself a lot of fudge factors and inviting a far greater error than direct measurements would give you.

On the quoted article, let me pull out an example:

Dr Evans has a theory: solar activity. What he calls “albedo
modulation”, the waxing and waning of reflected radiation from the
Sun, is the likely cause of global warming.

OK, so, which is it, solar activity or albedo modulation, cause they're not the same thing. The first takes place on the sun, the 2nd, the earth. This paragraph makes no sense to me.

He predicts global temperatures, which have plateaued, will begin to
cool significantly, beginning between 2017 and 2021. The cooling will
be about 0.3C in the 2020s. Some scientists have even forecast a mini
ice age in the 2030s.

Now, this paragraph is particularly devious. El Nino's tend to warm the earth, La Nina's cool it. The effect is temporary and not huge, but enough to cause yearly variation. A strong El Nino drove the big spike in global temperature for 1998 and we're in an El Nino now (edited my answer, since 2014 they've been talking about entering an El Nino, I gather it's officially started now).

We had more La Nina years than El Nino 2006-2013 with the only small El Nino coinciding with 2010, which set records for temperature. A lot of the hiatus in warming that is often talked about is related to there being only 1 small El Nino over 7 years.

Predicting 2017 as the time when the cooling will "begin" is devious because that could be around the time the El Nino has ended and the oceans could switch back to a La Nina (which usually follows El Nino). This will create a temporary cooling for a year or two, which he, no doubt, will take credit for if it happens. Now, he also predicts 2021 which could go either way and he gives an amount, but that doesn't change the fact that he's making a prediction and hoping the El Nino of 2015 will end and make it prediction look good.

Real global warming or cooling can't be measured in 1 year anyway, unless, maybe, if it's ocean current and the occasional mega-volcano adjusted - then, maybe you can get some measure of warming/cooling based on one year, but it's still only one year. That's a really really short period to make any predictions on and not something I'd trust very far at all.

and on the "scientists have predicted a mini ice age in 2030", that's not actually true. There was a study on sun-spots and they predicted that we could see a sun-spot low period around 2030, perhaps similar to the Maunder Minimum that may have caused the mini ice age, but the scientists who predicted that were very clear that they were not predicting a new mini ice age, they said the effect would be smaller than the effect of CO2.

Here's the mini ice age prediction, which a few people made (but not the scientists who did the research).

Here's an article that explains why it isn't true.

So, there's a lot of bad and a handful of false statements in that article you quoted, which, granted, wasn't written by Dr. Evans himself, but still, it's hard for me to take it seriously.

Until he publishes his results, he can't be proved or disproved but based on what I've read, I find it hard to take him seriously. My hunch is, he's not trying to reach scientists at all, but he's trying to reach his target audience. Those who question climate change and he gives them a name and an alternate argument that they can stand on. An argument doesn't have to be correct, it only needs to sound correct and with that, you can usually convince a percentage of people to agree with you.

Not sure how much that helps, but that's my take and I went through and tried to clean up my long answer a bit. If you'll forgive me, it reminds me of the the old joke. How can you tell Dr. David Evans is lying? He's talking or writing. :-)

gravity - How long until the Earth and Moon become a binary planet?

I'm not sure I agree with the double planet POV, but the calculation is pretty simple. The earth weighs 81 moons, so for the Barycenter to be outside the earth, the distance (center of Moon to surface of earth), = 81 earth radii.

or about 515,000 KM. It's current farthest distance is 405,000 KM, average distance 384,000 KM and closest 363,000 KM Source, so, it depends on whether you mean, temporarily outside, outside more than 50% of the time or always outside - each would provide different answers.

But moving away 4 CM per year (same source) or 1 KM every 25,000 years, it would need 2.75 billion years to move away the necessary 110,000 KM necessary to have the barycenter move outside of earth at the moon's furthest point. (less if the orbit gains eccentricity which is possible). But probably more than that, cause as the moon moves away from the earth, the tidal forces that continue to push it away grow weaker and the earth slows down a little. If the earth ever gets tidally locked with the moon, the reverse will happen and the sun's tidal forces will draw the Earth and Moon slowly towards each other, so a precise answer is too mathematically difficult for me, but longer than 2.75 billion years seems a pretty good guestimate.