observation - Why do spectroscopic binaries have approximate circular orbits?

So for an assignment I have to answer the question what I can conclude concerning the shape of the orbit, knowing that it is both an eclipsing and spectroscopic binary.

Now, I think the answer I supposed to be that, since we know it is a spectroscopic binary, the orbits will most likely be circular. And I've read on various places that this is true, because of the tidal forces that cause for circular orbits. These tidal forces are quite strong, because apparently spectroscopic binaries are quite close to each other.

So here is where I lose it, I don't understand why spectroscopic binaries should per se be close to each other. I have considered that this is because they are not visual binaries; if they were visual binaries we would be able to tell them apart. But we can't tell them apart, so they are not visual binaries. From this you could conclude that this means that they are close to each other, but I think it could just as well mean that they are very very far away, and that that is the reason we cannot tell them apart. So I can't find a proper explanation for this.

Many thanks if you can help me out!!

What is the formula to predict lunar and solar eclipses accurately?

The NASA sites have some very useful resources for this I will list them below:

Lunar Eclipses

This Link has an index for all lunar eclipses from -1999 to +3000, predominantly a statistics page but also has this page that contains how to calculate when lunar eclipses are.

There is more than one formula depending on which time frame you are trying to look in.

This is the formula for eclipses between the year 2005 and 2050:

$$Delta T = 62.92 + 0.32217 * t + 0.005589 * t^2$$

Where:
$$y = year + (month - 0.5)/12$$
$$t = y - 2000$$

Solar Eclipses

This Link has an index like above but for all of the solar eclipses from -1999 to +3000.

This link has the formula for calculating solar eclipses. This is the formula for between 2005 and 2050:

$$Delta T = 62.92 + 0.32217 * t + 0.005589 * t^2$$

Where:
$$y = year + (month - 0.5)/12$$
$$t = y - 2000$$

pluto - Closest point on Earth to a planet?

Your question is valid and is a common operation. When computing when an object will be visible, many observers want to be able to draw a globe or map that is marked with the position from which an object like Pluto is directly overhead. Such a position on the globe at a given instant (and you do say “at the current time”) is, all other things being equal, the best-situated place from which to observe the object.

Unfortunately, the “libastro” C library that PyEphem wraps seems to only provide one instance of this concept: Earth satellites have .sublat and .sublong attributes, because it is so common to want to draw the path of a satellite on a globe or map.

But libastro does not generalize the concept. From what I can see, there is no way to generate the point on the Earth that is directly below any other Solar System object. And since I have not been in the habit of trying to extend libastro, it is likely that PyEphem will not gain this ability.

However, I have been developing a replacement for PyEphem that is written in pure Python and that I will be free to extend, called Skyfield. I will hopefully have this concept working there soon, and when I do so I plan to make it work for all objects, and not limit it to Earth satellites!

(In the meantime, as pointed out in the comments: you could try asking PyEphem for the geocentric RA and dec and, if you could adjust RA by the Earth's current hour angle, then you could turn those into an approximate latitude and longitude.)

telescope - UTC to UT1 time corrections

Astronomic observations in geodesy.

It is well understood that sun and star positions may be expressed in Greenwich hour angle and declination. Or Right Ascension Declination. And may be published reckoned to Universal Time (UT), or TT.

An approximation of UT can be obtained by apply a correction (DUT) to UTC. That results in an expression of time described as UT1. DUT time offsets are published and broadcast (WWV shortwave radio) in 0.1" increments.

I'm looking for daily precise (better than 0.1") DUT corrections.

Current broadcast DUT correction is -0.2".

I have retrieved IERS Bulletin B, which has daily values of UT1-UTC in ms. Current value is -21.996 ms. Is it just me? Is -21.996 ms = -0.022"?

Or is there something in the IERS Bulletin B that I'm missing?

exoplanet - Is there an upper limit to the mass of terrestrial planets

The exo-planet Kepler-10c has a mass between 15 and 19 times the mass of the Earth (making it comparable in mass to Neptune), and yet is thought to have a density of about 7g/cm^3 and to be a terrestrial planet, with a substantial proportion of "hot ice"

Is there an upper limit to the mass of terrestrial planets, or can rocky planets form that are larger than Kepler-10c?

This, older, article in Universe Today suggests that terrestrial planets can't form more than 5-10 Earth Masses, substantially smaller than Kepler 10c.

nutrition - Has there ever been an attempt to create nutritionally tailored food for adult human consumption?

Shortest answer: there's nothing special in human biology, you could totally make it

Short answer: Bachelor chow!

I would totally buy this stuff if they made it. The closest I have now to bland, flavorless, zero thought/effort food is Wheaties.

longer answer:
Seriously though, dogfood for humans wouldn't be that hard to make. If you just took everything from the RDI guidelines (the Recommended Daily Intake from which they calculate the percentages you find on food labels) and mashed it all together into a thick brown paste you would get all of the macro and micro nutrients that you theoretically require. Precise proportions are irrelevant because:

a) everyone is a little different, and

b) to some extent, our bodies are able to fine tune our digestive tracts to fit our specific diets (presumably this tuning would occur even faster if you ate the exact same thing for every meal). You're a mammalian omnivore. Enjoy it!

You probably shouldn't try to live on something like this for the rest of your life, since there's plenty of research suggesting that there are other micronutrients, such as phytocompounds (i.e. plant stuffs), that are poorly understood but may be beneficial to humans. One thing to note here is that any health claims about any antioxidant that is not on the list of essential vitamins (vitamin C is an antioxidant on the list) are almost certainly bunk and/or hokum.

The upshot is that you would only need, at most, utterly minuscule quantities of these miscellaneous micronutrients. You would be fine(ish) for at least a few months. For related reasons, your statement:

The composition is also known for human babies. This is manufactured as "baby formula". Everything baby's organism needs to be healthy (and to grow).

is largely correct but somewhat flawed. While it is true that many babies are raised solely on formula and subsequently turn out just fine, there are some things that you can only get from mother's milk. For example, the mother passes on components of her own immune system to her baby through her milk, which then help to strengthen the baby's own immune system. These are the kind of subtle but useful compounds which are encountered in a "natural" diet but which could never realistically be included in people chow.

the sun - How many sun-like stars are there in the universe?

This is a question that concerns the initial mass function (IMF) - an empirical (that is, defined by observations rather than theory) function that describes the statistical distribution of stellar masses.

Edwin Salpeter (1955) was the first to describe the IMF, though if you read Chabrier (2003) there are some reasonably comprehensive explanations of the theory and history. However, these lecture notes are a fair bit more accessible.

From the approximations in the UCSC lecture notes I linked above, I get that around 4% of stars are between 0.7 and 1.3 solar masses (92% are between 0.1 and 0.7 solar masses!).

There are perhaps 100 billion stars in a galaxy and 100 billion galaxies in the observable universe, giving something on the order of $4times 10^{20}$ (400 billion billion) stars that are about ($pm 30%$) one solar mass.

constellations - Are there any bright non-stellar objects which make up asterisms?

Most of Messier's objects are too dim to be seen by the naked eye. The few that are visible were mostly listed as stars in older lists.

Edmund Halley lists 6 "luminous patches" which "discover themselves only by the telescope, and appear to the naked eye like small fixt stars" (Halley).

These include M42, in Orion, part of the sword (or what ever that is between the hunter's legs), and Omega Centuari, listed as a minor star in the centaur (roughly on the horse's back) in Bayer's list.

(The other objects on Halley's list were either known to be "cloud-like" [M31] or too faint for naked eye astronomy.]

amateur observing - Annual path polar chart for a star

I've often come across sun path charts like this (Wikimedia Commons) that show the position of the sun at any point of the year, at any time of day, for a specific location (or a set of locations), on an azimuth-elevation polar plot:

I'd like to get the exact same chart (or data, to create my own chart) for any given star, such as Aldebaran or Sirius, for a given point on Earth.

I'm looking into Stellarium scripting, as well as the HYG stellar database (I am not familiar with the mathematics of right ascension and declination, or how to convert this to azimuth-elevation like in the example). Is this a very unusual data product to ask for?

bioinformatics - How do I prepare a PDB for submission to the Protein Data Bank?

I have a couple structures that are nearly ready to be deposited in the PDB. Out of curiosity, I ran them through the ADIT's precheck tool and they failed with one error after another, as I lacked all sorts of additional records (TER, SEQRES, HETNAM, etc.) that my refinement and modelling tools don't seem to care about.

From what I understand/guess, the online submission tool will help fill in all the metadata (REMARKs, etc.), but how do I turn my coordinates into something acceptable?

biochemistry - Why should I degas my gel solution for polyacrylamide gels?

The reason for degassing your gels is to remove oxygen. Oxygen in the gel interferes with polymerisation, slowing it down and making it less consistent, so degassing makes it faster and more uniform.

From the EncorBio SDS-PAGE protocol:

Polymerization is quicker and more uniform if you degas the first three solutions for a few minutes in an Ehrlenmeyer flask on a house vacuum prior to addition of the last three reagents. Molecular oxygen inhibits polymerisation by reacting with the free radical SO₄^- ions, which is actually the reason why PAGE gels are poured in tubes or between plates and not in open top horizontal apparatuses, as can be done with agarose. Also it's a good idea to layer some isopropanol on top of the gel as this prevents oxygen getting in and inhibiting polymerisation.

Oxygen can also lead to oxidation of protein products, which might be crucial if you then want to extract the products and use them for something else (e.g. Sun & Anderson, 2004).

Finally, having bubbles in your gel can distort the results and make them less reproducible, as the bubbles will not form consistently with each repetition and they disrupt the physical medium of the polyacrylamide. So another purpose of degassing is to ensure repeatability.

The Bio-Rad acrylamide polymerisation info sheet has the best info I could find:

The formation of polyacrylamide gels proceeds via free radical
polymerization. The reaction is therefore inhibited by any
element or compound that serves as a free radical trap
(Chrambach 1985). Oxygen is such an inhibitor. Oxygen,
present in the air, dissolved in gel solutions, or adsorbed to
the surfaces of plastic, rubber, etc., will inhibit, and in extreme
cases prevent, acrylamide polymerization. Proper degassing
is critical for reproducibility. Therefore, one of the most
important steps in the preparation of polyacrylamide gels is
the evacuation, or “degassing” of gel solutions immediately
prior to pouring the gel. This is done by placing the flask of
gel solution in a vacuum chamber or under a strong aspirator.
In some cases, a vacuum pump may be required.

Buffer stock solutions and monomer stock solutions are usually
stored at 4°C. Cold solutions have a greater capacity for
dissolved oxygen. The process of degassing is faster and
more complete if the gel solution is brought to room
temperature (23–25°C)‚ before degassing begins.
Furthermore, if a cold gel solution is placed under vacuum,
the process of evacuation tends to keep the solution cold.
Pouring a gel with a cold solution will have a substantial
negative effect on the rate of polymerization and on the
quality of the resulting gel.

Polymerization in which riboflavin is used as one of the
initiators calls for degassing. The conversion of riboflavin from
the flavo to the leuco form (the species active in initiation)
actually requires a small amount of oxygen (Gordon 1973).

This explains why polymerization initiated primarily by riboflavin
can be completely blocked by exhaustive degassing. However,
oxygen in excess of that needed to convert riboflavin to the
active form will inhibit polymer chain elongation, as it does in
reactions initiated only by ammonium persulfate and TEMED.
Thus, while degassing is still important for limiting inhibition,
it must not be so extensive that it prevents conversion of
riboflavin to the active form. For polymerization initiated by
riboflavin/TEMED, or riboflavin/TEMED/ammonium persulfate
systems, degassing should not exceed 5 min.

A consequence of the interaction of riboflavin with oxygen is
that riboflavin seems to act as an oxygen scavenger. This is
supported by the observation that the addition of riboflavin
(5 µg/ml) to stacking gel solutions containing ammonium
persulfate/TEMED initiators results in cleaner, more uniform
polymerization at gel surfaces exposed to oxygen (such as
combs). The same effect could likely be achieved by more
thorough degassing of solutions without riboflavin.

Whether using chemical polymerization (ammonium
persulfate/TEMED) or photochemical polymerization
(riboflavin/TEMED or riboflavin/TEMED/ammonium persulfate
initiators), reproducible gel quality and separation
characteristics require careful attention to gel solution
temperature before degassing, and to degassing time,
temperature, and vacuum. These parameters should be
kept constant every time gels are prepared.

Sorry for the long quotes, but they are pasted here in case the original sources disappear.

References:

What is the direction of the movement of the solar system in relation to the galaxy's plane

I think your question is answered by the duplicate mentioned: but here are the relevant highlights.

Humphreys & Larsen (1995) suggest, using star count information, a distance of $20.5 pm 3.5$ pc above the Galactic plane; consistent with, but more precise than the Bahcall paper referred to by Schleis. Joshi (2007) is more guarded, investigating some systematic uncertainties in the estimation techniques and ends up with distances between 13 and 20 pc above the plane.

The Sun moves at about 15-20 km/s with respect to a local standard of rest defined by the general motion of stars in our vicinity around the Galaxy. In three-dimensions, this "peculiar velocity" is $U=10.00 pm 0.36$ km/s (radially inwards), $V=5.25 pm 0.62$ km/s (in the direction of Galactic rotation) and $W=7.17 pm 0.38$ km/s (up and out of the plane). (Dehnen & Binney 1998). Different authors arrive at velocities that differ by $sim 1-2$ km/s from these values and so this would probably be a more conservative estimate of the uncertainties.

The Sun executes oscillations around its mean orbit in the Galaxy, periodically crossing the Galactic plane. I borrowed this illustration (not to scale!) from http://www.visioninconsciousness.org/Science_B08.htm to show this oscillatory motion. The oscillations will not be exactly sinusoidal because the restoring force towards the plane does not very linearly with height above the plane.
As the Sun is currently above the plane and moving upwards, and each cycle takes about 70 million years with an amplitude of 100pc (Matese et al. 1995), it will be roughly 30 million years before we cross the plane again.

EDIT: Actually I'm glad I revisited this question because I think the picture is not very good at all. As the Sun takes ~230 million years to go around the Gaaxy, it should only execute 3 complete vertical oscillations in our Galactic year, whereas the picture implies many more. Secondly, the Sun executes a radial oscillation with a period of around 160 million years, which is not even indicated!

How large could gravitational waves get and what effect could they have on us?

Wave strain scales as $1/r$; the waves that were detected came from a source about a Gpc away and had a strain of $10^{-20}$.

To get a strain that would affect things at the 1% level, you would need to be $10^{18}$ times closer, or at about 30,000 km from the merging black holes.

The tidal force due to the black holes is roughly $GMmh/4r^3$, where $h$ is your height, $m$ your mass and $Msim 60M_{odot}$ the combined mass of the black holes. Assuming 1.8m height and 80kg mass, the tidal force stretching you is around 50N. So I think you would notice this more than the gravitational wave, and even more so if you were to get any closer...

orbit - Is it possible to have a star orbiting around a brown dwarf?

Yes

If a brown dwarf count as a star in this case, the solution is as easy as a smaller brown dwarf orbiting a larger one.

If not, it is still possible if you have two brown dwarfs orbiting close to each other, both just too small to be red dwarfs, together out-massing an orbiting red dwarf, just large enough to be counted as a star.

While I have no example of such a system, the Alpha Centauri system has to closely orbiting stars, with a more distant star orbiting, and Luhman 16 is a system with two brown dwarfs orbiting each other.

lunar eclipse - Does earth's Umbra reach Sun-Earth L2?

The Lagrangian point $L_2$ is very close to the most distant point from Earth with an umbra.
$L_2$ is like the radius of the Hill sphere at $r=asqrt[3]{frac{m}{3M}}$ for circular orbits, with $m$ the mass of Earth, $M$ the mass of the Sun, and $a$ the distance Earth-Sun. The ratio $frac{m}{3M}$ of the Earth and the triple mass of the Sun is almost exactly $10^{-6}$, the cubic root hence $0.01$.

The diameter ratio of Earth and Sun is about $1/109$. Therefore the umbra of Earth ends near $92%$ the distance to $L_2$.

The answer to another bonus question would then be: If Earth would be $9%$ larger in diameter, but with the same mass, its umbra would end almost exactly at $L_2$.

Earth's orbit isn't perfectly circular, but the aphel/perihel ratio of about $1.04$ is insufficient to question the result qualitatively.
The error of the implicite assumptions $tan x=x=sin x$ is negligible at the considered level of accuracy.

Is it safe to watch solar eclipse's reflection in water?

There will be a solar eclipse soon at my area... naturally I want to watch this with my son.

Is it safe for us to watch the solar eclipse's reflection in the water? There is a swimming pool near my house and I plan to watch it there.. is it safe to do so?

I remember vividly doing this with my dad when I was little, but some website says we are not supposed to watch it in a bucket of water..

So is it ok to watch it in the water?

communication - Does any satellite communicate only through laser?

The main reason for reduced received signal with radio communication in space is the spreading loss, that is intensity declines with the square of the distance from source to receiver. This is true even with beamed radio signals.

Laser communications beams suffer from exactly the same loss once the cross-sectional area of the beam exceeds size of the receiving sensor. Which for practical purposes will always be the case for space communications.

The comms. system you choose is more related to bandwidth, absorption losses, immunity to interference, the practicality of the technology, ... but not spreading losses.

For SETI laser communications signals will be more difficult to detect unless they are deliberated attempts to contact others by an alien civilisation own SETI program, as lasers usually have much narrower beam widths than radio transmitters.

staining - Recommended applications for commercial antibodies

Maybe WB is easier for them to test?

In general, however, my recommendation is to check if they provide refs to paper using those antibodies and look at the images on the paper.

Be careful putting your trust in the supplier's pictures, I have seen obviously photoshopped images on commercial websites.
Also always double-check the referenced papers, as sometimes they don't really use the same product that is sold...

Other than that, in my experience, the recommended application is just that: recommended. The antibody may as well work for other application.
That said, note that there is a rationale for an antibody working for WB and not IHC, for instance: perfusion with fixators such as PFA will change the 3D structure of the target protein and may mask (or reveal) certain epitops, so the antibody may be more or less efficient depending what form of the protein you're dealing with (native, fixed in PFA, fixed in glutaraldehyde, SDS-denaturated etc.)