# Tag Info

7

There are several ways to look at this. One rather simple way is to compute the Rayleigh number of Mars' mantle. The Rayleigh number is a dimensionless number defined as: $Ra = \dfrac{\rho g \alpha \Delta T d^3}{\eta \kappa}$ where $\rho$ is the mantle density, $g$ is gravitational acceleration, $\alpha$ is the thermal expansion coefficient, $\Delta T$ is ...

2

No orbits are strictly Keplerian, unless you really monkey about with your coordinate systems. However, a precessing orbit is pretty darned Keplerian. The relationship between the various orbital elements can be a little complicated, and precession is traditionally thought of as the rotation of the periapsis of the orbit around the more massive body. If we ...

1

Strictly speaking, pure Keplerian orbits can only occur with two bodies obeying Newton's law of gravitation. If there are more than two bodies then the orbits will be perturbed, to some degree. Kepler orbits are a reasonable first approximation to the orbits of the planets in the Solar System because the Sun is so massive compared to the planets, and the ...

3

The solar system barycenter (SSB) is sometimes inside the Sun and sometimes outside. As an observer outside the solar system could detect with Doppler spectroscopy, the Sun is what's wobbling around. The Sun's offset from the SSB is a vector sum of roughly: 0.00496 au ±5% away from Jupiter 0.00272 au ±6% away from Saturn 0.00083 au ±5% away from Uranus 0....

1

You can differentiate between planet (Did you mean asteroid? Planets are quite bright.) and noise quite easily. Just take three pictures of the sky and they will be probably quite noisy. If some spot changes its position on all three pictures, then this is some near object (astronomically near). You can additionally verify it if its trajectory is line or not....

6

The motions of the Sun, the planets and their moons and everything else in the solar system are well described by Newton's laws of motion and gravity (with some minor relativistic corrections needed to fully account e.g. for the perihelion precession of Mercury). These laws make absolutely no reference to a "barycenter" in any form, so the whole ...

15

The Sun's movement in the Solar System can be thought of as its movement around all the individual pairwise barycenters at once, or as a movement around the Solar System barycenter, which itself is constantly moving. Suppose Mercury was the only planet. The mutual barycenter of Mercury and the Sun is about 10km from the center of the Sun, which is inside the ...

29

The short answer is no; there is only one barycenter. Yes, you can count the Sun/Jupiter barycenter or the Sun/Saturn barycenter, or whichever barycenter you want, but the net effect of all Solar System bodies is to be considered when you calculate the actual barycenter of the Solar System. (And yes, that would include counting all the small asteroids and ...

0

Planetary orbits are normally described as heliocentric, but it is possible to describe them from a barycentric point of view. JPL Horizons (https://ssd.jpl.nasa.gov/horizons.cgi) provides for both possibilities.

2

According to JPL HORIZONS, the sub-solar latitude on Uranus has maxima on 1946-04-03 and 2030-04-11, and Uranus's heliocentric J2000 ecliptic longitude at those times is 77.54°. If I put the pole coordinates from the IAU WGCCRE report through the NED coordinate calculator, I get a J2000 ecliptic longitude of 77.65° for Uranus's south pole. The solstice ...

3

I have made an interactive simulation of Uranus and its seasons at https://ecliptiqc.ca/UranusEN.php. Playing with it, I get the north pole of Uranus pointing sunwards in early 1946 (my steps are 6 months, so I can’t be more precise for now). The actual date may be slightly different, as I use a “fixed” orbit instead of the osculating elements, but it would ...

1

tl;dr: Yes, but they must be small. The reason why is because large bodies, like major moons, break apart at a certain limit from their hosts, using the formula for the Roche Limit: $$d=r\Big{(}2\dfrac{M}{m}\Big{)}^\frac{1}{3}$$ $d$ is the Roche Limit, $r$ is the radius of the satellite, and $M$ and $m$ are the masses of the host and satellite objects, ...

3

You've answered you own question. If there are cohesive forces beyond that of simple self-gravitation, then objects can survive intact inside the self-gravitating Roche limit - as does every solid item on the surface of the Earth for example. Saturn's rings are made of ice, not rocks. The tensile strength of ice is about $10^6$ N m$^{-2}$. For a self-...

5

Does the earth spiral around the sun's path as it is shown in the video (exact time is 19:49) and screenshots? The Earth does spiral around the Sun's path, but not quite as it's shown in the video. It isn't clear if the video accounts for the 60 degree tilt of the Solar System with respect to the galactic plane. The sizes and distances are not to scale. ...

1

Is the helical or spiral model shown in the video real or just a theory. It's sort-of real but not necessarily the way it's shown. The Earth orbits the Sun in roughly a circle, and the solar system is moving relative to the center of the galaxy in roughly a straight line (on the time scale of thousands of years and more) but those squiggles shown ...

1

If (and it's a big if) your sample is unbiased, that is, random, and if the population sampled is large, then the percentage sampled is completely irrelevant - only the absolute number of samples matters. 100 drawn out of 1 000 000 is almost exactly the same statistically as 100 drawn out of 100 000 000 - or 100 drawn out of infinity for that matter! How ...

6

One way of estimating this is to consider the fraction of matter that is solid, and then make use of estimates of its density. The cosmic energy inventory includes a fraction 0.00036 ± 0.00008 in the form of white dwarf stars, 0.00005 ± 0.00002 as neutron stars, $10^{-6}$ as planets, $10^{-5.6}$ as other condensed matter. The rest is dark matter, gas, plasma ...

7

Any non-star is negligible, their volume is practically zero. Most matter in the Universe is gravitationally bound plasma, resulting that they do not have a well-specifiable volume, because their density continuously changes from the interstellar vacuum to many times of lead. But choosing a surface at any arbitrary point (for example, at 1 bar pressure) does ...

41

There are a number of methods of detecting exoplanets, but all of them favour detection of larger planets over smaller ones, albeit for slightly different definitions of large: Radial velocity measurement — this detects the small movement of the star towards and away from us as the planet and the star orbit their mutual barycenter. This movement is fastest ...

4

I've noted before that the IAU naming critera are guidelines rather than laws of nature. If applied to Jupiter's moons the four Galilean satellites would probably be "planets" (they don't share their orbits with anything else of comparable size and are large enought to be in hydrostatic equilibrium) There is a mean-motion resonance 4:2:1 between ...

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