Hot answers tagged

48

It’s because they are much smaller than Io. Tidal forces are differential forces, that is, they result from the difference in gravitational pull on one side of a body compared to the other. When an object is small, the difference in distance to the two sides of it is necessarily small as well. According to Wikipedia, Amalthea, the largest of those four ...


40

No. Besides the 13 Jupiter-masses required to ignite deuterium burning, and make Jupiter into a Brown Dwarf, there is a clear difference between the formation pathways of Brown Dwarves and Gas Giants. Gas Giants are planets, that form via processes in their parent protoplanetary disc. Contrasting this, Brown Dwarves form via direct fragmentaion of the ...


27

In an isothermal atmosphere, the exponential scale height of the atmosphere is $$ h \sim \frac{k_\mathrm B T}{\mu g},$$ where $g$ is the gravitational field, $\mu$ is the mean mass of a particle and $T$ is the temperature (in kelvin). i.e. The pressure/density of the atmosphere falls exponentially, with an e-folding height given by the above expression. I ...


22

Yes, you can see it (I have seen Io's shadow on Jupiter and we were happy it was visitor night so that we could share the view with guests) A 50cm mirror and 125x magnification allows you to see it when the air is not too disturbed and when you know where and when to look. Likely a somewhat smaller telescope will do, too, as light sensitivity is not too ...


17

Congratulations on your purchase. The first pictures dont' show anything much. Just a out-of-focus blur. The last one shows Jupiter and three of its moons. I've overlaid the image onto a simulated image from stellarium (at about 10pm BST, the moons move pretty quick so you need an exact time!): You can clearly see which moon is which, and why Europa is ...


17

There are four moons that are closer to Jupiter than Io with higher eccentricities, yet they don't seem to have any volcanism at their surface. Only one of those innermost moons (Thebe) has an eccentricity higher than that of Io. The other three have lower eccentricities. The reason they don't exhibit volcanism is because they are too small. The largest of ...


16

This has been an area of controversy for quite some time now, with no clear resolution (if you'll pardon the pun). There's been interest in supporting or refuting claims that observers were able to see the moons with the naked eye before Galileo. There are, of course, two major impediments (which you might well know about): brightness and angular separation ...


15

Short answer: no It all of course depends on how you define the term failed star. In general, a star should be able to generate heat by fusing atoms together, and it requires about 13 times the mass of Jupiter for conditions to be adequate for sustained deuterium fusion, and about 63 times the mass of Jupiter for fusion of lithium to take place. All other ...


13

You can make a ballpark estimate of Jupiter's synodic period if you know that Jupiter orbits the sun once every twelve years, while Earth orbits in the same direction in one year. So when your Earth calendar has completed a year, Jupiter has moved approximately one-twelfth of the way around its orbit, and Earth has to move approximately another month to ...


12

Yes, the shadows of the Galilean moons are easily seen in small telescopes. The technical name is "shadow transit". I saw one last summer (2019) in a 3" (76mm) refractor. My notes from the time say that it was much easier to see than the great red spot. I recall that I was not even looking out for this event, but it was unmissable.


11

Not in any way, no. The December solstice is the moment when the Sun reaches its southernmost point in its daily path in the sky (the June solstice, when the Sun reaches its northernmost point). It only depends on the tilt of the Earth on its orbit and the Sun. On the other hand, Jupiter and Saturn being in conjunction is a phenomenon that doesn’t depend at ...


9

You can simply startup Stellarium and have a look yourself. Choose any locations you are interested in. The Galilean satellites of Jupiter are inside the default list of locations. The attached image shows Jupiter as viewed from Io at the given time, Europa is the bright object to the right; with -9 mag it is considerably brighter than Venus when viewed ...


8

Consider the dimensionless parameter $k$ for some planet, given by $$k \equiv \frac{\Omega^2 a^3}{G M}$$ where $\Omega$ is the planet's equatorial sidereal rotation rate, $a$ is the planet's equatorial radius, $G$ is the universal gravitational constant, and $M$ is the planet's mass. The values for this dimensionless parameter are 0.003461 for the Earth, ...


7

The midpoint of an outer planet apparition is opposition, when it is directly opposite the Sun as seen from Earth. Jupiter is at opposition on 2019-06-11, 2020-07-14, 2021-08-20, and 2022-09-26. The average time between oppositions or solar conjunctions of another planet is its synodic period. Jupiter's synodic period is $$\mathrm{\frac{1}{\left|\frac{1}{365....


7

The Jupiter Trojans are not at the Sun-Jupiter L4 and L5 Lagrange points. They are instead in pseudo orbits about those points. These pseudo orbits appear from the perspective of a frame of reference that rotates at Jupiter's orbital rate (i.e., a frame in which the Sun and Jupiter are more or less fixed). Unlike central body motion, which results in planar ...


6

tl;dr The inner moons are much to small (and too stiff) to experience significant tidal work. They also cool down much faster though I don't think this is relevant in this case. There're two things to look at, here: The total amount of tidal work that is done on a moon The amount of energy that is radiated away over time When looking at the list of Jupiter'...


5

The initial impacts are brighter. The team that observed the shoemaker levy impacts described them as "fireballs" and the initial pictures showed spots glowing bright after impact, though I'm not sure how much those images used the infra-red range, so, grain of salt, because I think you're asking about the visible spectrum. Jupiter's bands are ...


5

This can easily be tested using software such as Stellarium, where you can visualize the field of view with given focal length. If you have the software installed, click on "ocular view" (the most left button in the upper right corner). The following view is what you culd expect on December 21 with a 10mm eyepiece and a focal length of 1000mm: And ...


5

Telescopes magnify, they don't bring you closer. So if from Earth Jupiter has an apparent radius of 0.01 degrees (measured as an angle because it is the apparent size) And if Saturn has an apparent angle of 0.005 degrees, then if you magnify 100x then Jupiter will have an apparent size of 1 degree, and Saturn would have a size of 0.5 degrees. Magnification ...


4

With any given star, Jupiter has 3 or 4 triple conjunctions per century, with either 12 or 71 years in between. These occur with Regulus in 1873, 1885, 1956, 1968, 2039, 2051, 2063, 2134, and 2146. Jupiter takes 11.9 years to orbit the Sun, advancing at a fairly steady ~30° per year. From Earth, Jupiter appears to move ~40° forward and ~10° ...


4

Let's first quickly estimate whether the numbers by HBO's TV series make sense: According to gesundheit.ch/strahlung the liquidiators were exposed to $2 \ldots 20 {\rm Sv}$ which compares to a life dose of $\approx 100 {\rm mSv}$ for usual people. I remember from my education that each worker was supposed to spent only ${\rm 40s}$ on the roof of the reactor. ...


4

Any time an image from telescope or even a camera lens with a central obstruction is out of focus, it will show "astronomical donuts". That's just the nature of the beast. It's sometimes a challenge to convince a phone to focus on an image through a telescope. One option is to use a camera application that runs on your phone and makes it work more ...


4

The rate of shrinkage is decreasing. The reason is that the core of Jupiter is becoming increasingly electron degenerate. Electron degeneracy pressure is almost independent of temperature. Once degeneracy sets in, an object can continue to cool with almost no change in its interior pressure, and hence no radius change. A cosmic example is a white dwarf. This ...


4

Direct viewing through an eyepiece When looking through an eyepiece, there is an apparent field of view. This answer says: Because I was lazy, I used the default telescopes and eyepieces. which means that the circles shown are what one would see if one had an eyepiece with a field of view equal to the default eyepiece used in the Stellarium simulation. If ...


4

Our eyes are not good enough to see the difference; Jupiter has an angular diameter of 29.8" to 50.1", while Saturn's is 14.5" to 20.1"; with rings, which are about 2.25 times as wide as the planet, this becomes 32.6" to 45.2". All well within the 60" (1 arcminute) angular resolution of the human eye. Now, when you use a ...


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 ...


4

we tried to answer to this question in this article: https://arxiv.org/abs/2001.01106 According to the current estimation of the tidal dissipation in the Jovian system, we expect that Callisto will be captured into resonance in about 1.5 billions of years, forming 1:2:4:8 resonant chain with the other Galilean moons.


4

Solar energy - from our Sun at Jupiter's focal distance? A negligible amount. Gravitational lensing doesn't have a focal point so much as a focal region that begins roughly at a point, and that point (which really shouldn't be called a focal point) can be calculated, ProfRob gives the formula here. Jupiter's gravitational lensing distance is so far (...


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