# Tag Info

37

Neither of those statements are true. It's an easy approximation to make: a neutron star has all of that 'space' removed from between nucleons --- so we just need to know how big a neutron star of mass equal to the solar system would be. Well, the only significant mass is the sun (jupiter is about 1% the mass of the sun---negligible). If the sun were ...

30

The answer kind of depends on how old you are. At a very introductory level, say, maybe middle school or younger, it's "okay" to refer to Jupiter as a failed star to get the idea across that a gas giant planet is sort of similar to a star in composition. But around middle school and above (where "middle school" refers to around 6-8 grade, or age ~12-14), I ...

28

We haven't ironed out all the details about how planets form, but they almost certainly form from a disk of material around a young star. Because the disk lies in a single plane, the planets are broadly in that plane too. But I'm just deferring the question. Why should a disk form around a young star? While the star is forming, there's a lot of gas and dust ...

25

Imagine two donut-shaped spaceships meeting in deep space. Further, suppose that when a passenger in ship A looks out the window, they see ship B rotating clockwise. That means that when a passenger in B looks out the window, they see ship A rotating clockwise as well (hold up your two hands and try it!). From pure kinematics, we can't say "ship A is ...

20

An orbit is stable because of conservation of angular momentum. Suppose we start with an object in an exactly circular orbit and slow it down slightly. That means it is moving at less than orbital velocity so it starts to fall inwards. However as its distance to the Sun decreases the tangential component of its velocity has to increase to conserve angular ...

17

yes, you may describe the motion from any reference frame, including the geocentric one, assuming that you add the appropriate "fictitious" forces (centrifugal, Coriolis, and so on). But the special property of the reference frame associated with the Sun - more precisely, with the barycenter (center of mass) of the Solar System, which is just a solar radius ...

17

When we say that the Moon rotates, we don't mean relative to an observer on Earth, because we're also rotating. Maybe best is to think of it from the perspective of the Sun. If you were at the centre of the solar system, looking at the Earth, you'd see the Moon rotates once every 28 days or so. That also happens to be the amount of time it takes for the Moon ...

16

There are a few things that keep Saturn's rings roughly the way they are. First, Saturn's D ring actually is "raining" down on Saturn currently. But, the phenomenon of shepherd moons prevents the vast majority of material from leaving the other rings: "The gravity of shepherd moons serves to maintain a sharply defined edge to the ring; material that ...

16

There are three main reasons. 1) While Venus is orbiting the Sun at 35.02 Km/s, the Earth is also orbiting the Sun in the same direction at 29.78 Km/s. This factor will decrease the relative transit velocity of Venus as seen from earth. 2) Venus is travelling at 35.02 Km/s an elliptical orbit. Hence the actual distance traveled by Venus during the transit ...

15

The leading theory is that at a distant point in its past, Uranus was struck by a very large object, which knocked it to its side, and current tilt. Imagine if you took a top, and smacked it with a rock. The top might be turning perfectly alright at first, but after it had been hit, the top would most likely be wobbling significantly. Similarly, after an ...

15

This web page has a nice discussion on it: http://archive.ncsa.illinois.edu/Cyberia/NumRel/EinsteinTest.html Basically the orbit's eccentricity would precess around the sun. Classical stellar mechanics (or Newtonian gravity) couldn't account for all of that. It basically had to do with (and forgive my crude wording) the sun dragging the fabric of ...

14

The night sky would certainly be spectacular from within a globular cluster! To get an estimate for the total brightness of the stars, let's say there are 6000 stars visible to the naked eye1. Let's make the globular cluster a big one, like Omega Centauri, with ten million stars2. The roughest approximation would be to say that those ten million stars ...

13

There are a variety of definitions, most of which can be put into the graphic below, grabbed from Wikipedia. Note that the scale is a log scale, so don't think the solar system is quite like is shown. The most commonly accepted is based off of what is known as the Heliosphere. Simply put, the Heliosphere is where the force from the solar wind equals the ...

12

A brief history of the misapplication of magnetohydrodynamics to the analysis of the solar wind: 1959: Soviet satellite Luna 1 directly observed the solar wind for the first time and measured its strength. http://en.wikipedia.org/wiki/Luna_1 So as of 1959, by direct experimental observation, it was known that the heliopause was at least the radius of the ...

12

I'm almost certain there used to be an answer to this question, but it seems to be gone, I'll write another one. The Earth and Sun both orbit their mutual barycenter (disregarding any other objects of course). That one point is a focal point of both ellipses, and all three focal points are collinear. It may appear asymmetric because the Sun's motion is so ...

11

There seems to be a known explanation. I quote from Composition, Structure, Dynamics, and Evolution of Saturn’s Rings, Larry W. Esposito (Annu. Rev. Earth Planet. Sci. 2010.38:383-410): [The] rapid collision rate explains why each ring is a nearly flat disk. Starting with a set of particle orbits on eccentric and mutually inclined orbits (e.g., the ...

11

The earth goes around the sun kind of like a ball on a string goes in a circle when you swing it around. Instead of the string holding the earth, the sun's gravity holds it. As the earth goes around the sun, it also spins. This makes day and night. You can see this with a flashlight on the ball when you spin it with your hand. Part 2: Since gravity is a ...

11

This correlation is known as Titius-Bode's law, which is often stated as $$d=0.4 + 0.3 \cdot 2^n$$ where d represents planet's mean distance from the Sun in Astronomical Units and n = -∞, 0, 1, 2... for Mercury, Venus, Earth, Mars, asteroid belt, Jupiter and so on. The rule is not satisfied exactly with Neptune's orbit (n=7) ...

11

Well, first of all, the entire site dedicated to the 2012 nonsense is a total hoax... I suggest that you check out this site for more information regarding the weakness and outright lies of that hoax. To address the copy/pasted nonsense... The charlatans at the site you reference have taken real terms, and mixed them up in a word salad as to make any lies ...

10

There are plans for encounters with Kuiper Belt Objects (KBOs) after it passes Pluto. (Pluto itself is a KBO.) Quoting the New Horizons Mission Timeline: Plans for an extended mission include one to two encounters of Kuiper Belt Objects, ranging from about 25 to 55 miles (40 to 90 kilometers) in diameter. New Horizons would acquire the same data it ...

10

If you could compress the mass into that small a space, it would collapse into a black hole, at which point the notion of "size" becomes harder to define, with space-time being so warped. The "event horizon" radius would be about 3 km, if I get the formula correctly. The idea of "there's a lot of space in atoms" comes from computations which state that the ...

9

Nic and Approximist's answers hit the main points, but it's worth adding an additional word on the reason the orbits lie roughly in the same plane: Conservation of angular momentum. The Solar System began as a large cloud of stuff, many times larger than its current size. It had some very slight initial angular momentum -- that is, it was, on average, ...

9

Water does not form a liquid at very low pressure. Here is the this phase diagram: (image from http://www.lsbu.ac.uk/water/phase.html and appears to be under the creative commons non-commercial no-derivatives) The lowest pressure at which water is liquid is that of the triple point: 273.15 K and 611.73 Pa. That pressure is about 0.006 atmospheres. ...

9

Coronal Mass Ejection is indeed a stream of particles that moves at velocities significantly lower than the speed of light. The speed of CME varies between 20km/s to 3200km/s with average velocity around 489km/s. Thus, observations of the Sun can notice changes in Solar activity hours or even days before CME hits Earth (85 hours for average speed of ...

9

The rings and moon system would disperse, but everything would remain in orbit around the Sun; nothing in the Saturn system is moving fast enough to break orbit, at least not immediately. Some moons might be gravitationally captured by other moons. The long-term interaction of the former moons could be quite complex, and close interactions just might eject ...

9

The NASA Dawn mission sees a craft headed toward the Asteroid belt as we speak - they intend to do well, navigating to Ceres and Vesta, and more. Our intuitive knowledge of the universe tells that there will be both semblance chaos and order, with bodies having dominating factors, creating mini, seemingly orderly systems which could or would eventually ...

8

Anglular momentum is conserved, so any tiny initial rotation that a the original ball of gas had becomes faster as the gas collapses down into a star and disk of planets. Planets near the sun rotate slowly for the same reason that the moon always faces the same side to the Earth - tidal braking Venus probably received a hit from a some lump of rock / ...

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