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36

Since we're talking about terminology, we need to remember that none of this really matters, outside of clarity when communicating. Still, some people tend to have rather strong opinions on it, thus confusion about how many planets are really in the solar system arises. The people The most trusted source in Astronomy would have to be the people that set ...


28

In addition to Undo's fine answer, I would like to explain a bit about the motivation behind the definition. When Eris was discovered, it turned out to be really, really similar to Pluto. This posed a bit of a quandary: should Eris be accepted as a new planet? Should it not? If not, then why keep Pluto? Most importantly, this pushed to the foreground the ...


12

You could confirm with taking an image of the star trails. They would form a circle with the apparent center at the zenith of the location. You do not need a pole star at all. Just a night of viewing. You would also be able to tell based on the height above the horizon that the sun is. On the equinox, the sun would be on the horizon at noon (when it is ...


11

I've also heard that people in the past knew about orbits even when they thought that Earth was at the center of the solar system. How did they figure this out in their times with their technology? The same celestial objects (stars, planets, the Moon) could be seen every year. So, people figured out there was a pattern to it. At first, geocentrism was ...


9

Gravity assists such as this are a form of elastic collision. There's a bit of number crunching here (hopefully no mistakes!), so you'll want to be familiar with the basics of momentum, kinetic energy, and the conservation thereof. Question: If Ceres (the largest known asteroid and nearly 500 km in diameter) used Earth to perform a gravity assist to ...


9

When an object is in orbit, there are two factors at play, not just one. The first, as you mention, is the force of gravity pulling the objects together. However, each object also has a momentum component which is generally (in the case of circular orbits) perpendicular to the direction of the gravity. If we look at the common situation of a small-mass ...


9

Assume the planet has a negligible mass compared to the star, that both are spherically symmetric (so Newton's law of gravitation holds, but this normally happens to a very good approximation anyway), and that there aren't any forces besides the gravity between them. If the first condition does not hold, then the acceleration of each is going to be towards ...


9

The answer is Mercury. Plutonium was made after Pluto was discovered.


8

Planets For a body to be classified as a planet it must have a few physical characteristics: Mass It must have enough mass to have a strong enough gravity to overcome electrostatic forces to bring it to a state of hydrostatic equilibrium. Hydrostatic equilibrium is important because early in a planets life it is nearly entirely fluid, crust and all ...


8

As the planets evolve during their protoplanetary stage and accrete materials from the protoplanetary disks, which are gravitationally collapsing interstellar dust and gases, these accreted particles retain some of the angular momentum from the materials they form from and being in constant motion.      ...


8

There is one known pair of moons in the solar system that seemingly swap orbits every couple of years. That's Saturn's Epimetheus and Janus. Their orbits are so close together that they interact gravitationally every couple of years (when the inner moon catches up to the outer moon), so that the outer moon is slowed down, and the inner moon is accelerated. ...


8

This is a very common question, yet very hard to answer if you prefer a clear, concise, uncontroversial answer that applies to all situations. So I'm not going to do that. Instead, I'm going to describe your main options, and let you choose. Be aware that you'll make the choice while still not knowing much about optics. So, in a sense, it will be just the ...


7

There are other formulas at work, but not any other forces. You need to take into account ont only the force, thus the acceleration, but also the current velocity of a body orbiting another. To put it simply: if you move a ball sticked to a rope around your head, the only forces are the tension of the rope and gravity towards the floor. Ignoring gravity, ...


7

It is posible for a planet to have a circular orbit, a circle, after all, is an ellipse where both foci are in the same place; this is known as having an eccentricity of 0. Eccentricity is defined in the following way: $$ e = \frac{r_{a} - r_{p}}{r_{a} + r_{p}} $$ where $r_{a}$ is the apoapsis (farthest point in the orbit from the center of mass), and ...


7

Quick calculation of Roche limit for two identical earthlike (fluid) bodies. Roche distance is about equal to 2.44 X radius X(density1/density2)^1/3. For identical bodies the density conveniently cancels, so we get Rd = 2.44 X radius. Earth radius is 6378 km. So Rd = 2.44 X 6378 = 15500 km (center to center). The distance surface to surface (15500- 6378) is ...


6

There are several potential explanation for the cause of the late heavy bombardment. One of them includes a passing star that disturbed the Oort cloud. A "stellar system collision" would have destroyed the solar system, though. While galaxies are mostly empty and a galactic collision won't involve that many actual collisions (if any), the gravitational ...


6

Answer to the NEW question: the Angular Momentum Conservation Law states that, for any moving body, its angular momentum does not change unless you exercise an external force different from the central force. For an orbiting body like a planet, this means that Sun's gravity, being the central force, does not modify Angular Momentum, but any other external ...


6

This is a low accuracy - yet simple - answer It allows you to calculate only radial alignment configuration of the planets. If you would like an approximation, let's say, you approximate the position of the planets as hands in a clock, you could work the math out by something like this. Assume $\theta_i$ is the initial angle for planet $i$ at time $t_0$ - ...


6

On 16 June 2014 at 11:45pm, it was up not far from 220az 30alt. It was actually 232az, 27alt, almost level and to the right of Spica. It would have been easily seen without binoculars being -0.2 visual magnitude. You can answer this sort of question yourself by getting a planetarium app like Stellarium, The Sky X, Sky Safari, or one of many others.


6

Short answer: 1) Yes and no; 2) Yes, there is a supercritical fluid, of hydrogen. Long answer: It's fairly hot deep down in Jupiter; estimates range from 10,000 K to 24,000 K. You would think that anything in the core would be liquefied, and you could be right. Many models predict that Jupiter's core is rocky, but others predict that it is liquid. Still ...


6

Yes, Pluto is still a dwarf planet. According to the IAU website, it still fits the criteria for a dwarf planet, fails to meet the criteria for a planet, and still carries the "dwarf planet" label, whatever its future status may be. I'm sorry I can't provide a longer or more detailed answer, but this is really a yes-or-no question. I hope this helps.


5

The answers provided to this question so far seem to be good. The easiest way I'd say is to take a long exposure picture in one of two places: 1) At the zenith (directly overhead), if you see that your picture looks like this: you are at either one of the poles. If the center of rotation of the stars is off by some angle (in degrees), you are that same ...


5

TidalWave says it perfectly: "What is the question? So far, all I can think of as an answer is explaining the meaning of scientific theory as an integral part of the scientific method. Of course it is possible it is not correct, science doesn't function on dogmatism. Theories are frequently corrected, expanded, or even completely dismissed. Having a ...


5

The gravitational pull of all the planets and the sun, and the rest of the galaxy and the universe, all play a part, but gravitational effects fall off with distance. For Earth's orbit, the Sun is far and away the single biggest influence. Jupiter perturbs our orbit slightly, but with it or without it we have a simple elliptical orbit round the sun. We can ...


5

There do exist somewhat trivial configurations, which are stable in the long term and which include arbitrarily many bodies. Consider, for example, a set of $N$ circularly moving bodies of the same mass $m$, which obeys the constraint $mN\ll M$, where $M$ is the mass of the star. So long as $mN\ll M$, the bodies move dominantly in the gravitational field of ...


5

The Saturn's Phoebe ring that has an orbital inclination 173° to the ecliptic so it is actually in a retrograde orbit and is tilted 27° to Saturn's inner rings, shows that there clearly isn't any limit to orbital inclination. It is feeding off the Saturn's moon Phoebe (possibly due to micrometeorite impacts) indicating that as long as the planetary ring has ...


5

The correct answer is 8 (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune). Pluto is not longer a planet since 2006 when it was adopted a formal definition of planet


5

There are a few major problems with calling something a small gas planet and of course depends on your definition of a planet. By currently accepted classification (that excludes Pluto), a planet has to be: massive enough to be rounded (i.e. held together) by its own mass, has to orbit a star, and has cleared its orbital neighborhood. All of these ...


5

Though it's probably very unlikely in a solar system as old as ours (~5 billion years), I would not put it out of the realm of possibilities. Most of the collisions of this scale and magnitude have already happened - the early days of the formation of the solar system was violent and full of collisions between protoplanets. In fact, one hypothesis for the ...



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