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Pluto was downgraded from planet to dwarf planet because the definition of a planet was changed.


Pluto was discovered by the astronomer Clyde Tombaugh in February 1930. It was given the status of the ninth planet of the solar system.

As telescopes, particularly in on satellites, improved, more objects were discovered which caused a problem that they were quite small and some astronomers didn't think they qualified as being planets.

The International Astronomical Union (IAU) had a vote which was very close. They defined three criteria which a planet must satisfy.

  1. It must be large enough for gravity to overcome structures of materials and make it spherical. Most bodies are flattened spheroids due to rotation.
  2. It must orbit the Sun.
  3. It must have cleared its orbit of other bodies other than moons.

The IAU created a new definition of an object called a dwarf planet which only satisfies the first two criteria. Pluto fails the third criterion, so it was demoted to a dwarf planet.

Many people, including myself, still consider Pluto to be the ninth planet.

To be pedantic, Jupiter has a lot of asteroids in its orbit at its two Lagrange points. They are called trojan asteroids. So, this means that Jupiter fails the IAU's third criterion and should be a dwarf planet, which it is certainly not!



The relative strengths of the four funndamental forces is very dependent on the distances involved.


The four fundamental forces are very different and each has an important role to play. In fact two of the fundamental forces are actually not forces.

Gravity behaves like a force as Newton described. In fact it is not a force. Mass, energy and momentum cause four dimensional spacetime to curve. Curved spacetime tells matter how to move. At the scale of large objects gravity rules supreme over the other forces. At the scale of an atom, the energies are so small that gravity has a negligible effect.

The electromagnetic force describes the interactions of charged particles. It also describes light. Stars and other massive objects output vast amounts of electromagnetic radiation. At the scale of the atom, electromagnetism rules supreme over the other forces. It describes how electrons move and the chemistry of elements.

When it comes to the atomic nucleus it is a battle ground between the electromagnetic and strong and weak nuclear forces.

Actually neither the strong or weak nuclear forces are true forces. The weak force is not a force in the usual sense. The strong force is actually a residual effect of the colour force which binds quarks into protons and neutrons which extends out of the protons and neutrons to bind them together.

At the scale of the atomic nucleus, the strong force has to be the strongest. The electromagnetic force is long ranged. Every proton in a nucleus repels every other proton. The strong force is able to overcome the electromagnetic force at short range. It can strongly bind adjacent protons and neutron and hold a nucleus together. All nuclei with more than 82 protons are unstable as the short range strong force loses to the long range electromagnetic force.

The weak force acts as a kind of peacekeeper between the strong and electromagnetic forces. It isn't really a force. It is also very slow acting as it is mediated by the heavy W bosons. A nucleus can be unstable because it has either too many neutrons or too many protons. The weak force corrects this by converting a proton into a neutron or vice versa. This is beta radiation.



They correctly asserted the sun was at the center of the universe, not the earth.


The above mentioned scientists were some of the greatest, if not the greatest minds of their eras. Their influence is still felt today, especially the fact that they were the astronomers who finally convinced everyone that the sun was the center of the solar system.

Now, before I get into their theories, I have to say something that is almost always overlooked per the scientific revolution. Most of us are familiar with the idea that is always pushed that the church was and is against science because at the time the church excommunicated or even imprisoned people who argued that the earth was not the center of the universe. This fact is often misconstrued to the point that the church hates science. It is actually quite the opposite.

Ever since the ancient Greeks, up until Copernicus, most every one believed the earth was at the center of the universe. It made perfect sense to them for many reasons. Namely, Ptolemy's GENIUS and I can't emphasize this enough, GENIUS, although not correct, geocentric theory based on epicycles.

Until Galileo and Copernicus there was no proof for a heliocentric universe. On the other hand, Ptolomy had already come up with a genius theory that the planets rotated around the earth, but also rotated around their orbit. This solved the main issue with geocentricity at the time, the retrograde motion of planets across our sky.

The first real proof came from Galileo, who, with his revolutionary telescope, discovered both Venus and Jupiter's moons experience phases like our moon, which is an observation that no geocentric model can explain.

Finally, Kepler's laws of planetary motion in conjunction with his belief that the universe was heliocentric produced theoretical planetary position predictions that matched the observations perfectly. Newton explained his laws with his idea gravity, and the rest is history!



A solar eclipse can only happen at a new moon and a lunar eclipse can only happen at a full moon.


A solar eclipse can only happen at a new moon when the Sun, Moon and Earth are aligned so that the shadow of the Moon falls on the Earth's surface.

A lunar eclipse can only happen at a full moon when the Sun, Earth and Moon are aligned so that the shadow of the Earth falls on the Moon surface.


  1. Every year there are at least 2 of each type of eclipse.
  2. There can be up to 5 of each type of eclipse in a year, but the total number of eclipses is at most 7.
  3. The Moon's orbit is inclined at #5^@# to that of the Earth. So an eclipse can only occur when the Moon is near one of its nodes where the two orbits intersect.
  4. All eclipses of both types can be predicted with great accuracy.


  1. Solar eclipses happen at new moon. Lunar eclipses happen at full moon.
  2. Solar eclipses are only visible from a narrow strip of land over which the Moon shadow passes. Lunar eclipses are visible from the whole nighttime hemisphere.
  3. Solar eclipses can be total, annular, hybrid or partial. Lunar eclipses can be total, partial and penumbral.
  4. Solar eclipses can only be safely be viewed with the naked eye during totality. Lunar eclipses can always be safely viewed with the naked eye.
  5. The Moon's surface can't be seen during a solar eclipse, but it can be during a lunar eclipse.


Short answer? We have absolutely no idea, and galaxies spin (way) too fast for their visible matter to hold them together.


We might be better dealing with these the other way round - firstly it was noticed, shortly after we discovered that the many ‘clouds’ (nebulae) we noticed in the night sky were actually galaxies, that they were spinning. This was discovered by using the Doppler effect on spectroscopic images of galaxies, which showed one side of a galaxy approaching us and the opposite side receding.

So far, so happy, they spin. Then Fritz Zwicky, whilst examining the Coma cluster of galaxies in 1933, spotted that the galaxies were spinning too fast for the visible matter to generate sufficient force to hold them together. This was confirmed in the 1970’s by Vera Rubin Cooper, specifically, the outer rim of the galaxies would be expected to rotate much slower than the centre according to Newtonian mechanics. This was not observed, the rotational curves were almost “flat” in the outer reaches of galaxies.

![] enter image source here

This allowed for two possibilities: (a) Newton’s theory of universal gravitation was wrong (b) there existed vast quantities of ‘dark’ matter. Nobody seriously believed (a) so we were left with the hypothesis that a dark matter “halo” surrounded each galaxy.

Now for the second part (your first) we have searched for the particles that might make up dark matter very hard (a Nobel prize almost certainly awaits the discoverers) but despite many year stars of searching (both in space and in particle physics labs) nothing has been found that could possible fit the bill.

Neutrinos of various sorts have been proposed, as have exotic matter, stable quarks groupings, dead stars, black holes etc etc. Martin Rees (Astronomer Royal at the time) even went so far as to suggest that it could be unread copies of the Astrophysical Journal! We genuinely have no idea and it bugs us. A lot.

It appears that this stuff outweighs visible matter by a factor of about 5:1 across the universe and you have as much idea what it is as they do. In my view this makes it an excellent time to be studying either cosmology or particle physics, because I hope major discoveries are just around the corner.



We are made of star dust. See below how that's true:


To understand where everything we see on Earth comes from, we first have to understand what it's all made of. The answer that is common to all of it (the atmosphere, hydrosphere, biosphere, and the material that is the Earth) is atoms (and so structurally we've gone more basic than chemicals and molecules).

Some of the more common atoms we have on and around Earth are Oxygen, Hydrogen (these are the constituent parts to water), Carbon (this is the basic building block upon which all life is made - you may have heard the term "carbon-based life form"), Nitrogen (this is what roughly 78% of the atmosphere is made of), Iron (this is what our blood uses to carry oxygen), and a whole host of other elements. In fact, there are 92 elements that can be found in, on, or around Earth (all the elements on the periodic table, up to and including Uranium).

And so the question is - where did all these different types of atoms come from?

To get to that answer, we have to start at the absolute beginning of time - just after the Big Bang. This was a time when there were no atoms at all - they were too big and the early Universe too energetic to exist - the bits and pieces that make up atoms would smash together and then be ripped apart. However, as the Universe expanded and it became less energetic, those bits and pieces started to come together and they formed the most basic atom there is - Hydrogen. One proton, one electron.

As the Universe expanded even more and became even less energetic, the Hydrogen began to form bonds and formed #H_2# - a stable Hydrogen molecule. And this is what the Universe was filled with for a very long time.

But over time, as gravity pulled and nudged the Hydrogen into larger and larger groupings, as the temperature rose to many thousands of degrees within these vast groupings of Hydrogen, they would start a process called Nuclear Fusion - in essence, the temperature within the star caused the Hydrogen to fuse together, creating Helium, the second element on the periodic table, and also cause energy to be released, keeping the star very very hot.

Stars were born and as they continued to smash atoms together, as they continued to fuse, more and more elements were created, up to the element Iron, number 26 on the periodic table. But some stars weren't done yet.

Huge stars go through a process of nova - essentially exploding. Some stars go through an even more massive explosion called a supernova. It's in these explosions where the 26 elements that were created in the belly of a living star are smashed together one last time, creating all the 92 naturally-occurring elements we talked about earlier.

The various atoms combine to form molecules, chemicals, and all the stuff that make up the air, the water, the solid Earth, and the life on, in, and around it.

It's sometimes said that we are made of star dust - and this is why.


Question #77d89

Mark C.
Mark C.
Featured 7 months ago


I think we can add a little detail here.


The Sun is an unconfined nuclear fusion reactor, converting some 600 million tonnes of hydrogen into helium each second. As a result of it not being confined like a reactor would be on earth (except for the effects of gravity) huge quantities of both charged and uncharged particles are emitted along with all the electromagnetic radiation. These particles form the “solar wind” and are ejected into space at very high speeds, even when the sun is quiescent (not particularly active.)

Now we need to understand a little bit about the Earth. Because of our active core, we have a powerful magnetic field around the Earth. This interacts with the charged particles (the uncharged ones are not deflected) causing them to swirl around and get pulled in towards the poles where our field strength is highest.

When these very fast, massive (in the Physics sense, having mass) charged particles collide with atoms/molecules in our atmosphere they cause ionisation (one or more electrons are ejected.) As these ions reconnect with electrons, the excess energy of the electrons is emitted as light. Because of quantum theory, we know the energy is not emitted randomly, but in fixed steps (like an uneven ladder) which means specific colours (or frequencies) of light are emitted, hence we see particular greens, yellows, reds and magenta colours in the night sky.

The colours are not easy to explain as you need to understand the difference between an excited atom (the electrons are lifted one or more “rungs” on the ladder) and an ionised one (where the electrons are lifted above the “top of the ladder”) and also that one incoming charged particle can cause multiple effects (excitations or ionisations) on atoms as it descends through the atmosphere.

Here’s a diagram that shows the main colours we see:

enter image source here

and one showing the effect of altitude:

enter image source here

Both taken from:

Before we leave colours, bear in mind that they eye is (much) more sensitive to green/yellow than other colours, so we see these preferentially.

Finally, here’s a diagram showing the whole process that is hopefully more than eye-candy:


Question #57f3c

Mark C.
Mark C.
Featured 6 months ago


OK, we need to understand a bit of quantum theory and the standard model.


The universe is best understood by reducing everything to the smallest possible number of entities (the goal of physics.) The “standard model” is our current best attempt and is based on quantum theory (more specifically, quantum electrodynamics or Q.E.D or it’s rather more bohemian cousin quantum chromodynamics, Q.C.D.)

The standard model is much like a family tree the way I teach it ... and the first split down from “everything” is into two groups called ‘fermions’ and ‘bosons’. Fermions include all matter. That means solids, liquids, gases, ceramics, polymers, you, me etc. Every single particle we know of including electrons (and there are hundreds) is a fermion. They all have one fundamental thing in common - they obey something called the “Pauli exclusion principle.”

The other group, the bosons, concerns us less in the context of your question, but includes all the known forces (called strong, weak, electromagnetic and gravity, though this last one is troublesome.)

All the known particles can be described as having a set of “quantum numbers” that determine all the properties that exist in quantum theory. Effectively this states that energy, momentum, spin and possibly even space and time exist as quantised states, meaning they cannot take any value, but only discrete ones (i.e. they are like an uneven ladder where you have to be on one rung or another, you cannot exist in between, so we would say your height is then “quantised”). The opposite would be something like an escalator, where your height could vary continuously.

The Pauli exclusion principle simply states that any fermion (particle to us, including the electrons in your question) cannot exist in the same state (complete set of quantum numbers) as another fermion. In other words (rather loosely stated, but it helps) they can be in the same place, but not at the same time, or they can be in the same time, but not at the same energy etc. It is forbidden for two or more to occupy the same complete set.

This explains why two electrons can be in the same orbit (same energy, possibly even the same position at the same time) but then cannot have the same spin. One must be spin “up” and the other spin “down” to prevent violation of this fundamental principle.

Finally, no you don’t get anything like a black hole, it is not even dangerous, just forbidden by the laws of physics.



They are: Gravity, electromagnetism, strong and weak. More follows...


We are familiar with gravity, which acts on the property of a body known as mass. It is a force of essentially infinite range, meaning masses at very great distances will still interact. It's strength varies inversely as the square of the distance between the masses, and it is always attractive. Therefore, all masses attract all other masses.

(Note: I am describing gravity as it was known to Newton. This is not a general relativity treatment.)

Electromagnetism is similar to gravity is some ways. It also varies as the inverse square of distance, although the property that it acts on is charge. It can be either attractive or repulsive, and so, its effects will often be cancelled out, even for large collections of charge. E-M force, like gravity, also has infinite range.

In modern theories, this force is described as a gauge force, meaning there is a particle (called a photon) that "mediates" the force - the charges actually exchange photons as the basis of how the interaction works.

In your question, you ask how these forces interact, but in fact they do not! Gravity and electromagnetism are distinct and separate, although, like any forces, one can be applied to oppose or support the effects of the other.

The same is true of the two nuclear forces, They are both gauge field forces. However, the range of these forces is very short. Hence, they are limited in their effects to the nucleus of the atom.

The exchange of particles that occurs in these forces bring about changes in the nature of the particles on which they act - quarks in the case of the strong force, and quarks and leptons in the case of the weak force.

The weak force utilizes massive particles known as W and Z bosons to cause quarks to change their "flavour" meaning the "up" "down", "strange", etc characteristic. This is able to cause an up quark to become a down quark (or vice versa, depending on the W involved), meaning that at the same time, a proton becomes a neutron, and the affected atom changes into a different element.

The strong force holds quarks and nucleons together, with a mediating particle called a gluon. It acts on the "colour" of the quark.

That's probably enough info for now. You should draw your own conclusions as to whether the above connects these forces together in any manner. The gauge field nature comes to mind as one example of this.