Hot answers tagged

151

This picture (source) should pretty much answer your question: The train's destination is not above the ground, but rather far away, and perspective means that the tracks appear not to be parallel but instead to converge to the vanishing point. The same applies to the beams of light above them. The Sun is very far away and the beams are pretty much ...


83

The bottleneck in Solar fusion is getting two hydrogen nuclei, i.e. two protons, to fuse together. Protons collide all the time in the Sun's core, but there is no bound state of two protons because there aren't any neutrons to hold them together. Protons can only fuse if one of them undergoes beta plus decay to become a neutron at the moment of the ...


79

Human perception is generally logarithmic. For example, the perceived loudness of a sound is measured using decibels, where an decrease of $10 \text{ dB}$ divides the sound intensity by $10$. So if the eclipse were heard instead of seen, "90% coverage" might mean reducing the intensity from $120 \text{ dB}$ to $110 \text{ dB}$, a small change. Perceived ...


75

This is an answer that I made, as suggested by John Rennie, by cutting and pasting his answer and dmckee's and adding a little more material. There are four factors involved: Velocity distribution of the nuclei Small geometrical cross-section for head-on collisions of nuclei Quantum-mechanical tunneling probability For the p-p reaction, a weak-force ...


61

The symmetry of the Sun has got very little to do with any symmetry in its formation. The Sun has had plenty of time to reach an equilibrium between its self gravity and its internal pressure gradient. Any departure from symmetry would imply a difference in pressure in regions at a similar radius but different polar or azimuthal angles. The resultant ...


57

Yes. For example, on October 8th 1970 Earth was in the Sun's radiofrequency shadow with respect to quasar 3C 279. In other words, quasar 3C 279 was occluded by the sun. Observation from just before and after the occulation permitted measurement of the bending of radiowaves as a test of general relativity. The sun would also block other frequencies of ...


48

Clean dry air lets sunlight through; dirty moist air scatters it. Aerosols (small air borne particulate contamination) are more prominent near areas of dense population - due to power plants, cars, fires, ... These particles form nucleation sites for moisture - and these small water drops become very effective scatterers of sunlight. The humidity is high ...


42

The graph looks exponential because the vertical axis is logarithmic! If you were to re-plot it as linear lumens per square meter, it would be much more v-like, or even u-like. It so happens that a logarithmic plot matches our subjective perception of light intensity better than a linear one would. That's a result of our eyes having evolved to work well in ...


41

The sun does rotate. We can see the rotation of the sun by the doppler shift of the light we get from the sun. . (Image from this page.) Since we know the characteristic spectrum of light from a hot body of a given temperature, we can use the same effect to determine if other stars rotate as well. Note that this only gives the spread in velocities along ...


41

I refer you to the picture below, taken from Ciocca & Wang (2013). This clearly shows that the spectrum of the moon (normalised to have a similar overall strength as sunlight) is redder than sunlight and so has a lower "colour temperature". This is a fact, not a perception. EDIT: Just to clear up some confusion - the OP talks about "yellower" because ...


40

Briefly: Because the moon's orbit "wobbles" up and down, so it isn't always in the plane of the earth's orbit around the sun. There's a 2D plane you can form from the ellipse of the earth's orbit and the sun. This plane is known as the ecliptic. The moon's orbit is not exactly in the ecliptic at all times; see this (slightly overcomplicated) picture from ...


40

This is from the Physics FAQ article that I wrote 15 years ago: If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high ...


40

Describing the sun as an average star is probably more of a reaction against the idea that there is something unique about it. Obviously there is for us, since it is the star that we happen to be in orbit around, and much closer to than any other star, and hence historically the sun has been considered rather unique. But over the centuries we've discovered ...


36

No, the main reason the poles are colder is because the surface is angled with respect to the sun rays. That is the same reason why in winter it is colder than in summer. You can think that in the poles the winters are harder and the summers are softer, while in the equator it is the other way around. That is also the reason why some solar panel are ...


35

You probably know that the mass of the Higgs boson is around $125$ GeV, which means the energy it takes to create a Higgs boson is around $125$ GeV and therefore that the temperature at which significant numbers of Higgs bosons will be created will be given by $kT = 125$ GeV. One GeV is $1.602 \times 10^{-10}$J, so the corresponding temperature is around $10^...


34

The Sun isn't "made of fire". It's made mostly of hydrogen and helium. Its heat and light come from nuclear fusion, a very different process that doesn't require oxygen. Ordinary fire is a chemical reaction; fusion merges hydrogen nuclei into helium, and produces much more energy. (Other nuclear reactions are possible.) As for rockets, they carry both ...


33

This is very rough and based on eyeballing without careful measurements: I've got a four-watt nightlight. I can read by it (not comfortably) at a distance of about a meter. The sphere of radius 1 meter has a surface area of about 12 square meters, so it appears that 1/3 of a watt per square meter will (barely) suffice for reading. The earth gets about ...


32

Yes, the sun and nearly all other stars do rotate. One can see the rotation of the sun by looking at the motion of sunspots on its surface. Over time, the sunspots will move across the sun's surface - proof of its rotation. Furthermore, the rate of the sun's rotation is not constant throughout the sun; it is higher near the equator and slower near the poles....


31

In addition to the excellent answers already given, here is another famous picture of an anticrepuscular ray (or better: shadow). It is not a coincidence that the shadow of the plume extends almost exactly towards the full moon. It is a consequence of the Sun, Earth and full Moon being almost aligned. So all shadows must point towards the moon at such a ...


30

The conditions at the core of the Sun are very different from those in a thermonuclear bomb. The first thermonuclear bomb used deuterium as the secondary. The Sun has to create deuterium before getting to this stage. It's the creation of deuterium that's the bottleneck in the fusion that occurs inside the Sun. Later bombs used lithium deuteride, which is ...


30

Whether the sun "rises" in the east depends on your position on earth, and the time of the year. In northern latitudes, during the summer, the sun rises significantly North of East, and in the winter it rises in the South. For example, today's sunrise/sunset directions in Umeå Sweden, look like this (source: www.suncalc.net) The yellow line shows the ...


27

The premise that the sun has the same conditions all throughout is incorrect. For the most part the conditions (Temperature and Pressure) necessary for nuclear fusion to occur are only found within a small region in the core. For example, when hydrogen fusion occurs and creates helium, since that helium is heavier it tend to coalesce as the core. In ...


24

A quick look at the Del Sol How it Works page shows their explanation, HOW DOES IT WORK? The Spectrachrome® crystal reveals color upon irradiation by ultraviolet waves; i.e., sunlight. When a flower blooms, the result is the exposure of the inherent color of the flower. A Spectrachrome® crystal is similar in that an energy-shift occurs causing the ...


23

It just happens to be a coincidence. The current popular theory for how the Moon formed was a glancing impact on the Earth, late in the planet buiding process, by a Mars sized object. This caused the break up of the impactor and debris from both the impactor and the proto-Earth was flung into orbit to later coallesce into the Moon. So the Moon's size just ...


22

When you're trying to understand the mechanics of a system it's usually convenient to choose coordinates that reflect the symmetry of the system. The solar system is roughly centrally symmetric because the Sun is by far the largest mass in it, and the coordinates that reflect this symmetry are polar coordinates with the Sun at the centre. For example in ...


22

One point, the difficulty of seeing colors in dim light is due to properties of the human vision system. Most cameras will not have the same effect and will be able to show vivid colors in even dim light (as long as the light is sufficient for imaging). But as a good guess, with accommodation, you can read (to some extent) under a full moon. The sun (...


21

This is a really rough calculation that doesn't take into account the realistic direction of the bow shock, or calculation of the drag force. I just take the net momentum flow in the solar wind and direct it so as to produce the maximum decceleration and see what happens. Apparently the solar wind pressure is of the order of a nanoPascal. As I write this it'...


21

According to Opacity of an Ionized Gas, "light from regions [of the sun] where the pressure is greater than 0.01 atm. is cut off completely, so that all we see comes from a spherical shell of rarefied gas". There is no real surface of the Sun. Instead, the density and pressure of gas/plasma progressively increase from an infinitesimal value far from the ...


20

The reason being closer to a heat source makes you warmer is the inverse square law. Think of it this way: If you have a $1~\mathrm{m}^2$ piece of material facing the Sun and located at Mercury's orbit, it will be quite hot. What does the shadow of this square look like at Earth's orbit (about $2.5$ times further away than Mercury)? Well, it will be $2.5$ ...



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