# Tag Info

69

Photons can't have a perspective. If we have a particle with mass, we can imagine taking a frame of reference in which that particle is at rest. We can then see things "from the particle's perspective". But there is no frame of reference in which a photon is at rest. Photons always move at the speed of light in every frame of reference. If I try to set ...

65

The oldest light in the universe is the cosmic microwave background. Roughly 380,000 years after the Big Bang, protons and electrons "recombined"1 into hydrogen atoms. Before this, any photons scattered off the free electrons in the plasma filling space, and the universe was essentially opaque to light. Once recombination occurred, however, photons were able ...

62

The first handful of hits on Google actually return incomplete and even wrong answers (e.g. "Because the Moon is much brighter" which is plain wrong, and "Because the Moon is closer" which is incomplete [see below]). So here's the answer: As you mention, when light enters our atmosphere, it goes through several parcels of gas with varying density, ...

56

The light from the Sun spreads, at least initially, in an isotropic fashion into the universe. As it gets further from the Sun, some of that light will interact with the interstellar medium (ISM) and therefore some of the energy emitted by the Sun will be used to excite atoms and molecules or even ionise some atoms. This will be the fate of almost all the ...

51

There are two reasons that often — but not always — light from galaxies millions and even billions of lightyears away make it through the Universe and down to us: Particle number and particle size First, the intergalactic medium (IGM) is extremely dilute. The number density of particles out there is of the order $n\sim10^{-7}\,\mathrm{cm}^{-3}$, or roughly ...

50

Another way to answer this question is to apply the Equivalence Principle, which Einstein called his "happiest thought" (so you know it has to be good). The equivalence principle says that if you are in an enclosed box in the presence of what Newton would call a gravitational field, then everything that happens in that box must be the same as if the box was ...

40

That's what it really would look like if you were there with DSCOVR. The albedo of the Moon is only about 0.136, about half of the Earth's average albedo. Of course the part with clouds is higher. I was shocked too, but it was explained in written copy that accompanied the release of the original image. Shouldn't the Moon appear as bright as a full ...

38

Well, there's two things we'll need for this: apparent magnitude (the brightness that an object appears to have) and absolute magnitude (the actual brightness an object has). Both of these scales are logarithmic, with brighter objects being lower and dimmer objects being higher. Astronomers have determined that the Sun's absolute magnitude is 4.83. Knowing ...

33

You want nature to be frugal and efficient. You want all the energy of the sun to have a purpose. However what you want nature to be like has no bearing on what it is. The light from the sun is a colossal amount of energy in human terms, but very minor in comparison to the rest of the universe. The light that didn't fall onto anything left the solar system ...

33

Ole Rømer did not measure a change in the frequency of light. He measured an apparent change in the orbital period of Io, one of Jupiter's moons. The orbit of Io can be measured very accurately by observing when it enters or leaves the shadow of Jupiter. When the Earth is moving away from Jupiter, Rømer noted that the orbit of Io appeared to be very ...

31

There are a couple of ways one could approach your question: Black holes are regions of space that have been deformed by a sufficiently concentrated mass. Light waves/particles always travel in a straight line at a constant velocity ($c$). Although a photon approaching a black hole will continue traveling in a straight line through space, space itself has ...

28

It's brighter on Pluto than you think. NASA developed a tool called Pluto time, which tells you when at your place the ambient light conditions are similar to the ones on Pluto. This occurs when the Sun is only 2° below the horizon! That's quite shortly after sunset, and considerably before the end of civil twilight, which is when it's 6° below. All of ...

28

The simple answer is that it does, but it's not bright enough to be visible to the naked eye. Earth's atmosphere scatters the moon light just like sunlight. The full moon (like the sun) fills about 1/2 of 1 degree of the sky, the entire sky being 180 degrees, give or take, so the full moon fills less than 1 part in 100,000 of the night sky, so there ...

27

By convention, astronomy uses the Julian Year for the computation of a light year: Although there are several different kinds of year, the IAU regards a year as a Julian year of 365.25 days (31.5576 million seconds) unless otherwise specified. Wikipedia gives the length as $31 557 600 s \times 299 792 458 m/s = 9 460 730 472 580 800 m$ (exactly) The ...

24

The local dark matter density is actually quite tiny, on the order of $\rho\sim10^{-19}\text{ g/cm}^3$ (see e.g. Bovy & Tremaine (2012)). This means that there is roughly $0.001$-$0.01M_{\odot}$ of dark matter per cubic parsec - a staggeringly small amount. 1000 cubic parsecs would contain about one solar mass of dark matter - and that's a cube 10 ...

24

As Rob Jeffries says, the universe is mostly empty space. A photon can easily travel thousands of light years without interacting with anything. Most of the interaction would occur when photons entered the earth's atmosphere. The Hubble avoids this. These photos were most likely from combining several viewing sessions giving basically an extended time period ...

23

There's a misconception in your question I don't think the other answers have addressed. If light emitted from the galaxy travels in all directions, then how is it that we can still map out the entire galaxy Light is emitted from the galaxy in all directions. Only a tiny, tiny fraction of it is directed to Earth, and of that, an even tinier fraction ...

23

Stars twinkle because they are effectively a point of light. This point of light can be distorted and magnified by movement of patches of varying density in the atmosphere. These act as lenses causing the twinkle effect. If you get to space, the stars don't twinkle. Stars appear as a point only because they are so very very distant. Planets twinkle less ...

21

Yes, the speed of light in vacuum (or c) is 299,792,458 m/s and one light-year is the distance the light travels in one Julian year (365.25 days), which comes out as 9.4605284 × 1015 meters. Since c is the maximum speed at which all energy, matter, and information in the Universe can travel, it is the universal physical constant on which the light-year (ly) ...

21

Surely if you stared long enough, the light from them would eventually hit your eye? Collecting light over a long span of time is how telescopes can see very dim objects. The human visual system doesn't work that way. For one thing, even when you think you are staring at something, your eyes still dance around a bit. It's a built-in response called ocular ...

21

It is quite correct that a black hole has so much mass that light cannot escape from a region around the black hole. The edge of this region is called the event horizon. If you cross an event horizon you are never coming back. That applies equally to light, and matter. Around the black hole there may be matter in orbit. Since the Black hole has such strong ...

20

Dark matter, is just a name for something we know nothing of. It was named to account for an extra gravity source for which there have been indirect observations, but yet we cannot explain. The force of gravity exerted by light is negligibly small yet we have measured the gravitational pull of Dark Matter to be big enough to affect whole galaxies; it is ...

18

There was a object, apparently flying above you, that you couldn't identify. By definition this is an unidentified flying object. However this does not imply that it was an extra-terrestrial spacecraft. UFO reports can be explained by a combination of: Not recognising a known natural object, such as Venus, or unusual clouds. Planes, drones, Chinese ...

18

Nothing forbids this, and it is actually observed astronomically. You need a very bright source of light: such as a supernova (which isn't a beam, but radiates in all directions) and very large distances. The flash of light can be seen spreading out from supernova in a circle, as it illuminates the dust and gas ejected by the supernova progenitor star in the ...

17

Rainbows would lack most blue, and some green for red stars. For a blue star, the blue part of the rainbow would be more intense. For more complex colors, the rainbow may show some gaps. A rainbow is essentially a spectrum of that star light portion, which is visible to our eyes, and to which the atmosphere is transparent. Stars vary in brightness. A blue ...

16

It's called sunglint. This can be problematic for Earth-observing satellites in low Earth orbit. Such satellites typically don't take a "picture". They instead continuously scan the Earth a line at a time. This means the sunglint moves with the satellite. You can see this effect yourself while flying in an airplane. Little ribbons of rivers and lakes can ...

15

I think the question is referring to situating a very large mirror in space facing earth. If we were to put it several light minutes away, then events occurring opposite the mirror could be reviewed de novo with more preparation upon the warning we received upon the first light of the event arriving at earth. For example, a supernova going off in M31 might ...

15

Gravitational lensing is the effect that large amounts of gravity have on the path of light. Here's an example of the effect: The animation shows a black hole passing in front of a galaxy (simulated). In theory any object with a surface mass density larger than the critical surface mass density would be able to produce similar such effects. What's ...

15

At the center of our galaxy is a powerful radio source named Sagittarius A*, which is believed to be a super massive black hole (SMBH). This blackhole would contain far more mass than your run-of-the-mill supernova remnant. Our galaxy is believed to contain a SMBH containing the mass of likely a bit above 4 million times the mass (Gillessen) (2) (Ghez) of ...

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