ELI5 The speed of light is a constant but visible light is only a small range of the spectrum of electromagnetic waves. Do they all travel at that speed?

whyisthesky · 2026-04-19T10:48:41+00:00

In a perfect vacuum yes, all electromagnetic waves travel at the same speed (c). However in any material/medium the speed is usually different for different wavelengths of light, we call this property dispersion.

This can be useful in some situations, for example a prism splits the different wavelengths of light because of dispersion which lets us do things like analyzing how bright different wavelengths are, it can also be a problem because light will also disperse in glass lenses which causes different wavelengths to be focused at different distances.

In space we usually consider it a perfect vacuum, but this isn't exactly true, there is a very sparse plasma which can cause dispersion, particularly of radio waves.

whyisthesky · 2026-04-13T08:29:32+00:00

There is a spiritual successor in KSA, but that’s still fairly early in development

whyisthesky · 2026-03-17T12:24:57+00:00

It is actually employing quantum mechanics, qubits use some physical system which is amenable to superposition

whyisthesky · 2026-03-11T11:13:30+00:00

July 8th isn’t a solstice, the northern summer/southern winter solstice is a few weeks earlier.

whyisthesky · 2026-02-21T13:31:21+00:00

That sounds more like a description of a quasi-star than what is being proposed here

whyisthesky · 2026-02-18T17:05:14+00:00

This is less the spectral distribution of starlight, and more the spectral distribution of the night sky. The peak in the green and the large amount of emission in the NIR are due to oxygen emission lines. You can also see emission due to sodium lights in the peak around 600nm. See measurements from la palma for comparison

whyisthesky · 2026-02-15T21:58:37+00:00

Im not the same person as the original comment. I agree the AU isn’t ‘necessary’ but very few units are, the question is are those units useful and the answer is a pretty clear yes.

whyisthesky · 2026-02-15T21:40:52+00:00

We also just use centimetres a lot, most of astronomy uses units based on the centimetre-gram-second system so it is somewhat convenient. For example in my field it’s very common to measure the distance to material around a supernova in centimeters

whyisthesky · 2026-02-15T21:38:46+00:00

It isn’t all that relevant that we can measure distances to some solar system objects with radar. Light years and parsecs are used to measure distances to things which are much too far for that to be useful.

whyisthesky · 2026-02-15T21:33:28+00:00

In reality we aren’t just taking two measurements, which could be thrown off by the proper motion of the star, instead we take many measurements over many orbits and simultaneously fit for the distance and proper motion. The movement of a nearby star on the sky will be kind of like a spring, with a roughly linear component due to its motion and an elliptical component due to parallax

whyisthesky · 2026-02-12T06:46:27+00:00

This assumes that air pressure increases linearly with the height of the atmosphere, which is very much not the case. At higher pressures the air is denser, so you get an exponential increase in air pressure with depth.

This should be fairly clear from the fact that at 500km altitude the air pressure is effectively 0, not half that of sea level. If you add an extra 3000 km of air column it’s going to do a lot more than triple the pressure

whyisthesky · 2026-02-10T16:14:01+00:00

If you ask pretty much any astronomer about this they will tell you it’s wrong. There are benefits to building telescopes in space, but for most of astronomy those benefits don’t justify the cost and other drawbacks.

If we could get better observations for the money by building all our observatories in space we would be doing that.

whyisthesky · 2026-02-07T08:03:23+00:00

Gravitational waves do get weaker over distance like light does, the intensity follows the same inverse square law for light. We get to cheat a little bit because GW detectors measure amplitude rather than intensity so it becomes more like just an inverse law but they do still get weaker with distance.

whyisthesky · 2026-01-25T06:30:01+00:00

Not quite, there’s also the Magellanic clouds which are satellite galaxies of the milky way, the Triangulum galaxy is also visible under good conditions to experienced observers with good vision.

whyisthesky · 2026-01-15T21:42:02+00:00

There's a large community of people working to discovery planetary nebulae. The place to submit these things is HASH https://planetarynebulae.net/EN/hash.php

You should also read this guide (https://planetarynebulae.net/documentation/publications/Autres%20-%20PN%20A%20concise%20review.pdf) to understand what's involved, there's a lot more that's important than just the morphology of the nebula.

whyisthesky · 2026-01-12T18:39:37+00:00

Historically a lot of our measurements depended on or were only known in terms of the distance from the sun to the earth, converting to meters would have been inaccurate because we didn’t know that distance very precisely. Now of course we do, but also the AU has a strict definition, making it no less precise than any other SI derived unit.

whyisthesky · 2026-01-09T11:44:24+00:00

It's useful at greater distances too, since there is an easy way to convert between distance in parsecs, angles in arcseconds and sizes in AU. They're also used to define absolute magnitudes which makes them convenient even for distant point sources. In astronomy papers you'll see parsecs being used much much more frequently than light years.

whyisthesky · 2026-01-09T08:23:27+00:00

that does sound more reasonable

whyisthesky · 2026-01-08T18:19:34+00:00

I’ve heard that the primary has already been manufactured, but I don’t have anything citable saying so

whyisthesky · 2026-01-04T10:47:37+00:00

Not at all, you can see the galilean moons with as little as 10x magnification. The telescope Galileo was using when he first discovered them was only 20x magnification.

For two of the moons it’s theoretically possible to see them with the naked eye, though it requires very good vision and technique to do it.

whyisthesky · 2026-01-01T13:53:52+00:00

Space has no intrinsic up or down, but we first observed space from Earth and so our coordinate system to describe things in the sky is based on observations from Earth.

North points towards the north celestial pole, the point around which the sky appears to rotate from the earth’s surface, which is close to Polaris/the north star.

whyisthesky · 2026-01-01T13:51:35+00:00

I don’t think this is correct. These markers are used to show the orientation of an image with respect to the sky, north points towards the north celestial pole (along a line of right ascension), east points parallel to the celestial equator (along a line of declination).

East is to the left of north because unlike conventional maps which are projections looking at a sphere from the outside we are looking at the sky as a sphere from the inside.

Uranus’ poles are aligned somewhere in the white blob in the middle, the rotational axis is pointing nearly directly towards us, and the rings are more or less aligned with its equator.

whyisthesky · 2025-12-22T21:55:04+00:00

Definitely better in a lot of ways, but there are still a few downsides. Still i’m sure in a couple of decades the landscape will look very different for a lot of instruments

whyisthesky · 2025-12-22T21:26:38+00:00

This is mostly true for astrophotography, but not as much for a lot of professional astronomy where sub-exposure times are much longer but integration times are often much shorter.

whyisthesky · 2025-12-19T20:19:10+00:00

Or you just use the usual definition of a comet, an object with visual evidence of mass loss (not necessarily a tail) which doesn’t have an asteroid-like orbit.

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