# Tag Info

73

Sometimes we do, and the phenomenon is called a light echo. What you're looking at there is NOT moving gas. It's an "echo" exactly as you describe. The problem is that you need a pulse of light. If you have a constant stream of light, the "light echos" will be exactly like what you see in fog on earth.

68

The problem with finding a new planet in our solar system is not that it is too faint, but knowing where to look in a big, big sky. This putative planet 9 is likely to be in the range 20-28th magnitude. This is faint (especially at the faint end), but certainly not out of reach of today's big telescopes. I understand that various parts of the sky are ...

60

I've made this into an answer because it's too long for a comment, and I really want to show the pictures. It is tempting to think of visible light as "close enough" to (near by wavelengths) and to conclude that "yes, actually, the yellow does affect it. I want a mirror without an obvious tint" However you are wrong, Physics will slap you down. Exhibit A ...

50

If you look at the reflectivity of gold (vs silver or aluminum) you can see a plateau at wavelengths below 500 nm source: If blue wavelengths are not reflected as well as other colors, the resulting image will look "more yellow" - which is what you see. At longer wavelengths, gold is a very good reflector (better than the other two above 600 nm). It also ...

42

The sun is still surprisingly bright at Pluto. While it is approximately 1500 times less bright than at Earth, this is still approximately 250 times brighter than a full moon. If you consider the effect the difference between a full and new moon has on star viewing I expect that the reflected sun at Pluto is still bright enough to make it difficult to see ...

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 ...

39

Barring whatever fantastic energies would be required to stop the mass of the Earth from rotating and then changing the direction of the rotation, one of the major things I can see changing would be the expectations of weather patterns. Part of what affects our weather is known as the Coriolis Effect. While there would certainly be effects from the ...

34

You can look up the camera settings behind these images. Here is the link to one of the raw images. The exposure time of the camera was 100ms. New Horizon's LORRI camera is a Ritchey-Chrétien telescope with a 20.8 cm diameter primary mirror and a focal length of 263 cm. That gives us approx. f12.6, which is a rather long i.e. fairly slow optical system (but ...

33

I stumbled across this question, and while it is an old question and there are some halfway decent answers, I think it deserves a more in-depth response. I completely understand the question, which I think is am excellent question. Trying to understand the phases of the moon is much more difficult than most think. The short answer of it is that you cannot ...

30

The highest resolution 3d printers I know of are around 1600dpi, which is a resolution of about 15$\mu m$. Telescope mirrors have to be smooth to fractions of a wavelength of light, so the resolution of current printers is nowhere near good enough. Whether 3D printers could one day be good enough is a different question, but given that the improvement in ...

29

Sort of. As Space.com writes, The raw Hubble images, as beamed down from the telescope itself, are black and white. But each image is captured using three different filters: red, green and blue. The Hubble imaging team combines those three images into one, in a Technicolor process pioneered in the 1930s. (The same process occurs in digital SLRs, except ...

28

Not quite like in the photo above, which shows more than what the naked eye can see, but yes, absolutely! Our galaxy (well, the chunk of it visible from these parts) is a naked-eye object. The fact that your question even exists shows how much time is now spent by people under light-polluted skies. It will not be visible from the city, however. You need to ...

28

A typical giant galaxy, such as the one you've provided a picture of, has a radius of something like $10\;\rm kpc$ (kiloparsec - $1\;\rm pc \approx 3.2\;ly$). A supermassive black hole hosted in such a galaxy has a mass of something like $10^6-10^9\;\rm M_\odot$ (solar mass, $1\;\rm M_\odot \approx 2\times10^{30}\; kg$). The monstrous billion solar mass ...

28

I generally regard NASA as authoritative, and they report the orbital parameters on their Earth Fact Sheet. I note that they disagree with Wikipedia about the aphelion though they agree on the perhelion, semi-major axis and eccentricity: NASA Wikipedia Aphelion 152.10 151.93 Perhelion 147.09 147.095 Semi-major 149.60 ...

27

Money and willpower. With any program (scientific, military, public works, etc.) it all depends on the amount of money someone is willing to put to it, and how much backing and protection that program has from getting money re-prioritized to other projects. You are making a false dichotomy of attempting to present our past actions as a justification for ...

25

The estimates I've read are similar to yours: 200 to 400 billion stars. Counting the stars in the galaxy is inherently difficult because, well, we can't see all of them. We don't really count the stars, though. That would take ages: instead we measure the orbit of the stars we can see. By doing this, we find the angular velocity of the stars and can ...

25

The relevant Science summary is "Radio Bursts, Origin Unknown" by Cordes, abstract here. Briefly, it notes that a burst of radio waves will undergo dispersion in the interstellar and intergalactic medium (ISM and IGM), the amount of which is indicative of how much matter the signal has passed through. Some further background not given in that summary: In ...

24

First of all, the universe is most certainly not shaped like an hourglass. It simply looks that way because the gas and dust in the plane of our galaxy obstruct our view of anything outside the galaxy in those directions. So we can only see other galaxies (and similarly distant objects) by pointing telescopes at some angle to the galactic plane. That gives ...

24

There are a variety of methods used to measure distance, each one building on the one before and forming a cosmic distance ladder. The first, which is actually only usable inside the solar system, is basic Radar and LIDAR. LIDAR is really only used to measure distance to the moon. This is done by flashing a bright laser through a big telescope (such as ...

24

One cannot tell by the light spectra. Hydrogen and antihydrogen would give the same lines in the spectrum. The prevalence of matter over antimatter from other evidence indicates matter is predominant in the observable universe, and here is a nice review. How do we really know that the universe is not matter-antimatter symmetric? The Moon: Neil ...

24

It would be possible to see the progress of photons through space if the light pulse were exceedingly intense, and if the dust cloud from which they reflect were positioned and shaped to reflect the light toward us. Rather than shooting a beam from Point A to Point B, it would be better if the light source were between us and the dust cloud, as light ...

24

It depends what you mean by day and night. The day and night are not of equal lengths now, where I live at latitude 53N. The tilt of the Earth's rotation axis with respect to the ecliptic plane means that this is generally true. The situation you describe would have to be considerably more extreme. If the planet was in a highly eccentric orbit and had a ...

24

I know that the opposite could happen. There is an old book called "Night Fall" about a planet that had three stars. Because there was always a star shinning on all sides it would never get dark. About every 500 years everything lines up just right so that all the stars were on one side. Then as the planet rotated on its normal spin, night fall would come ...

23

The reason why we can see exoplanets 13,000 light years away but not a planet 200 AU away (about 30 light-hours) is because these planets are found using different techniques. The planet discussed in the article I linked was discovered using a technique known as "microlensing," which requires a star to pass behind another star with a planet around it. The ...

22

I have operated public observing nights at various universities for eight or ten years. These are the objects that have stood out for me. Saturn and Jupiter are the "stars" of the night sky. Bar none. Spec. Tac. U. Lar. If you can look at Saturn with nothing between it and your eye besides a few panes of glass, and see it hanging in the sky in all its glory,...

22

First, the speed of other galaxies isn't too helpful. For example, the radial velocity of the Andromeda galaxy relatively to us is 300 km/s, i.e. 0.1% of the speed of light only. Moreover, internally, everything in that galaxy moves by pretty much the same speed and is confined to the vicinity of that galaxy which makes us pretty sure that no piece will ...

22

To address your last point, there are several stars of which we have been able to resolve images i.e. see the star as more than just a featureless point. There is a list of these stars on Wikipedia (I love that they put the Sun at the top of the list - true but pedantic :-). The farthest away of the stars in the list is Epsilon Aurigae at about 2000 light ...

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