# Tag Info

89

Impacting solar system objects would have relative closing speeds from around 11 to 72 km/s. We could take the optimal case that the asteroid approaches whilst fully lit by the Sun (which I think precludes the minimum and maximum speed in the range quoted above) and then scale from another similar body - say the asteroid Vesta. This has a diameter of around ...

66

Yes and no. Yes, it's true that the apparent size of the Moon is 30 arcmin. It's true that the visual acuity of most people is 1 arcmin. So it's true that if you take the angular size of the smallest detail you can see on the Moon, and you put a bunch of those lined up straight in a row, you could span a Moon diameter with only a few dozen of them. In that ...

48

It doesn't seem so far-fetched to me. Sure, you might be off by a few pixels, due to differences between the human eye and a computer monitor, but the order of magnitude seems about right — the detail in your images, viewed closely, more or less matches what I see when I look at the full moon. Of course, you could fairly easily test it yourself: go ...

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 ...

21

When you gaze at the moon "live", you are not seeing a still image. You're seeing a "video": your retina is gathering multiple images over time. Those pixels have to be taken into account; they amount to extra pixels. Suppose that 60x60 pixel images are taken of a scene using a tripod-mounted camera which slightly jitters. From the multiple images, a higher-...

13

Well, having seen Neptune and identifying Neptune are two totally different things. Let's tackle this one step at a time. Your link says: From these observations it would appear that, if seen upon a perfectly black background, a star of magnitude approximately $8.5$ would be at the limit of unusually good vision. The problem with this is that there are ...

13

I'll add to Wayfaring Stranger's comments. In fact most of the time you would be able to see fewer stars in the night sky of Mars, than in a good dark night sky on Earth, because of dust obscuration. Even in favourable conditions, the optical depth of the Martian atmosphere is usally somewhere between 0.5 and 1 per airmass. (Petrova et al. 2012; Lemmon et ...

9

A carbonaceous condrite has the same reflectivity as the moon at around 7-13%. If there was ice, if the tail was 10 times smaller than hail bopp, it would have auspiciously covered half of the sky. it could have made an incredible display in the 1-2 days preceding the collision, because it was as close to the sun as hale bopp, the brightest astronomical ...

8

In the best sky conditions, the naked eye (with effort) can see objects with an apparent magnitude of 8.0. This reveals about 43,197 objects in the sky. There are 9 galaxies visible to the naked eye that you might see when observing the sky, and there are about 13 nebulae that you might see. Sources: The Bortle Dark-Sky Scale - John E. Bortle How many ...

8

You can tell a lot about Galactic structure by just looking. The ~5000 stars that can be seen with the naked eye have a roughly "lognormal" distribution of distance. I show plots below which were generated from the most recent version of the Hipparcos parallax catalogue. Fig.1 shows results for all stars with $5.5<V<6.5$ (i.e. very faint naked eye ...

7

First Shooting Star Seen from Mars space.com 2005 The background image shows the meteor near the top-left and the horizon at the bottom. A red arrow shows the direction of travel. The inset is a larger version of the meteor itself. The graph is a "light curve" that aided in tying the meteor to comet Wiseman–Skiff. (Image: © Nature/NASA/Spirit/F. ...

6

The apparent magnitude classification was done arbitrarily by Ptolemy. His thought was to set the 20 brightest stars to the first position, the less bright stars to the second position and so on up to the faintest stars which were given the sixth position. After the use of Pogson's law we were able to give to stars not only natural numbers, but also numbers ...

6

We have pictures of this, thanks to Curiousity! (Source) “A human observer with normal vision, if standing on Mars, could easily see Earth and the moon as two distinct, bright “evening stars,” said NASA in a statement issued today. So yes, you can see our moon from Mars. The apparent brightness of our moon from Mars is +0.9. The apparent brightness of ...

6

There is an article here that describes the visible effects quite well. In essence, within a week or so, it would be comparable in brightness to the moon and therefore visible during the day. Betelgeuse would then start a phase of final, rapid dimming and again reach its current brightness level after possibly three years. After six years, it would be ...

5

The resolved stars (those that can be seen as individuals) are all part of the Milky Way Galaxy (unless there are any interlopers that have been captured!). The distances to the next nearest galaxies of any size are more than 100,000 light years. Andromeda is 2 million light years away. Unless one goes supernova, there basically aren't any types of star ...

5

Alpha Centauri A and B happen to be rather similar to Sol, and their absolute magnitudes are 4.38 and 5.71 respectively (Wikipedia). Add them together and you get absolute magnitude 4.10 (the scale is logarithmic, and backward). Sol, with absolute magnitude 4.83, should look 0.73 magnitude dimmer than αCen at the same distance, so magnitude +0.46, ...

5

In theory, yes; in practice probably not ... i) It's perhaps helpful to take the Andromeda galaxy as a first working example. It is somewhat bigger than our own Milky Way galaxy but not so big as to be a problem. When we look at Andromeda, it appears to occupies an area about 3 degrees by 1 degree. By comparison, the Moon appear about 0.5 degree across. ...

5

Taking Mars' average distance from the sun of 1.52 AU, the sun would be 43% as bright. Phobos has an albedo of about 0.071 which is pretty dark. Darker than the Moon with an average albedo of about 0.12, so it reflects about 59% as well as the Moon does. That gives it a brightness to area of just over 25% the brightness of the moon. Phobos is an odd ...

5

The aurora depicted in the artist's conception is of ultraviolet light. It would not be seen with the naked eye. "NASA's Hubble Space Telescope observed a pair of auroral belts encircling the Jovian moon Ganymede. The belts were observed in ultraviolet light by the Space Telescope Imaging Spectrograph and are colored blue in this illustration. They are ...

4

Depending on your eyesight, the faintest stars you can see in a perfect environment have an apparent magnitude (brightness) of 6-6.5 (though some people have managed to see magnitude-7). According to this site, that translates into about 10,000 stars. As for how it would look, I have seen the sky from the stratosphere (about 3 miles up, in a plane, where ...

4

Because light with higher frequency, that is bluer colors, scatters more in Earth's atmosphere and celestial objects thus look redder (yellow is towards red) while the atmosphere looks blue. See Rayleigh scattering. Here is video with a lengthy and somewhat entertaining explanation: https://www.youtube.com/watch?v=SRh75B5iotI

4

After all these astronomic answers, I will add a computer one. Pixels are not the same on all monitors. Take a 1990's monitor and take the latest smartphone screen, the 60 pixels won't be the same. How did you calculate the pixel size according to the vision accuracy ?

4

The key to this is the so called Absolute Magnitude, which represents the visual magnitude from a distance of 10 parsecs (about 32 light years). The sun is much brighter than Proxima Centauri. It has an absolute magnitude of 4.8, and at a distance of 4 light years (the distance of Proxima), it would be be somwhat brighter than 1st mag, and so very easily ...

3

At any one time, an average observer can see about 2,500 stars in a clear dark sky. Note that eyesight varies and sharp-eyed individuals may be able to see a half-magnitude dimmer stars than the average eye (apparent magnitude is a scale in which each integer is $2.51$ ($100^{0.2}$) times brighter or dimmer than the next consecutive integer.) A very dark sky ...

3

Kaufman & Freedman's Universe is a very basic astronomy book aimed at non-astronomy science students or astronomy 1st year students, although for the latter it is perhaps a bit too basic. It covers introductions to all the topics you mention, except perhaps exoplanets (but my edition is from the late 90's; my guess is they have it covered too by now). It ...

3

Firstly, the galaxy is only about 1000ly thick. We are fairly close to the galactic plane, maybe around 65 ly 'above' it if we call the direction we are moving away from 'down'. So on your assumption that visible objects are all within about 1000ly, we can suppose that we should see more stars in the plane than above and below the plane, as there is only ...

3

Good previous answer, just to add. The Moon can also look shades of yellow and red when there is a high amount of pollution in the air. I live in Australia and the moon can look Yellow, orange and red when the bush fire season is about us and there is a lot of smoke in the air. I have also noticed in cities like New York on windless days that the moon can ...

3

Assuming you mean Bortle class 4, then the Andromeda galaxy (M31) should be a fairly easy object with the naked eye. It won’t be particularly impressive, but rather will look like a hazy blob. On the other hand it is more than 2 million light years distant, so it’s impressive to be able to see it at all.

3

A truly negligible amount. You only need compare the brightness of the planets as viewed from the Earth with the brightness of the sun. A very rough calculation (considering the relative magnitudes of the planets and the Sun) suggests that light emitted directly by the sun is 100 million times brighter than light reflected off planets. You note that the sun ...

2

There is a rule about average surface brightness: it is conserved as you change distance or magnification. For the gory details see. Telescopes make unresolved objects brighter, but for resolved objects, a telescope shows you the actual surface brightness (assuming that the exit pupil diameter is well matched to your eye's pupil (~7mm) and is not throwing ...

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