# Tag Info

22

The moon does have a night and a day, but this isn't as fully connected to your question as you might think. The moon is tidally locked with the earth, meaning that the same side always faces earth. Since the moon also orbits around the earth (with a period of a lunar month), this means each side changes, over the course of a lunar month, between facing ...

12

With the atmosphere of the moon being $10^{14}$ times less dense than that of Earth, there is negligible scattering, so whereas on Earth, approximately 25% of direct solar radiation is scattered around (making the sky light up and appear blue), there is no mechanism for this on the moon, and all light from the sun travels (essentially) unaffected to the ...

5

In addition to the lack of an atmosphere (the primary reason, already addressed in other answers), something that compounds the effect even further is that the lunar surface is quite reflective, and because the people who are taking the images of lunar surfaces often want to have Earth and/or astronauts in the shots and also be able to make out detail in the ...

5

One piece of physics that you've missed is the most pulsars spin down due to the emission of magnetic dipole radiation. For instance, the crab pulsar has a period of 33.5028 (plus a few more sig figs) milliseconds, but slows down by 38 nanoseconds per day. Furthermore, the size of several more increasing order derivatives is known accurately. So in ...

3

The positions of stars on the sky are defined against a co-ordinate reference frame that is ultimately defined by the positions of very distant radio sources (quasars at high redshift) that are assumed to be "stationary" in terms of their celestial grid co-ordinates (see International Celestial Reference Frame) All stars can have their positions precisely ...

2

Spectroscopic parallax is the technique whereby you estimate the absolute magnitude (i.e. the brightness it would have if it were placed at 10 pc) by estimating what "type" of star it is using information fro a spectrum. It can be applied to any kind of star where (a) you have a reasonable chance of determining the type of star from its spectrum and (b) ...

2

You are correct. If you worked through the same steps on the distance ladder in a universe with a different speed of light, you would find the answer in light years would be different. Your question on whether light travels one light year in one year, though trivial sounding, opens the door to difficulties that are inherent in the nature of space-time. Your ...

2

Space is mostly black, because most of space doesn't have light coming from it for you to see. The moon, being basically a big rock, unsurprisingly does not change this: You would expect that standing on a rock would not affect what you see. Earth is unusual in having a sky because it has an atmosphere. The atmosphere is a bunch of gas that changes the ...

1

The primary reason for the asymmetry of the Ly$\alpha$ line is bulk motion of neutral hydrogen, i.e. accreting gas (causing a blueshifted line) or galactic outflows (causing a redshifted line). Mechanism of the Ly$\alpha$ double peak In general, for Ly$\alpha$ photons produced in the center of a blob of neutral hydrogen (i.e. a galaxy), the photons must ...

1

You have to distinguish between stellar mass and gravitating mass. The quoted Milky Way mass includes dark matter. Despite searching in the literature, I have yet to find reliable apparent magnitudes and good distance estimates for NGC 1097. The total Blue luminosity of this galaxy is near to Minus B-band absolute magnitude -21, but only ballpark ...

1

See, first of all our eyes are not a good device to determine the main color of a group of photons. the main color that we see is actually the intensity of a specific range of frequency in the light wave. it means there are all kind of photons coming out of the sun, but the amount of "Yellow" photons are much much more. that's because we see the sun ...

1

Sort of. This is demonstrated clearest in barred spiral galaxies, which make up about 1/2 to 2/3 of all spiral galaxies. A dramatic example is NGC 1365: Image courtesy of Wikipedia. Others, such as M95, have spiral arms that wrap even further around while still retaining the central bar: Original image courtesy of Wikipedia; color added by me in ...

1

$\theta$ is half the difference in direction of 2 measurements of the position of the star taken 6 months apart.

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