# Tag Info

21

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 ...

11

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 ...

4

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 ...

4

By estimating the distance is the most obvious method, but you are correct, the parallax will be too small to measure. If we can tell what type of star it is (by measuring its spectrum, or using its colour(s)), then we know roughly how intrinsically luminous the star is. The actual brightness then tells us how far away it is. Fortunately, the diameter of ...

3

The sodium line that is being referred to is a reasonably sharp, dark absorption feature that is seen in the orange part of the visible spectrum of the bulk of stars that make up the light from a distant galaxy. The absorption is caused because stars are hotter in the middle than they are on the outside. The relatively cool gas on the outside has sodium ...

3

If by fire you mean flames as seen at room temperature by the combustion with oxygen of various materials the answer is, we do not know in the solar system of a planet with an oxygen atmosphere at a level that given combustibles flames will appear given the trigger. Hydrogen vents for example would need an atmosphere with oxygen to start flames. In ...

3

The width of a SN light curve is changed due to a time dilation between the source and the observer. If the source emits light with wavelength $\lambda_\text{em}$, it will be observed with wavelength $\lambda_\text{ob}$, so that its redshift is $$1 + z = \frac{\lambda_\text{ob}}{\lambda_\text{em}}.$$ We can also write everything in terms of frequencies ...

2

Spectroscopy: you pass the light through (or reflect from) a dispersive element (a prism or diffraction grating) and then you record the dispersed light. You have a record of the intensity of the light as a function of wavelength. Advantage: potentially you can record the light of one or more objects over a very wide wavelength range and have excellent fine ...

2

You raise two issues: ΔT, and the HORIZONS timescales. Let us tackle each in turn. 1. What is Delta-T? You are correct that HORIZONS is using a confusing term here. What the HORIZONS menu calls Delta-T is an entirely different quantity than the ΔT you will see defined and used in many other references on astronomy. Briefly: What HORIZONS calls Delta-T ...

2

It would be remarkable if there were no dark matter in the vicinity of the Earth. As far as we know, the hypothesised properties of dark matter mean that it should be quite smoothly distributed. If we can't find any here on Earth, why should we find it anywhere else? Therefore it makes perfect sense to push sensitivity limits as low as possible here on ...

2

I'm assuming you mean the region $\{(\alpha,\delta) : \alpha_1\leq \alpha\leq \alpha_2, \delta_1\leq \delta\leq \delta_2\}$. Measuring $\alpha$ and $\delta$ in radians, we can find the desired formula by integrating in spherical coordinates: \begin{align*} \int_{\alpha_1}^{\alpha_2} \int_{\delta_1}^{\delta_2} \cos \delta\ d\delta\ d\alpha ...

1

In respect to above answer, although you are correct but there is a little discrepancy in your last line " a bigger leap was to try the Sun's location at a focus and not the center." A circle is a ellipsoid with a special case having the two foci at same point, so there could not be so chance putting in centre because it was observed that planets near ...

1

Some studies have suggested that the Earth may have had a second tiny moon, which later crashed into and merged with the bigger current Moon. This might explain the lopsided back and front faces of the Moon. The second moon may have orbited the Earth for between 10 million and 100 million years. This little moon was likely about 750 miles wide, which ...

1

It is a standard line in the atomic spectrum of sodium, that is used as a standard The line you refer to in nmeters is a standard line because of its strength for determining refractive indices of materials in the lab. A reference wavelength of 589.3 nm (the sodium D line) is most often used. T The evolution of the line due to the motion of the ...

1

Let's jointly go through the motions of calculating our signal strength in detecting electromagnetic radiation from an object in space. This means starting at the emitter and going through the relevant steps until the light/radiation reaches our detector: Emission. Only if the source emits (appreciably) at a particular wavelengths and towards you, there is ...

1

You are entirely correct: when assigning JD real numbers to UTC calendar dates, it is simply impossible to name any moment during the leap second — while an analog rendering of a UTC time can say “23:59:60.25”, the JD will provide no name for any moment of that entire second. This can be seen if you visit the standard JPL HORIZONS system: ...

1

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 ...

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