# Tag Info

13

On what basis is the information about the distance and velocity of the Voyager probes determined? For distance: round-trip travel time of radio signals For velocity: Doppler-shift of round-trip radio signals, and by the rate of increase in distance as discussed above. The Voyagers as well as many other deep-space spacecraft before and after carry what's ...

8

Background/Video Starting around minute 6 of the video frozen orbits are discussed, and some example GMAT simulations are run to explain how lunar orbits can be simulated including up to order 160 in the Moon's gravitational spherical harmonics, which have been very accurately mapped by the GRAIL mission. The video then links to the study itself: arXiv: ...

8

Bond et al. (2017) measure the orbital period of the Sirius system to be $50.1284 \pm 0.0043$ years. I believe this is the most precise and accurate value (I cannot find any more recent papers, with new determinations, that cite this paper). An earlier, comprehensive study by Gatewood & Gatewood (1978) gave $50.090\pm 0.056$ years; consistent with the ...

7

In Observing Handbook and Catalogue of Deep-Sky Objects, Luginbuhl and Skiff describe how hundreds of deep-sky objects look in different apertures. For example, for galaxy M63 (NGC 5055) in Canes Venatici: Messier 63 is an easy object for 6 cm, located 3'.5 E and a bit S of a mag. 8.5 star. It is elongated E-W, passing just S of the star. The broadly ...

7

There are a couple of inconsistencies with theory presented in the video and the paper it is based on. Apparently, the author(s) are suggesting that Eagle did not crash as it was/is in a 'frozen' orbit. However, frozen orbits are thought to occur only for orbits with higher inclinations (in fact only for 4 specific inclinations: 27,50,76 and 86 deg) (see ...

7

When you consider an expanding universe, the meaning of "distance" becomes difficult, and different reporters use different definitions. In many newspapers the distance used is the "light travel time distance". This is the time that light has been travelling from the galaxy multiplied by the speed of light. So a galaxy that is 300 million ...

7

Right ascension is usually given in hours, minutes and seconds, but declination is usually given in degrees, arcminutes (') and arcseconds ("), with one arcminute being 1/60th of a degree and one arcsecond being 1/60th of an arcminute. This means that the declination of the source you list should be 40$^{\circ}$08"55.6', in accordance with the SDSS ...

6

There are really two type of "dark matter" in the universe and two "dark matter problems". The first problem is that most of the "normal matter" in the universe is dark and difficult to find (a.k.a. the missing baryon problem). This dark matter could consist of gas between galaxies (most likely) or in the form of objects that ...

5

The theorem says that black holes' surface areas are directly related to their entropies, and can therefore never decrease, only (possibly) increase... The logic is really the other way around. The Hawking area theorem says that the area can only increase. This leads to the idea that we can form an analogy between area and entropy, or possibly find some ...

5

According to Formation of the UV Spectrum of Molecular Hydrogen in the Sun (S. A. Jaeggli et al. 2018 ApJ 855 134, also here) molecular hydrogen in the sun was first spectroscopically discovered in 1977. The model calculation in this reference give a ratio molecular/atomic hydrogen of around $10^{-5}$ at a height of about 650 km (where the $H_2$ emissions ...

5

Some of the comments here seem to be suggesting that there should not be any residual charge of the Sun at all because of the fact that in a conducting medium no electric fields can exist. This argument ignores the crucial point here, namely that there are unequal numbers of positive and negative charges, because electrons, unlike ions, can easily escape ...

5

I was wondering why the absorbtion lines of the template are broader then those of the galaxy, since it actually should be the other way arround. You are correct that it should be the other way around. The reason the plot looks confusing is that you are not actually plotting the galaxy spectrum in the top panel; you are plotting some combination of noise ...

4

It seems that the planet is marked with longitudinal lines every 10 degrees, and so you can measure off the video that the planet rotates by about a little less than three longitudinal lines, or about 30 degrees. As Jupiter has a rotation period of just under 10 hours, the length of time can be estimated as about 10×30/360 hours: 50 minutes, or possibly ...

4

Straight line, though it wont make much difference, The point about the size is that it tells you about how much light the telescope can gather. This is the "projected-flat" area, not the curved surface area. However, the difference is not much, as telescope mirrors are not highly curved. I suppose it makes more of a difference when considering ...

3

You may want to look at "Stellar Structure and Evolution" by Kippenhahn, Weigert and Weiss. Paragraph 36.3.1 "Simple collapse solutions" contains just what you are looking for. In particular, figure 36.5 shows the velocity profile as a function of the radial mass coordinate. I have reproduced the figure here: The reasoning is more or ...

3

You are asking about the Moon's diurnal libration. While it is small (about 1°), it is easily measurable. It is also overwhelmed by the much larger librations due to the Moon's non-circular orbit and due to the Moon's orbit being inclined with respect to the Earth's equatorial plane.

3

Sources above: https://www.facebook.com/NASAWebb/photos/10158883840795049/ below: https://jwst-docs.stsci.edu/jwst-observatory-hardware/jwst-telescope Do telescope measurements (in meters, usually) measure in a straight line, from edge to edge, or follow the curve of the mirror? The short answer is @JamesK's; it's the straight-line diameter of whatever ...

3

You'd be completely right that a single Fourier transform would be needed if the interferometer were able to sample the entire $(u,v)$ plane. Unfortunately, that's not the case; we only have a fairly small, finite number of dishes and a finite amount of time. Coverage of the plane will increase as time passes and the Earth rotates, but it's going to be ...

3

The closest that I know of would either be Nightwatch (Terence Dickinson) and/or The Backyard Astronomer's Guide (also Terence Dickinson). I don't think either of those cover your last requirement of "telescopes of all regions of the EM spectrum like Ultraviolet Telescopes, Infrared Telescopes, etc.", though.

3

Both statements are correct: aperture is the most important factor, and many large aperture telescopes are resold due to lack of use. The brightness of any object you observe is determined by the aperture of the telescope ("light grasp"). With larger objects, such as planets, the aperture also determines the resolution: the amount of detail you're ...

2

That question made me curious too! The grazing can be quantified with the impact parameter b, easy to understand in this schematic from Winn 2011. When looking at exoplanet.eu diagrams with one axis set as the "Impact Parameter b", some planets show as having b~1.7 and 1.6, however these are from TTV (Transit-timing variation) measurements. I'm ...

2

The surface brightness of a source is by definition the flux density per solid angle; the surface brightness sensitivity of a telescope is, analogously, its point source sensitivity divided by the beam solid angle.$^{\dagger}$ Interferometers have smaller beam solid angles than dishes with the same area (Frayer 2017). Given that the solid angle is \$\Omega\...

2

If the signal is detected at the same time at two sites, then the source must be in the plane bisecting the sites. This constrains the source to a great circle in the sky. Generally with two detectors, the direction can be limited to a circle in the sky. With "clever techniques" (perhaps comparing the quality of the signal in the two detectors) ...

2

I first wanted to edit this into my other answer here, but since this, even though being highly relevant for the issue, does not directly address the OP's question(s), I decided to add this as a separate answer : The orbital elements used in the cited work for simulating the orbit of the Eagle ascent stage after being jettisoned from Apollo 11 are actually ...

1

Spectral wavelengths usually are defined as vacuum wavelength. The index of refraction of the medium influences the wavelength, and as such 'in vacuum' is both easier to define and measure: You'd need to define and specify composition, pressure and temperature in addition to the wavelength for each measurement in air - all of which is unnecessary if you ...

1

Arrive whithin a year = almost surely already within the heliosphere. I assume a stellar remnant black hole (bigger ones will be even more noticeable, smaller ones are still unknown). It will be a rather bright shiny x-ray source in the sky (we do have enough matter around to support a bright accretion disk). It will distort our outer planet's orbits for ...

1

A black hole approaching our solar system would have been detected hundred of years in advance due to nearby stars shifting position, although it's true nature wouldn't be known until decades ago, when the concept of black holes was first postulated and then confirmed. Even if the black hole were to enter the solar system, rather then pass nearby it, it ...

1

Here is a brief explanation of the various techniques that can be used to detect exoplanets. The scatter plot suggests that for a planet to be detected by direct imaging, it has to have (roughly speaking) an orbit as large as Jupiter's, and a mass as great as Jupiter's. So Jupiter looks like the best candidate. As for your second question, the declination of ...

1

The peaks in the temperature and polarization spectra determine the angular size of the sound horizon at the time of recombination fairly accurately: $$\theta = \frac{r}{D(z)}$$ The sound horizon, which is represented by r, is the comoving distance a sound wave could travel from the beginning of the universe to recombination and is a standard ruler is any ...

Only top voted, non community-wiki answers of a minimum length are eligible