# Questions tagged [units]

Use for questions about standards of measurement used for different types of quantities, and for conversions between units.

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### How do I create a comoving quantity using YT?

I can assign a unit to a scalar using: x = YTQuantity(scalar, 'kpc') but how would I specify that the unit should be kpccm? Perhaps it doesn't even make sense to do so? Something as simple as this is ...
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### What does par. lines mean in relation to a telescope aperture in 19th century German astronomical publications?

I am trying to understand what is meant by par. lines in an 1867 article "The Aberration of the fixed stars after the wave theory" by Prof. W. Klinkerfues of Royal Astronomy Works of ...
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### Astropy units issue

hope you're all doing well. I'm not very familiarized with AstroPy and I'm having a problem computing the conversion between angular momentum units. If I define the ...
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### How to calculate an infrared SED from the specific intensities?

I have values of total infrared fluxes (SED) in literature expressed in Jy (Jansky) for galaxies. I want to assure that my infrared radial profiles of the same galaxies in MJy/sr (and other in Jy/...
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### Convert from Jy/beam km/s to W/m$^2$

I need to compare some ALMA observations of a protoplanetary disk to disk-integrated fluxes obtained from a model. The ALMA observations are upper limits of non-detected spectral lines, where the RMS ...
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### Flux Density Unit Conversion [closed]

I am confused how to convert from ${\rm cm^{-2}\,s^{-1}\,Å^{-1}}$ to ${\rm erg\,cm^{-2}\,s^{-1}\,Å^{-1}}$. How do I do that?
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1 vote
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### Rayleighs units conversion

I have a question regarding the units conversion from photon counts on a detector to Rayleighs. I am pretty new to the subject but I would like to know more. The problem is the following : I know that ...
1 vote
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### RA expressed in degrees

Using the Python package Skyfield I thought that I could get the RA expressed in degrees simply by doing ra.degrees, but I get the following message this angle is usually expressed in hours, not ...
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### Units and some equations

I'm a grad student and I feel like this is an extremely elementary question that I should know, but I admit that I don't. Occasionally in science I find myself solving an equation for some answer ...
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### Mathamatica vs Sympy and Astropy to solve basic astronomy problems: solving linear equations and unit conversion

I realize this may be slightly off topic, but I also believe this is probably the best way to get an answer to my question: What is the best combination of software packages / programming languages to ...
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### Unit problem regards to proton

In a textbook, we have an example: The density of matter in the interstellar medium is of order one proton per ${\rm cm^3}$, then the total mass of the interstellar medium is of order (The $10^6$ ...
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### 'Little h' usage in cosmological simulations

I am running a cosmological simulation and am having some trouble putting things into code units. The physical distance units in my simulation are in terms of $\text{Mpc/h}$, where $h$ is the ...
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### Units confusion while calculating flux of a star using distance and luminosity, Python

I am trying to calculate the flux of various stars in Python. I am using the information from a database containing all stars in Hipparcos, Yale Bright Star, and Gliese catalogs (almost 120,000 stars)....
669 views

### Correct units to use while plotting mollweide projection of sky chart with RA and Dec values, matplotlib

I am trying to plot a mollweide projection of a sky chart in Python using the matplotlib package. I am using the RA and Dec values from a database containing all stars in Hipparcos, Yale Bright Star, ...
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### Finding the radius of a star in arcsec

If i know the radius r of a star in cm, how can I convert it to arcseconds? For example, if I have a star with r = 3.18e13 cm, and distance to the star d = 220 parsecs, what is the relation to ...
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### How to convert density from physical unit to comoving unit?

I am learning co-moving units and I am interested to see how the conversion works. If Hubble constant is assumed to be 0.7, then I want to convert $1gm/cm^3$ into comoving unit. If I want to express ...
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### Units of integrated flux density (irradiance) along longitudes

If I've taken an image of the Galactic bulge with the pixel units in MJ per steradian: and I've looked down each column of the image and added up the values: then do the units change? My thinking is ...
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### Astrodynamic Canonical Units [closed]

Maybe this is more appropriate for the Physics or Maths, but anyway. I am struggling with applying Canonical units with formulas. I am struggling specifically with how they are used, am I to convert ...
1 vote
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### Natural units of star clusters angular momentum and energy

I'm calculating the total angular momentum of a bunch of theoretical stars of a few solar masses, separated by a few light years, moving at velocities of about a few dozens of km / sec. Mechanical ...
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### How are people converting intensities in Janskys to Kelvin?

I'm reading some ALMA proposals and I am often seeing a conversion from Jy to Kelvin when, for example, people quote noise levels or source flux. For example on their sensitivity calculator (https://...
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### Why don't astronomers use meters to measure astronomical distances?

In astronomy distances are generally expressed in non-metric units like: light-years, astronomical units (AU), parsecs, etc. Why don't they use meters (or multiples thereof) to measure distances, as ...
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### wavelength-based IFU data cubes VS velocity-based IFU data cubes

I have 2 simulated IFU data cubes of the same observation, let's say A and B. The 3rd dimension of cube A is in wavelengths. In order to create cube B I just rebinned cube A in log-wavelengths and ...
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### ${\rm W/m}^2$ to Jansky [duplicate]

Lets assume I have a source and receive $8 \times 10^{-20}$ ${\rm Wm}^{-2}$ in (400-750) nm filter. How do you convert this received flux to the unit Jansky, is there a formula? (Ultimately, I want to ...
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### Why levels of radio contours maps are given in mJy/beam and what does it mean?

I know what a jansky is, but I don't understand what beam (in $\frac{1 Jy}{beam}$) is. If jansky is a unit of flux density, what kind of unit is $\frac{1 Jy}{beam}$?
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### Does the age of the universe take into account General Relativity / Special Relativity?

It is generally accepted that the age of the universe is approximately 12-15 billion years old based on the speed of the expansion of the universe. Since everything is moving very fast away from us, ...
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### Converting ångström spectral dimension to galaxy speed (km/s)

I have a spectral cube (in FITS format) whose spectral dimension is in ångströms. The sampling along the spectral dimension is 0.28A (CDELT=0.28). The observation in the cube is Ha emission of a ...
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### Is the Earth-Sun distance 1.012 AU?

Wolfram Alpha's query: Sun Earth distance in AU gives: 1.012 au (astronomical units) They definitely use this value in other places too - e.g. Earth-Sun $L_1$ distances from Earth and from Sun ...
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### Astrophysical "unitised" version of the Gravitational constant

This question relates formalising the gravitational constant, $G$, (which in S.I. units is usually quoted as $\sim6.67\times10^{-11}\,\mathrm{m}^{3}\,\mathrm{kg}^{-1}\,\mathrm{s}^{-2}$) in units which ...
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### What is projected separation and how can I make sense of its unit (h^-1 kpc)?

I have been reading this paper on the separation of a binary quasar in which it states: At a projected separation of 8.3/h kpc (Omega_matter = 1), CTQ 839 would be the smallest projected separation ...
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### How to scale down solar system data to simulatable values

Okay, I am seriously ashamed for asking this especialy when I geniualy study on physics but there is something that bugs me with the simulation I am working on. I am re-creating the solar system in an ...
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### What are the units of distance in this subtended angle calculation?

I'd like to find the distance at which a 2.4 cm coin is subtended by an angle of 10". I've drawn my diagram and found that $D=\dfrac{d}{\alpha}$ using the small angle approximation. Since \$10" = 4.85\...
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### How often and over what period is Earth’s rotation averaged to compute UT1?

I understand that UT1 (and for that matter UT0, UT2, etc.) are based on averages of actual earth rotation, and serve as a form of mean solar time. However it’s not clear to me when these averages are ...
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### What are some applicable problems with the correct usage of G?

In some fields of astrophysics, where distances are measured in parsecs (pc), velocities in kilometers per second (km/s) and masses in solar units (), it is useful to express G as approximately 4.302 ...
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### What is frequency range?

Using the NED, you can search via frequency range corresponding to the selected pass band filter. I'm a beginner here and don't really understand. Here's the quote: NOTES: Objects with observations ...
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