# 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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### When analyzing a particle moving at Kepler speed, √(GM/R) in a planetary magnetic field, do we need to convert it to angular speed, √(GM/R^3)?

A charged particle is moving in a planetary magnetic field, in a spherical polar coordinates which is corotating with the magnetic field. To solve the equations of motion numerically what would be the ...
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### How to convert mW/m^2 to W/m^2/nm?

I have a vizier catalog with H-alpha fluxes as logF(Haplha) in units as mW/m^2. See sample data here: http://vizier.cds.unistra.fr/viz-bin/VizieR-3?-source=J/MNRAS/431/2/fluxes How do I convert ...
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### Using Gaia Passbands with L_nu coordinates

I am trying to find the coordinates of my bundles of gas on the gaia ($G, G_{bp}-G_{rp}$) hr diagram. I have its spectrum in $\nu L_\nu$ coordinates. Thus far, my attempts have mainly been to ...
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### How do nuJ(nu) units work?

I'm working with (Nublado) Cloudy, which is essentially a spectral modelling tool for stellar matter. Using the software, I model my own stellar object and obtain its continuum. According to Hazy, ...
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### What is the unit of the correlation function between logarithmically rebinned galaxy spectrum and stellar template spectrum?

Assume I have a logarithmically rebinned galactic spectrum $G(x)$ and a stellar template spectrum $S(x)$, where $x = \ln{\lambda}$. When $[\lambda] = 1 Å$, then $[x] = 1 Np/Å$. The fourier transformed ...
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### How to convert Flux Density to Flux?

I have a radio catalogue with flux density values. (Jy/bm) I'm confused converting this value to flux. Because I want to calculate luminosity, I need flux values. The unit made me confused. Since I ...
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### Conversion to Jy of Hubble WFC3 units (electrons/s) and of Spitzer IRAC (MJy/sr)

I am trying to figure out how to convert the units of some FITS files to Jy. These files refer to observations in different bands using different instruments. I list them below with my way of ...
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### Units in distance parallax relation

Relation between distance d and paralax plx is: $$d (pc) = 1 / plx(arc)$$ When I want to express plx in mas, will it be $$d (pc) = 1/[plx(mas)*1000]$$ or $$d (pc) = 1000/plx(mas)$$ Thank you very much
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### What does using these units (c=G=M=1) in MHD simulations mean?

I was reading some papers on numerical HD/MHD simulations of black hole accretion disks, and most of them scale their physical quantities to this form of units: c = G = M = 1. I know c = 1 and G = 1 ...
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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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### 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)....
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### 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 ...
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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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