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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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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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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A "strange" unit radio astronomy
I'm reading up on radio astronomy, and I came across this paper from 1964. At the bottom of page 193, the author uses a unit that I've not seen before in discussing radio power emission from stars:
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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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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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Converting from $\mathrm{W \: m^{-2}}$ to $\mathrm{Jy \: km \: s^{-1}}$
Suppose I observe a spectral line with rest frequency $\nu_0$ and integrated flux density $F = 10^{-26} \mathrm{W/m^2}$. Is the following line of reasoning correct?
$1 \: \mathrm{Jy} = 10^{-26} \: \...
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Convert from Jy/beam km/s to W/m$^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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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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apparent size of an astronomical body
What's the name of the measure of apparent (visible) size of a body? e.g. the Sun and the Moon appear to be roughly the same size, despite the Sun being vastly bigger - because the Moon is much closer....
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What does BJD-2450000 mean?
In different papers I've encountered time indications in BJD-2450000. I know that BJD stands for Barycentric Julian Date, but I don't understand the meaning of the suffix -2450000.
This is an example ...
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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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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 Jy/beam to Jy?
Maybe its a dumb question, but to convert Jy/beam to Jy, I just have to multiply it by the beam size in sr right?
Being $\Omega$ the beam size:
$\Omega = \frac{\pi \theta_{maj} ~~ \theta_{min}}{4 \...
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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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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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