Redshift Velocity Calculator
Redshift, written as z, is the fractional stretch of a spectral line as a source moves away from us, and it is the single most important number in extragalactic astronomy. At small values the recession velocity is simply z times the speed of light, but as z grows the naive formula overshoots and you must use the relativistic Doppler expression, which keeps the velocity below the speed of light no matter how large z gets. This calculator reports both results in kilometers per second and as a fraction of light speed, so you can see where the simple approximation breaks down. Enter your measured z below. All computation runs in your browser with nothing sent to a server.
How to use Redshift Velocity Calculator
- Enter the redshift value z from a spectrum.
- Read the Doppler and relativistic recession velocities.
- Note the result in km/s and as a fraction of light speed.
Frequently asked questions
How does it turn redshift z into a velocity?
It applies both the simple Doppler relation, velocity equals z times the speed of light, and the relativistic formula that keeps the result below light speed at high z.
When do the two formulas disagree?
At small z below about 0.1 they agree closely, but at large z the simple form overshoots and can exceed light speed, so the relativistic result is the physical one.
What does the result mean physically?
A positive z means the source is moving away, and the tool reports the recession velocity in km/s and as a fraction of the speed of light.
What is Redshift Velocity Calculator?
Redshift, written as z, is the fractional stretch of a spectral line as a source moves away from us, and it is the single most important number in extragalactic astronomy. At small values the recession velocity is simply z times the speed of light, but as z grows the naive formula overshoots and you must use the relativistic Doppler expression, which keeps the velocity below the speed of light no matter how large z gets.
Is Redshift Velocity Calculator free to use?
Yes. Redshift Velocity Calculator is completely free, with no sign-up and no usage limits.
Does Redshift Velocity Calculator work in a web browser?
Yes. Redshift Velocity Calculator runs in any modern web browser. There is nothing to download or install.
Is my data private with Redshift Velocity Calculator?
Yes. Redshift Velocity Calculator runs entirely on your device in your browser, so nothing you enter is uploaded to a server.
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