<p>According to <a href="https://www.lexico.com/en/definition/modulus" rel="noreferrer">Oxford Dictionary of English</a>, the word "modulus" is the diminutive of the Latin <em>modus</em>, meaning <strong>measure</strong> (<em>modus</em>, in turn, comes from Proto-Indo-European <em>mod-os</em>, <a href="https://accademiadellacrusca.it/it/stazione-bibliografica/letimologico-vocabolario-della-lingua-italiana-con-cdrom-e-online/2658" rel="noreferrer">Nocentini &amp; Parenti 2010</a>.</p>

<p>Hence, the distance modulus is a "measure of distance", just like the modulus of a vector is a way of measuring its size, and Young's modulus is a way of measuring the stiffness of a material.</p>

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2019-11-07T09:02:39.543Z
2019-11-07T09:08:15.783Z
2019-11-07T09:08:15.783Z
pela
<p>The difference of apparent (m) and absolute (M) magnitudes of an object is called "distance modulus", which is related to the distance <span class="math-container">$d$</span> to the object in parsecs with equation</p>

<p><span class="math-container">$$m - M = 5 \log \Big(\frac{d}{10 \ \textrm{pc}}\Big) \tag{1}.$$</span></p>

<p>I'm trying to understand why <span class="math-container">$m - M$</span> is called "distance modulus". The word "distance" makes sense, because there is a distance d term in Eq. (1). But what is "modulus" word doing there? In math, modulus means the remainder after division of two numbers. For example, modulus of 5 and 3 is 2. </p>

<p>Another meaning of "modulus" is length of a vector. I don't see how either of these two meanings can be relevant to Eq. 1. </p>

<distances><apparent-magnitude>
Why is there "modulus" word in the "distance modulus" term?
Evgenii
# Why is there "modulus" word in the "distance modulus" term?

(Asked by: username_0 on 2019-11-07)

The difference of apparent (m) and absolute (M) magnitudes of an object is called "distance modulus", which is related to the distance $d$ to the object in parsecs with equation

$$m - M = 5 \log \Big(\frac{d}{10 \ \textrm{pc}}\Big) \tag{1}.$$

I'm trying to understand why $m - M$ is called "distance modulus". The word "distance" makes sense, because there is a distance d term in Eq. (1). But what is "modulus" word doing there? In math, modulus means the remainder after division of two numbers. For example, modulus of 5 and 3 is 2.

Another meaning of "modulus" is length of a vector. I don't see how either of these two meanings can be relevant to Eq. 1.

**Tags:** distances, apparent-magnitude


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## 1 answers

### Answer by username_1 on 2019-11-07. Score: 6 | Accepted answer

According to [Oxford Dictionary of English](https://www.lexico.com/en/definition/modulus), the word "modulus" is the diminutive of the Latin *modus*, meaning **measure** (*modus*, in turn, comes from Proto-Indo-European *mod-os*, [Nocentini & Parenti 2010](https://accademiadellacrusca.it/it/stazione-bibliografica/letimologico-vocabolario-della-lingua-italiana-con-cdrom-e-online/2658).

Hence, the distance modulus is a "measure of distance", just like the modulus of a vector is a way of measuring its size, and Young's modulus is a way of measuring the stiffness of a material.


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