<p><strong>annum (ka, Ma, Ga)</strong></p>

<p>Time before present. A point of time in the geological past, measured backwards from now. Often, “now” is defined as the year 1950.</p>

<p><strong>year (kyr, Myr, Gyr)</strong></p>

<p>A measure of duration, not necessarily age and not necessarily measured from the present time.</p>

<p><strong>examples</strong></p>

<ol>
<li>The fossil is dated to 110±2 Ma.</li>
<li>According to zircon dating, the granite crystallised at ~30 Ma.</li>
<li>The volcano was erupting intermittently for 10 kyr.</li>
<li>The Jurassic period lasted for 56 Myr, starting at 201 Ma and 145 Ma.</li>
<li>The Jurassic period lasted for 56 Myr, starting 201 Myr ago and ending 145 Myr ago.</li>
</ol>

CC BY-SA 4.0
2018-05-26T05:58:46.313Z
2020-02-23T03:00:42.240Z
2020-02-23T03:00:42.240Z
Gimelist
<p>Annum and year are not the exact same thing. The exact length of a year depends on what exactly you are measuring, whether distance between perihelions, distance between solstices, etc. It also varies because of the gravitation of other planets. These effects are minor, but enough that measuring units of years is tricky. So an annum is defined as 31,557,600 seconds exactly.</p>

CC BY-SA 4.0
2020-11-19T15:40:12.260Z
2020-11-19T15:40:12.260Z
James
<p>Yes and no. One annum ago is defined as 365.25 days. But a Julian year is not defined in seconds and so is not an SI unit. We live with a mixed system. And how do we pronounce 1 Ma ? If you are french or italian you may think it is 'il y a un million d'années'/'un milione di anni fa'. As the unit is latin, anglophones should/could say 'a million annos ago', using the accusative plural.
Further:</p>
<ol>
<li>many think the latin is in the nomiminative, so 1 Mega annus/anni. With units we typically write them in the singular but pronounce them in the plural, viz. 10 km.</li>
<li>one finds proposals to distinguish points in time from geological durations, the latter requiring yrs for clarity.</li>
<li>one notes the anglophone bias of such discussion.</li>
</ol>

CC BY-SA 4.0
2024-05-11T06:37:30.160Z
2024-05-15T06:45:23.307Z
2024-05-15T06:45:23.307Z
hillwalker
<p>I've noticed that geology and related fields use mega-annum (Ma) instead of mega-year (Myr) as the unit for 1 million years.  Why is that?  Is an "annum" different from a "year" in geology?</p>

CC BY-SA 4.0
2018-05-25T14:37:48.503Z
"The metric unit for year is annum." Do you have a source for this?  I couldn't find anything stating this.  In astronomy (my field) we use the metric system (cgs though instead of mks) and we never use "annum", always "year".
NeutronStar
CC BY-SA 4.0
2018-05-26T09:18:31.117Z
[Uni of North Carolina](https://www.unc.edu/~rowlett/units/dictY.html#year) & [PetroWiki](http://petrowiki.org/Nomenclature_for_SI_metric_conversion_tables). It was something I was taught years ago & all scientific literature, regardless of field, that I've read since has always used annum (a).
Fred
CC BY-SA 4.0
2018-05-26T12:55:10.533Z
@Fred Thanks for sharing that.  It's worth the mention though for clarity for other reads that the metric unit for time is the second, and that's it.  Any unit representing a time duration other than that is not overseen or mandated by the SI.
NeutronStar
CC BY-SA 4.0
2018-05-26T22:07:29.480Z
The real question is, why is the accusative form "annum" used rather than the nominative "anno"?
Spencer
<geology><time>
Why do the earth sciences use mega-annum instead of mega-year?
NeutronStar
# Why do the earth sciences use mega-annum instead of mega-year?

(Asked by: username_0 on 2018-05-24)

I've noticed that geology and related fields use mega-annum (Ma) instead of mega-year (Myr) as the unit for 1 million years. Why is that? Is an "annum" different from a "year" in geology?

**Tags:** geology, time

> **Comments:**
> **username_0**: "The metric unit for year is annum." Do you have a source for this?  I couldn't find anything stating this.  In astronomy (my field) we use the metric system (cgs though instead of mks) and we never use "annum", always "year".  
> **username_1**: [Uni of North Carolina](https://www.unc.edu/~rowlett/units/dictY.html#year) & [PetroWiki](http://petrowiki.org/Nomenclature_for_SI_metric_conversion_tables). It was something I was taught years ago & all scientific literature, regardless of field, that I've read since has always used annum (a).  
> **username_0**: @username_1 Thanks for sharing that.  It's worth the mention though for clarity for other reads that the metric unit for time is the second, and that's it.  Any unit representing a time duration other than that is not overseen or mandated by the SI.  
> **username_2**: The real question is, why is the accusative form "annum" used rather than the nominative "anno"?  

---

## 3 answers

### Answer by username_3 on 2018-05-26. Score: 7

**annum (ka, Ma, Ga)**

Time before present. A point of time in the geological past, measured backwards from now. Often, “now” is defined as the year 1950.

**year (kyr, Myr, Gyr)**

A measure of duration, not necessarily age and not necessarily measured from the present time.

**examples**

1. The fossil is dated to 110±2 Ma.
2. According to zircon dating, the granite crystallised at ~30 Ma.
3. The volcano was erupting intermittently for 10 kyr.
4. The Jurassic period lasted for 56 Myr, starting at 201 Ma and 145 Ma.
5. The Jurassic period lasted for 56 Myr, starting 201 Myr ago and ending 145 Myr ago.


---

### Answer by username_4 on 2020-11-19. Score: 4

Annum and year are not the exact same thing. The exact length of a year depends on what exactly you are measuring, whether distance between perihelions, distance between solstices, etc. It also varies because of the gravitation of other planets. These effects are minor, but enough that measuring units of years is tricky. So an annum is defined as 31,557,600 seconds exactly.


---

### Answer by username_5 on 2024-05-11. Score: 1

Yes and no. One annum ago is defined as 365.25 days. But a Julian year is not defined in seconds and so is not an SI unit. We live with a mixed system. And how do we pronounce 1 Ma ? If you are french or italian you may think it is 'il y a un million d'années'/'un milione di anni fa'. As the unit is latin, anglophones should/could say 'a million annos ago', using the accusative plural.
Further:

1. many think the latin is in the nomiminative, so 1 Mega annus/anni. With units we typically write them in the singular but pronounce them in the plural, viz. 10 km.
2. one finds proposals to distinguish points in time from geological durations, the latter requiring yrs for clarity.
3. one notes the anglophone bias of such discussion.


---

