# mya (time scale)

The term mya is an acronym and stands for English. million years ago "million (s) years ago", "x million (s) years ago" (see millions of years ). mya is mainly used in archeology and geology ( geological timescale ) to indicate a specific point in time in the past. The reference point for the indication of time is the present, whereby the time passing now (e.g. today or this year) can be viewed as a moment.

More precisely, mya is just a - albeit the most common - sub-form of ya ( years ago ) with a prefix . It is thus in a row with the far less used forms tya ( thousand years ago ) and kya ( kilo years ago , "thousand years ago") and bya ( billion years ago , "billion (n) years ago").

## Conflicts to the international system of units and possible solutions

The abbreviation mya does not correspond to the International System of Units (SI) in three points, each of which can be assigned to one of its letters :

1. ( m illion): Neither t (for tya) nor b (for bya) are internationally recognized prefixes for units of measurement . In the International System of Units (SI), the small m (for mya) stands for milli , i.e. thousandths , and is therefore also not SI-conform.
In the English-speaking world in particular, there is therefore an attempt to approximate the International System of Units by using kya ( kilo years ago ) instead of tya and Gya ( giga years ago ) instead of bya .
2. ( y ear): y is not a coherent SI unit or even an SI base unit .
Instead of y or yr (for year , year), the derived SI unit a (Latin annus , year ) can be used, a multiple of the SI base unit second . This leads to the following formulations:
• 1 tya = "1,000 years ago" = "1 ka ago" (English "1 ka ago")
• 1 mya = "before 1 million years" = "1 Ma " (Engl. "1 ma ago")
• 1 bya = "1,000,000,000 years ago" = "1 Ga ago" (English "1 ga ago")
3. Unlike time units , this scale does not designate a period of time , but a point in time in the past.
Since the context often shows that it must be a point in time in the past, mya is often simply replaced by Ma . Physically, this means that a point in time is replaced by a period of time , but this does not necessarily pose a problem if one agrees on a definition (such as the ISO standard 8601 for specifying dates). Here it becomes clear why the abbreviation mya is still so popular: It stands for the phrase “ so and so many years ago”, which means, even without a definition, that it is a point in time so and so many years ago in the past got to.
If it is not apparent from the context that the past is involved, a bp (for " before present ") is appended to the latter when mya is replaced by Ma . This also makes it clear at the same time that it is a point in time and not a period of time. Ma bp does not have to mean exactly the same as mya (see reference point ).

## Reference point

It is not entirely clear when specifying times with mya whether the abbreviation is to be taken literally and whether it actually refers to the respective now of the viewer or whether there is an exact reference date or an exact zero point. For the latter, for example, the year 1950 is mentioned (as with Before Present ), possibly the year zero can also be used as a reference point. Since times with mya are usually times from several million years ago, at which the dating uncertainty is already in the range of a few 10,000 to 100,000 years, the span of around 2000 years in which the three mentioned here " possible “reference points are almost always neglected.

## Individual evidence

1. ^ North American Commission on Stratigraphic Nomenclature: North American Stratigraphic Code. Retrieved March 9, 2011 : " Such qualifiers as" ago "or" before the present "are omitted after the value because measurement of the duration from the present to the past is implicit in the designation. "
2. NACSN: North American Stratigraphic Code. Retrieved March 9, 2011 : " The" present "refers to 1950 AD. "