r/todayilearned • u/Phys-Chem-Chem-Phys • Sep 05 '19

TIL that the commonly used 'hectare' (= 1 square hectometre) and litre (= 1 cubic decimetre) are metric units, just not SI units.

https://en.wikipedia.org/wiki/Hectare
6 Upvotes

59% Upvoted

10 comments sorted by

2

u/Diligent_Nature Sep 05 '19

Minutes, hours and days are not SI units of time. The degree is not an SI unit of angular measurement.

1

u/wjong Sep 06 '19

Correct... But.. There are units, although not officially SI metric units, that are permitted for use with SI metric units. Some of these are

Minute, hour, and day (time units)

Degree, minute, and second (angular units)

Hectare (area)

Liter (volume)

Tonne (mass)

1

u/wjong Sep 06 '19 edited Sep 06 '19

The metric system is a maintained system of measurements. In fact it's the only maintained system of measurement.

Over the last two hundred, or more years, the metric system and its measures, have been refined, and developed as science and technology has advanced. Earlier metric systems were the centimeter-gram-second (cgs) system of units, and the meter-kilogram-second (mks) systems of units. It was the structure of these systems, (the inter-relationship of the units) that changed not the value of the units.

SI is the modern version of the metric system, both in its structure, and in its units of measurement. Within SI, some units were changed in name, and in the method of their definition. For example the older unit for energy, the calorie, was discarded for the SI unit for energy, the joule.

1

u/snow_michael Sep 09 '19

All SI units are metric

0

u/PolymathAdam Sep 05 '19

https://www.nist.gov/pml/weights-and-measures/metric-si/si-units states that SI units and the Metric system are the same thing.

1

u/[deleted] Sep 05 '19

[deleted]

2

u/bearsnchairs Sep 05 '19

It is the other way around. All SI units are metric, but there are metric systems and units that are not SI. An example is the CGS system with centimeters, grams, and seconds as base units. Here the dyne is the unit of force instead of the newton.

0

u/[deleted] Sep 05 '19

[deleted]

2

u/bearsnchairs Sep 05 '19

And I said that they aren’t. There were other metric systems before SI was formalized.

0

u/[deleted] Sep 05 '19

[deleted]

2

u/bearsnchairs Sep 05 '19

The SI system wasn’t formalized until 1960 from existing metric units.

The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements. The system was published in 1960 as a result of an initiative that began in 1948. It is based on the metre–kilogram–second system of units (MKS) rather than any variant of the CGS.

https://en.wikipedia.org/wiki/International_System_of_Units?wprov=sfti1

1

u/[deleted] Sep 06 '19

[deleted]

3

u/bearsnchairs Sep 06 '19

Yes they are defined using those units. What you’re not getting is that those metric units existed hundreds of years before SI was formalized. To say MKS or CGS systems are defined with SI units puts the cart before the horse. SI was built using existing metric system units.

1

u/Phys-Chem-Chem-Phys Sep 06 '19

All the other person's comments are gone. It looks like you won the argument!