Sascha,

The following is a link to download a Mathematica UnitsHelper application. Its main palette has drop down menus for all the Mathematica physical constants, also all the units sorted alphabetically or by magnitude and that will paste into your notebook. Also links to all the Mathematica units help pages and NIST pages. There are also extra routines for implementing reduced units such as atomic units or geometrical units.

UnitsHelper Download

Ah, you mean

In[3]:= Quantity["earth's gravity"]
Out[3]= Quantity[1, "StandardAccelerationOfGravity"]

then you can

In[9]:= QuantityForm[
 Quantity[1, "StandardAccelerationOfGravity"], "LongForm"]
Out[9]//QuantityForm= StringForm["`1` `2`", 1, "standard acceleration \
due to gravity on the surface of the earth"]

In Quantity it reads

Supported units include all those specified by NIST Special Publication 811.

And here it is

In[25]:= Take[
 EntityValue["PhysicalConstant", {"Name", "Value"}], {187, 187}]

During evaluation of In[25]:= Quantity::unkunit: Unable to interpret unit specification EddingtonConstant4.

During evaluation of In[25]:= Quantity::unkunit: Unable to interpret unit specification ElectronProtonElectricGravitationalForceRatio.

During evaluation of In[25]:= Quantity::unkunit: Unable to interpret unit specification (SpeedOfLight)^2/(MONDConstant).

During evaluation of In[25]:= General::stop: Further output of Quantity::unkunit will be suppressed during this calculation.

Out[25]= {{"standard acceleration of gravity", 
  Quantity[196133/20000, ("Meters")/("Seconds")^2]}}

For the sake of diversity it has four different textual representations

• standard acceleration of gravity
• StandardAccelerationOfGravity
• standard acceleration due to gravity on the surface of the earth
• g

see QuantityForm.

In Mathematica, you may enter Quantity["earth's gravity"] and get a valid result. However, I haven't found "earth's gravity" anywhere. The function call provided by you doesn't deliver that quantity either.

You could try whether the FormulaLookup feeded with the newly accepted fundamental constants following As of Today, the Fundamental Constants of Physics (c, h, e, k, NA) Are Finally… Constant! delivers all known data. This can be considered as the systematic, canonical way.