Chemistry of life - carbon-14 dating question

Hi,

In the Chemistry of life article "Atomic number, atomic mass, and isotopes" the following appears:

After a half-life of approximately 5,730 years, half of the carbon-14 that was initially present will have been converted to nitrogen-14. This property can be used to date formerly living objects such as old bones or wood. By comparing the ratio of carbon-14 to carbon-12 concentrations in an object to the same ratio in the atmosphere, equivalent to the starting concentration for the object, the fraction of the isotope that has not yet decayed can be determined.

I understand this as follows: we establish a ratio of carbon-12 to carbon-14 in the current atmosphere. Then we compare this ratio to the ratio of carbon-12 to carbon-14 in the sample. By establishing how much of the carbon-14 in the sample has decayed relative to the carbon-14 in the atmosphere we know how long it has been decaying.

If this is correct, does this mean that we assume (know?) that the ratio of carbon-12 to carbon-14 in the atmosphere has been relatively stable throughout time, since otherwise this would skew the comparison?