I am working through a derivation in relativity and find the statement that A (unit is 1/distance) is small. I don't see how we can make such a statement when A could have the unit or equally the unit could be .

Some background:

The equation that we are solving is:

We propose a solution of the form:

Where is the distance between a planet and its sun, A and are constants. is a slowly varying function of . Clearly A could be, numerically, small if we use light years as the unit of measure.

Does the given DE tell me that u (and hence A) is small?