Full Sun vs Part Sun

Maybe the western exposure of my balcony has changed over the years. It seemed I had more direct sunshine years ago. For a dozen years, I planted predominantly Geraniums and Petunias with very good success.

Maybe the difference is that more of my plantings are inside my railings and therefore more protected from the sun.

Full Sun: Full sun exposure is defined as six (6) or more hours of direct sun. At this point, which is nearly 3 weeks into my summer planting, I have regrets for some of the Full Sun plants I have purchased including Ageratum, Marigolds, Petunias, Lilies and others.

Part Sun: It does make me wonder how to understand the concept of part sun. Less than 6-8 hours of direct sun. Does this mean that the shade plants would be over-exposed? Does it mean that my full-sun plants will fail to thrive?

It will simply be a matter of learning. I know that I will be moving some of the "sun" plants out to a sunny portion of the common grounds. I'll do some shifting and some cutting. Er, eliminating.