Answer: As you have noticed, the amount of sunlight can vary dramatically due to ovehanging trees as well as the angle from which a location faces the sun. Luckily, many plants are tolerant of a range of lighting conditions within some general parameters and will adapt.
In general, a location in uninterrupted sun all day long (for example the middle of an open lawn)or a location with uninterrupted sun for at least six hours a day with those hours including noon, would be considered full sun.
Locations with sun all afternoon or all morning could both be considered full sun, but the location with afternoon sun will be by far the hotter and harsher of the two and as a result you would nearly always select plants with a preference for full sun (rather than partial sun) for that location.
A spot with good morning sun could be considered partial shade or partial sun, and may require some experimentation to see which plants do best there.
Overhanging trees can cast a solid shade or a dappled shade. Dappled shade can be counted as partial shade to shade and will again require some trial and error.
Many shade loving plants need a moist soil, so keep in mind too that trees can affect the planting location by stealing moisture from the soil. In a shadier location plants may tolerate a slightly drier soil than they would in a sunnier location. Again, trial and error will be the best way to determine the best plants.
Seasonal changes can also affect lighting because the sun is higher in the sky during the summer. This makes shadows less, and so a location can be sunnier in the summer than in the winter.
Finally, sometimes a symmetrical planting of matched pairs is nearly impossible due to differences in growing conditions. You might take a look at other houses in your neighborhood with similar conditions to see what is doing well (or badly) for them and use that information in your decision making process.
I hope this helps, shade is truly one of the most interesting planting conditions to address and its nuances nearly defy description.
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