If you grew up in L.A. during the '80s/'90s like I did, appreciating the sun wasn't exactly encouraged by the world around you. Sun meant smog alerts and "indoor recess." Sun meant the beach where you risked getting a rash from swimming in the polluted ocean. Sun meant skin cancer, and then when the sun went down, you had to hurry indoors so you wouldn't breathe in the malathion spray/aerosol mixture sprayed by fleets of helicopters to combat the Mediterranean fruit fly. Sun can also mean global warming ... and off away from the sunshine and down a rabbit hole goes my brain.
The sun hasn't exactly held exalted status for me, so awhile back when I began to read the instructions for a chili pepper plant that called for direct sun, I squinted up at the sky and frowned. It seemed so simple, but if you asked me what direct sun meant, I wouldn't have been able to give you a straight answer. I had held the gut reaction that sun = bad for so long and spent so much time avoiding it I didn't realize I had no real reference point from which to get this answer: What was direct sun? I'm outside, and it's bright directly because of the sun, so is that direct sun?
"Direct sunlight is sun shining right on the plant at least 6 hours a day, not through a window," Renee Shepherd of Renee's Garden graciously explained. This means if you put your hand over the plant and sunlight hits your hand, that's direct sunlight. "Shade is literally shady, like under an umbrella, but partial or dappled shade depends on the plant," she says. "Partial shade can mean morning sun and afternoon shade, or morning shade and afternoon sun, which means sunshine in the afternoon will be hotter." As a good rule of thumb, dappled shade is when the sun gets filtered through a trellis or through leaves.
"When considering which crops to grow in shady areas," Mother Earth News puts succinctly, "Think of them in terms of leaves and roots. Crops we grow for their leaves (kale, lettuce, spinach) and those we grow for their roots (beets, carrots, turnips) will do fairly well in partially shady conditions. The crops we grow for their fruits -- such as eggplants, peppers and tomatoes -- really do need at least six hours of full sun per day."
A good quick way to see if your plants aren't getting enough sun is if you see them bending toward a light source, if they're growth is especially slow, or if the plants look "leggy," meaning they look kind of spindly or elongated.
To get more specific, another way of measuring the light a plant needs is by foot-candles, or fc. Basically it's good to have a basic understanding of foot-candles if phrases like "partial shade" feel amorphous and make you nervous for lack of control over the specific meaning of such things. I won't get too much into this because I don't want anyone to be like, "Foot candles? This is just great," and throw their laptop on the floor. If you'd like to know a little more about foot-candles, the Seattle Post-Intelligencer has a tiny primer. Very tiny.
One more thing: Renee raised an important point about the connection between heat and sunshine. This is where climate zones come in. It's good to know what climate zone you're in because it helps figure out how your plants are going to hold up. There are 2 climate zone maps you can look at, the USDA Plant Hardiness Zone Map and Sunset's Climate Zone map. While the USDA map is a good comprehensive map, it only measures the average minimum temperatures for a particular region. So while true, it's not likely to snow in Los Angeles anytime soon, it still doesn't take into account the difference between say, the microclimates along the coast and in the Valley. Punch in your zip code and how much sunlight you have to see what plants will grow best under your given conditions.
Sun: not so bad after all. Not exactly simple ... but not bad.