Erice?

If the light that is important to one of you is for photographs, then early morning and late afternoon/early evening are best, especially on a sunny day (mid day sun is not good for photos). If the weather is overcast but bright enough for shadows then that isn't as important. If the need for light is for some other reason, you'll have to expand on that info.

We like to have the sun at our back, not in front of us (or overhead). More important, though, is: can we get there by bus from Palermo?

You can get to Erice from Trapani by both bus and funicular. You can get to Trapani from Palermo by bus. But not sure of Palermo to Erice by bus. There might be a tour that would work. Or possibly you could find a driver to take you there (expensive).

I’m a little puzzled by “ruins”. I don’t recall anything I would call a ruin in Erice. And temple? Are you perhaps thinking of Segesta instead of Erice??

Public transportation into that corner of Sicily does exist, but it is rather slow. I suspect it will take 3 hours or more just to get from Palermo to Trapani. You would then need to get to the bottom of the Erice funicular (or a bus stop that provides service to Erice). I'd estimate about 4 hours one way, and I don't know how often the buses run from Palermo to Trapani.

If your interest is actually Segesta, I can't help with that, because I haven't been there.

I think the poster wants to visit Segesta which she called ruins???

In May 2014 I stayed 2 nights in Erice and absolutely loved it! Great ancient hill town, great food, very nice people. Especially wonderful in the early morning and evening mist, without day trippers around. However, I rented a car at Palermo Airport, and started my trip by driving to Erice.
I then took a driving day trip to Segesta (certainly a highlight, the temple, the theater and the walk down from the theater to the temple. I was there mid-to-late late morning, the light for photography was excellent, and on to the more expansive ruins at Selinunte. I came back up the coast, planning to stop in Marsala and Trapani, but traffic and parking were so congested In those towns that I never found a parking space in either town, and didn’t stop.
The owner of the apartment where I stayed in Erice cautioned me about depending on the funicular between Erice and Trapani, because: (1) if you take the trip either way, and then the weather changes or wind picks up, the funicular can stop running and you need to find a different way to return; (2) at the Trapani end, the terminus of the funicular is well outside of the center of town.
You might find a day tour from Palermo that includes both Erice and a Segesta.

From the funicular in Trapani to the historic center is about 2 miles. Up towards the funicular end there are fig palms that line the street. When we were in Agrigento, the light, to photograph the temples, was nice around dusk. After Agrigento and Selinunte, we skipped Segesta. To visit Segesta from Palermo I believe there is a bus, but you’d need to return to Palermo to catch a bus to Erice.

Thank you all so much! These responses are very helpful in my planning.