A very good question. You have doubtless been taught that unfinished periods of time, such as 'this week' and 'today', require the present perfect. However, in this case, you need to use the past simple, not the present perfect. 'When did the sun rise today?' Here's the reason. By asking 'When...?', you are focusing in on a specific point of time several hours ago. "When did the sun rise today?" "It rose at about 6.30 a.m." This conversation specifies the period of around 6.30 am - a time which is now in the PAST. That is why you have to use the past simple. The best way to understand the present perfect is to bear in mind that it is a present tense - it always tells you something about the PRESENT MOMENT. If you look at the conversation above, this doesn't tell you anything about the present moment, does it? It tells us about an event that happened many hours ago. Compare it with these conversations in the present perfect. Note that it should be 'has risen', not 'have', by the way: "Has the sun risen yet?" "No, it's still dark outside." In this conversation, we use the present perfect because the fact that the sun hasn't risen yet has an effect on the present moment - meaning that it is still dark. "What have you done today?" "I've cut the grass and swept the path. I haven't washed the car yet, though." In this conversation, what the person has done so far today tells us about the present situation - it tells us that the grass is now tidy and the path is now clean, but the car is still dirty. It's all about the present. I hope that makes sense.

When did the sun rise. When did the sun rise today. They both work.

I think it sounds good , with or without the word "today "