Since I critiqued the companion piece to this and saw that no one has reviewed this one yet, I thought I would review this one too.
Again, I think the clouds are done very well with the tapering edges. Unlike Moon, the clouds are layered nicely and the use of the reds, oranges, and yellows of the setting (rising?) sun gives them dimension. I can understand how it would be difficult to give the clouds in Moon the same dimension as the one in Sun because there are less colors to work with. Like moon, the colors are muted nicely, but instead of being muted with blue-grey, this one is muted with purple and orange which is a nice touch. I like how the mane and tail merge together and the tail sort of fades away, at least that what it looks like to me. Then again it can be just the mane. The mane is very translucent looking and it seems that there are colors writhing just below the surface. Her mane and tail truly looks like the dawn. I also like how the day transitions into the night in the background.
Now for the awkward things. The sketch lines need to be erased, especially the ones by her forelegs because they are out of place and a bit distracting. There is also a pink thing in the small space between her mane and rump which I assume is the base of her tail. To me, its positioned awkwardly and I think it would look a bit better if it was slightly lowered. Celestia's wings look better than Luna's, but I suggest making the feathers closer to the joint a bit fuller and longer. Speaking of wings, where is the other one? Judge from the position of her body and the perspective, wouldn't the wing be a bit visible? I know the whole wing won't show, but I believe that there should a little bit of it peaking out from behind her back. Also, where are her back legs? The lower half of her body just seems to vanish.
As for the sun, I find it to be both good and bad. I like it because it is Celestia's cutie mark that is lighting up the scene, and it is both muted and colored well. The symbolism is a good choice. The other side of it is that my need for symmetry is making me twitch. What I mean is that in "Moon" you have the moon in the sky, but in this one you have the cutie mark. It's just me nitpicking, but I would like to see "Moon" have Luna's cutie mark in the sky or replace the cutie mark in "Sun" to be an actual sun. Again it's just my need for symmetry that is making me squirm.
Overall, I enjoy this piece and "Moon" is a good companion piece for this. My favorite part is her mane and tail. The use of purple for creating the shadows on her body is a good choice versus black which aid in the muteness of the colors. I look forward to more of your works.