What is the significance that his seed will always be?
Radak: They will always be in kingship.
What is the comparison of his throne to the sun?
Radak #1: Just like the sun exists forever, so his seed will always be in kingship. Similarly, "Lifnei Shemesh Yinun Shmo" (72:17) - as long as the sun endures.
Radak #2: The good ones of his sons will be bright and illuminate like the sun.
Malbim: Even though the throne will cease sometimes when they are not worthy, it will not cease permanently. This is like the sun and moon (verse 38); the sun's light is constant, and never lacking 1 ; it is from itself.
I.e. even though there are times when we do not see it, e.g. night. (PF)
