My first line of defense for cracks like this is:
https://www.amazon.com/Captain-Tolle...s%2C161&sr=8-1
If you RT celestial windows are in a very poor gasket condition you may have to do the whole replacement of the gaskets route. But, being basically cheap and lazy, I usually just do what is necessary to provide a fix.
Boat folks really like this product. If it works, it works. If it doesn't you are out less than fifteen bucks and a little time to treat all three windows. Then, you can step up the ladder to the more involved repairs that will put the seals back like new.
If you search this forum or do a general web search, you will find others who have dealt with this .