Is this a troll post?! So, Rhaegar saw a red comet when his son was conceived. Not when he was born. Rhaegar went missing AFTER his son was born.
From what we know Aemon and Rhaegar's last correspondence was sometime after Aegon was conceived and before he ran off/kidnapped Lyanna.
Everyone & all their domesticated animals in Westeros know Rhaegar kidnapped/ran off with Lyanna. From what we know from the series, news of the realm reaches the Wall. So Aemon learning about what transpired is not farfetched.
Also, if Jon is Robert's son, why doesn't Robert know?! It's not like Robert doesn't have bastards from Nobel Familes in Westeros.
Or are you claiming Jon was a legitimate Baratheon? Because that makes it even less logical.
Either way, Lyanna did not like Robert. She thought he was a womaniser & didn't mainly find it appealing to marry Robert.