I keep on noticing that I get the slow sown message when I pass a planet or star, even though I am selecting another astrological body. My thinking is that the message is set to go off whenever the ship is being effected by a gravitational force, so I think it would be helpful just to let Commanders know they are being effected by said gravity instead of telling them to slow down. This would also get Commanders to do more gravitational whips, making Supercruse even more fun.