The words astrologers, stargazers, and monthly prognosticators refers to people who studied the stars and the movement of the moon in order to foretell the future. This practice was especially popular among the ancient Babylonians.
In Paul’s time, slaves were branded with distinctive marks to show that they belonged to their masters, much as cattle are branded in modern times (Psa 40:6). Paul declared that his body bore marks from the persecution he had endured in Christ’s service. These showed the he belonged to the Lord Jesus.