It means in the 17th century (1600 until let's say 1620, 1630...). It makes the reference of the "begging" of the 17th century. 1640-1650 would be considered as the middle of the 17th century and so on. If by "in the past" you mean in the entire century, you could say "back in the 17th century" for example.

The 17th Century ends in the year 1700 1601 is the first year of the 17th Century The 19th Century, for example begins in 1801 and it ends in 1900 1901 is the first year of the 20th Century

It means "in the earlier years of the seventeenth century." Galileo discovered the moons of Jupiter early in the 17th century. (1610). Isaac Newton published his theory of gravitation late in the 17th century (1687) If you wanted to combine the ideas of "seventeenth century" and "long ago," for example if talking to young students, you would use some other phrase, like "Galileo and Newton lived long ago, in the seventeenth century."