Caesar Augustus is the Emperor of Rome, and he is the ruler of the land at the time. 

He ruled a lot of area including Italy where he lived and Israel where all the Hebrews lived. Because of all this land and people he ruled over, they wanted to have an accurate count of all the people in the area so they could collect taxes for the government. Because both Joseph and Mary's way back great-grandfather was David, they had to travel to the town of Bethlehem which was David's birthplace.

They have to pack up all their things, or as much as they can, and move from their new home to an even newer area.

Now it's time for Mary and Joseph to travel to Bethlehem #9

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Luke 2:1-5

1 And it came to pass in those days, that there went out a decree from Caesar Augustus, that all the world should be taxed.

2 (And this taxing was first made when Cyrenius was governor of Syria.)

3 And all went to be taxed, every one into his own city.

4 And Joseph also went up from Galilee, out of the city of Nazareth, into Judaea, unto the city of David, which is called Bethlehem; (because he was of the house and lineage of David:)

5 To be taxed with Mary his espoused wife, being great with child.