“Look!” The Greek word is idou (#2400 ἰδού), and it is used to get our attention. Although the literal meaning of idou relates to visual perception (seeing), it was used idiomatically, and thus should be translated in ways appropriate to the context, such as “look,” “listen,” “pay attention,” “take notice,” “consider,” “remember,” etc. Many translations of the English Bible (cp. NIV, NRSV, HCSB) do not translate idou, but in doing so miss the meaning that it is bringing to the context. See commentary on Matthew 1:20.
“The star that they had seen when it rose.” The fact that this verse points out that this “star” (or planet) was the one “they had seen when it rose” in the eastern sky when they were in the area of Babylon seems to indicate that they had not seen it on their journey from Babylon to Jerusalem, which could have been the case. For one thing, they may have been traveling during the day.
“was going ahead of them until it came and stood over where the child was.” As the Magi headed south to Bethlehem (Bethlehem is 7 miles south of Jerusalem), the “star,” the planet Jupiter, was rising in the southern sky. So it appeared in the sky ahead of them, and in the hours that they traveled, likely about three hours, it rose in the sky, seeming to go ahead of them toward Bethlehem, and it was “before” them, always in the southern sky. As the Magi were arriving, the “star,” Jupiter, reached its zenith in the southern sky and thus was directly over Bethlehem. The astronomical phraseology used in the Bible is still used by astronomers today.
The timing of the star in the sky shows us the Magi would have gotten up in the early morning, before light, and traveled south. This was very common. Often travelers got an early start and then stopped traveling and rested when the sun was hot. They would have arrived at Bethlehem right around dawn.
The fact that the “star” was the planet Jupiter, the “king planet,” would have been obvious to the Magi, who were astronomers and who rejoiced greatly, but entirely unnoticed by the average person. If the “star” had been an unusual celestial event, thousands of people would have been watching and waking their neighbors, and Bethlehem would soon have been overrun by curiosity seekers. Tradition is silent on why only the Magi seemed to notice the mysterious star that stood over Bethlehem, but the answer is logical and beautiful.