Why is there a nine-branched menorah used for Chanukah, when there was a seven-branched menorah used in the Temple?
By and large in Judaism, there is a prohibition on recreating objects used in Tabernacle/Temple worship to be used in the local synagogue. This tradition developed during the time when the Temple was still in operation, and the synagogue was largely a place of assembly for teaching. As Chanukah developed as a holiday, the chanukia was formulated as an emblem that looked substantially similar to the seven-branched menorah, but it was intended to be lit for eight days to memorialize the miracle of the oil, mimicking the menorah, but not to be exactly like it. Today, of course, there are many kinds of chanukias, which range from traditional ones looking similar to the Temple menorah, to others that only allow eight candle spaces for lighting that are anything but traditional.