Answer
Alexandria was north of Syene, at a higher latitude. To take an extreme example, if your friend is at the equator when the sun is directly overhead and you are at the North Pole, a vertical pole at your location would cast a long shadow, because the sun would be very low on the horizon, not directly overhead. So the shadow would be casted dependent on the position of the sun.
Work Step by Step
See the diagram on page 4.
