The length of a shadow depends on the position of the Sun in the sky. During the summer, the Sun is higher in the sky than it is in the winter, so the shadows that it casts are shorter in the summer than in the winter.

The diagram below illustrates this concept:

[Image of a diagram showing the position of the Sun in the sky in the summer and the winter, and the corresponding lengths of shadows.]

As you can see from the diagram, the Sun is much closer to the horizon in the winter (left side of the diagram) than it is in the summer (right side of the diagram). This means that the shadows cast by objects in the winter are much longer than the shadows cast by objects in the summer.

So, in summary, the reason why shadows are long in the winter is because the Sun is lower in the sky.