Shadows change length throughout the day because the angle at which the sun shines on stationary objects changes with the Earth’s rotation. For example, early in the morning, when the sun is near the horizon, it casts long shadows when an object blocks the light. Conversely, when the sun is high overhead during the middle of the day, the shadows become shorter, as the angle of the sun has changed.
The angle at which the sun’s rays strike the Earth changes over the course of the day, but the sun’s position in the sky also varies throughout the year. In the summer, the sun traces a very high arc in the sky, while the winter sun traces a very low arc through the sky, which barely rises above the horizon at extreme latitudes. This change in its path also causes the temperature and day length to change over the course of the year.
The fact that the sun’s position changes over the course of the day, thus causing a change in the shape of the shadows its light creates, is the principle behind a sundial. A sundial uses a central shadow-casting object to cast shadows in a circular pattern that changes throughout the day.