The time when the sun is at its highest point in the sky is called Afternoon.

The position of the sun in the sky changes throughout the day due to the rotation of the Earth. At noon, the sun is at its highest point in the sky, casting the shortest shadows. This time is known as afternoon.

Explanation:
During the course of a day, the sun rises in the east, reaches its highest point at noon, and then sets in the west. This movement of the sun is due to the Earth's rotation on its axis.

The Earth's Rotation:
The Earth rotates on an imaginary line called its axis. It takes approximately 24 hours for the Earth to complete one full rotation. As the Earth rotates, different parts of the planet are exposed to the sun's light, resulting in day and night.

The Sun's Movement:
As the Earth rotates, the sun appears to move across the sky. It rises in the east, reaches its highest point at noon, and sets in the west. This daily path of the sun is called its apparent motion.

Afternoon:
Afternoon refers to the time of day when the sun is closest to the zenith, which is the highest point directly overhead. This occurs around noon, depending on one's location and the time of year. During the afternoon, the sun is at its highest point in the sky, providing the most direct and intense sunlight.

Significance of Afternoon:
Afternoon is often associated with the warmest part of the day, as the sun's rays are more directly overhead. It is a time when shadows are shortest, and the sun's energy is at its peak. People may take advantage of this time to engage in outdoor activities, such as picnics, sports, or gardening.

Conclusion:
The time when the sun is at its highest point in the sky is called afternoon. This is when the sun is closest to the zenith and provides the most direct and intense sunlight. Afternoon is a significant time of day, often associated with warmth and outdoor activities.