Final answer:
Objects cast the shortest shadows or none when the Sun is directly overhead, typically at noon. This happens when sunlight hits the Earth more directly and the angle of the Sun to objects is smaller, resulting in minimal shadow length.
Step-by-step explanation:
Objects cast short shadows or no shadows at all when the Sun is at its highest point in the sky, which generally occurs at noon. This is because the Sun's rays are hitting the Earth more directly, and the angle of the Sun relative to objects on the ground is smaller. A vertical pole or post will have its shortest shadow at this time. When the Sun is at this position in the sky, and we reference traditional timekeeping, this moment is referred to as 'noon by the Sun', to distinguish it from the 'noon by the clock', which might be different due to various factors including time zones and daylight saving adjustments. The casting of shadows is affected by the angle at which sunlight reaches the Earth. When the Sun rises in the morning or sets in the evening, shadows are long because the Sun is near the horizon and the light has to travel through more of the Earth's atmosphere, hitting objects at a lower, more oblique angle. On the contrary, at noon when the Sun is directly overhead, the light travels the shortest path through the atmosphere, and shadows are at their minimal length, or may not be present if the Sun is directly overhead.