Here's why:

* Solar intensity refers to the amount of solar radiation (energy from the sun) that reaches a particular point on Earth's surface.

* At night, the Earth is facing away from the sun, meaning no direct sunlight is reaching the Earth's surface.

* While there might be some reflected light from the moon or stars, this is negligible compared to the direct sunlight received during the day.

Therefore, the solar intensity at night is essentially zero.