The angle at which the sun's rays hit the Earth most directly is 90 degrees, also known as solar noon.

Here's why:

* Angle and Intensity: The angle of the sun's rays determines how much energy is concentrated on a given area of the Earth's surface. A higher angle (closer to 90 degrees) means the rays are more concentrated, leading to more intense heat.

* Solar Noon: At solar noon, the sun is at its highest point in the sky for a particular location. At this moment, the sun's rays hit the Earth at the most direct angle possible for that location.

Important Note: While 90 degrees is the most direct angle, it only occurs at the equator on the equinoxes (spring and fall). For other locations and times of year, the angle will vary depending on latitude and time of year.