The sun's rays strike the Earth at the equator at a 90-degree angle (also known as directly overhead) twice a year, during the equinoxes.

Here's why:

* Earth's Tilt: The Earth is tilted on its axis at approximately 23.5 degrees. This tilt causes the sun's rays to strike different parts of the Earth at different angles throughout the year.

* Equinoxes: During the equinoxes (spring and autumn), the Earth's axis is neither tilted towards nor away from the sun. This means that the sun's rays hit the equator directly, resulting in a 90-degree angle.

Important Note: While the sun's rays hit the equator directly during the equinoxes, the exact location where this happens changes throughout the day as the Earth rotates.