A solar eclipse can only occur during the day because it happens when the Moon passes directly between the Earth and the Sun, blocking the Sun's light and casting a shadow on Earth. For this to happen, the Sun must be visible in the sky, which only occurs during daytime. If the Sun is not visible (i.e., at night), there is no sunlight to be blocked by the Moon, so a solar eclipse cannot be seen. More specifically, a solar eclipse occurs during the new moon phase when the Moon is aligned on the same side of Earth as the Sun. However, because the Moon's orbit is tilted about five degrees relative to Earth's orbit, the Moon usually passes above or below the Sun and does not cause an eclipse every new moon. The Moon's shadow during the eclipse has two parts: the umbra, where the Sun is completely blocked, causing a total eclipse, and the penumbra, where the Sun is only partially blocked, causing a partial eclipse. This shadow travels across the Earth's surface during the day as Earth rotates and the Moon moves in its orbit. Therefore, a solar eclipse is inherently a daytime event since it involves the alignment and blockage of the Sun, which is only visible during the day.