How would the total amount of insolation change at the equator if Earth's axis was vertical instead of tilted?

• A. insolation would increase
• B. insolation would decrease
• C. insolation would stay the same
• D. insolation would vary seasonally more

Answer: (A)

Insolation at the equator is set by how high the Sun gets in the sky and how long the day lasts. With Earth’s tilt, the Sun’s declination changes through the year, so the noon Sun at the equator climbs from about 66.5° up to 90°. That means the Sun isn’t always directly overhead, and sunlight travels a longer path through the atmosphere at lower sun angles, reducing the average flux a bit and creating seasonal variation, even if small near the equator.

If the axis were vertical, the Sun would be directly overhead at noon every day, giving the maximum possible solar intensity with 12-hour days year-round. The atmosphere’s path length is minimized at noon, so more solar energy reaches the surface on average over the year. That’s why the total insolation at the equator would increase.

The other options don’t fit because there wouldn’t be more seasonal variation with a vertical axis, and it wouldn’t be constant or decrease—overhead noontime sun would boost the average insolation.