An eccentric billionaire wants to build a mansion in a location on Earth where the effect of the seasons is the least pronounced -- where summer and winter are not that different. Where should his personal astronomer advise him to build?

near the equator

Explanation:

as you move further away from the equator, seasons are less pronounced. in tropical regions for example there are less season variations and temperature is pretty much the same. Countries like Brazil, Peru, Venezuela and Ecuador

His personal astronomer should advise him to build at the tropical region which are areas close to the equator of the earth.

Explanation:

Seasons are factor of earth's movement relative to the sun. When the earth tilt through its axis, it results to change in season. Earth's axis tilting is far pronounced in the Northern and Southern Hemispheres and less pronounced along the equator. At different times of the year, the sun's rays hit different parts of the globe more directly. The angle of the Earth's axis tilts the Northern Hemisphere towards the sun during the summer.

Without the tilt of the earth's axis, we wouldn't have seasons. Instead, the areas around the equator would receive the most sun and the northern and southern hemispheres would be stuck in a gradual gradient of hot to cold. The seasons would not change, it would be about the same temperature year round and there would be no seasons.