Why is temperature relatively uniform throughout the year in the tropics along the equator?

The temperature is relatively uniform throughout the year in the tropics along the equator because of the variation of the net radiation of the sun in that area. At daytime, the sun warms the Earth and, at night, the heat is radiated back to space. The balance of these is the net radiation. At the equator, this value is almost always positive which means there would be more heat than being lost.