Obliquity of the Equinoxes
The Tropic of Cancer and the Tropic of Capricorn are defined as the 23.5° north and south latitudes, respectively, because Earth's 23.5° axis tilt means those are the points where the sun is directly overhead at the vernal and autumnal equinoxes, respectively. At least that is what early observations determined. In reality, gravitational perturbations caused by the sun, planets, asteroids and other minor objects causes the axis tilt to vary between 22.1° and 24.5° during a cycle of about 41,000 years. Current obliquity is decreasing by just under 0.4 arc seconds per year. This effect can account for a solar heating variation of about +/-1% for the +/-1° of tilt. Far larger climate effects are caused by the long-term variations (due to the same forces) in the eccentricity of Earth's orbit, which can vary between 0.0005 (almost circular) to 0.0607 (currently 0.0160) over a period of about 10,000 years. Keep this in mind when reading the next report on global warming; remember in that in the 1960s, and 1970s, the environmental "experts" were in a tizzy over what they were certain was a coming ice age, and were given prime print space in major publications like Fortune, Time, and Newsweek.