Earth does not rotate on its perpendicular axis. Instead, it is tilted by an angle of 23.5 degrees as measured from the line that is perpendicular to the planet’s orbit around the sun. The tilt in the earth’s axis causes the seasons. As the earth revolves around the sun, the different hemispheres are closer to the sun at different times.
In June, the earth’s tilt causes the northern hemisphere to be closer to the sun. June 21st marks the June solstice, when the sun is directly above the Tropic of Cancer. After this day, the tilt of the planet causes a shift and the sun begins to move closer to the southern hemisphere. By December 21st, the December solstice, the sun is directly above the Tropic of Capricorn, which is 23.5 degrees south of the equator.Learn more about Months & Seasons