Earth’s Rotation and Tilt The Earth’s axis of rotation is tilted at 23.5º (pre-
cisely, 23.47º), also known as its declination. The Earth travels in the plane of the ecliptic (Fig. 6.2) in its orbit around the sun. The tilt of the Earth is toward the sun (June +23.5º) or away from the sun (December −23.5º). This tilt is responsible for the seasons. In the northern hemisphere in June, the sun’s direct radiation is perpendicular to the Earth’s surface; in December, there are fewer hours of sun, and the radiation passes through a longer, nonperpendicular, length of atmosphere to reach the Earth. The rotation of the Earth (once every 24 hours) and its tilt determine the length of atmosphere that solar radiation passes through and consequently how much energy the Earth’s surface receives.