We all know that on earth the sun appears to rise from the East and sets on the West. But in fact, the sun truly rise from the East and sets on the West two day per year, which is during the fall and spring Equinox.
It is because the earth have a axial tilt (obliquity) of its rotational axis from its orbital plane. The Earth’s axial tilt oscillates between 22.1 and 24.5 degrees on a 41,000-year cycle. Currently it have mean obliquity of 23°26′13.1″ (or 23.43697°).
The axial tilt cause the four seasons as we know of.
During the Northern Solstice in June, the sun will be directly above (subsolar point) the Tropic of Cancer (Northern Tropic). Tropic of Cancer is the most northern latitude on Earth where the sun can be directly overhead. It noted the beginning of hot summer of the Northern Hemisphere. The southern hemisphere at the same time will experience the start of winter.
During the Southern Solstice in December, the sun will be directly above the Tropic of Capricorn (Southern Tropic). It noted the beginning of Summer in to Southern Hemisphere and Winter in the Northern Hemisphere.
By knowing the axial tilt of the earth, we can know where we should face our solar PV panels. It is important when comes to fix solar PV panels in compare to motorized solar PV panel that can target the sun direction directly. A fix solar PV panel comes with easy installation, lower cost and easy maintenance.