Eversince equinoxes are of exceptional importance for astronomers. These are the only days under identical circumstances all over the world. Day and night are exactly of the same length. The sun rises stricly in the east and sunset is strictly in the west. During equinoxes both poles of the earth lie over the ecliptic so both northern and southern hemispheres of the earth are lighted equally. In the course of the year the northern and southern hemispheres are lighted unequally. Beyond the polar circle the day and night side is permanently lighted for six months on each hemisphere (polar day).