The shape of Earth's orbit, known as eccentricity; the angle Earth's axis is tilted with respect to Earth's orbital plane, known as obliquity; and. The direction Earth's axis of rotation is pointed, known as precession. Precession is the cyclic change in Earth's rotational axis, amounting to roughly 1° every 72 years. One major effect of precession is that, at different times during the cycle, the seasons will be either more or less extreme in the northern or southern hemisphere. Greater eccentricity increases the variation in the Earth's orbital velocity. Currently, however, the Earth's orbit is becoming less eccentric (more nearly circular). This will make the seasons in the immediate future more similar in length. The role of eccentricity on Earth's seasons might be more prominent were the southern hemisphere to have continental area like in the northern hemisphere: since perihelion occurs near northern hemisphere winter solstice and aphelion during northern hemisphere summer solstice, seasonal contrasts in the southern hemisphere. Eccentricity is the reason why our seasons are slightly different lengths, with summers in the Northern Hemisphere currently about 4.5 days longer than winters, and springs about three days longer than autumns. As eccentricity decreases, the length of our seasons gradually evens out. If Earth's orbit had a higher eccentricity, the seasons would be more extreme. This is because the amount of sunlight that a particular region receives depends on its distance from the Sun. When Earth is closer to the Sun, it receives more sunlight, and when it is farther from the Sun, it receives less sunlight.