Before you watch the video (or read the explanation underneath), try answering the question:
What causes the seasons on Earth?. You'll be surprised that even the brightest minds – many of whom have been educated at one of the world's leading universities – can have the same misconceptions as everyone else.
Many believe that the Earth's orbit is an exaggerated ellipse, as pointed out by the narrator. Although the Earth’s orbit is elliptical, it is so close to round that Earth’s distance from the Sun does not vary enough to affect the seasons. In fact, its orbit is only 1.7 percentage points from being perfectly round.
Indeed, Earth is closest to the Sun each year on January 4, when it is winter and cold in the Northern Hemisphere. The seasons are caused by
Earth’s tilt on its axis (see figure below). Earth’s tilt is constant with respect to the stars, and as Earth goes around the Sun each year sometimes the Northern Hemisphere is tilted toward the Sun and sometimes it is tilted away. The Southern Hemisphere, of course, is tilted the opposite way. When one hemisphere is tilted toward the Sun, it is summer there and winter in the opposite hemisphere.
Earth’s tilt affects the seasons because a beam of sunlight 1 m (3 ft) in diameter, for example, warms a patch of ground 1 m across if the beam shines directly down on Earth’s surface. But when the beam hits a part of Earth’s surface that is tilted, that same beam has to warm a larger patch of ground. Thus, the energy in the beam is diffused, and each square centimeter of Earth’s surface receives less energy per second. For example, at the equator (0 degrees latitude), the sun angle at the
equinox is 90° - 0° = 90 degrees, or directly overhead. In comparison, Washington, DC is at about 39 degrees north latitude, so the noon sun angle would be 51 degrees. The closer you live to the equator, the less seasonal variation you will experience throughout the year.
