Understanding the Seasons
The reason that the weather is warmer in summer and colder in winter for the northern hemisphere is not due to the fact that the earth is nearer to the sun in summer and farther from the sun in winter. In fact, just the opposite is true. The seasons are the result of the differing amounts of solar energy which are received at the earth's surface. This changing insolation is a result of the 23 ½° inclination of the earth's axis to the perpendicular of the ecliptic, and the revolution of the earth around the sun. The earth's axis is the imaginary line about which the earth rotates. Expressed in another way, the ecliptic (path of the sun in the sky) is tilted to the plane of the celestial equator by 23 ½°. This causes the sun to not only move eastward as the earth revolves around this star, but to also move northward or southward with respect to the celestial equator. This change in the declination of the sun results in three yearly cycles which can be readily observed as the seasons progress:
In winter, the northern hemisphere is tilted away from the sun. The daily duration of sunshine is restricted, and the sun is lower at noon. The sun's energy strikes the ground at a shallower angle, and thus less energy is received per unit area. The temperature becomes colder. In summer, the northern hemisphere is tilted toward the sun. Not only is the daily duration of sunshine longer, but the sun also climbs to a higher altitude, so that its energy strikes our location more directly, and we receive more energy per unit area. It's time to party in the warmer weather.
The fact that the weather is warmer in the northern hemisphere in summer and colder in winter is NOT because the sun is nearer to us in summer and farther from us in winter. In actuality, the summer sun is three million miles farther from the earth than its winter distance. The two factors which produce the seasonal effects are (1) the duration of sunlit hours experienced during each season and (2) how directly the sun's rays strike the earth's surface. These two factors are in essence due to a single factor, namely the inclination of the plane of earth's equator with the plane of its orbit around the sun. Put another way, the axis of the earth is inclined 23 ½° from the perpendicular to its orbit.
In summer, the northern half of the earth's
axis is tipped toward the sun. Not only is the duration of daylight
longest at any place in the northern hemisphere, but the sun transits
the meridian at a higher altitude, so that its rays are more nearly
vertical and more concentrated. In winter, the northern hemisphere of
the earth is tilted away from the sun. The duration of daylight is shorter
and the sun transits the meridian at a lower altitude. The sun's rays
strike the earth more obliquely and thus impart to the surface of earth
less energy per unit area.