Earth revolves around the sun on an incline of 23.5 degrees and because of this the hours of sunlight received at a location varies from one season to the next. In the northern hemisphere, the sun's rays are more direct in summer and more oblique in autumn and winter, thus less effective in heating the atmosphere. Due to conservation of energy, the coldest part of winter comes about a month after the shortest day and the hottest part of summer comes about a month after the longest day. Earth's inclination is responsible for wide temperature variations within a single region. Mercury and Venus have slight inclinations or none at all and do not have such seasonal variations.

Originally released by QA International, 2005. Streaming video file encoded with permission for digital streaming by Infobase on May 06, 2015.

Seasons, The (1:00); .