Today, on winter solstice 2018, my post discusses the seasons. Four seasons occur because of shifting sunlight, not changes in temperature. The astronomical start of a season is based on the position of the earth in relation to the sun. A solstice or an equinox marks the start of each season.
Twice a year the earth’s axis is tilted neither toward nor away from the sun. The result is an almost equal amount of daylight and darkness at all latitudes. These events are called equinoxes. At the equator, the sun is directly overhead at noon on the two equinoxes.
Solstice literally means ‘Sun Stands Still’. For a few days around the time of the winter solstice the sun appears to stand still in the sky; its elevation at noon does not seem to change.
- Winter Solstice A solstice is the point during the earth’s orbit around the sun at which the sun is at its greatest distance from the equator. In the Northern Hemisphere, the winter solstice is the darkest or shortest day of the year. As solstices are opposite on either side of the equator, our winter solstice is the summer solstice in the Southern Hemisphere and vice versa. The first day of the winter season usually happens on December 21 or 22. It occurred on December 23 in 1903 but will not recur on that date until 2303.
- Spring Equinox The first day of the spring season occurs when the sun is at its closest distance to the equator. This vernal or spring equinox commonly takes place on March 20. So far in the 21st century, however, it happened twice on March 21 and once on March 19. The first day of spring is the only time of year when the sun rises in the east and sets in the west for everyone across the world. The spring equinox in the Southern Hemisphere happens at the exact opposite time of the year; it’s the fall equinox for people on the other side of the world from us.
- Summer Solstice The first day of the summer season takes place when the sun is directly over the Tropic of Cancer, which is located at 23.5° latitude North, and runs through Mexico, the Bahamas, Egypt, Saudi Arabia, India, and southern China. For every place north of the Tropic of Cancer, the sun is at its highest point in the sky, making it the longest day of the year. The days get shorter after this solstice. In 2019 it will occur on June 21 but often happens on June 20.
- Fall Equinox The September equinox occurs the moment the sun crosses the celestial equator — the imaginary line in the sky above earth’s equator — from north to south. During the autumnal equinox, day and night are balanced to about 12 hours each across the world. It will happen on September 23 in 2019 but can also occur on September 22 or 24.
Because the earth does not move at a constant speed in its elliptical orbit, the seasons are not of equal length. For the sun to move from the vernal equinox to the summer solstice, to the autumnal equinox, to the winter solstice, and back takes approximately 92.8, 93.6, 89.8 and 89.0 days respectively. Consequently, spring and summer last longer than autumn and winter in the Northern Hemisphere — making campers happy and skiers sad. •