Earth’s seasons based on its axis, not orbit
Although our home is heated by gas, we have a wood-burning stove in our kitchen. During the winter months our thermostat is set at 62 degrees at night. Each morning I build a fire and turn the thermostat up to 68 degrees before Mary Alice crawls out of bed and comes downstairs. We eat our breakfast at the table in front of the wood fire, where it is warmer.
Most people believe the closer you get to the heat source the warmer it will be. However, this is not the case with Mother Earth. The Earth, which gets its heat from the Sun, is closest to the sun in early January. This makes no sense. If Earth is closest to the sun each year, in January, shouldn’t it be warmer? Well, it is, if you live in the Southern Hemisphere.
The Earth’s elliptical orbit around the sun has nothing to do with the seasons. The reason for seasons has to do with the tilt of the Earth’s axis.
The Earth’s orbit around the sun is not a perfect circle, but an elliptical (slightly oval-shaped). This means there is one point in the orbit when the Earth is closest to the sun; however, here in the Northern Hemisphere, Earth was tipped its maximum away from the sun, when the winter solstice occurred. This year Earth reached perihelion (its closest point to the Sun) on Jan. 2. The perihelion usually occurs approximately two weeks after the December solstice (the first day of winter).
The path of the Earth varies due to the gravitational influences of other planetary objects, particularly the moon. Approximately every 100,000 years, the Earth’s orbital path changes from being nearly circular to elliptical. The difference of the Earth’s orbital shape, from a perfect circle, is known as its eccentricity. An eccentricity value of zero is a circular orbit, while values between zero and one describe an elliptical orbit.
On Jan. 2 at 8:50 a.m. the Earth was 91,399,454 miles from the Sun (perihelion), and on July 5 at 6:27 p.m., the Earth will be 94,510,886 miles from the sun (aphelion).
Due to variations, the elliptical path of the Earth’s orbit, the dates when the Earth reaches its perihelion or aphelion are not fixed. In 1246, the December Solstice was on the same day as the Earth reached its perihelion. Since then the perihelion and aphelion dates have drifted by a day every 58 years. In the short-term, the dates can vary up to two days from one year to another.
Mathematicians and astronomers estimate that in 6430, over 4400 years from now, the perihelion will coincide with the March equinox.
Our words perihelion and aphelion come from ancient Greek words, peri meaning close; apo meaning far, and helios meaning the sun.
Although not responsible for the seasons, the Earth’s closest and fartherest points to the sun do affect seasonal lengths. During the year, the Earth comes closest to the Sun in January. At this time (January), our world is moving the fastest in orbit around the Sun. The Earth is rushing along at almost 19 miles per second, moving about 0.6 miles per second faster than it is in early July when the Earth is fartherest from the sun. Thus, winter in the Northern Hemisphere simultaneously occurs with the summer in the Southern Hemisphere, making winter the shortest season as Earth rushes from the solstice in December to the equinox in March.
In the Northern Hemisphere, the summer season (June solstice to September equinox) lasts nearly five days longer than the winter season. Of course, the corresponding seasons in the Southern Hemisphere are opposite, with their winter season being nearly five days longer than the Southern Hemisphere summer.
Well, old Father Time has taken his sickle and marched off, ending 2020. Not many of us are sad to see him go. We hope that the 2021 New Year’s Baby will treat us much better.