The equinox is one of the oldest solar celebrations in human history. Some cultures have been celebrating the official start of Spring for more than 3000 years, proof that knowledge of solar (Sun) cycles precedes modern day science. The word Sun is derived from the old English word sunne. The Ancient Greeks called their God of the Sun Helios, and the Roman personification of the Sun was called Sol. To this day we still use the words Sun, Sol & Helios to describe seasonal patterns & to mark the annual orbital path of the Earth around the Sun.
On the Vernal (spring) Equinox (Saturday, March 20th, 2021) the sun is directly overhead, if you are standing on the equator. Those of us north of the equator are already enjoying longer days than nights, while those on the southern hemisphere experience longer nights with each passing day. If you are on the north or south pole, the line of shadow that divides day & night cuts directly through the spinning planetary pole. While the Earth shifts into northern hemisphere summer, the north pole will will have to wait until the autumnal equinox in September to experience another sunset.
While the northern hemisphere summer approaches, Earth is also moving further away from the sun. As Earth orbits around Helios (Greek), the point at which Earth is furthest from Helios is called the aphelion, which we will experience on July 5th of 2021.
This is good news, because northern hemisphere summers are not quite as intense due to the greater distance from the Sun. The day that Earth is closest to the Sun is called the perihelion, which occurred on January 2nd, 2021. On the aphelion the Earth is 3 million miles further from Helios than it was on the perihelion, so less solar energy arrives at Earth in July. Conversely southern hemisphere summers are hotter because they coincide with the perihelion.
The vernal equinox marks a half way point between solar extremes, and the transition into half a year of increased insolation (incoming solar radiation) reaching the northern hemisphere. Much of that solar energy will be absorbed by the northern equatorial ocean waters, which will more readily evaporate and allow clouds to form aloft. That warm water flows west and north along the western sides of the ocean basins. This will fuel the rainstorms and humidity that characterize the eastern seaboard of the USA. Cooler water from the north pole flows south along the eastern side of the ocean basins, and is more resistant to evaporation, so the weather is drier along the Pacific Coast of the USA. The weather changes because the Earth's position relative to the sun shifts, and determines where the majority of the solar energy is absorbed and how it circulates around the atmosphere and oceans. In the coming months those of us in the northern hemisphere look forward to warmer weather & the sun rising higher above the horizon. Thousands of years ago, our northern hemisphere human ancestors were aware of the approaching warmth and agricultural bounty, and it called for celebration...and it still does.
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