The arrival of spring is imminent, at least according to the calendar. The vernal equinox occurs this Thursday, heralding the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere.
At the equator, the sun will be positioned directly overhead at midday. Equinoxes are unique periods when both the North and South Poles experience simultaneous sunlight.
For centuries, these astronomical events have been commemorated globally. Nowruz, the Iranian New Year, is timed to coincide with the spring equinox. Furthermore, at Chichen Itza, a Mayan archaeological site in Mexico, observers gather during the equinox to witness a solar shadow that resembles a serpent descending the El Castillo pyramid.
But what exactly occurs in the cosmos to bring about these seasonal changes? Understanding how Earth’s orbit dictates our yearly calendar is key.
The Earth’s journey around the sun is not perfectly aligned; it travels at an angle.
Throughout much of the year, the Earth's axis is inclined either towards or away from the sun, resulting in unequal distribution of sunlight and warmth between the northern and southern halves of the globe.
During the equinox, the Earth's axial tilt and orbital position align in such a way that both hemispheres receive an equal share of sunlight.
The term "equinox" originates from Latin roots meaning "equal night." This is because, during the equinox, the duration of day and night is nearly identical, with slight variations depending on one's location on the planet.
The vernal equinox in the Northern Hemisphere can occur between March 19 and 21, varying slightly from year to year. Similarly, the autumnal equinox falls between September 21 and 24.
Solstices mark the points in Earth's orbit when the axial tilt is most extreme, either towards or away from the sun. This results in the greatest disparities in daylight hours between the hemispheres.
During the summer solstice in the Northern Hemisphere, this half of the Earth is tilted towards the sun, leading to the longest day and shortest night. This event takes place between June 20 and 22.
Conversely, the winter solstice sees the Northern Hemisphere angled away from the sun, resulting in the shortest day and longest night, occurring between December 20 and 23.
These are just a couple of the ways we delineate the year.
While astronomical seasons are determined by Earth’s movement around the sun, meteorological seasons are defined by prevailing weather patterns. These divide the year into three-month intervals based on temperature cycles, with spring commencing on March 1, summer on June 1, fall on September 1, and winter on December 1.