The vernal equinox marks the official start of spring in the Northern Hemisphere on March 19, when daylight and darkness are evenly distributed across the globe.
To simplify recordkeeping, meteorologists and climatologists designate March 1 as the commencement of spring. However, from an astronomical perspective, the vernal equinox signifies when the Earth's equator aligns directly with the sun. In 2024, this event will take place on March 19 at 11:06 p.m. EDT.
Astronomical seasons are determined by the Earth's position relative to the Sun during its yearly orbit around the nearest star.
Due to the Earth's tilt of about 23.5 degrees off its vertical axis, the Southern Hemisphere receives the most direct sunlight during our astronomical winter, while the Northern Hemisphere experiences this during our astronomical summer.
The two solstices and two equinoxes mark precise moments when the Sun aligns directly with three specific bands of latitude.
Their dates may vary by a day or two each year due to the Earth taking approximately 365 days and 6 hours (365.25 days) to complete one orbit around the Sun, necessitating a leap year every four years.
During the winter solstice in December, the Sun's most direct rays fall upon the Tropic of Capricorn (23.5 degrees south latitude).
During June's summer solstice, the Sun's most direct rays align with the Tropic of Cancer (23.5 degrees north latitude).
On the vernal equinox in March and the autumnal equinox in September, the Sun is directly aligned with the equator (0 degrees latitude).
Consequently, every location on Earth experiences approximately 12 hours of daylight and darkness during these equinoxes as the Sun rises directly in the east and sets directly in the west.
During the summer, the Sun rises in the northeastern sky and sets in the northwestern sky, leading to extended daylight hours and shorter nights.
At solar noon (typically around 1 p.m. local time due to daylight saving time) on the summer solstice, the Sun reaches its highest and northernmost position in the sky.
This results in the most direct solar radiation of the year, causing greater heating of the Earth's surface and consequently warmer temperatures.
During winter, the sunrise occurs in the southeastern sky and the sunset in the southwestern sky, resulting in a considerably shorter path across the Northern Hemisphere sky. As a consequence, days are short while nights are long.
On the winter solstice, the solar-noon Sun angle is at its lowest and furthest south in the sky. Consequently, we experience the least direct solar radiation of the year on the first day of winter, leading to colder temperatures due to reduced heating of the Earth's surface.
It's noteworthy that Earth's orbit around the Sun is elliptical rather than perfectly circular. As a result, Earth is closest to the Sun in January (perihelion) during the Northern Hemisphere winter and farthest from the Sun in July (aphelion) during the Northern Hemisphere summer.
