Vernal (Spring) Equinox

GS1 – Geography

Context:

The Vernal Equinox, occurring on March 20, signifies the official beginning of astronomical spring in the Northern Hemisphere and astronomical autumn in the Southern Hemisphere.

• The term “vernal” translates to “spring,” indicating the transition from winter to longer, warmer days in the north, while the southern regions begin shifting toward cooler autumn nights.
• On this day, the Sun crosses the celestial equator, marking a seasonal shift.
• As per NASA, day and night are not exactly equal on this date. The exact duration of daylight and darkness depends on latitude and atmospheric conditions.

Understanding the Equinox

An equinox happens twice annually—once in March and again in September—when the Sun’s center aligns with Earth’s equatorial plane.

• March Equinox: Marks longer and warmer days in the Northern Hemisphere (spring) and shorter, cooler days in the Southern Hemisphere (autumn).
• September (Autumnal) Equinox: Signals shorter, cooler days in the Northern Hemisphere (autumn) and longer, warmer days in the Southern Hemisphere (spring).

What Occurs During an Equinox?

According to NASA, during an equinox:

• Observers at the equator witness the Sun directly overhead at noon. Both hemispheres receive nearly equal amounts of sunlight, making day and night approximately equal (with minor variations due to latitude and atmospheric effects).
• After the March Equinox, the Sun shifts northward each day until it reaches its highest point at the June Solstice, after which it starts moving south again.
• During the September Equinox, the Sun crosses the equatorial plane southward and continues this movement until the December Solstice, when it begins moving northward again.
• This movement leads to the March Equinox being called the Northward Equinox, while the September Equinox is known as the Southward Equinox.
• Although Earth’s orbit is slightly elliptical, equinoxes and solstices are primarily caused by the fixed axial tilt of 23.5°, rather than Earth’s distance from the Sun.
• Equinoxes mark a balance point in this celestial journey, whereas solstices indicate the extremes of this cycle.

During an equinox, the Sun’s rays illuminate both the Northern and Southern Hemispheres equally. This results in a unique phenomenon where, for a brief moment, both the North and South Poles receive sunlight simultaneously—an event that only happens twice a year.

Seasons and Earth’s Tilt

The seasons arise due to the 23.5° axial tilt of Earth. As the planet orbits the Sun, this tilt remains constant, leading to seasonal variations:

• In June, the Northern Hemisphere tilts toward the Sun, bringing summer.
• In December, the Northern Hemisphere tilts away from the Sun, resulting in winter.