The 2018 Vernal Equinox, marking the changing of the astronomical Seasons from the last day of Winter to the first day of Spring, occurs on Tuesday, March 20 at 12:15 p.m. Eastern Daylight Time (4:15 p.m. Universal Time), according to the U.S. Naval Observatory.
The Vernal Equinox can occur on March 19, 20, or 21, depending on whether the year is a Leap Year and other calendar calculations.
Equinox means “equal night” in Latin, capturing the idea that daytime and nighttime are equal lengths everywhere on the planet. That is true of the Sun’s presence above the horizon, though it does not account for twilight, when the Sun’s rays extend from beyond the horizon to illuminate our gas-filled atmosphere.
It is the equal amount of day and night in the northern hemisphere for locations like Melbourne, Florida, but also in the southern hemisphere for locations like Melbourne, Australia. The apparent change in location of the sun and moon marks important dates for hunting, fishing, and farming.
Of course, it is not the Sun that is moving north or south through the seasons, but a change in the orientation and angles between the Earth and its nearest star.
As pictured in the NASA Earth Observatory photo above, the Spring Equinox is caused by the tilt of the earth’s rotating axis. The axis of the Earth is tilted 23.5 degrees relative to the Sun and the ecliptic plane. The axis is tilted away from the Sun at the Winter Solstice and toward the Sun at the Summer Solstice, spreading more and less light on each hemisphere. At the equinoxes, the tilt is at a right angle to the Sun and the light is spread evenly.
How can it be Spring if it is still snowing in parts of the United States?
Blame the oceans, which heat up and cool down only slowly.
By March 20 they are still cool from the winter time, and that delays the peak heat by about a month and a half. Similarly, on the first day of Fall in September, the ocean water still holds warmth from the Summer, and the hottest days are still (on the average, but not always) a month-and-a-half ahead.