Spring officially begins in the Northern Hemisphere on the vernal equinox on March 19, when the entire world experiences equal amounts of day and darkness.
For ease of record, meteorologists and climatologists consider March 1st to be the first day of spring, but astronomically speaking, the Earth’s equator aligns directly with the sun at the spring equinox. In 2024, it occurs on March 19th at 11:06 PM EDT.
Astronomical seasons are based on the Earth’s position relative to the Sun as it orbits its nearest star each year.
The Earth is tilted approximately 23.5 degrees from the vertical axis, and because of this tilt, the most direct sunlight falls on the southern hemisphere during astronomical winter and on the northern hemisphere during astronomical summer.
The two summer solstices and the two vernal equinoxes are precise moments when the sun directly aligns with three different latitude zones.
It takes the Earth 365 days and 6 hours (365.25 days) to orbit the sun, so the date can change by 1-2 days each year. Therefore, there is a leap year every four years.
At the winter solstice in December, the sun’s most direct rays are located over the Tropic of Capricorn (23.5 degrees south latitude).
During the summer solstice in June, the most direct sunlight coincides with the Tropic of Capricorn (23.5 degrees north latitude).
On the vernal equinox in March and the autumnal equinox in September, the equator (0 degrees latitude) is in a straight line with the sun.
Therefore, the sun rises due east and sets due west, so everywhere on Earth experiences the same 12 hours of daylight and darkness.
In summer, the sun rises in the northeast and sets in the northwest, resulting in longer days and shorter nights.
The sun reaches its highest and northernmost point in the sky at solar noon on the summer solstice (approximately 1:00 pm local time due to daylight saving time).
This provides the most direct solar radiation of the year, which further heats the Earth’s surface and, as a result, increases temperatures.
In winter, the sunrises in the southeast and sunsets in the southwest, which means the distance across the Northern Hemisphere sky is much shorter, resulting in shorter days and longer nights.
The sun’s angle from noon to noon is the lowest and most southerly in the sky at the winter solstice. This means that on the first day of winter there is the least amount of direct solar radiation of the year, which means less surface heating and therefore cooler temperatures.
Interestingly, Earth’s orbit around the sun is elliptical (not perfectly circular), so it is actually closest to the sun in January (perihelion) during the northern hemisphere winter, and during the northern hemisphere summer. It is furthest from the Sun in July (aphelion).
