The vernal equinox occurs in the Northern Hemisphere when the center of the Sun crosses the celestial equator. This occurs around March the 21st of every year. The word equinox comes from the Latin word aequus, which means equal and nox, meaning night. On the date of the equinox the day and night are of nearly the same length. The vernal equinox marks the first day of spring. The equinox is caused when the earth’s axis and the Earth´s orbit around the Sun combine into one single point at the equator.
An equinox occurs when the plane of the ecliptic intersects the celestial equator. On this exact time, the center of the Sun coincides with the Earth´s equator. This occurs twice a year. Once on the vernal equinox on March 21st and the other on the autumnal equinox on September 23rd. The Sun crosses the equator, moving northwards at the vernal equinox, and it crosses the equator moving southward at the autumnal equinox. At the time when the Sun crosses the equator, the Sun can be seen exactly overhead at that region.
Earth´s tilt at 0° latitude
The equinox occurs simultaneously on both hemispheres. The vernal equinox marks the beginning of spring in the Northern Hemisphere, while the autumnal equinox marks the beginning of fall in the Southern Hemisphere. This is due to the fact that the Earth´s orbit around the Sun does not follow an upright position, but instead, its orbit is tilted by 23° 26´, so that each Hemisphere receives the Sun´s light during six months of every year. An equinox is experienced twice a year during spring or fall when the Earth´s tilt is at 0° altitude and the Sun´s light falls directly over the equator.
During an equinox, the Northern and Southern Hemispheres receive The Sun´s light evenly, except at the Poles, and the day and night are of equal length in both Hemispheres. Since the Earth continues to follow its orbit around the sun, one Hemisphere starts receiving more sunlight each day, while the other receives less sunlight. Over time, the solstices, the time of more sunlight on one Hemisphere, and the time of less sunlight on the other hemisphere, occur, and the cycle reverses until we experience the equinoxes again.
Halfway between the solstices
The arc of the Sun across the sky continuously shifts from one side to the other on the horizon. In the Northern Hemisphere at the day of the vernal equinox, the Sun rises due east and sets due west, intersecting the Earth´s equator. Over time, the Sun can be seen moving slightly northward on the horizon each day. When it appears to stand still, it has reached the day of the solstice, then it turns back in a southward direction. Over the course of one year, it moves back and forth along the horizon. Halfway between these paths on the horizon are the equinoxes.
The day and night are not exactly of the same length on the day of the equinox. This is due to atmospheric refraction. Atmospheric refraction shifts the apparent position of the Sun by about the size of its diameter, which makes the Sun appear some minutes earlier at sunrise and some minutes later at sunset. These effects add approximately seven minutes to an equinox day, making the day of 12 hours seven minutes and the night 11 hours 53 minutes. According to earthsky.org, bird migration follow the path of the Sun either northward or southward.