This evening, we will have a change of the seasons: The occurrence of the vernal equinox, marking the official start of spring. In fact, it will be a rather auspicious occurrence. The earliest that the equinox has occurred nationwide in 128 years. More on that in a moment.
At the equinox, the Earth will have reached the point in its orbit where its axis is at right angles to a line from the sun. Thus, the sun will then be directly over a specific point on the Earth's equator moving northward. In the sky, it's where the ecliptic and celestial equator cross each other.
