Come Thursday (March 19), we will have a change of the seasons: the occurrence of the vernal equinox, marking the official start of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. In fact, it will be a rather auspicious occurrence: the earliest that the equinox has occurred nationwide in 124 years. More on that in a moment.
The exact moment of the equinox will occur Thursday night at 11:49 p.m. EDT (0349 GMT on March 20), according to the astronomy reference book “Astronomical Table of the Sun, Moon and Planets” (Willmann-Bell, 2016). At that time, the Earth will reach the point in its orbit where its axis isn’t tilted toward or away from the sun. Thus, the sun will then be directly over a specific point on the Earth’s equator moving northward. On the sky, it’s where the ecliptic and celestial equator cross each other.
A not-so-equal equinox
On the day of the equinox, the sun will appear to rise exactly east and set exactly west. Daytime and nighttime are often said to be equally long with the equinox, but this is a common misconception — the day can be up to 8 minutes longer, depending on your latitude.
The sun is above the horizon half the day and below for half — but that statement neglects the effect of the Earth’s atmosphere, which bends the rays of sunlight (called refraction) around the Earth’s curvature when the sun lies close to the horizon. But, because of this bending of the sun’s rays, the disk of the sun is always seen slightly higher above the horizon than it really is.
In fact, when you see the sun appearing to sit on the horizon, what you are looking at is an optical illusion; the sun at that moment is actually below the horizon. So, we get several extra minutes of daylight at the start of the day and several extra minutes more at the end.