You may have noticed that it is getting more and more difficult to drive east in the morning and west in the evening. The reason is because we are approaching what is called the spring, or vernal, equinox. This year the vernal equinox is March 20th. On the calendar, this is noted as the first day of spring, but this day is special because of what is going on astronomically. As the Earth orbits the Sun in a near perfect circle, it also rotates. This rotation of the Earth is what provides us with day and night, and the time it takes for the Earth to orbit the Sun is our year. The rotation of the Earth, though, is tilted 23.5 degrees relative to the plane that the Earth orbits the Sun. This means that there are months during which the northern hemisphere is tilted towards the Sun, and these are our summer months, and there are months that the northern hemisphere is tilted away from the Sun, and these are our winter months. But this means there must be a couple of days when we transition from being tilted away from the Sun to being tilted towards the Sun and vice versa; these are the days of the vernal and autumnal equinoxes, respectively. On these days, the Earth is neither tilted towards nor away from the Sun, which means that the Sun will rise due east and set due west for everyone on Earth (thus the difficult driving), and this also means that for everyone on Earth the day will be exact 12 hours long and the night 12 hours long; that is what the word “equinox” means – “equal night” and equal day for all.
--
Follow your curiosity to the Fred G. Dale Planetarium at Wayne State College.
