Today marks the vernal equinox, often described (in the Northern Hemisphere) as the first day of spring. The word equinox refers to “equal night”: for a lot of Earth, today marks roughly 12 hours of day and 12 hours of night. In the Northern Hemisphere, we’ll start seeing increasingly more than 12 hours of daylight from here until the summer solstice.
Contrary to common belief, the seasons on Earth are not due to how close we are to the Sun. If that were true, both hemispheres would experience summer at the same time! Instead, the seasons of Earth are because our axis of rotation is tilted compared to our orbit, so part of the year the North Pole is pointed more towards the Sun and the other part of the year the South Pole is pointed more towards the Sun. The equinoxes (vernal and autumnal) mark the transitions between these two: for today, neither pole is pointed more towards the Sun than the other. Before today, the South Pole was more towards the Sun; after today, the North Pole will have that honor.
Galileo's Pendulum 
3 responses to “Equinox”
[…] the Sun (or else both the Northern and Southern hemispheres would have summer at the same time!); instead, it’s the tilt of Earth’s axis relative to its orbit. However, a planet with a highly eccentric orbit would experience seasonal variation between the […]
[…] day of the year in the Northern Hemisphere. It’s also the Southern Summer Solstice, and we are close to perihelion, the closest Earth gets to the Sun during its elliptical orbit. However, our orbit is nearly circular, so the difference between perihelion and aphelion (the […]
[…] as it would overlay any seasonal variation the planet has. Earth’s seasons are due to the tilt of our axis, but with larger changes in sunlight over time, things might be different on the imaginary […]