I missed the actual date, but figured I’d post this anyway – this is sciencelearntech, after all….
Simple explanation: equinoxes are the points in the calendar when the plane of the Earth’s equator passes the center of the sun. It’s also the point when the earth’s axis isn’t tilted either toward or away from the sun. Check out Wikipedia’s article on equinoxes – it’s decent reading. And any planet with a significant tilt of its axis can have an equinox – it’s not an event restricted to the Big Blue Marble on which we live.
So what’s a solstice? Solstices happen twice a year, too – when the sun reaches it’s “highest” or “lowest” point in the sky, above the horizon, when viewed at local solar noon. (I’m using the terms highest and lowest, with the assumption that reference points have been defined – in physics, we always define our system of reference!) As we chew through the calendar, there’s an observed movement of the sun’s path with the seasons – until it comes to a stop and then reverses direction. That stopping point is one of the two solstices observed each year. The annual equinoxes and solstices are tied to our observance of winter, spring, summer and fall.
… and that’s your bit of science for the day – carry on…