In Northern California, Fall will officially begin at 8:09 this evening, with an astronomical event called an equinox. This equinox is special because it will also occur on the same night as a full moon. This is the first time these two things have happened together since 1991. Traditionally, the full moon closest to the September equinox is called the Harvest Moon.
An equinox occurs twice a year. The first day of Spring (which happens in March) is called the Vernal Equinox, and the first day of Fall (today) it is called the Autumnal Equinox. The first days of Summer and Winter are called solstices.
During an equinox, Earth’s axis points straight toward the sun. It doesn’t tilt away from, or toward, the sun. In this way, the center line of the sun is directly over the center line of the Earth, or the equator.
Because equinoxes happen when two round objects align – the Earth and our sun – they actually happen at different times around the globe. The time for an equinox is first calculated in Coordinated Universal Time (UTC) which is standardized for the entire Earth, based on atomic time and adjusted for the Earth’s rotation. However, you can use a seasons calculator to figure out exactly when the equinox is happening in different parts of the world.
Today, in the United States, many people won’t notice or celebrate the equinox. However, in ancient civilizations, astronomical events such as equinoxes and solstices were very important. Entire monuments were built to help people appreciate how big and special these celestial events are. In some cultures, these events are still celebrated. (Watch the National Geographic video to learn more)