We place a lot of importance on the seasons. In the US, autumn is a time for colorful trees, sweater weather and Starbucks Pumpkin Spice Lattes. Spring is about flowers blooming and the days lengthening after winter. There are specific times when the astronomical seasons officially turn over. For fall, it's the autumnal equinox. For spring, it's the vernal equinox.
Fall equinox in the northern hemisphere is in September and it marks a welcome shift in the seasons for many after a long, hot summer. But what exactly is an equinox? It's all about Earth and its relationship with the sun. Here's how to understand, visualize and celebrate the autumnal equinox.
Understanding equinox
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The meaning of equinox is right there in the name, a combination of the Latin words for equal and night.
"There are only two times of the year when the Earth's axis is tilted neither toward nor away from the sun, resulting in a 'nearly' equal amount of daylight and darkness at all latitudes," the National Weather Service said in an explainer about the seasons.
A NASA illustration shows the angle of Earth's tilt in relation to the sun.
NASA's Goddard Space Flight CenterEarth spins on an axis (imagine a line running from pole to pole) and the planet sports a jaunty 23.5-degree tilt. The tilt is what gives us seasons. Earth orbits the sun, so the tilt means some areas of the planet get more direct sun than others. That's how it can be summer in the northern hemisphere (tilted toward the sun) and winter in the southern hemisphere (tilted away from the sun). During equinox, the sun shines straight at the equator and gives equal love to the two hemispheres.
Other planets that are also tilted on their axes of rotation experience equinoxes. The time scales can be very different, however. An equinox on Saturn only comes around about every 15 Earth years. That means each season lasts for over seven years on Saturn. It's even wilder on Neptune, which has seasons that last decades. We can be grateful for the relatively short seasons on our planet.
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When is the fall equinox?
The autumnal equinox in the northern hemisphere falls on Sept. 22 this year. If you want to be extra specific and put it on your calendar, then mark it for 5:44 a.m. PT. Dates can shift slightly for equinoxes depending on the year, but it's always around this time in September. The next vernal equinox in the northern hemisphere takes place on March 20, 2025 and will mark the start of spring.
How is equinox different from solstice?
This National Weather Service graphic shows Earth's tilt, how it orbits the sun and when the equinoxes and solstices occur in the northern hemisphere.
NWS/NOAAAs with equinoxes, solstices are also associated with Earth's tilt, but instead of daylight and nighttime being even, the days and nights are at their extremes. Winter solstice is the shortest day of the year, while summer solstice is the longest. Winter solstice for the northern hemisphere falls on Dec. 21 this year.
See equinoxes from space
Earth-watching satellites up in orbit have a unique view of equinox. A NASA Earth Observatory video shows Earth from space and how the positioning of the sun's light shifts with the changing of the seasons. It's a great way to visualize what's happening during our planet's orbit around the sun.
How to celebrate fall equinox
Fall means it's time for autumn foliage sightseeing adventures.
Amanda Kooser/CNETEquinoxes aren't like eclipses or meteors. There isn't a big wow moment when you see something spectacular. Fall equinox this year will look like any other day, but it's a handy way to mark the changing of the astronomical seasons. You can go around and declare, "It's officially the first day of autumn!" How you celebrate is up to you. Here's a suggestion: Put on your favorite sweater, go for a scenic foliage drive and sip a pumpkin spice latte to welcome fall in style.
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