Freelance writer Amanda C. Kooser covers gadgets and tech news with a twist for CNET. When not wallowing in weird gear and iPad apps for cats, she can be found tinkering with her 1956 DeSoto.

It's almost time to fall into fall, though in many areas the leaves are already turning. 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 folks 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.

The autumnal equinox in the Northern Hemisphere falls on Sunday, 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.

Wherever you are on Earth on the day of the equinox (and whether it's a spring or fall equinox, depending on which hemisphere you live in), the sun will rise as close to east and set as close to west as possible, making it a foolproof compass. Go outside and watch sunset and sunrise, and make note of the landmarks in front of the sun -- that way, you'll always know what exactly is west and east.

The two annual equinoxes also feature the fastest sunrise and sunset of the year, with the sun taking just a few seconds to appear and disappear. That's because this is the steepest angle at which the sun rises and sets during the year.

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.

The Earth 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. As the Earth orbits the sun, 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) at the same time. 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 also 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.

As with equinoxes, solstices are 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.

Earth-watching satellites up in orbit have a unique view of the 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.

Equinoxes 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.