Summer heat has overstayed its welcome for many in September. But if you’re looking forward to cooler weather, Thursday’s autumnal equinox is a reminder that fall is finally on our doorstep. 
 
 The 2016 fall equinox arrives Sept. 22 at 10:21 a.m. Eastern. What happens on the equinox — and why are day and night not quite equal? Check out these five questions (and answers) to learn more. 
 
 1. What happens on the equinox? 
 
 Like the spring equinox in March, the autumnal equinox marks an astronomical turning point of the seasons. 
 
 [Take the quiz — Which season are you?] 
 
 Since the Earth is tilted on its axis of rotation by 23.5 degrees, the Northern and Southern hemispheres receive different amounts of amounts of sunlight throughout the year. On the equinox, however, the orbital plane of the equator becomes geometrically aligned with the center of the sun so that neither hemisphere is tilted away or toward the sun (visualize Earth’s axis going into the screen in the image below): 
 
 From our Earthbound perspective, the sun appears directly overhead along the equator, and day and night are roughly equal in length (emphasis added — see No. 4). 
 
 2. Where does the sun rise and set on the equinox? 
 
 (Daniel Schroeder/Weber State University) 
 
 The fall equinox is one of only two days each year when all points on Earth — apart from the polar regions — see the sun rise due east and set due west along the horizon. Until the winter solstice in December, the sun continues to rise and set farther to the south. 
 
 As the sun takes a shorter, lower path across the sky, we experience more darkness than daylight, and cooler temperatures as a result. 
 
 3. How fast are we losing daylight around the equinox? 
 
 Rapidly! Though the days have steadily been getting shorter since the summer solstice, the earlier arrival of dusk becomes especially noticeable by September. That’s because the Northern Hemisphere loses daylight at its fastest pace around the September equinox. The District loses 2 minutes and 30 seconds of daylight each day, while cities across the northern tier lose close to three minutes or more. 
 
 Data from timeanddate.com (Justin Grieser) 
 
 The rate of daylight change depends on your latitude, or distance from the equator. In Miami — located just north of the tropics — daylight dwindles by only 90 seconds a day. At the opposite extreme lies Fairbanks, Alaska. Located just 200 miles south of the Arctic Circle, residents there see nearly 7 minutes of daylight loss around the September equinox — not exactly uplifting! 
 
 4. Why are there more than 12 hours of daylight on the equinox? 
 
 Though equinox means “equal night” in Latin, both of Earth’s hemispheres get slightly more than 12 hours of daylight on the equinox. In the District sunrise on the fall equinox is at 6:56 a.m. and sunset at 7:04 p.m., bringing a total of 12 hours and 8 minutes of daylight. 
 
 There are two reasons for this. One is atmospheric refraction. This optical phenomenon bends the sun’s light as it passes through Earth’s atmosphere and causes the sun to appear slightly higher in the sky than it actually is. 
 
 (NOAA/Earth System Research Laboratory) 
 
 The other is how we define sunrise and sunset. The sun appears as a disk, not a single point. Sunrise is defined as the moment the sun’s upper edge appears on the horizon, while sunset doesn’t occur until the sun’s upper edge disappears from the horizon. Together, these factors add about 10 minutes of daylight to the equinox, depending on one’s distance from the equator. 
 
 The map below — by Alaska-based meteorologist Brian Brettschneider — shows when we can expect equal daylight and darkness. In most of the lower 48, the exact 12-hour day occurs three or four days after the fall equinox. 
 
 5. Why does it get dark so quickly around the equinox? 
 
 We observe the sun setting much earlier as September progresses, but what about how quickly it gets dark? If you’re an astute observer, you may have noticed that daytime transitions to night a bit faster than it did a month or two ago. Indeed, the fastest sunsets of the year occur around the two equinoxes. 
 
 This happens because the sun crosses the horizon at a slightly steeper angle than it does on the solstices. As a result, we see the sun appear and disappear from the horizon more quickly. The difference is almost imperceptible closer to the equator, but is much more pronounced at higher latitudes, such as the cities across the northern tier, Canada and Alaska: 
 
 Comparison of twilight length in four U.S. cities throughout the year. Civil twilight refers to the time before sunrise and after sunset when the sun is 6 degrees or less below the horizon. (Justin Grieser) 
 
 Notice that at all times of year, twilight lasts longer the greater your distance from the equator (compare Seattle vs. Miami, for example). Yet each location sees faster sunsets — and shorter twilights — around the spring and fall equinox. In March, we usually don’t notice the shorter twilight because the days are already getting longer. In September, on the other hand, we are more likely to observe not just how much earlier it gets dark, but also how quickly nighttime falls. 
 
 As we plunge into the darker months of the year, it’s won’t be long before temperatures fall as well. So if you’re ready for crisp mornings, pumpkins and hot cider, the fall equinox is a day to celebrate. ||||| Although there’s nothing official about it, it’s traditional to say the March or vernal equinox signals the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. This equinox does provide a hallmark for the sun’s motion in our sky, marking the passage of the sun across the celestial equator, going from south to north. The March 2019 equinox happens on March 20 at 21:58 UTC, which is 4:58 p.m. Central Daylight Time for us in the central U.S.; translate to your time zone. 
 
 In the Northern Hemisphere now, we’re enjoying earlier sunrises, later sunsets, softer winds, sprouting plants. Meanwhile, you’ll find the opposite season – later sunrises, earlier sunset, chillier winds, dry and falling leaves – south of the equator. 
 
 For all of us on this March 2019 equinox, the full moon will come less than 4 hours after the equinox: on March 21 at 1:43 UTC. What’s more, this will be the final in a series a 3 full moon supermoons. 
 
 The equinoxes and solstices are caused by Earth’s tilt on its axis and ceaseless motion in orbit. You can think of an equinox as happening on the imaginary dome of our sky. 
 
 The Earth-centered view is that the celestial equator is a great circle dividing Earth’s sky into northern and southern hemispheres. The celestial equator wraps the sky directly above Earth’s equator. At the equinox, the sun crosses the celestial equator, to enter the sky’s Northern Hemisphere. 
 
 The Earth-in-space view is that, because Earth doesn’t orbit upright, but is instead tilted on its axis by 23 1/2 degrees, Earth’s northern and southern hemispheres trade places in receiving the sun’s light and warmth most directly, as Earth orbits the sun. We have an equinox twice a year – spring and fall – when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun. 
 
 At the equinox, Earth’s two hemispheres are receiving the sun’s rays equally. Night and day are often said to be equal in length. In fact, the word equinox comes from the Latin aequus (equal) and nox (night). For our ancestors, whose timekeeping was less precise than ours, they likely did seem equal. But we today know it’s not exactly so. 
 
 Read more: Are day and night equal at the equinox? 
 
 Here’s another equinox truism. You might hear that the sun rises due east and sets due west at the equinox. True? In fact, this is true. And that’s true no matter where you live on Earth. At the equinoxes, the sun appears overhead at noon as seen from Earth’s equator, as the illustration above shows. This illustration shows the sun’s location on the celestial equator, every hour, on the day of the equinox. 
 
 No matter where you are on Earth, you have a due east and due west point on your horizon. That point marks the intersection of your horizon with the celestial equator – the imaginary line above the true equator of the Earth. 
 
 That’s why the sun rises due east and sets due west for all of us. The sun is on the celestial equator, and the celestial equator intersects all of our horizons at points due east and due west. 
 
 Read more: Sun rises due east and sets due west 
 
 Since Earth never stops moving around the sun, the position of the sunrise and sunset – and the days of approximately equal sunlight and night – will change quickly. 
 
 The video below was the Astronomy Picture of the Day for March 19, 2014. APOD explained: 
 
 At an equinox, the Earth’s terminator – the dividing line between day and night – becomes vertical and connects the north and south poles. The time-lapse video [below] demonstrates this by displaying an entire year on planet Earth in 12 seconds. From geosynchronous orbit, the Meteosat satellite recorded these infrared images of the Earth every day at the same local time. The video started at the September 2010 equinox with the terminator line being vertical. As the Earth revolved around the sun, the terminator was seen to tilt in a way that provides less daily sunlight to the northern hemisphere, causing winter in the north. As the year progressed, the March 2011 equinox arrived halfway through the video, followed by the terminator tilting the other way, causing winter in the southern hemisphere — and summer in the north. The captured year ends again with the September equinox, concluding another of billions of trips the Earth has taken — and will take — around the sun. 
 
 Where can you look to see signs of the equinox in nature? Everywhere! Forget about the weather for a moment, and think only about the daylight. In terms of daylight, the knowledge that spring is here – and summer is coming – permeates the northern half of Earth’s globe. 
 
 Notice the arc of the sun across the sky each day. You’ll find it’s shifting toward the north. Responding to the change in daylight, birds and butterflies are migrating back northward, too, along with the path of the sun. 
 
 The longer days do bring with them warmer weather. People are leaving their winter coats at home. Trees are budding, and plants are beginning a new cycle of growth. In many places, spring flowers are beginning to bloom. 
 
 Meanwhile, in the Southern Hemisphere, the days are getting shorter and nights longer. A chill is in the air. Fall is here, and winter is coming! 
 
 Bottom line: The 2019 equinox comes March 20 at 21:58 UTC; translate to your time zone. It’s an event that happens on our sky’s dome – and a seasonal marker in Earth’s orbit around the sun! 
 
 A Chinese perspective on the spring equinox ||||| These San Francisco sunbathers may not know it, but spring is losing about 30 seconds every year. 
 
 Spring arrives on Friday, and you might want to make the most of it. The season of flowers and showers actually gets shorter every year by about 30 seconds to a minute, due to astronomical quirks, researchers say. 
 
 This year, spring officially starts at 6:45 p.m. EDT on March 20, according to the U.S. National Weather Service (NSW). At that exact moment, which is called the vernal equinox, the Earth's axis will reach a halfway mark, where it points neither toward the sun (as it does on the summer solstice) nor away from the sun (as it does on the winter solstice), said Gavin Schmidt, the director of the Goddard Institute for Space Studies in New York City. 
 
 But for thousands of years, spring has been losing time in the Northern Hemisphere. This year, summer is the longest season, with 93.65 days, followed by spring with 92.76 days, autumn with 89.84 days and winter with 88.99 days, said Larry Gerstman, an amateur astronomer in New York. (Gerstman got his values from "The Astronomical Tables for the Sun, Moon and Planets," second edition, written by Jean Meeus and published in 1995 by Willmann-Bell, Inc.) 
 
 As the years go on, spring will lose time to summer, and winter will lose time to autumn. In the year 3000, the seasonal lengths will have shifted in the Northern Hemisphere: summer will be 93.92 days, while spring will be 91.97 days, autumn 90.61 days and winter 88.74 days, Gerstman said. [6 Signs Spring Has Sprung] 
 
 But why is this happening? 
 
 The Earth's seasons are caused by the tilt of the Earth on its axis (not by how close the planet is to the sun). This tilt of 23.5-degrees from the straight-up-and-down position means that for six months of the year, the Earth's Northern Hemisphere is leaning slightly toward the sun, whereas during the other six months, the Southern Hemisphere leans toward the sun. 
 
 The main reason spring is getting shorter is that the Earth's axis itself moves, much like a wobbling top, in a type of motion called precession. 
 
 Spring ends at the summer solstice, and because of precession, the point along the Earth's orbit where the planet reaches the summer solstice shifts slightly. Next year, the planet will reach the point in its orbit of the solstice slightly earlier. 
 
 Spring will end, and summer will begin, just a little bit earlier in the year. [50 Interesting Facts About The Earth] 
 
 Over thousands of years, the shift in the time of the vernal equinox becomes more apparent. For instance, spring will be shortest in about the year 8680, measuring about 88.5 days, or about four days shorter than this year's spring, Gerstman said. (After that point, spring will lengthen again.) 
 
 Why spring is shorter than summer 
 
 Other aspects of Earth's movements also affect the exact length of the seasons. One is that Earth's orbit around the sun is not a perfect circle, but instead is elliptical. This means that the planet is not always the same distance from the sun. These days, Earth reaches the point in its orbit where it is closest to the sun, an event called perihelion, in early January. (In Greek, peri means near, and helios signifies the sun.) 
 
 At the point of perihelion, the Earth is about 91.6 million miles (148 million kilometers) away from the sun. When the Earth is farthest from the sun — in early July, during aphelion — the distance is about 94.8 million miles (153 million km). 
 
 This graphic shows an exaggeration of Earth's elliptical orbit around the sun. The planet is closest to the sun in January, during the Northern Hemisphere's winter. Credit: NASA 
 
 This change in distance from the sun, of about 3.2 million miles (5 million km), isn't much compared to Earth's total distance from the sun, according to NASA, and people don't notice it. But the difference is large enough to change the speed of the Earth as it travels in its orbit. The Earth moves fastest when it's closest to the sun, and slowest when it's farthest, Schmidt said. 
 
 "We go through winter quite quickly, whereas in the summertime we're far away from the sun, and so we're going to go slower,"Schmidt told Live Science. 
 
 This change in speeds affects the length of the seasons. The Earth moves the fastest along its orbit path between December and March, hence both winter and spring are shorter than summer and autumn, he said. 
 
 In the year 1246, Earth reached perihelion on the day of December solstice, said Joe Rao, a New York based meteorologist and astronomer (Rao is also a contributing writer at Live Science.) This year, Earth reached perihelion on Jan. 4. 
 
 "Near the year 3000, perihelion will take place near Jan. 20, and near the year 4000, it will take place near Feb. 7, and so on," Rao told Live Science in an email. 
 
 At these distant dates, the Earth will be moving at its fastest speed later during the year, making spring even shorter. Perihelion will occur on the March equinox in the year 6430. 
 
 But spring will be shortest when perihelion occurs halfway through the season. And eventually, as perihelion and precession continue to change Earth's speed and wobble, spring will lengthen again, Schmidt said. 
 
 However, these changes are so minute that most people won't notice a difference during their lifetimes, Schmidt said. In fact, since most people associate spring with warm weather, he said, they will be more likely to notice that warm weather is beginning earlier in the year, because of climate change, than to be aware of the changing orbit. 
 
 "It's not something that anyone is going to notice unless you're an astronomer or a paleoclimatologist," Schmidt said. 
 
 Follow Laura Geggel on Twitter @LauraGeggel. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.
– An eventful day on the equinox front: We've entered autumn, as of 10:21am EDT, per National Geographic. That means that Sept. 22 will see 12 hours of day and 12 hours of night in the Northern Hemisphere, and our part of the Earth will start to lean further away from the sun as we head into winter. If the arrival of autumn is a depressing signal for you that we're hurtling toward the upcoming cold-weather solstice, the magazine points out it could be worse, a la Game of Thrones: The angle of Uranus' axial tilt is so extreme that its winters last 42 years at a time. Some other tidbits as the leaves start to fall: The Washington Post offers a scientific Q&A for all things autumnal. Don't miss Google's main page, where, per SearchEngineLand.com, its Doodle continues the story of a bunch of rocks who were there with us to witness March's vernal equinox. The Sun does some myth-busting about first-day-of-fall egg balancing. Bustle offers some ideas—some practical, some hedonistic—for celebrating the change of season. Circle back to National Geographic for a list of celestial viewings this week, including a brightly shining Saturn.