An unusually early spring equinox arrives late Thursday night, as days continue to grow longer as sunsets arrive later.

The shifting seasons are marked by the vernal equinox. Equinoxes occur twice a year — once to usher in fall, and once to herald the arrival of spring.

During most years, the spring equinox falls between March 20 and 22. But for those in the United States, that’s not the case this year. In fact, reports that the March 19 equinox is earlier than any in the past 124 years.

Perhaps the early equinox is fitting in a year when springlike weather arrived weeks in advance in many parts of Lower 48 states. The USA National Phenology Network, which tracks the blooming of plants, reported that trees were leafing three to four weeks early in many parts of the southern and eastern United States because of the mild weather. In some areas, “leaf out” started in February.

When and what is the vernal equinox?

The vernal equinox isn’t a day; it’s a precise moment that strikes at 11:49 p.m. Eastern time on Thursday night. In that instant, the sun’s most direct rays will cross the equator from the Southern Hemisphere into the Northern Hemisphere. And after that, they’re ours for the taking over the next six months.

And with that, spring will arrive — spelling an end to a winter that for many on the East Coast was over before it began.

Earth’s tilted axis is the reason for the season. Our planet is 23.5 degrees off kilter from the vertical, meaning that at certain times of the year some of us receive more direct sunlight than others.

During the spring and summer months, the Northern Hemisphere leans toward the sun. That allows us to soak up the most intense sunlight, with increasing temperatures as we approach June, July and August.

We experience the greatest solar heating in June around the time of the summer solstice, but because of a phenomenon known as “seasonal lag” many of us don’t experience our warmest temperatures until July.

Once we pass the autumnal equinox in September, our tilt, combined with Earth’s position amid its annual orbit around the sun, means our lean is now away from the sun. It’s the Southern Hemisphere’s turn to soak up extra vitamin D.

It’s also why Christmas is a summer holiday in Australia.

Equal day and equal night? Not so fast.

Despite the term “equinox,” originating from Latin words that translate to “equal night,” the length of day vs. night is not in balance on the equinox.

Sunrise is defined as the moment the upper limb of the sun pokes over the horizon — the first hints of sunlight. Sunset occurs when the last glimmers of sunlight disappear below the horizon. Because we’re taking the first-up, last-down approach to defining day length, rather than tracking when a single point on the sun is above the horizon, our day is a couple of minutes longer than 12 hours.

Moreover, our day can sometimes be stretched a bit by mirages. No, mirages don’t only occur in the desert. When the air’s temperature (and therefore its density) and amount of bending light change significantly in various levels of the atmosphere, we can sometimes see the sun even after it has set below the horizon.

The result? Some folks could experience a day as much as 12 hours 20 minutes long on the equinox if they lived near the North Pole. In Washington, sunrise Thursday is at 7:12 a.m., and sunset comes at 7:19 p.m. — the day will be 12 hours 7 minutes 16 seconds long.

The days are lengthening rapidly

The spring equinox also marks the point at which day length is growing the fastest. It’s kind of like a pendulum; with one extreme representing winter and the other summer, the pendulum is moving the fastest as it swings past the center point — the spring equinox.

In the nation’s capital, the days are growing longer by 2 minutes 32 seconds every day. In Boston, that figure jumps to 2 minutes 52 seconds. In Miami, the shift is a little less impressive — only about 90 seconds daily.

Areas closer to the equator experience the least day-to-day variation. As you go poleward, it’s more extreme. That’s why the Arctic Circle is sunlit all night in the summertime, but you can see two months of straight darkness as “polar night” settles in each winter.

The position of sunrise and sunset change

The spring and fall equinoxes are the only times when every place on Earth should experience a sunrise that is due east, as well as a sunset perfectly west.

Over the coming three months, sunrise and sunset positions in the Northern Hemisphere will continue to scoot to the north as the sun traces a longer path throughout the sky. During the winter, the sun retreats farther south, and so do the points at which it’s visible over the horizon.

The bottom line?

Spring has sprung! Summer is just a few short months away.

Of course, winter doesn’t want to go down without a fight. During the spring transition, vestiges of winter cold will wage war with summerlike air sourced from the tropics. This occurs most often over the Deep South and Great Plains. The clashing air masses are likely to spark severe thunderstorms, tornadoes and flooding.

Nonetheless, as the birds start chirping, the landscape becomes greener and the weather continues to warm, it’s hard not to smile.

Jason Samenow contributed to this report.

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