(Jarrett Hendrix via Flickr)

Spring arrived early in many parts of the United States this year, making it easy to forget that we’re still in the final days of winter. On Sunday, however, we can really say spring has begun with the arrival of the vernal (or spring) equinox, which occurs March 20 at 12:30 a.m. Eastern time.

On the equinox, neither of Earth’s hemispheres tilts away from or toward the sun, and there are nearly equal amounts of daylight and darkness across the globe. It’s one of only two days each year when you can see the sun rise due east and set due west from anywhere on Earth outside of the polar regions.

Each year we hear a few misconceptions about the meaning of the spring equinox and why it occurs. These are the most common:

1. The equinox is the “official” first day of spring

Humans have long celebrated the equinoxes and solstices as seasonal turning points. Yet while people often refer to the equinox as the first “official” day of spring, there is no universally agreed upon definition for the seasons. Astronomers and physicists define seasons by the solstices and equinoxes, while for meteorologists and climate scientists, spring is defined by annual temperature cycles, and begins on March 1.

[Is it spring yet? Debating the definitions]

Spring may be one of the hardest seasons to define, as wintry weather often extends well beyond March in colder climates. For some places, the date of the last freeze or measurable snow may be a more useful metric. Alternatively, if we define spring only by daylight — following the Irish Celtic tradition — then spring began back on Feb. 1. By this reckoning, winter marks the three darkest months of the year (November to January), and spring comprises the three months with the biggest increase in sunlight (February through April).

2. You can balance an egg on its end on the equinox

Urban legend has it you can keep an egg balanced upright at the exact moment of the equinox. This practice likely has roots in Chinese Lunar New Year traditions but in reality has no scientific basis. With a bit of luck (and patience) you can balance an egg upright on any day of the year.

 

3. Sunrise and sunset are exactly 12 hours apart

On the equinox, all latitudes receive equal amounts of daylight and darkness. (timeanddate.com)
At the equinox, all latitudes receive nearly equal amounts of daylight and darkness. (timeanddate.com)

Though equinox means “equal night” in Latin, both of Earth’s hemispheres get a bit more than 12 hours of daylight. Take D.C., for example: On March 16 — four days before the equinox — sunrise and sunset were exactly 12 hours apart. Yet by March 20, the sun is already up for 12 hours and 9 minutes.

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.

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.

Why, then, is the spring equinox not on March 16 or 17, when we get exactly 12 hours of daylight? First, the date with exactly 12 hours from sunrise to sunset varies with distance from the equator. Second, the equinox occurs when the sun appears directly overhead along Earth’s equator, at zero degrees latitude, and that happens around March 20.

4. The spring equinox is always on March 20

The exact time of the equinox varies from year to year, which means so can the date. To avoid confusion between time zones, the time of the equinox is usually based on Coordinated Universal Time (UTC), which is four hours ahead of Eastern Daylight Time. By this metric, the vernal equinox can occur anywhere from March 19 to March 21.

This year’s spring equinox is the earliest since 1896. For the rest of the 21st century, the March equinox will arrive continually earlier with each passing leap year. Beginning in 2044, the equinox will be on March 19 (UTC) on every leap year until 2100. The earliest equinox of the 21st century will be in 2096, arriving midday on March 19.

Since 2100 isn’t a leap year, the equinox will eventually drift later again — though March 20 is the most common date of the equinox for a while. The last March 21 equinox was in 2007, and it won’t happen again until the year 2102.

5. Spring is generally warmer than fall

After the cold days of winter, we tend to expect spring weather to be warm and sunny. While it often feels warm compared to winter, spring isn’t as warm as you might expect. Not only is the spring equinox typically colder than the fall equinox in September, but in most of the Lower 48, spring on the whole is a few degrees colder than fall (based on the meteorological seasons).

(Justin Grieser)
(Justin Grieser)

The reason for this is seasonal temperature lag. There’s a lot more sunlight in spring compared to fall, but much of the sun’s energy goes toward heating the ground and large bodies of water first. The difference is noticeable in more northern cities along the coast. In Boston, for example, the spring months of March through May average more than six degrees cooler than the fall months of September through November.

Fortunately, while spring can be slow to arrive in some places, we’ll all be seeing much more daylight than darkness for the next six months.