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Why is Monday considered spring? The vernal equinox, explained

The Boston Globe logo The Boston Globe 3/20/2017 By Dave Epstein
Spring crocuses bloomed in South Natick on the first day of spring in 2016. (Photo by Dave Epstein) © Provided by Boston Globe Media Partners, LLC Spring crocuses bloomed in South Natick on the first day of spring in 2016. (Photo by Dave Epstein)

Here in the northern hemisphere, spring in the form of the vernal equinox is scheduled to start Monday at 6:29 a.m. Although there’s still snow on the ground and cold weather in the forecast, one can’t deny the new season has arrived.

Meteorologists start spring after the coldest 90 days of the year, so we began it on March 1st. But astronomical spring arrives when the sun reaches a certain height over the equator at noon each year. This is typically what most folks celebrate as spring’s arrival.

While nearly everyone knows spring arrives around March 20, what’s actually occurring in terms of the relationship between the earth and the sun still isn’t universally understood.

On both the vernal and autumnal equinoxes, the sun will be directly overhead at noon at some place along the equator of our planet. This year, the first of these two occurrences happens at 6:29 a.m Eastern standard time on March 20. Basically, if you were in central Africa and looked up at noon, the sun would be directly above your head, forming a 90-degree angle with the ground. On Friday, September 22, at 4:02 p.m., the same thing will occur at a different spot along the equator to begin the fall season.

Just before sunrise, the sun is even with the horizon and then rises during the day, reaching the maximum height exactly between sunrise and sunset. The highest point the sun will reach around Boston Monday is about 48 degrees above the horizon. This angle will continue to increase until the first day of summer, when it’s at about 71 degrees. From then on it falls, reaching a minimum of 24 degrees as winter begins. Other places on the planet have different maximum heights. There’s always someplace where the sun is directly overhead, but it only occurs exactly at the equator on the equinox.

Down at the South Pole this week, the sun will go from being above the horizon all the time to being just below. As the days go on, it will become progressively darker until it’s pitch black for many months. The opposite is occurring at the North Pole.

The word “equinox” is derived from the Latin words meaning “equal night.” While nearly all spots on planet Earth have about 12 hours of darkness and light as spring begins, it’s not exact. You can see from the chart below, depending on where you live on the planet, the day on which you have equal day and night shifts on the calendar. Multiple factors cause this, including the shape of the planet and the way the sun’s light is bent as it passes through the atmosphere.

In a world in which many of us are overextended and stressed, the fact that the planet is spinning around a tilted axis at over 1,000 miles per hour can be lost. As spring arrives Monday, take a moment to ponder the changes occurring, and how the increase in light and strength of the sun will soon be melting the snow, warming the earth, and bringing about that magical and dramatic metamorphosis to our landscape known as spring.

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