Here in the Northern Hemisphere, on the first day of spring, you are likely to see all sorts of images and gifs welcoming in the vernal equinox. Spring arrived Monday at 6:29 in the morning and while 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 and those in the media herald as spring’s arrival.
While nearly everyone knows spring arrives around March 20th, what’s actually occurring, in terms of the relationship between the Earth and the sun, can still be poorly 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 daylight time March 20th, spring. Practically, 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 Sept. 22nd at 4:02 p.m EDT the same thing will occur on different spot along the equator as fall begins.
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 Portland on Monday is about 46 degrees above the horizon. This angle will continue to increase until the first day of summer when it’s at about 70 degrees. From then on, it falls reaching a minimum of 23 degrees as winter begins. Other places on the planet have different maximum heights. There’s always somewhere where the sun is directly overhead, but it only occurs exactly at the equator on the equinox.
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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 progress, it will become progressively darker until it’s pitch black for many months. The opposite is occurring at the North Pole.
The sun is up all day at the south pole during our fall and winter, but down all day during our spring and summer. (TimeandDate)
The word equinox comes from the Latin 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 including the shape of the planet and the way the sun’s light is bent as it passes through the atmosphere cause this.
In a world which many of us are overextended and stressed, the fact that the planet is spinning around a tilted axis at over 1,000 mph can be lost. As spring arrives Monday, take a moment to ponder the changes occurring, 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.
Follow Dave Epstein on Twitter @ growingwisdom