“Happy Spring!” Our household thermometer recorded an overnight low temperature of -5ºF on March 3. The median temperature on that day was thirty degrees below normal. “Take heart,” we tell our friends. “In several days we will enjoy two weeks of normal temperatures, according to the long range forecast.” We do not cite these figures to become a source for weather statistics in the upper Midwest. Rather, our desire is to improve general weather literacy. We are now in meteorological spring, but we may experience surprising vagaries of the season no matter which season we claim to inhabit!
As a prelude to our discussion of spring and other seasons we recommend you review the link from our post of 10/27/17:
Just when we thought we understood pertinent facts about the spring season, we discover scientists have defined two spring seasons: meteorological spring and astronomical spring. Calendar-wise, there is some overlap between the two. Meteorological spring runs from March 1 to May 31. Those three months are characterized by generally rising, pleasant temperatures on average. By calling our three Gregorian calendar months—March, April, and May—meteorological (weather-related) spring, we may be certain which time frames we compare from one year to another. There is no guarantee that temperatures will always rise from one day to the next, or that we will never receive a bitter atypical cold spell or warm spell. We deal with average conditions as meteorological spring progresses.
Our above link to “Earth Without Seasons” stressed the effect of Earth’s seasons on Earth’s living things, including the occurrence of unpredictable weather. Beyond those there are many other more astronomical effects of Earth’s axial tilt, Earth’s elliptical orbit, and the 41,000 year wobble in Earth’s axis of rotation. Below we discuss why the astronomical seasons commence approximately March 21 (spring), June 21 (summer), September 21 (autumn), and December 21 (winter). As an example, why does the first day of astronomical spring occur at precisely 5:58 PM EST on March 20 in 2019? Meteorological spring (the “getting warmer” season) correlates with dates on the man-made 16th century Gregorian calendar.
Astronomical spring is related to precise points, times, speeds, motions, and paths in space as Earth rotates on its axis and revolves around the Sun. These are all mathematically determined. Specific times of occurrence of events correlate with dates on the Gregorian calendar. The first day of meteorological spring is about three weeks prior to the vernal (spring) equinox—the first day of astronomical spring.
What is the vernal equinox? If you live in the northern hemisphere you have noticed that on December 21 the sun does not rise very far above Earth’s horizon at noon. By March 21 its noon position has gradually risen about halfway to its maximum, and by June 21 it has risen to its highest point. The Sun follows a path through the sky called the ecliptic. It does not really move, but it appears to move because we are moving around the Sun viewing it from constantly changing positions. The Sun, traveling along the ecliptic, traces a complete 360º path through the sky in one year. On about March 21 each year the apparent path of the sun crosses an imaginary circle called the celestial equator, made by extending the Earth’s equator infinitely out into celestial space. It intersects the imaginary celestial sphere at an infinite distance. On this date the Sun is exactly halfway to its highest point in the sky (or on the celestial sphere). The result is day and night are of equal length on that day. Before or after that date, day length is shorter or longer than night. This occasion is called the spring equinox (equinox = day and night of equal length). The term also refers to a date on the calendar or a point in space.
On June 21 when the sun reaches its highest northern point on the ecliptic, we enjoy longest days and shortest nights. We have reached the date of the summer solstice (solstice = sun stands ‘still’). After this date the Sun gradually begins to descend toward the celestial equator. This marks the end of astronomical spring, and the beginning of astronomical summer. Likewise, on about September 21 astronomical summer ends; astronomical autumn begins. We then observe the autumnal equinox. On about December 21, astronomical autumn ends; astronomical winter begins: we have reached the winter solstice. Our planet’s well-known axis tilt is responsible.
The meteorological grouping of seasons in three month blocks conveniently highlights environmental effects of our weather cycles. The astronomical season groupings of three month blocks of time beginning, respectively, on about the 21st of March, June, September, and December are, instead, based entirely upon celestial points, times, speeds, motions, and paths in space.
Our universe is coherent and orderly. Meteorological and astronomical seasons both amply illustrate coherence and cyclical orderliness. Both sciences, meteorology and astronomy, manifest the glory of the Creator and His loving physical provisions for our temporal existence.