Our unusual summer has slipped away and March signals the start of autumn. What is this in-between season?
Our planet Earth goes around the Sun in an elliptical (sort of egg-shaped) path once each year. Seasons are caused by the Earth always leaning a little to one side.
Scientists imagine that there is an invisible line running through the Earth from the North Pole to the South Pole, around which the Earth spins constantly (once each day).
They call this imaginary line the Earth’s AXIS.
Instead of the axis running straight up and down, it’s tilted about 23.5 degrees (about the difference between the small and big hands on a clock at four minutes after noon).
This tilt means the northern and southern hemispheres receive different amounts of the Sun’s energy at different times of the year. It also means the strength of the sunshine is different.
IT’S GETTING CHILLY
About March 20 or 21 of each year, the Earth reaches a place on its orbit called the VERNAL EQUINOX — here day and night are about the same length all over the world and if you stood at the equator, the Sun would be directly overhead.
As Earth moves away from the equinox, the southern hemisphere’s nights grow longer and days grow shorter. The sunshine we do get arrives at more of an angle so is less strong.
With less energy reaching the southern hemisphere, the weather becomes cooler and the decrease in energy changes other climate patterns such as wind, ocean currents and rainfall. The season we call AUTUMN will have well and truly set in, with the months we call WINTER not far behind.
THE LONG AND SHORT OF IT
At the very same time, the exact opposite pattern happens in the northern hemisphere: days get longer, nights get shorter and all that extra energy from the Sun means the weather becomes warmer.
They experience what is usually called spring and then summer.
This pattern continues until about June 20 or 21 which we call the winter solstice (but if we lived in the northern hemisphere, we would call it the summer solstice). This is the day each year with the longest night and, so, the least amount of daylight.
After the solstice, the pattern starts reversing. The southern hemisphere’s days gradually get longer and the extra energy warms the weather.
And all because the Earth spins lopsided!
WHEN ARE …
Equinoxes: Day and night are the same length (both hemispheres) — usually March 21 and September 23.
Solstices: Shortest day or night (depending on which hemisphere) — usually June 21 and December 22.