Many people chose the autumn season as their favorite time of the year, and look out…it’s about to fall right in your lap!
Earth officially reaches the point in its orbit around the Sun called the autumnal equinox on Friday, Sept. 22. Equinoxes…solstices…they are words that we use freely when the season merits. But we give little thought to what is happening.
Such a simple visual tool as that classroom globe of Earth has a lot of meaning as to the cause of the seasons. On that Earth globe are two lines that straddle the equator on a globe that is appropriately tilted 24.5 degrees from vertical. The invisible north line is the Tropic of Cancer and the south line is the Tropic of Capricorn. From a celestial perspective, the Sun appears to move between these two latitudinal benchmarks
The four distinct seasons have everything to do with the tilt of Earth’s axis. And has nothing to do with how close or far away from the Sun the Earth is. In fact, Earth is farthest from the Sun, 95 million miles, in mid-June, and closest, 91 million miles, mid-January.
When tilted toward the Sun in the summer, the solar rays are more direct and therefore much hotter than the slanted Sun rays of winter—when our Northern Hemisphere is tilted away from the Sun. That’s why the Southern Hemisphere seasons are just the opposite of our lands above the equator. At the equinoxes, the North and South Hemispheres are going through the opposite transition of warm and cold seasons.
The Sun stands directly over Earth’s equator this Friday at 4:02 pm. in what’s called autumnal equinox. Remember, it’s the first day of spring in Australia, South Africa and South America. The Sun will rise at 7:17 am and sunset will be at 7:27 pm. But the word “equinox” means “equal day” meaning the same hours of daylight and night. The 12 hours of daylight and 12 hours of night occurs on Wednesday Sept. 26th when the sunrises at 7:21 am and sets at 7:21 pm. The reason this equal day and night doesn’t happen on the equinox is accounted for by errors in our Gregorian Calendar.
Because the celestial mechanics of physics can’t be altered, our imperfect, human calendars often get out of sync for a variety of reasons. Remember, our 365-day earthly year is actually more than one-fourth a day longer than the actual solar journey. And even though corrected with extra day every four days, there are still errors that compile over time. But let’s not get bogged down with too many heady celestial physics concepts of celestial motion in the Solar System. Just believe that the seasons are the result of Earth’s tilted axis, and not anything to do with the distance from the Sun.
Our approximately 8,000-mile-wide globe doesn’t spin straight up and down. We are knocked over from vertical by 24.5 degrees. As Earth orbits the Sun, there is a period when our Northern Hemisphere is tilted toward the Sun (summer), and away from the Sun (winter).
It was thousands of years ago when ancient stargazers noticed how the Sun’s arc across the sky moved during the season from north to south in its daily motion of rising in the east and setting in the west. In those ancient days thousands of years ago, the day of the solstices saw the Sun in Cancer in the summer and Capricorn in the winter, thus the imaginary latitude 24.5 degrees from the equator were called the Tropic of Cancer and Tropic of Capricorn.
The axis of Earth wobbles like a toy top at the end of a spin, the North Star changes over thousands of years. In fact, the axis has swung from 22.1 degrees to 24.5 degrees in 40,000-year cycles, altering global weather and the night sky. Polaris is the North Star now; it was Thuben a faint star in Draco the Dragon when the Pyramids of Egypt were built. And in 12,000 years it will be the very bright star Vega, high in the northwest during autumn. This wobble of Earth is called “precession,” and the moving dates of our calendar are called “precession of the equinoxes.” This changes the background of stars where the Sun is located during celestial events when comparing the stars in the sky of the 21st Century to times throughout antiquity.
The 2017 autumn equinox finds the Sun in Virgo, and the spring equinox had the Sun in Pisces. If we were renaming the lines that define the tropics on our classroom Earth globe, the north point would be the Tropic of Gemini, and the south line would be the Tropic of Sagittarius.
And now, I’ve lost you…so, where’d I put that box of sweaters?