Celestial News: The reason for the seasons
For Steamboat Pilot & Today
In a famous study from the 1980s, a simple question was posed to Harvard University graduates, right after they walked off the stage with diplomas in hand. The question: What is the cause of Earth’s seasons? You can look up the video of the interviews for yourself and see that graduate after proud graduate confidently answered that the reason it is hotter in the summer is because the Earth is closer to the Sun in the summer than it is in the winter.
So much for higher education.
In fact, the Earth is closest to sun in January every year — during the dead of winter for the northern hemisphere. Earth is farthest from the sun in July every year, in the heart of the northern hemisphere summer.
Clearly, the reason we have seasons is not due to Earth’s distance from the Sun.
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It is due, instead, to the 23.5-degree tilt of the Earth on its axis. This causes the midday sun’s rays to strike the Earth’s surface at a low, glancing angle in the winter and a high, more nearly vertical angle in the summer. It also causes the number of daylight hours to decrease in the winter and increase in the summer. These two effects of the Earth’s tilt create our changing seasons.
Thanks to that 23.5-degree tilt of the Earth’s axis, the Sun spends half of the year shining directly onto the northern hemisphere and the other half of the year shining directly onto the southern hemisphere. It reaches its highest point in our sky around June 21, the June solstice, and its lowest point around Dec. 21, the December solstice.
Separating these two extremes are two days during the year called the equinoxes, six months apart, when the sun shines vertically down on the Earth’s equator. The equinoxes occur on or near March 21 and Sept. 23 each year.
Equinox is a word that means “equal nights” and is used to describe these two special days of the year when every location on Earth experiences 12 hours of sunshine and 12 hours of darkness. These are also the only two days of the year when the Sun rises exactly in the east and sets exactly in the west.
Here in the northern hemisphere, we have just endured six months of cold, short days, but now, it’s the southern hemisphere’s turn. Our season of spring begins the moment the sun crosses over the equator and enters the northern hemisphere.
This year, that moment occurs on March 20 at 3:58 p.m. We are rapidly exchanging minutes of darkness for minutes of daylight each day as we move toward the June solstice. After the solstice, the sun will begin moving southward once again and cross the equator on Sept. 23, the September equinox, ushering in the season of fall.
So, if you are ever given the opportunity to go on record about why it is hot in the summer and cold in the winter, be smarter than a Harvard graduate and say, “Because the Earth is tipped 23.5 degrees on its axis.”
Jimmy Westlake retired from full time teaching at Colorado Mountain College Steamboat Springs in 2017, after 19 years as a professor of physical sciences. “Celestial News” appears monthly in the Steamboat Pilot & Today. Check out Westlake’s astrophotography at jwestlake.com.
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