Spring Equinox a Welcome Change in Weather

       Spring will finally officially be sprung this Thursday, March 20th as our part of the Earth begins tilting toward our life-sustaining star.

       It’s easy to appreciate the change in seasons, but hard to understand why the weather changes from cold to hot to cold again. In fact, it’s just in the last 400 years that humans have figured out the rhythm of our seasons.

       Seasons happen only because the Earth is a little whacked over on its side.  Instead of spinning like a toy top straight up and down, Earth is tilted 23.5 degrees with a little wobble.  Something whacked us good in the early days of our Solar System, maybe ripping the Moon out of our sides.  And it’s the Moon’s gravity that keeps us tilted the way we are, otherwise, we’d keep wobbling all over the place! We do wobble a little, completing a circle every 26,000 years—and changing North Pole stars in the process.    

       The physics of the seasons is this:  in the Summer our Northern Hemisphere is tilted toward the Sun, and in the Winter we are tilted away.  Sunlight is hotter when striking a surface more direct than at an angle.  Just feel the sun rays on your skin at 10 am, than again at 1 pm.  The higher the Sun angle, the hotter, like around mid-day. 

       That’s what’s happening this week, as Earth reaches a point in its orbit when the angle is between Winter and Summer.  Called the equinoxes, the Spring and Autumn events welcome an equal day and night, with daylight getting longer as the Sun’s arc climbs higher northward. 

       At the Vernal Equinox, the Sun crosses the imaginary line of the ecliptic from the south side to the north, and the Sun is directly overhead at noon. At precisely 12:57 pm DST, the Sun is directly overhead at the Earth's equator. The Sun's daily arch will continue northward, reaching its farthest point north around June 21, the Summer Solstice. The farthest point southward of the Sun's arch against the celestial sphere is the Winter Solstice, around Dec. 21^st.  In between are the Spring and Autumn equinoxes. On Earth, the point where the Sun reaches these points are called the Tropic of Cancer and Tropic of Capricorn—each 23.5 degrees from the equator.

       So, during Winter, our hemisphere is tilted away from the Sun, the indirect rays not being very warm.  But as we move in our orbit and begin to nod toward the Sun, we warm up.  Keep in mind that the Southern Hemisphere experiences just the opposite seasons—so our Vernal Equinox is the first day of Autumn “Down Under” in Australia.

       People find it hard to believe that the Earth is actually farther away from the Sun in the summer, and closer to the Sun in the winter.

       That’s because we live with a Northern Hemisphere bias. The Earth will be farthest from the Sun, called aphelion, on July 3 at 94.5 million miles.  We’re closest to the Sun, perihelion, in the first week of January at around 91.3 million miles. 

Now, a few facts about our Sun, after all, it’s the star of this Vernal Equinox show!

       Just an average star in many ways, our Sun is 865,370 miles across and is basically 99 per cent hydrogen. The Sun is so huge that is contains 99.86 per cent of everything in our Solar System.  Incredibly, all the eight planets, all their moons, tens of thousands of asteroids and millions of comets make up just 0.14 per cent of the mass in our star family!

       So, how hot is the Sun?  The surface temperature is around 10,000 degrees F.  But the center, where nuclear fission splits hydrogen atoms into stellar energy, the temperatures must approach 50 million degrees F. 

       The surface we see of the Sun, the photosphere, is granular like boiling oatmeal.  Electromagnetic storms create the dark and cooler sunspots, and flames of hydrogen lick off the surface, taking three days to reach Earth.  These solar gases are magnetically drawn to the magnetic poles of planets Earth, Jupiter and Saturn and create glowing crowns of aurora.

Sun in Hydrogen Light by Solar Dynamics Observatory

       The Sun rotates once every 33 days with some variances at different latitudes.  Like all stars, it emits many dangerous wavelengths of energy like ultraviolet and x-rays, most are blocked out by our atmosphere.  But some of the UV rays sneak through; toasting gently our skin if exposed too long.

       Nothing travels faster than light, and those sunrays leave the surface of our favorite star at 186,000 miles a second, or about 670 million miles in an hour. Traveling the 93 million miles from the Sun’s surface to Earth takes more than 9 minutes. So when you’re laying on the beach soaking up the Sun, you are looking back in time at our star. 

       Want to see the Sun close up? It is being watched every minute of every day by four powerful space satellites and several major solar observatories on Earth.  Check out the World Wide Web for the McMath-Pierce Solar Telescope at Kitt Peak in Arizona and Big Bear Solar Observatory in California.

In outer space are observatories in a permanent orbit a million miles ahead of the Earth and a million miles behind us at the “Lagrangian Points”. The two unique satellites revealing each side of the Sun are simply called Stereo A and Stereo B. The die-hard sun watcher is the orbiting Solar Heliospheric Observatory (SOHO), which began working in 1996 on a two-year mission that has lasted more than 18 years.

But the real workhorse watching the Sun is the Solar Dynamic Observatory, costing $2 billion and providing the most detailed look at the Sun with its special instruments.  All these great solar observatories have websites devoted to their images, and many are pictures of beauty as well as scientific data. Another great website to daily follow the Sun is Space Weather, which monitors the solar activity in layman’s terms.

       Enjoy our favorite star as it climbs higher in the sky each day, bringing Spring warmth and new vegetative growth to our Northern Hemisphere. And don’t forget to lather up with the sunscreen!