Astronomy: The Summer Solstice Explained

                                              Astronomy: The Summer Solstice Explained



    Regrettably, far too many people shy away from attempting to visualize and understand such grand celestial events as the upcoming Summer Solstice of June 20th. Well, if you're among them, leave that computer mouse alone! It's high time for your primer on the basic celestial mechanics of our planet.

    We fortunate billions live on a planet which circles a star or should I say "ellipses a star" once each year. In fact, one Earth year equals one complete journey of the planet around our star, the Sun. Our path is not a perfect circle around the Sun, but rather more of an egg-shaped plane of geometry called an ellipse where the Sun remains stationary inside the narrow portion of our egg-shaped orbit. When the planet is farthest from the Sun it is at the widest portion of the elliptical orbit opposite the narrow end of the ellipse where the Sun always resides. Conversely, our least distance from the Sun coincides with winding our way around the narrow end.

    If you were to stop reading here, you might leave with the false impression that summer must occur while we are passing around the narrowest portion of our ellipse - at the least distance from our star. You would be incorrect.

    While we orbit our Sun, the planet is also spinning like a child's top. The Sun appears to rise from the east while we stand stationary. The celestial reality is that the Sun remains still and our planet turns into the direction of the sun from west to east, but we'll use the everyday expression of the sun rising in the east and setting in the west.

    Finally, to begin to understand the mechanics of the Summer Solstice, we have some more points to consider.

    Under current theory, an object about one third the mass of our planet had the audacity to give a glancing smash to the early Earth some billions of years ago. The remnants coalesced to form the moon. The force of the impact, though, was of such a magnitude that it actually tilted the planet a couple dozen degrees off kilter. Prior to the impact, if you could imagine a rod stuck into the South Pole and out through the north, the rod would have been "straight up and down" or vertical. That imaginary rod is called the Earth's axis. After the impact, the north pole of the axis (and so the northern hemisphere) tilted forward toward the sun a bit. The planet continued to spin around its imaginary axis just as it did before and (thankfully) continues to do so today.

    Now, we live on a planet which spins around this imaginary, tilted axis as it moves around the Sun in an egg-shaped orbit where the Sun remains closer to the narrow end of the ellipse. The Earth's axis always points in the same direction. As it turns out, the northern hemisphere is tilted towards the Sun by those two dozen degrees when we are at the spot in the orbit which is farthest from the Sun. The Sun's rays make the most direct, warming hit of the year on the northern hemisphere. This is summertime north of the equator!

    If we did not have the tilt, the equator would always receive the most direct sunlight. The sun's rays would always become less direct the farther north or south away from the equator one traveled. If you were to stand at the north or south poles the sun would be so low on the horizon as you looked toward the equator that you probably would not see it. With our tilted planet, however, the poles do receive some sunlight at opposite times of the year, and when the northern hemisphere is pointed toward the Sun on what we call the Summer Solstice, the sun rises to its highest elevation in the sky for people living north of the equator because we're leaning toward the Sun. It also appears to hang high in the sky longer than usual before continuing on its way to setting. The meaning of "solstice" actually derives from the Latin - "sun standing still." All of this provides the most daylight hours of the year.

    Though I've concentrated on the details of the northern hemisphere's Summer Solstice, similar mechanics work for the southern hemisphere when the Earth is at the other end of the elliptical orbit, where lands south of the equator are leaning toward the sun. At that time of the year, the north experiences winter.