Sunday at 3:49 p.m. is the summer solstice, which I’ve avoided writing about for years. I remember studying these matters in college and saying to myself: “English major. Yeah, that’s the ticket.”
However, by popular demand, I’ll give it a go, knowing full well that my fellow astronerds will bomb me with email for leaving something out.
On a practical level, the solstice is that day when the daylight hours are at their longest. Combined with daylight saving time, the solstice means that evening twilight doesn’t end until well after 11 p.m., a backyard barbecuer’s dream. Those of us who work the night shift can’t do any serious stargazing until nearly midnight.
To the celestial cartographer, the solstice is an imaginary point on an imaginary sphere. Given how much we know about the universe, it’s ironic that for the practical purposes of sky mapping, we still imagine the universe as a great sphere that encloses the Earth. Of course, we can see only half the celestial sphere at one time because Earth blocks our view of the other half.
On the celestial sphere we draw several imaginary circles. One of them is the celestial equator, the extension into space of Earth’s equator. Another line, called the ecliptic, marks the apparent yearly path of the sun across the sky.
Because Earth’s axis is tilted 23 degrees with respect to its orbit around the sun, the celestial equator and the ecliptic are tilted by the same amount with respect to each other.
As a result, the two circles touch each other on the ecliptic at opposite points, which also happen to be the points along the sun’s path where day and nights are equal in length. Those points, which also correspond to the first days of spring and autumn, are called the vernal and autumnal equinoxes.
Following this so far? Me neither. In any case, the summer solstice is the point on ecliptic where the sun is at equal distances between the vernal and autumnal equinoxes, or in other words, as far as its going to get from the celestial equator.
In effect, the sun is at its farthest point north. For those of us in the NORTHERN hemisphere (get it?), the sun reaches its highest point in the sky at local noon, which is about 12:38 pm EDT on June 2. Why 12:38 and not, as expected, noon as measured by the clock?
Several factors influence the time. Essentially, our clocks and calendars are only approximations of “real” time as measured by the sun. The year is 365¼ days long, not 365. Also, the time of, say, daily sunrise is measured from the eastern edge of our time zone. The sun won’t rise for us until about half an hour after it does on the east coast. Add daylight saving time, and solar time and clock time are essentially out of sync with each other.
The solstice also means that the sun rises and sets at its most northern points, north of east for the rising and north of west for the setting. That means it stays up as long as it’s going to all year, making for the longest period of daylight.
During the spring, the sun rises higher and higher in the north each day, and the daylight time is therefore longer each day. After the solstice, Sol will be a little lower each day, and the daylight will get shorter and shorter.
Since the summer solstice happens on the longest day, one might expect that it is also the day of the earliest sunrise and the latest sunset. Oh that the universe were that simple, fellow skygazers. The earliest sunrise occurred on June 14, a week before the solstice. The latest sunset will occur on June 27, about a week after. Sadly, the reasons for this strangest of celestial phenomena are too mind-meltingly complex for an old English major to explain completely here.
In short, they have to do with the tilt of the Earth on its axis and the fact that Earth’s orbit around the sun is an ellipse and not a circle.
Because of that orbital path, Earth actually speeds up and slows down as it orbits the sun. Both the length of a day and when a day starts change from day to day if you measure them against the apparent motion of the sun across the sky.
Solar time will not do as a civil measure of time. We need a fixed measure by clock and calendar to make civilization work. Thus, we call a day exactly 24 hours, which on any given day is inaccurate. We divide the orbit of the sun into 365 equal parts, which is also inaccurate since the sun is constantly slowing down or speeding up in its orbit.
Civil time is uniform. Solar time is not. So if you try to use civil time to determine solar events, they won’t match up.
If you’d like a more detailed explanation, go to your local library and look up the “equation of time.” Or you can try an Internet site like http://aa.usno.navy.mil/faq/docs/eqtime.php. However, don’t blame me if your brain melts a little. Mine did, and that’s how I ended up an English major. Thanks, equation of time.
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