By Keith Johnson
The quiz.
It's time for another pop quiz, class. Today's question: Why is it getting colder outside at this time of year? (Please check off all correct answers.)
(A). The Earth is farther from the Sun in its orbit.
(B). The Sun is lower in the sky now.
(C). The Sun's nuclear reactions slow down at this point in its cycle.
(D). The Sun is in the sky for fewer hours.
(E). It's the result of a law passed by the New Jersey legislature many years ago.
The right answers.
Everyone finished? All right, put down your pencils and let's discuss the answers. We'll start with the correct choices first.
(B). The Sun is obviously lower in the sky in the fall and winter. This means its rays have to spread out to a greater degree, and must cover more ground than is the case in the summer. They don't illuminate the Earth as strongly.
There's another, less obvious factor. Sunlight is absorbed slightly by our atmosphere. When the Sun is low in the sky, its rays are passing through more atmosphere than when it's high, and the air absorbs more light. This is why the Sun appears so much dimmer at sunset than at noon: its rays must push through a much greater mass of air.
(D). The Sun does stay in the sky for much less time now than it does in the late spring and summer. On June 21, the date for the summer solstice (often considered the first day of summer), we have more than 15 hours of daylight. Six months later, on December 21, the winter solstice, the Sun is up for only 9 hours and 20 minutes. Clearly it's going to heat us up more effectively if it's got more hours to do the job!
The wrong answers.
Now let's look at the wrong choices.
(A). The Sun is not farther away from us in the winter. In fact, the Earth's closest approach to the Sun occurs a few days after New Year's Day! We're farthest from the Sun around the Fourth of July!
But the difference isn't much: the Earth's orbit is very close to being a circle. The change in distance amounts to about three percent from January to July. It does make our Northern Hemisphere winters just a tiny bit milder, and our summers a little cooler, than would otherwise be the case, but the difference is too small to notice.
(C). The Sun's nuclear reactions don't change that quickly, and probably don't vary at all. Every so often you read of an astronomer that believes he's discovered that the Sun's hydrogen "burning" has shut down altogether, but the evidence has not been convincing so far. Even if it were to happen today, we'd not know about it for about a million years, as it takes that long for energy to percolate upwards from the Sun's center (where all the action is taking place) to its surface. Finally, the Sun is large enough that any such changes couldn't happen on time scales as short as a year. And there would be no reason for this to happen at the same time each year.
There are changes that do occur on the Sun, though. There is a cycle of solar activity, the famous "sunspot cycle," where about every eleven years we see a peak of those darker spots on the solar surface. But the cycle isn't very regimented: it can vary from 9 years to 14 years.
There seem to be unpredictable variations of a greater sort, too. During the years 1645 to 1715, there were practically no spots visible at all, an interval now called the "Maunder Minimum" after its discoverer. This was a time when the climate changed: it became noticeably colder, and the period is often termed the "little ice age." It's not clear how a lack of solar activity causes climate change, but it seems that it does.
It would be good to find out the details. For example, we have been going through a period of almost no sunspots for a year or two, and some experts were starting to worry, until a couple tiny sunspots from the new solar cycle finally appeared earlier this year.
A good place to keep track of the Sun is http://www.spaceweather.com. We have the Sun under constant observation by space telescopes such as SOHO, and you can find out more, along with fabulous images and videos of the Sun, at http://sohowww.nascom.nasa.gov/.
(E). Even the New Jersey legislature wouldn't try this. Maybe in Nevada...
Edelman Winters.
The final performances of the star show "The Rowan Universe" and the laser show "Laser Floyd" will be given next Saturday night at 7 and 8:30 p.m., respectively. See the link below for the planetarium Web page for more information.
We are starting a new tradition at the Edelman Planetarium next month. The public star show "Season of Light" will play on Saturday nights at 7 p.m. the first three weekends in December. Winter may be cold and dark, but we humans have found ways to make it bright and cheerful.
This show will portray holiday customs from various cultures, explain why winters happen in the first place, and offer some ideas about the Star of Bethlehem. Narrated by Noah Adams of public radio fame, with music by Mark Petersen of Loch Ness Productions, "Season of Light" is a festive presentation that can be enjoyed by everyone over the age of about six.
We'll also offer a holiday laser show! With music by Andy Williams, Bing Crosby, Dean Martin, and the Mannheim Steamroller, "Holiday Magic" is not your standard laser rock show. Unlike most of our laser shows, this one is perfect for a family outing (though we won't guarantee folks under six will enjoy it as much as their older siblings.) It's playing on Saturday nights at 8:30, December 6-20. We plan to run these two shows each December from now on. For more information, take a look at the planetarium Web page, http://www.rowan.edu/planetarium/.
Keith Johnson enjoys warm winters in the star theater of Edelman Planetarium at Rowan University, where he is the director. He's glad to answer questions about astronomy, space exploration, and stargazing, though not about astrology, at johnsonk@rowan.edu and (856) 256-4389. Teachers can schedule field trips to the planetarium at those locations, too.