Monday, December 20, 2010

Astronomy 101

I have seen more scrambled explanations of the solstice and lunar eclipses in the press this week than I would have imagined. I saw one and blew it off, then another, and finally Richard Cohen in the New York Times:
WHAT is the winter solstice, and why bother to celebrate it, as so many people around the world will tomorrow? The word “solstice” derives from the Latin sol (meaning sun) and statum (stand still), and reflects what we see on the first days of summer and winter when, at dawn for two or three days, the sun seems to linger for several minutes in its passage across the sky, before beginning to double back. [emphasis mine]
The Santa Fe New Mexican had something on Saturday about the sun and the moon and the earth being opposite each other. (It's gone to recycle limbo in the garage.)

Another said something about the earth adjusting its tilt.

And some have noted that the full moon and the eclipse are happening at the same time. Quelle surprise!

The New Mexican formulation seemed to be a clumsy translation into words of what the writer might have understood. Cohen simply doesn't understand, nor do those seeing the earth adjust its tilt or being amazed at the conjunction of full moon and eclipse.

So let me explain lunar eclipse and winter solstice.

A lunar eclipse happens when the moon enters the earth's shadow. So the sun, earth, and moon are lined up. Usually at full moon, the moon is above or below the earth's shadow, so not every full moon is an eclipse, but every lunar eclipse is at a full moon.

And here's how it will look at various times, earth's shadow indicated.

Diagrams from Our Universe and Sky and Telescope.

Now the solstice. Cohen has the derivation right. It's about the sun standing still. Sort of.

The earth is tilted in its orbit. That tilt remains in the same orientation relative to the solar system, which means that it points more or less toward the sun at different times. At the solstices, the tilt is toward or away from the sun. At the equinoxes, it is at right angles to the sun.

[From Boquete Weather.]

So what does that have to do with the sun standing still?

Well, the sun doesn't stand still. It keeps doing what it's been doing all year and before that. We know (I think) that the sun doesn't actually move around the earth to provide day and night, but rather that the earth's rotation makes it look that way. So if we start with the fall equinox (from the Latin for equal night, because the day and night are of equal duration), then as the earth moves toward the winter solstice, the sun rises a little later every day and sets earlier. If you watch the sunrise, you can seen the sun rising a bit further south every day. It also sets a bit further south. On the solstice, that changes. The sun then rises and sets a bit further north every day after the solstice, and the days get longer.

So on the winter solstice, the sun stands still in its journey south.

"[T]he sun seems to linger for several minutes in its passage across the sky, before beginning to double back." And just what had you ingested before you saw that, Richard Cohen?

Update: Props to The Guardian for getting it right!


Peter said...

Nice clarification! I haven't noticed the errors in the popular presses, but that's because I haven't been paying attention.

Small errata:
"So if we start with the fall equinox ... as the earth moves toward the winter solstice, the sun rises a little earlier every day and sets later."

You've reversed later and earlier.

Should read:
So if we start with the fall equinox ... as the earth moves toward the winter solstice, the sun rises a little later every day and sets earlier.

As for the sun standing still, it's important to remember that early astronomers (read: all early sedentary human groups) payed close attention to where the sun set and rose on the horizon all year long. Some times of year the sun rose from, at the most northerly, say, the cleft of a valley on the horizon; by the time of the equinox the sunrise might occur in line with a distant mountain peak further south; other times of year the sun rose furthest to the south, maybe behind the limb of a hill. That hill, peak, and valley then take on huge significance in the way the local people track the seasons.

The solstice marks that particular day when the sunrise stops marching further south, appears to rise in the same place it did the day before, and then on the next day will rise slightly to the north... and day after day the location of sunrise speeds towards the point on the horizon of the Equinox (the peak mentioned above). And as this continues, the days get longer and longer.

I was not a humanities student in college, but one of the only memorable seminars I attended focused on sundials, the seasons, duality in human thought, (cyclisism??) all related to the analog clock face - in contrast to the 'water under the bridge' linearity of digital time.

By this point you should have realized that this is among the only days of the year that my wife and I make a point to celebrate. Another year complete, another year about to begin, as our Earth continues around the sun for the umpteen-millionth time.

Cheryl Rofer said...

Thanks, Peter!

I've fixed the "earlier and later" problem. You're correct.

And yes, it's those early observers who saw the sun stand still in its march south (or north in the summer) to whom we owe the term "solstice." Too bad too many reporters don't do that kind of observing.

I like the winter solstice, because then the days get longer. And we don't have to worry about keeping the larder through the winter and spring starvation like people once did.

Peter said...

I've read that in DPRK April is still called 'The Dying Month'.

We went outside last night around 3am Eastern. Here in NYC there is, as one can imagine, massive light pollution. But it was an especially clear night - low humidity, still winds. Still, all told there were probably only about 30 or 40 stars visible in the entire sky... and those were the ones not blocked by 6 story apartment blocks.

Orion was blazing in the south, and the Rusty Moon was interfering with his aim at Taurus.

It was easy to imagine pre-astronomic people being blown away by the spectacle.