Post by Joanna on Aug 1, 2015 1:53:45 GMT -5
The Night Sky of Lammas
Today is Lammas Day, or Lughnasadh, probably not high on your morning agenda. But in medieval Britain it was a big deal: the festival of the first wheat of the harvest. It’s also one of the four “cross-quarter” days: the midpoints between the solstices and equinoxes, and thus the centers of astronomical summer, fall, winter and spring. So is summer half over? Not quite. The calendar has shifted a bit with respect to the seasons since the cross-quarter days were set, so the first half of summer continues until August 7. The tipping point arrives that morning at 8:29 a.m. EDT. This makes the seasons seem so exact. Astronomically speaking, they are. They’re determined by Earth’s position along its orbit and the direction that Earth’s axis tilts, things that are predictable with extreme precision. But it hardly feels that way.
If summer already feels like it’s winding down, you may be thinking of “cultural summer,” which for Americans runs from Memorial Day to Labor Day. The peak of “temperature summer” depends on where you live; in Massachusetts the hottest days center around July 23 on average. We’re already two-thirds of the way through “meteorological summer,” which is defined simply as June, July and August. Very different is “solar summer,” centering on the highest sun and the longest day: the solstice on June 21. Which makes more sense the farther north you live. The British Isles are farther north than we are and used to go by the height of the sun. That’s why Shakespeare called our beginning of summer Midsummer’s Night.
Lammas was the beginning of fall. Astronomy aside, seasons are a social construct. Equatorial cultures often recognize just two: rainy and dry. Others have three: planting, harvest and winter. Where I lived in northern Vermont, there were seven: spring, summer, fall, almost winter, winter, still winter and mud.
The not-so-eternal stars. Meanwhile, the sun and stars stay their courses with unhuman precision, ignoring all such concerns. Always, we think we believe, the western evening sky of late summer will display bright Arcturus. It shines pale ginger-ale in color, like a stray drop from a picnic elevated to shine with the gods. Arcturus is the fourth brightest star in the sky after the sun, and at a distance of 37 light-years, it is the nearest orange-giant star. It is godlike indeed in its uncaring, material way, lording over its realm of space with 115 times the light of our sun – a supernal ball 25 times wider than the sun and 28,000 times wider than Earth. But it’s just hot gas. Forever paired with Arcturus for summer stargazers is the Big Dipper, hanging to its right in the northwest. This is the season when the Dipper dips down as if to scoop up imaginary water. It dumps the water back onto the world in the evenings of spring.
Arcturus itself is the brightest star of Boötes the Herdsman. His brightest stars form a narrow, tilted kite. Amateur telescope users know Boötes as a rich area for double stars, a fine globular star cluster that looks like a sugar pile in moonlight and a smattering of faint galaxies.
Are these things really forever? Well, that depends. Across a human lifetime, yes. But over millennia, no. Remember that part about the direction Earth’s axis points? The direction gradually changes, carrying our seasons along with it, in a 26,000-year cycle that drags the constellations entirely around the seasons and back to where they started. So the stars drift out of synch with human notions of forever. Our reliable warm August panorama of Arcturus and the Dipper in the west, for instance, displayed itself over the icy landscapes of February in 9,000 BC. And Orion was a constellation of summer. That will happen again in the millennia around 17,000 AD. In the year 28,000, the stars will be back to rights again by 21st-century standards . . . or will they? The stars themselves are slowly drifting through space in their own random directions. By then the Big Dipper itself will have become distorted. So will most of our constellations. It turns out you can’t go home again.
After a million years, almost all the naked-eye stars making up our night sky will have traveled far out of sight in various directions, as new ones continually arrive and pass through. The starry scene over the Earth will be completely remade from scratch. And even that is fast compared to the age of the Earth and solar system. Earth, in its 4.6-billion-year history, has seen several thousand such complete turnovers of its nighttime stars. Eternity is relative.
Source: Alan M. MacRobert, The Boston Globe, August 1, 2015.