Thursday, May 04, 2006

Antipodal Moon

Yesterday evening, Jay & I were walking along a high ridgeline in the parkland behind our house as the sun was setting. As we came around a bend, my eyes focused on the quarter moon's brilliant white semi-circle directly ahead of me, and I stopped dead in my tracks. My brain struggled to make sense of what I was seeing. The moon was facing the wrong way!

Now some of you may not understand what I mean by this. You may also wonder how it is that I've lived in NZ for over two months without ever noticing the moon. Let me explain: when I was little, I learned a handy rule for remembering the moon's phases when it is waxing and waning. "If you draw a vertical line closing the moon's crescent, it will form either a 'p' for premier - first - or a 'd' for dernier - last." As a budding francophile, I immediately learned this by heart, and I have never been at a loss to know whether I am seeing the first quarter or the last quarter of the moon.

Since arriving in NZ, I have seen moonrise in the dusky rose of sunset, with the full moon blossoming fat on the far ridges of the Rimutakas. I have noticed the increased brilliance of the city's already bright stars on the nights of the new moon. I have glimpsed the crescent moon here and there in the sky without pausing to consider its celestial context. It was only last night that I finally, viscerally, realized that I am seeing the moon from a new perspective.

[The graphic you see moving above is how I have been used to seeing the moon. In the southern hemisphere, imagine playing the film backwards. See chart at right.]

A similar experience occurred soon after I arrived here. One of the things I looked forward to with the greatest anticipation was the prospect of seeing the stars of the southern hemisphere. The New Zealand of my imagination nestled beneath strange stars, entirely unknown to me. On a first clear night, I was shocked to see familiar Orion above me, doing a handstand, upside down! For a few moments I felt entirely disoriented, not because of Orion's novel position, but because he was visible at all! I had looked at the same stars above my home in Minneapolis just a week before. Traveling all this way had failed to take me out of their sight.

This makes sense once you think about it - we see more than just the sky directly above us at 45 degrees latitude.
If you live at 40 degrees north latitude, and if you have an unobstructed view of the southern horizon, it's theoretically possible to see a star that would be overhead for someone at 50 degrees south latitude (40 + 50 = 90). From our view, such a star would be kissing the horizon. ...The point is, just because we're in the Northern Hemisphere, it doesn't mean we can't see stars in the Southern Hemisphere. We just can't see the ones that are more than about 80 degrees or so of our latitude. [source]
Our angle of view isn't so different from what we left behind, so we continue to share many of the same stars with those of you back home. However, the Big Dipper is gone, as is Cassiopeia. Perhaps you are curious to learn: What does our sky have that yours doesn't? For one thing,
All-in-all, the northern sky is rather devoid of light. By contrast, in the Southern Hemisphere, there are 11 constellations which are circumpolar and 6 first order magnitude stars! [source]
We have noticed that the stars are incredibly bright here, with the Milky Way visible even inside the central city. Whether this is due to brighter stars or cleaner air is hard to say. If you're interested, here's a link to get you started on learning the stars down under. And maybe you'll even be inspired to pack your bags to come see the antipodal sky.


Thunderstorms and Volcanoes


The front pages today are full of criticisms of New Zealand’s civil defense, which should have responded more efficiently to the tsunami threat yesterday morning. As it turns out, there was no tsunami, despite a powerful 7.9 magnitude earthquake about 2000 km from Auckland near the Tonga Islands. Apparently, the rest of the world had heard about our nearby quake before most of us in NZ. Word got out in the town of Gisbon, NZ when friends and family living abroad called town residents after hearing reports of the tsunami potential on the BBC. It surprises me that New Zealand’s warning systems are a bit rusty. We are located within the ring of fire, a region notorious for volcanic and seismic activity, and the country is well-equipped to monitor these conditions. (Photo above depicts a 6.6 magnitude quake that hit Edgecumbe in 1987. Photo: L. Homer)

Living in this fiery ring thing is new to me, though. I grew up in Virginia and Minnesota where the primary forces of nature to be reckoned with are thunderstorms and damaging winds. I’ve always enjoyed watching as the sky darkens; the wind gusts and the sounds of thunder grow steadily closer. I can remember countless occasions of standing out under the roiling clouds and running to cover only when the torrential downpours began. With real-time weather data now widely available on the internet I often monitored the storm’s approach and could tell exactly when it was time to head outside and watch the show.

What thrills me about thunderstorms, and strong weather in general, is the opportunity these events provide. Aside from the sheer exhilaration of physically recoiling from a deafening clap of thunder or gazing skyward to see branches of white light climb across the clouds, there's something more. Observers have the chance to understand the world in new ways. Nature puts on a demonstration, much like a professor does in a science lab. We’ve all heard the faint crackle of static electricity; how big those bolts must be to cause the ground shaking thunder we hear overhead!

I was saddened to discover, after a night of distant rumbling and occasional flashes, that Wellington sees only 2 or 3 thunderstorms a year. No snow, no thunderstorms… weather changing so quickly that keeping up with it is pointless! (What am I to do?) I’ve slept through the only tremor I was ever present for—one night when I was living in Oakland, CA, for a brief time. As I said earlier, I’m new to the restless character of the land I’m now living on. It goes without saying, then: I’ve never been frightened by an earthquake, and the same can be said for any weather related event, which is why the sound of thunder still delights me and I stay out watching the storm even when Dacia is shouting to run for cover (it’s fair to say that Minnesota has a much higher tornado potential than Virginia, and I spent most of my time growing up in VA).

I got my first inkling of what's going on down beneath the earth one evening, a few weeks ago. I was sitting against the wall in my room, reading and minding my own business. I felt a bump, as though a bus had nudged the corner of the house. My head was resting against the wall and it got a gentle knock as the entire wall moved against me. Dacia and I both looked at each other, “Did you feel that?” Whoa…Yeah...

That little grunt from deep within the earth captivated my imagination for days. I can still recall the sensation even now. My awareness traveled deep into the earth while entertaining the notion of not just our house having moved but the entire Wellington region “bumping” in that moment. It was hard to get my head around, which is the reason I’m compelled to ponder it so intently. The same fascination I have with lightning, or hail storms (how fast must the winds be blowing up above to push those hails stones higher and higher as they grow heavier?) has found an outlet in New Zealand—how far down underground did that movement of earth occur to yield such a sensation here on the surface?

I will leave it to Dacia to explain the tectonic conditions that are at work under ground here in New Zealand. I want to point out some of the fascinating earthquake information that is now freely available to NZ residents and the world at large through the Internet.

After our tremor experience we wanted to find out if there was any seismology information online. We discovered a website called Geo-Net which “provides real-time monitoring and data collection for rapid response and research into earthquake, volcano, landslide and tsunami hazards.” The site includes up to the hour images of the 4 major volcanoes and also short video clips of the steaming craters. As you can see from the map of strong motion recording sites the islands here are well monitored.

Once we found Geo-Net we were able to locate the report on the particular tremor we experienced—note the big spike, which was the bump we felt. If your explore the site further you’ll find real-time seismograph records, magnitude, depth and epicenter information, as well as fascinating seismic history reports that date back to the 1400’s. GeoNet even offers a "Felt Earthquake Report" link that Dacia used to submit her account of the April 17th tremor.

We decided to investigate the earthquake history of the Wellington region. Using the QuakeSearch feature we identified all quakes over 5 on the Richter scale. Whoa…too many to know what to do with. We raised the bar to show quakes over a 7. Yikes—still a lot. We kept raising the limit until we got to 8 and above. Now there were only 2 earthquakes shown on the map. Can you guess where they were?

It’s well known that Wellington is situated directly on top of a significant fault line. New Zealand is full of fault lines, actually. Never-the-less, the fact that in the past 600 years there have been two major earthquakes right here in Wellington… it’s a bit concerning. The continued tremors, however, are signs that all is well beneath our feet. Slight movements indicate that the colliding plates are moving together smoothly. A major earthquake occurs when friction builds up between the plates and releases all at once. Let’s all hope we feel some more rumblings soon.

Eruption of Mount Ruapehu, 1945. Source: Matapihi