Sunday, December 20, 2009

New Marble Outcrop in Brookfield, CT

Generally speaking, I'm not a fan of man-made landscape alteration, but fresh highway roadcuts provide a great unweathered view of the local geology.
A few weeks ago, after years of proposals followed by years of construction, the northern extension of "Super 7" (US route 7) north of Danbury CT finally opened - and the exposures of the Stockbridge Marble along the road are spectacular!  The Stockbridge Marble was formed as the rocks of the carbonate bank (limestones deposited off the coast) of ancestral North America were metamorphosed during the Taconic Orogeny ( see blog on 12/2/2009 and the diagram below).

The rocks were further deformed, and actually overtured, in the subsequent Acadian Orogeny. The extent of those deformations are evident in the complex folds pictured here.
Carbonate rocks, including marbles, generally weather faster than silicate rocks, and in SW Connecticut and SE New York many of the regional valleys are underlain by less resistant marbles.  Along route 7 between Danbury and New Milford the Stockbridge Marble underlies the Still River valley, and is bounded by harder rocks on either side of the valley.  The same rock underlies the Saw Mill River valley in New York  where is known as the Inwood Marble (there's a "Marble Avenue" exit on the Saw Mill River Parkway in Pleasantville/Thornwood, and the Shop Rite plaza in Thornwood is built on an old marble quarry there - great outcrops of snowy white "snowflake" marble are still visible on the quarry walls behind the stores, but they're weathering fast!).
Riders of the Hudson Metro North commuter rail can get a good look at the Inwood Marble at the Marbledale stop on the way into Manhattan.

Tuesday, December 8, 2009

Earliest Sunset of the Year

Well, we've made it.  Tonight is the earliest sunset of the year!
The daylight period is still getting shorter (most folks know that shortest day is the Winter Solstice around December 21), but not a lot of people can explain tonight's early sunset.  It turns out that the rate at which the Sun travels across the sky is not constant - the tilt of Earth's axis and its elliptical orbit conspire to push the Sun ahead of our clocks, and then slow it down again, twice every year.  Astronomers call the difference between time told by the Sun (apparent solar time) and clock time (mean solar time) the "equation of time".(If you're interested, you can get the sunrise and sunset times for your location at the US Naval Observatory site.)
The chart on the left above, called the analemma, combines the equation of time with the position of the Sun relative to the equator.  Click it for a larger view, and notice that through most of the fall the Sun has been running ahead of the clock, but in December it began to slow dramatically.
It's the Sun slowing down relative to the clock that's moving the daylight period later into the day even as the days get shorter!
This photo composite was made by Tom Matheson over the course of a year, snapping a picture of the Sun at exactly 8 AM (by the clock) each day.  Here is a labeled image of  Tom's photo.
(This blog isan edited  re-post from December 2008)

Wednesday, December 2, 2009

Rocks on the Shore of Lake Champlain Tell an Interesting Story

An early May paddle on Lake Champlain turned into an interesting geology field trip (for me at least ;-) ) when we passed this rocky shoreline north of Burlington, VT.  The rocks here record a significant event in the formation of North America.
To understand what happened here, imagine a deck of cards spread out on a table. Now imagine that you use your arms to bring the cards together into a pile. As the cards slide together some will end up on top of others, and the mass of cards will become shorter in the horizontal direction, but thicker in the vertical direction. In a similar way, colliding crustal plates (“drifting continents”) produce thick masses of earth’s crust pinched between them.  Around 440 million years ago, eastern North America was deformed as it collided with volcanic islands to the east as the ocean between North America and Europe was closing (and as the super continent of Pangea was assembling).  This event, called the Taconic Orogeny, resulted in the rocks of western New England piling up and forming the Taconic Mts. (which have been reduced from their former grandeur by 400 million years of erosion!)
So what's happening in this photo?

Essentially you're looking at the boundary between 2 of the cards you imagined above.  The rocks in the top half of the photo have been thrust westward (to the left) up and over the rocks at the bottom of the image along a "low angle thrust fault" (traced with a red line).  It's hard to judge just how far the top rocks have moved relative to the bottom rocks right here, but along a major thrust fault east of here, the displacement is estimated to be on the order of 50 miles! (See this Earth Science Picture of the Day).  And you can see that the rocks themselves have been squeezed, too - notice that the shortening and thickening is evident on a very small scale in the deformation of the light colored vein at "A".
You can see more pics of this outcrop, and pics of the entire paddling adventure if you have nothing better to do.
And here's a pretty nice cartoon of the Taconic Orogeny from oldest at the top to present day at the bottom.

Tuesday, November 24, 2009

Pikes Peak in the Clear

The morning of June 3, 2009 brought overcast skies and drizzle to Colorado Springs, CO - dampening my hopes of a clear day on a field trip to nearby Pikes Peak (at 14,110 ft, Pikes Peak is one of Colorado's 54 "14ers"). Assured by the folks who run the Pikes Peak Cog Railroad that the summit was clear, we boarded the train for the ride to the top.
It wasn't long before we broke free of the clouds, and soon were way above treeline in a few inches of new snow! From the summit we were able to look back down toward the cloud covered Colorado Springs and the plains to the east, where it remained cloudy and wet for the rest of the day.

Tuesday, November 17, 2009

Crepuscular Rays

We've all seen friend Donald calls them "God Lights". The the rays of sunlight that seem to radiate out from the sun through breaks in the clouds are more properly known as "crepuscular rays". We see them because sunlight is scattered by dust and other particles in the atmosphere making the air lit by the sun appear brighter than the air that is in the shadow of the clouds. Though the rays are virtually parallel to each other, perspective causes an apparent convergence toward the Sun in the same way parallel railroad tracks seem to converge in the distance.
This image of some bright crepusculars was made late one afternoon in Bedford, NY. Note the particularly dark shadow on the right side of the photo, also parallel to the light rays, cast by the clouds there.

Tuesday, November 10, 2009

Upcoming Leonid Meteor Shower 11/17-18/2009

This year's Leonids should put on a good show, as the New Moon will not be in the nighttime sky. Peak viewing will be in the wee hours of the night of November 17-18. Here's some good information on how to view meteor showers, and information on all the periodic meteor showers we experience throughout the year.
See the Westchester Astronomers November newsletter for more upcoming sky related events.

Thursday, November 5, 2009

Boring Clams are Interesting!

While attending the Earth Science Information Partners conference at UC Santa Barbara last summer, I enjoyed a nice walk along the beach from the dorms to the conference center each morning. I was struck particularly by the vast number of holes apparently drilled into the rocks along the shore, many of them containing clam shells that just fit into the holes. A closer look at those shells reveals that it's the clams themselves that make those holes!
Examination of the clam shells reveals the sharp ridges visible in the close-up picture on the right. By persistent grinding and rotation of the cutting edges of their shells against the rock, these "rock boring clams" create safe homes for themselves in solid rock, and in doing so accomplish significant weathering of the shoreline rocks, and contribute significant sand to the beach. You can read more about boring clams here:

Sunday, February 1, 2009

The "tilted" crescent Moon

Last week, a friend noticed that the new crescent Moon seemed to be illuminated more nearly on the bottom of the disk of the moon, rather than on the side, and wondered if there was some relationship between that observation and the solar eclipse a few days before.
It turns out that there's no relationship to the eclipse, but rather to the time of the year.
Pretty much, the horns of the crescent coincide with the north and south poles of the Moon, so the waxing crescent pretty much illuminates the "right" side of the Moon (what we call the western limb when viewed from earth), and is centered pretty much on the orbital path of the Moon, and the Moon's equator (not quite, because the Moon's orbit is not quite on the ecliptic). But the illuminated portion of the waxing crescent will appear to be nearer the "bottom" of the Moon as it sets, near the right side of the Moon as it crosses your meridian, and near the "top" of the rising Moon. See the screen shots from below:

The Waxing Crescent Moon setting on 1/29/09 (note that the ecliptic is making a fairly steep angle with the western horizon, and that the Moon is north of the ecliptic. The north pole of the Moon is on the "right" side of the moon here, and the Moon's equator is almost vertical:

This was the Moon at meridian crossing on 1/29/09. View is to the south, of course. You could have seen it naked eye if you looked in the right place (you can see Venus naked eye, too!) Notice that the illuminated crescent is pretty much on the right (western limb) side.

Here's the Moon rising on the morning 1/29/09. Notice that the illuminated portion of the Moon is kind of "on top", but that the angle the ecliptic makes with the horizon is less steep than the angle it makes at sunset, so the crescent is not quite as "on top" as it was "on the bottom" when it set.

The places where the celestial equator (blue) and the ecliptic (red) cross are the equinoxes, and the equinox in these images is the vernal (spring) equinox. When the vernal equinox is setting, the angle between the ecliptic and the western horizon is large (the ecliptic is more nearly vertical). When the sun is near the vernal equinox (March and April), the angle between the ecliptic and the horizon at sunset is at its greatest, and Venus and Mercury, if they're east of the Sun (and therefore setting after it), will be high in the sky right after sunset.

Similarly, in September and October as the autumnal equinox rises, the angle between the ecliptic and the eastern horizon at sunrise is the greatest, and around the autumnal equinox Venus and Mercury, if they're west of the Sun (and therefore rising before it), will be high in the sky and well placed for viewing.

The following screen shots show, in order, the rising vernal equinox, the setting vernal equinox, the rising autumnal equinox, and the setting autumnal equinox.

Rising Vernal Equinox (low angle ecliptic)

Setting Vernal Equinox (high angle ecliptic)

Rising Autumnal Equinox (high angle ecliptic)

Setting Autumnal Equinox (low angle ecliptic)

You should also note that the celestial equator makes an angle with the horizon of (90-latitude) (I have Stellarium set to White Plains, NY at 41N), and it remains constant throughout the year.

If you've never imagined it, think of the ecliptic wobbling across the sky each day. And if you can't imagine it, run Stellarium at high speed, and watch it.

Now, back to my friend's question! Eclipses occur as the Moon crosses the ecliptic on its way north or south (called the "nodes" of the Moon's orbit). That's why solar and lunar eclipses come in pairs about 2 weeks apart (a lunar at one node, and a solar at the other), and the eclipse pairs occur every six months or so when the nodes line up with the Sun! The nodes drift with respect to the sun, but in recent years the eclipses have been occurring in the winter and summer. Winter waxing crescents, for the reasons described above, tend to look like they're lit on the "bottom" But watch the crescent moon after the July 22, 2009 solar'll look like this: