Wednesday, January 20, 2010

The Oblate Sun

This shot of the setting sun taken from Seamans Neck Park in Seaford, NY illustrates an interesting atmospheric optical property. Earth's atmosphere causes refraction of the sunlight (and moonlight, and starlight) that passes through it. Near the horizon, refraction will raise an object on the horizon (like a star or the Sun) to an apparent position about 0.5° above the horizon. When the 'bottom' of the Sun is raised more than the 'top', the rising or setting sun can look quite flat! This refraction hastens the apparent rising of the Sun and delays the apparent setting of the Sun, effectively giving us a few more minutes of daylight than we'd have if there was no atmosphere at all.  You can read a little more about the effect this refraction has on the length of day here.

Sunday, January 10, 2010

Revealing New York City Skyline

This photograph of the New York City skyline was taken from the top of Bear Mountain in the Hudson Highlands some 40 miles (64 kilometers) due north of the city.  It reveals something interesting about the interplay of physical and cultural geography as New York City developed.
Notice how the large, tall buildings are clustered in the Midtown and Wall Street areas while the buildings in "The Village" (Greenwich Village and Chinatown)are smaller and lower.  The pattern is not simply a matter of coincidence - it is controlled by the underlying geology of Manhattan Island.  The tall, heavy buildings are built where strong, crystalline bedrock is available at the surface to support them.  The Village, on the other hand, is underlain by unconsolidated glacial deposits incapable of supporting such massive skyscrapers without extensive and expensive engineering of their foundations.

The annotated Google Earth screen shot to the left here (south is to the TOP of the image)is a "top view" of what you're seeing in the photograph.
The Midtown - Village - Wall Street pattern is also apparent looking east from the New Jersey Turnpike, and you can read more about the development of NYC at this US Geological Survey page.