Tree of Science, January 12

Today, the temperatures increased and sublimation has started to fill the valley with fog.  Tonight, if the temperature approaches the freezing point, black ice will be the result.  Meanwhile, temperatures in San Diego have been, for most of the day, 10 degrees cooler than in Boston.

Tonight's picture was taken at 8:15.  It's 41 degrees Fahrenheit.  The Tree of Science sees a lot of activity, even tonight, but long exposure times allow a faculty member and his dog to pass across the scene and several students to enter the atrium doors, all without capture.

The blue light—an interesting source of light pollution on our campus—causes reflections off the surface of my lens onto the UV filter, thus the blue and yellow lights above the building.  (Blue lights on campus signify call boxes, or, as it happens, objets d'art.)   The four skylights visible across the building point northwest, away from the telescopes that are atop Physics, just to the left.  At night, windowed areas aren't lit by the sky, but light the sky, contributing, generally, to the large amount of light pollution even small towns contribute to our atmosphere.   Light pollution is a real problem for astronomers at night.  No one, of course, is observing tonight.

Thinking about temperature, a blog is probably the right place to inform you of a common misunderstanding about the definition of our standard units of temperature.  In high school, we learn that 0 degrees Centigrade is defined to be the freezing point of water, and 100 the point at which it boils.  Actually, it's a little more complex. All measurements are made relative to the first defining point, absolute zero or 0 degrees Kelvin, or -273.15 degrees Celsius.  The second point that defines the standard temperature scale is the triple point of water, the point where water can be found in three states: a solid, a liquid, and a gas.  (A reason for this, I suspect, is that the triple point of water is easier to verify than the freezing point.)  This point is taken to be exactly 273.16 degrees Kelvin and 0.01 degrees Celsius.  Thus, neither the freezing or boiling points are defining temperatures, and, to compound the problem, neither of those happens at exactly 0 or 100 degrees in Celsius.  This may not be surprising, but now, it turns out, the triple point is ever so slightly higher than 273.16.  Our high school friend, Centigrade, is an archaic term, supplanted by Celsius in standards definitions.  To this day, Centigrade appears more accurately defined than its 'equivalent', official scale, Celsius.  Your mileage, of course, may vary.