Everyone living in the middle latitudes experiences the Earth’s seasons—and people have observed these changes for thousands of years. The sun is highest in the sky during the summer months and lowest in winter. Early people weren’t dumb—they could see that as the seasons changed, so did the time of sunrise and sunset. Constructing a sundial to track the daylight hours is an easy matter: put a stick upright in the ground and mark the points that the shadow of the stick’s tip casts in the course of a day. Many cultures developed elaborate ways of tracking the annual cycle of the seasons and keeping time, including Sumerians, Egyptians, and Mayans, to name only a few—and these cycles were often based on observations of the Sun, Moon, and sky. The term “analemma” was used in Alexandria and Rome for a diagram or “sketch” attributed to the Roman architect Vitruvius. This complex geometrical diagram allowed one to construct a sundial at any latitude without having to make measurements of shadow length. These days, analemma refers to the curve the Sun appears to draw in the sky if its position is captured at the same time of day over the course of an entire year. This curve was difficult to obtain by observation until we developed photographic techniques for multiple exposures. The Sun’s position doesn’t trace a straight up-and-down line as it moves through the seasons. Instead, the analemma turns out to be a lopsided figure 8 or infinity symbol. Below is an analemma photographed by Frank Zullo. He took multiple exposures of the Sun at 8:00 a.m. on 37 days spaced throughout a single year. He then superimposed the analemma on a photo of the giant sundial at Carefree, Arizona, to produce this photo. (You can learn more about how he made this photo here.) The analemma below is a calculated curve that was then superimposed on a photo taken at Croton-on-Hudson, New York. This one is by Daniel Cummings; he used the Stellarium program to plot the curve. (Read more, including how to use Stellarium to make a plot, here.) The analemma is a result of two characteristics: The tilt of the Earth’s axis (which causes the seasons), and the Earth’s elliptical orbit—it is not a perfect circle. The result is that at some times, the Sun slides ahead of schedule, as measured by a steady, mechanical clock, and at other times, it lags behind. These changes cause the figure 8 pattern. If you want to get more technical, see the Equation of Time graph below. “Equation” doesn’t mean a formula in this case, but an “equating” or reconciliation of different characteristics. In this case, apparent solar time (sundial time) is plotted against days of the year. The curve shows the time values ahead of or behind mean solar time (that steady mechanical clock). Above the horizontal line, the sun is ahead, and below, it's behind. The peaks and valleys correspond to the top loop (smaller peak and valley) and bottom loop (larger peak and valley) of the analemma. I’m thrilled by the analemma because it’s a reminder that the universe contains mystery. Nature doesn’t follow perfectly straight lines, nor does it produce perfect circles or operate only by nice, neat rational numbers—although many cultures have wanted to think so.
For further exploration The first photographic analemma was created by Dennis Di Cicco in Watertown, Mass., in 1978 and 1979. You can read about it here. The straight lines in the photo are exposures of the Sun until it reached the time point, to show how it was moving across the sky. If you're interested in a technical stroll through Vitruvius's diagrams of the traditional analemma, here's a link to an article (you can download a pdf). This article is rather odd, possibly due in part to translation.
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Imagine that you’re struggling in the “death zone” on Everest, above 23,000 feet—taking one more step, gasping for breath. It’s called the death zone because once you’re up that high, you start dying; you can stay for only a limited length of time. Before this was well understood, many climbers died in the death zone.
Mammalian lungs never empty completely—a residual volume of gas always remains, and so the lungs never contain totally fresh air. This is called a “reciprocating” system. By contrast, bird lungs are “flow-through.” They don’t expand and contract; instead, birds have air sacs that expand and contract, moving fresh air through their rigid lungs in one direction.
The birds’ respiratory system makes it possible both to fly high and to fly long distances without stopping during migration. This efficient respiration also helps birds, which are warm blooded, to keep their metabolism high. They get plenty of oxygen. Birds evolved from the theropods, a dinosaur group that included such lovely beasts as Velociraptor and Tyrannosaurus. Mammals evolved from the cynodonts, a rather undistinguished, non-dinosaur reptile group. These groups had different breathing systems, which later resulted in the mammal–bird differences. What about bats, though? They are mammals, but they fly quite successfully, although not as high as some birds, nor for as long a period of time. As you’d expect, bats have several adaptations to support flight: very large hearts and lungs (compared to body size), a larger number of red blood cells than in other mammals, and more hemoglobin in those red blood cells. They, too, get plenty of oxygen. The Sherpa people of the Himalayas appear to have gene adaptations for better oxygen utilization, allowing them to perform much better at altitude than lowlanders do. As a group, they have lived at high altitudes (an average of 14,700 feet) for at least 6,000 years. For further reading:
Chase D. Mendenhall, “How Birds Breathe with Their Butts.” Carnegie Museum of Natural History. Click here “How Air Sacs Power Lungs in Birds’ Respiratory System.” July 2020. Click here J. N. Maina, “What it takes to fly: the structural and functional respiratory refinements in birds and bats.” J Exp Biol (2000) 203 (20): 3045–3064. [Note: This is a highly technical article.] Click here Emily Sohn, “The Science Behind The Super Abilities Of Sherpas.” May 28, 2017. NPR.org, Goats and Soda. Click here Jennifer Jordan, The Last Man on the Mountain: The Death of an American Adventurer on K2. W.W. Norton & Co., August 1, 2011. [The story of Dudley Wolfe, a climber who died at high altitude before the dangers were fully understood. Available at Amazon, Barnes & Noble, Thriftbooks, and probably your local library.] |
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