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A sand pendulum that creates a beautiful pattern only by its movement.

But  why does the ellipse change shape?

The pattern gets smaller because energy is not conserved (and in fact decreases) in the system. The mass in the pendulum gets smaller and the center of mass lowers as a function of time. Easy as that, an amazing pattern arises through the laws of physics.

Mood

History of pandemics

Thousand-year-old Viking fortress reveals a technologically advanced society

When archaeologists uncovered four ancient ring-shaped fortresses in Denmark in the 1930s, the find profoundly changed the way they thought about the Vikings that built them. Rather than mindless marauders, Vikings in the Middle Ages must have been a complex, technologically advanced people to build these fortifications. Now, Danish archaeologists have described a fifth ring fortress—the first such discovery in more than 60 years—revealing even more about these architecturally gifted warriors.

The new fortress, called Borgring, was found principally using an aerial, laser-based surveillance method called LIDAR, which returns an extremely high-resolution 3D ground map. It’s located on the Danish island Zealand, south of Copenhagen. The stronghold is a perfect circle with an outer diameter of 144 meters, and has four main gates crisscrossed by wood-paved roads. Read more.

Now we finally know what Ancient Greek music sounded like

In 1932, the musicologist Wilfrid Perrett reported to an audience at the Royal Musical Association in London the words of an unnamed professor of Greek with musical leanings: “Nobody has ever made head or tail of ancient Greek music, and nobody ever will. That way madness lies.”

Indeed, ancient Greek music has long posed a maddening enigma. Yet music was ubiquitous in classical Greece, with most of the poetry from around 750BC to 350BC – the songs of Homer, Sappho, and others – composed and performed as sung music, sometimes accompanied by dance. Literary texts provide abundant and highly specific details about the notes, scales, effects, and instruments used. The lyre was a common feature, along with the popular aulos, two double-reed pipes played simultaneously by a single performer so as to sound like two powerful oboes played in concert.

Despite this wealth of information, the sense and sound of ancient Greek music has proved incredibly elusive. Read more.