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The Melting Point

Physical properties of a material fall into two categories: intrinsic properties determined by the structure of the particular molecule, and bulk properties characteristic of quantities of molecules together as bulk solids, liquids, or gases. The melting point of a pure substance is a bulk property and represents the point of transition of physical ...

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TheMeltingPoint
Astronomy

Ancient Planet

Long before our Sun and Earth ever existed, a Jupiter-sized planet formed around a sun-like star. Now, almost 13 billion years later, NASA's Hubble Space Telescope has precisely measured the mass of ... Continue reading

AncientPlanet
Biology

Hey Nose-Brain!

Sex, food, and smell are linked in our brain by ancient pathways governing appetite, odor detection, and hormones. In fact, another name for the brain's limbic system (a primitive ... Continue reading

NoseBrain
Physics

Galileo Thermometers

Every substance has the property of 'mass', which is the basic physical presence of matter. Matter occupies space. A physical mass contained within a physical space produces the physical property of ... Continue reading

GalileoThermometers
Chemistry

Spontaneous Combustion

Most of us know if we leave oily rags or papers in an enclosed area, we risk a fire. The process of burning is called oxidation. Oxidation is the same process that causes iron to rust or a banana to ... Continue reading

SpontaneousCombustion

Sundials, Ancient Clocks

SundialsAncientClocksThe earliest and simplest form of sundial is the shadow stick. The time of day is judged by the length and position of the stick's shadow. Some nomadic peoples still use this method for timekeeping. The technical name for a shadow stick is a gnomon. As the sun moves through the sky from sunrise to sunset, the shadow of the gnomon rotates 'clockwise.' The shadow is shortest when the sun is directly in the south, defining local noon.

As early as 3500 B.C. the Egyptians began building slender, tapering, four-sided obelisks which served as timepieces. The moving shadow of the obelisk formed a type of sundial, and markers arranged about the base separated the day into divisions as well as indicating the longest and shortest days of the year. However, because of the earth's tilt, the sun's path through the sky changes slightly from day to day, so the shadow cast by the gnomon is not the same every day. Many sundials overcome this problem by angling the gnomon and aiming it north. This type of gnomon is called a style. Because its alignment compensates for the Earth's tilt, the hour marks remain the same all year round.

In the quest for accuracy, many types of sundials evolved, including some very complex portable sundials. In about 30 B.C. Marcus Vitruvius, a Roman architect, described 13 different sundial designs used in Greece, Asia Minor, and Itay. The invention of more accurate mechanical clocks and the standardization of time using time zones made sundials obsolete. Now sundials are used mostly for ornamental purposes.