## Sunday, November 29, 2009

An acre was the descriptive name given in about 1300 AD to the amount of land that one man with his oxen and plough could plough in one day.  This amount tended to vary since some land is easier to plough than others but now, in the U.S., one acre is 4,840 square yards; 640 acres is one square mile.

Sound travels at different speeds through different media.  Through dry air at sea level and at freezing point (0 degrees Centigrade/Celsius), sound travels at about 5 seconds per mile.  Sound travels a bit faster as the temperature of the air rises above 0 degrees Celsius but never travels anywhere near as fast as light.  Water is more dense than air but is more elastic and sound travels faster in water than in air. Sound cannot travel through a vacuum.

Light travels through the vacuum of space at 299,793 km per second (which is 1,079,252,960 km per hour); or 186,282 miles per second (which is 670,619,880 miles per hour). The speed of light is not constant; it travels about 3 percent more slowly through air and much more slowly through glass or water.  In a thunder and lightning storm you see the lightning before you hear the rumble of the thunder unless the storm is right overhead. If you can count 5 seconds between the lightning and the thunder you will know that the storm is about a mile away.

A day is 86,400 seconds or 24 hours long which is the time it takes Earth to rotate once on its axis. This is measured by observing the movment of the Sun relative to a fixed point on Earth.  Interestingly, when we measure the length of a day by observation of the movement of a star other than the Sun, we find that the time taken by a complete rotation of Earth is shorter. This shorter day is called a sidereal day.

A year is the length of time that it takes Earth to make a complete revolution around the Sun. Technically this is exactly 365 days, 5 hours, 48 minutes and 45.51 seconds.

Because the measurement of the passage of time is important in the affairs of man, the period of time that we call a year has been recognized since the earliest times. At first it was probably divided up into useful chunks of time simply by noting the changing seasons. Much later, the first Roman calendar appears to have divided the year into ten months. An early Greek calendar organized the year into 12 months.

The mile was originally a Roman unit measurement of distance and was 1,000 double-paces; one Roman mile is generally considered to have been what we would now describe as 1,618 yards.  However, in the modern world one (statute) mile is longer than that, it is 1,760 yards or 5,280 feet. While this is longer than the old Roman mile, it is shorter than the old Scottish or Irish miles.

While a statute mile (on land) is 1,760 yards, a nautical (at sea) mile is approximately 2,028 yards by UK standards, and approximately 2,025 yards by US standards.