J

jansky (Jy)

a unit used in radio astronomy to measure the strength, or more precisely the flux density, of radio signals from space. In measuring signal strength, it's necessary to take into account both the area of the receiving antenna and the width of the frequency band in which the signal occurs. Accordingly, one jansky equals a flux of 10^-26watts per square meter of receiving area per hertz of frequency band (W/m²Hz). Although it is not an SI unit, the jansky is approved by the International Astronomical Union and is widely used by astronomers. It honors Karl G. Jansky (1905-1950), the American electrical engineer who discovered radio waves from space in 1930. The jansky is sometimes called the flux unit.

jar

a traditional unit of electric capacitance, approximately equal to the capacitance of one of the Leiden jars used in electrical experiments as long ago as the eighteenth century. Benjamin Franklin is said to have measured the storage power of his electrical equipment in jars. There are 9 x 10⁸ jars in a farad, so 1 jar is approximately 1.1 nanofarad.

jerib or djerib

a traditional unit of land area in the Middle East and southwestern Asia. The jerib originally varied considerably from one area to another. In modern times it has become identified with the hectare in many countries, including Turkey and Iran. In Afghanistan, however, it is usually equal to 1/5 hectare (2000 square meters or 0.494 acre).

jerk

a unit of change in acceleration sometimes used by engineers. This is not as silly as it sounds, because when we're in a vehicle and feel a jerk, we are in fact experiencing a change in the acceleration of the vehicle. One jerk is equal to a change in acceleration of one foot per second per second in one second, that is, 1 ft/sec³. One jerk equals 0.3048 m/s³ or about 0.03108 g/sec.

jeroboam

a large wine bottle holding about 3 liters, 4 times the volume of a regular bottle.

jiffy

a unit of time used in computer engineering. A jiffy is the length of one cycle, or tick, of the computer's system clock. In the past, this was often equal to one period of the alternating current powering the computer: 1/60 second in the U.S. and Canada, usually 1/50 second elsewhere. More recently the jiffy has become standardized, more or less, as 0.01 second (10 milliseconds). The word jiffy, with its ordinary meaning of an instant or very brief time, appeared in English during the eighteenth century, but its origin is not known.

jigger

a unit of volume for liquor, usually considered equal to 1.5 (U.S.) fluid ounce or 44.360 milliliters.

jin

a traditional unit of weight in China, comparable to the English pound. During the European colonial era the jin was identified with the catty, a Malay unit widely used in various forms throughout East and Southeast Asia. Like the catty, the jin was then equal to 1 1/3 pounds or 604.79 grams. Traditionally, it was divided into 16 liang. In modern China, however, the jin has been identified as a metric unit equal to exactly 500 grams (1.1023 pounds), and it is divided into 10 liang. The spellings chin and gin also have been used for this unit.

jo

an informal unit of area used in Japan to measure the size of rooms in houses and apartments. One jo is the area of a traditional tatami mat, 180 by 90 centimeters or 1.62 square meters (1.94 square yards).

joch

a traditional unit of area in German speaking countries, especially in Austria. One joch is the area of a square 40 klafters (about 83 yards) on a side. This comes to 0.5755 hectare or about 1.422 acres. The plural is joche. Joch is also the word for a yoke in German, so presumably this unit represents an area that could be plowed in a day by a yoke of oxen.

joule (J)

the SI unit of work or energy, defined to be the work done by a force of one newton acting to move an object through a distance of one meter in the direction in which the force is applied. Equivalently, since kinetic energy is one half the mass times the square of the velocity, one joule is the kinetic energy of a mass of two kilograms moving at a velocity of 1 m/sec. This is the same as 10⁷ergs in the CGS system, or approximately 0.737 562 foot-pound in the traditional English system. In other energy units, one joule equals about 9.478 170 x 10^-4Btu, 0.238,846 (small) calories, or 2.777 778 x 10^-4watt hour. The joule is named for the British physicist James Prescott Joule (1818-1889), who demonstrated the equivalence of mechanical and thermal energy in a famous experiment in 1843. Although Joule pronounced his name "jowl", the unit is usually pronounced "jew'l".

journal

a traditional unit of land area in France, equal to the area that could be plowed in a day ( jour is the French word for day). The unit varied from one region to another, generally in the range 0.3-0.45 hectare (0.75-1.1 acres).

JTU

abbreviation for Jackson turbidity unit, a unit formerly used in measuring water quality. The turbidity of water was measured by lighting a candle under a tall glass tube and filling the tube with the water sample until a observer looking down the tube could no longer see the candle flame through the water. The height of the water column determines the turbidity in Jackson turbidity units, using a table constructed for this purpose. Turbidity is usually measured now in a different unit, the NTU, using an instrument called a nephelometer.

jug

an informal name for the Scots pint, a unit of volume equal to about 1.791 U.S. liquid quarts or 1.695 liters.

Julian day (JD)

a continuous count of days beginning with January 1, 4713 BC (-4712 CE). Julian days are named for the French scholar Julius Scaliger (1540-1609), who introduced the idea in 1582 (the same year the Gregorian calendar was proclaimed). They are often used by astronomers and sometimes used by historians to provide a precise date for an event, independent of all calendar systems. Scaliger picked 4713 BC for the start of the count because this was earlier than all known historical records and happened to be a convenient starting point for several chronological and astronomical cycles. The Julian day is taken to begin at noon Universal Time. The first moment of the year 2001 CE, at the International Date Line, is also the start of JD 2 451 910.

Julian epoch (J)

a measure of time used in astronomy. The word epoch comes from Greek, and means a fixed or standard instant of time. Other times are stated with reference to this fixed time using years and fractions of years. In 1984, astronomers agreed to fix the standard epoch at 12 hours Universal Time of 2000 January 1 (JD 2 451 545.0). This instant is designated J2000.0. Other times are specified with reference to this time using a year of length 365.25 days, the average length of the year in the Julian calendar. This means that J2001.0, for example, is 18 hours Universal Time of 2001 January 1 (exactly 365.25 days after J2000.0).

Julian year

the average length of the year in the Julian calendar, equal to exactly 365.25 days. See year [2].

Jupiter

a unit of mass, now being used in astronomy to express the masses of new planets being discovered in orbit around various stars. It's equal to the mass of the planet Jupiter, estimated to be about 1.899 x 10²⁴ metric tons, or, if you please, 1.899 yottatonnes (Yt). By coincidence, this is approximately 0.001 Sun (0.000 955 Sun, to be more exact).