 eclipse year

a unit of time used in astronomy in the prediction of eclipses. The plane of
the moon's orbit around the earth and the plane of the earth's orbit around the
sun intersect in a line called the line of nodes. Eclipses of the sun or moon
can only occur at times when the earth, moon, and sun all lie very close to
this line. The eclipse year, has a length of 346.62 days.
 e

a mathematical unit used as the base of "natural" logarithms and exponentials.
The real number e is irrational, which means that its decimal expansion
is infinite and nonrepeating. To 25 significant digits e equals 2.718
281 828 459 045 235 360 287. Function e^{x} : the behavior
called "exponential growth." The Swiss mathematician Leonard Euler (17071783)
introduced the symbol e , because it is the first letter of the word
"exponential." It is sometimes called the Euler number.
 el

the Dutch ell, a traditional unit of length equal to about 6870 centimeters
(roughly 27 inches). See the ell below.
 electric horsepower

a unit of power, equal to exactly 746 watts (550.221 foot pounds per second),
used in the electric industry. This is slightly larger than the ordinary or
mechanical horsepower of exactly 550 foot pounds per second.
 ell

a traditional unit of length used primarily for measuring cloth. In the English
system, one ell equals 20 nails, 45 inches, or 1.25 yards (exactly 1.143
meters).
 elle

a traditional unit of distance in German speaking countries. The elle varied
considerably, but it was always shorter than the English ell or French aune. A
typical value in northern Germany was exactly 2 fuss (German feet), which would
be close to 24 inches or 60 centimeters. In the south, the elle was usually
longer, about 2.5 fuss. In Vienna, the elle was eventually standardized at
30.68 inches (77.93 centimeters).
 energy factor (EF)

a measure of the energy efficiency of an appliance. In the U.S., the Department
of Energy has defined energy factors for a variety of appliances. For
dishwashers, the energy factor is the number of cycles per kilowatt hour of
power input. For clothes washers, it is the capacity of the washer in cubic
feet divided by the number of kilowatt hours of power input per washing cycle.
For clothes dryers, it is the number of pounds of clothes dried per kilowatt
hour of power consumed.
 ennead

a unit of quantity equal to 9, coined from the Greek word for nine, ennea.
 eon

alternate spelling for aeon, a unit of time equal to 1 billion years.
 ephah

a ancient unit of volume for grains and dry commodities, used in the Bible. The
ephah is believed to equal about 40.32 liters or 1.4239 cubic feet, about 1.144
U.S. bushel.
 epoch

a measure of time used in astronomy.

a unit of time equal to 19 years, used in predictions of the tides. In this
use, an epoch is another name for a Metonic cycle. All possible alignments of
the sun and moon occur in this 19year cycle, so tidal heights and other tidal
phenomena are averaged over this period.
 equivalent or equivalent weight (Eq)

a unit of relative amount of substance used in chemistry. One equivalent weight
of an element, compound, or ion is the weight in grams of that substance which
would react with or replace one gram of hydrogen. Since one gram of hydrogen is
very nearly equal to one mole and since hydrogen has one electron free to react
with other substances, 1 Eq of a substance is effectively equal to one mole
divided by the valence of the substance.
 erg

the unit of work or energy in the CGS system, equal to the work done by a force
of one dyne acting through a distance of one centimeter. Equivalently, one erg
is the kinetic energy of a mass of 2 grams moving at a velocity of 1 cm/sec,
equal to 0.1 microjoule, or about 7.375 x 10^{8} footpound. The name
of the unit is from the Greek word ergon, work.
 estadio

a traditional unit of distance in Spain and Portugal, like the English furlong.
The Spanish estadio is equal to 1/8 milla or 625 pies; this is about 571 feet
or 174 meters. The Portuguese unit is 1/8 milha, which is much longer: about
856 feet or 261 meters.
 ett or etto (h)

Italian spelling for the metric prefix hecto (100). The hectare, for example,
is ettaro in Italian. The International System allows national
variations in spelling of the names of units, but not in the symbols used for
them; thus the symbol for etto is h.
 exa (E)

a metric prefix denoting 10^{18} (one quintillion in U.S.
nomenclature). The Latin and Greek prefix ex means "out of," and is often used
to indicate "a long way," as in the words "expanse" or "extreme." The
prefix suggests the Greek hexa, meaning 6, this being the sixth prefix
(n = 6), in the list of SI prefixes for multiples of 10^{3n}.
 exajoule (EJ)

a metric unit of energy. One exajoule equals 947.817 (U.S.) trillion Btu,
277.7778 petawatt hours, or about 9480 megatherms. The unit is often used in
discussing global energy production, which is measured in hundreds of exajoules
per year.
 exameter (Em)

a metric unit of distance equal to 10^{15} kilometers. This is
equivalent to about 621.371 trillion miles, 105.7 light years, or 32.408
parsecs. One exameter is approximately the distance from the earth to the
Hyades star cluster in Taurus.
 exposure value (EV)

a unit used in photography to describe relative exposure. EV 0 is assigned to a
specific combination of exposure time and lens aperture, such as 1 second at
f/1. The difference between two exposure values is equal to the number of stops
separating the two exposure settings.
 eyesight

a term often used with a Snellen fraction in phrases such as "20/20 eyesight."
