An Etymological Dictionary of Astronomy and Astrophysics
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The first successful measurement of the → speed of light carried out by the Danish astronomer Ole Rømer in 1675 at Paris Observatory. Astronomers knew that the mean period of revolution for Jupiter's innermost satellite → Io (Jupiter I) was 42.5 hours. During this period Io was sometimes eclipsed by Jupiter. Astronomers expected that if Io was visible at some time it must be visible 42.5 hours later. But Ole Rømer discovered that there were many irregularities in Io's orbital period. Sometimes Io appeared too early and other times too late in relation to the expected times. The irregularities repeated themselves precisely at a one-year interval, which meant that they must be connected to the Earth's rotation around the Sun. Rømer attributed this difference in time to the additional distance the light from Io had to travel at different times, and used this information to calculate the speed of light. He found that it takes light 22 minutes to traverse the Earth's orbital diameter; the correct figure was later determined to be 16 minutes and 40 seconds. Rømer was able to measure the speed of light to be 230,000 km s^-1. Although this figure was very close to the currently accepted value of 300,000 km s^-1, it was rejected by the scientific community of the time, who assumed it to be much too high a figure.

Ole Rømer (1664-1710); → measurement.

The → radiation pressure of solar photons, which pushes a comet's dust outward to form a → dust tail.

→ solar; → radiation; → pressure.

The periodic fluctuation above and below atmospheric pressure created by an oscillating body which provides the → sound power. Instantaneous sound pressure is the peak value of air pressure.

→ sound; → pressure.

The sum of → static pressure and → dynamic pressure in the → Bernoulli equation.

→ stagnation; → pressure.

1) The most commonly used definition is temperature of 273.15 K (0 °C) and pressure of 1 → atmosphere.
2) Chemistry: Temperature of 273.15 K (0 °C) and pressure of 10⁵  → pascal (Pa)s (1 → bar). International Union of Pure and Applied Chemistry (IUPAC) recommends that the former use of the pressure of 1 atm as standard pressure (equivalent to 1.01325 × 10⁵ Pa) should be discontinued.

→ standard; → temperature; → pressure.

In → fluid mechanics, the → pressure felt by an object suspended in a → fluid and moving with it. This pressure is called static because the object is not moving relative to the fluid. See also → dynamic pressure.

→ static; → pressure.

Free from doubt as to the reliability, character, action, etc., of something (Dictionary.com).

M.E. sur(e), from M.Fr. sur, O.Fr. seur "safe, secure; trustworthy," from L. securus "free from care, untroubled, safe," from *se cura, from se "free from" + cura "care."

Tenz, from (Fine-e Bandar Abbâs) tenz "firm, fixed, solid," variant tereng; (Tabari) tereng, təreq "firm, fixed;" (Baxtiyâri) teng "firm;" cf. Baluci tranj-, dranjit, tranjit/drannj-, draht, dratk "to hang up;" ultimately from Proto-Ir. *dra(n)j- "to fix, fasten, hold;" Av. drənj- (draž-) "to fix, fasten, hold;" Parthian (*abi-) 'bdrynj- "to be sure, secure; make certain;" Khotanese drys- "to hold;" cognate with Gk. drassomai "I hold (with the hand), take;" O.Irish dringid "he climbs;" M.Welsh dringo "to climb;" PIE *dreg^h- "to hold, fasten."

Without doub; certainly; to be sure; for sure.

→ sure; → -ly.

1) Security against loss or damage or for the fulfillment of an obligation, the payment of a debt, etc.; a pledge, guaranty, or bond.
2) The state or quality of being sure (Dictionary.com).

M.E. surte, from M.Fr., from O.Fr. seurte "a promise, pledge, guarantee; assurance, confidence;" from L. securitas "freedom from care or danger, safety," from securus, → secure.

The ordinary pressure in a gas that is due to motions of particles and can be attributed to the object's → temperature.

→ thermal; → pressure.

The sum of → static pressure, → dynamic pressure, and → hydrostatic pressure in the → Bernoulli equation.

→ total, → pressure.

The equilibrium pressure of a vapor above its condensed phases (liquid or solid). In other words, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container. The vapor pressure varies with the temperature. As the temperature increases its the vapor pressure also increases, and conversely. The temperature at which the vapor pressure is equal to the atmospheric pressure is called the → boiling point. For example, at 20 °C, water would boil at a pressure of about 0.023 atm, or about a fortieth of atmospheric pressure.

→ vapor; → pressure.

→ dynamic pressure.

→ velocity; → pressure.