An Etymological Dictionary of Astronomy and Astrophysics
English-French-Persian

The average → sound power passing through a unit area perpendicular to the direction that the sound is traveling. It is usually expressed in watts per square meter.

→ sound; → level.

The expression of sound intensity in decibel units. The sound intensity level (L_I), in decibels, is computed as: L_I = 10 log (I/I₀), where I is the measured sound intensity and I₀ is the reference intensity (1 x 10 ^-12 watt per square meter).

→ sound; → intensity; → level.

A large area of the south polar region of → Mars which is covered with layers of → water ice and → dust. The SPLD, like the NPLD, has a maximum relief relative to the surrounding terrain of ~ 3.5 km and ~ 1,000 km across. Above the SPLD lies a very thin temporary (1-10 m) cap of → carbon dioxide ice/frost that snows out in the winter and sublimates over the spring and summer seasons. It is believed that the rhythmic nature of the deposits is related to oscillations in Mars' → orbital parameters (J. J. Plaut et al., 2007, Science 316, 92).

→ south; → polar; → layer; → deposit.

A program aimed at monitoring the near-Earth environment for recognizing and preventing space hazards by means of radar and optical observations from either space or the ground. The objective of the → European Space Agency initiative is to support the European independent utilization of, and access to, space for research or services, through the provision of timely and quality data, information, services and knowledge regarding the space environment, the threats and the sustainable exploitation of the outer space surrounding our planet Earth. The SSA Program was authorized at the November 2008 Ministerial Council and formally launched on 1 January 2009. The mandate was extended at the 2012 and 2016 Ministerial Councils, and the program is funded through to 2020. The program comprises three segments: 1) Space Surveillance and Tracking (SST), which is the monitoring and tracking of every object orbiting the Earth, such as satellites, space stations and debris. The objective is the prediction and warning of collisions and re-entry events. 2) → Space Weather (SWE), which aims at detection and forecasting of space weather and its effects through monitoring of the Sun, solar wind, magnetosphere, radiation belts, ionosphere and disturbances in the geomagnetic field. 3) → Near-Earth Objects (NEOs), which provides warning services against potential asteroid impact risks, including discovery, identification, orbit prediction and civil alert capabilities.

→ space; → situation; → -al; → awareness.

Same as → relative density.

→ specific; → density.

A measure of the amount of radiation received per unit solid angle per unit time per unit area normally from an element of surface.

→ specific; → intensity.

For a specified → bandwidth of radiation consisting of a continuous → frequency spectrum, the total → power in the bandwidth divided by the bandwidth. Spectral density describes how the power (or variance) of a time series is distributed with frequency. Also called power spectral density.

→ spectral; → density.

An → atomic transition that gives rise to a → spontaneous emission.

→ spontaneous; → transition.

The total amount of energy emitted by a star per unit time. According to the → Stefan-Boltzmann law, the stellar luminosity is given by: L_* = 4πR_*²σT_eff⁴, where R_* is radius, σ is the → Stefan-Boltzmann constant, and T_eff is → effective temperature. A star's luminosity depends, therefore, on two factors, its size and its surface temperature. Stellar luminosity is measured either in ergs per second or in units of → solar luminosity or in → absolute magnitude. See also → luminosity class.

→ stellar; → luminosity.

1) The act of placing upon; the state of being placed upon.
2) Math. The act of placing (one geometric figure) over another so that all like parts coincide.

→ super- + → position.

Barhamneheš, from bar- "on, upon, up" (Mid.Pers. abar; O.Pers. upariy "above; over, upon, according to;" Av. upairi "above, over," upairi.zəma- "located above the earth;" cf. Gk. hyper- "over, above;" L. super-; O.H.G. ubir "over;" PIE base *uper "over") + -ham- "together, with," → syn-, + neheš, → position.

1) Math.: The principle concerned with homogeneous and non-homogeneous → linear differential equations, stating that two or more solutions to a linear equation or set of linear equations can be added together so that their sum is also a solution.
2) One of the most fundamental principles of → quantum mechanics which distinctly marks the departure from classical concepts. It holds that any linear superposition of → wave functions is also a possible wave function. Simply put, the state of an object is all its possible states simultaneously, as long as we do not look to check. It is the measurement itself that causes the object to be limited to a single possibility. The superposition principle is rooted in the linearity of → Schrödinger's equation. Hence if two solutions of the wave function, ψ₁ and ψ₂, are known, other solutions, of the form: ψ = a₁ψ₁ + a₂ψ₂ also represent possible states of the system.

→ superposition; → principle.

The amount of a quantity distributed over a surface area divided by the area, such as a surface-charge density.

→ surface; → density.

A generalization of the tensor concept that like a tensor transforms, except for the appearance of an extra factor, which is the → Jacobian matrix of the transformation of the coordinates, raised to some power, in transformation law. The exponent, which is a positive or negative integer, is called the weight of the tensor density. → weight of a tensor density. Ordinary tensors are tensor densities of weight 0. Tensor density is also called → relative tensor.

→ tensor; → density.

1) An event where one astronomical object appears to move across the face of another. As seen from Earth, the planets Venus and Mercury are seen to transit the Sun. We can also observe natural satellites transit the face of their host planet. Similarly exoplanets have been observed to transit their host stars. See also → planetary transit, → transit method.
2) The passage of a heavenly body across the meridian of a given location.

M.E., from L. transitus, p.p. of transire "to go or cross over," from → trans- "across" + ire "to go."

Gozar "passage, transit, passing," from gozaštan "to pass, cross, transit," variant gozâštan "to put, to place, let, allow;" Mid.Pers. widardan, widâštan "to pass, to let pass (by);" O.Pers. vitar- "to pass across," viyatarayam "I put across;" Av. vi-tar- "to pass across," from vi- "apart, away from" (O.Pers. viy- "apart, away;" Av. vi- "apart, away;" cf. Skt. vi- "apart, asunder, away, out;" L. vitare "to avoid, turn aside") + O.Pers./Av. tar- "to cross over;" → trans-.

An observing instrument provided with a graduated vertical scale, used to measure the declinations of heavenly bodies and to determine the time of meridian transits. Same as → meridian circle.

→ transit; → circle; nimruzâni, adj. of nimruzân, → meridian.

An instrument mounted so as to allow it to be pointed only at objects in the sky crossing the local meridian. Also known as → transit telescope.

→ transit; → instrument.

A method for detecting → exoplanets that is based on the decrease of star → brightness when the exoplanet passes in front of its star. As the planet transits, a portion of the light from the star is blocked causing a decrease in the → magnitude of the star. The amount of decrease (typically between 0.01% and 1%) depends on the sizes of the star and the planet. The duration of the transit depends on the planet's distance from the star and the star's mass. This change must be periodic if it is caused by a planet. In addition, all transits produced by the same planet must be of the same change in brightness and last the same amount of time. Once detected, the planet's distance from its star can be calculated from the period and the mass of the star using → Kepler's third law of planetary motion. The size of the planet is found from the depth of the transit and the size of the star. From the orbital size and the temperature of the star, the planet's characteristic temperature can be calculated. Knowing the star's mass and size, the planet's size and distance can be estimated. Also the composition of a → transiting planet's atmosphere can, in principle, be determined.

→ transit; → method.

The crossing the face of the Sun by the planet Mercury, as seen from Earth. Because the plane of Mercury's orbit is not exactly coincident with the plane of Earth's orbit, Mercury usually appears to pass over or under the Sun. On the average it occurs 13 times each century when the Earth is near the → line of nodes of Mercury's orbit. The three last transits were on 2003 May 07, 2006 November 08, and 2016 May 09. The next one will be on 2019 November 11. The first observation of a transit of Mercury was on November 7, 1631 by Pierre Gassendi. On June 4, 2014 NASA's Mars rover Curiosity photographed a transit of Mercury, marking the first time such a phenomenon has ever been imaged from the surface of a planet other than Earth. See also → black drop.

→ Mercury; → transit.

A rare phenomenon that happens when the planet Venus passes directly between the Sun and Earth and is therefore seen against the solar disk. Such a passage occurs every 122 or 105 years and when it happens the next occurrence is after 8 years. Only seven transits of Venus have occurred since the invention of the telescope: in 1639, 1761, 1769, 1874, 1882, 2004, and 2012. The next one will be in 2117. The reason for this rarity is that the Earth and Venus do not orbit the Sun in the same plane. Their orbital planes have a relative inclination of about 3°. The first observation of the Venus transit was in 1639 by the English Jeremiah Horrocks (1618-1641). See also → black drop.

→ Venus; → transit

Same as → transit instrument.

→ transit; → telescope.