An Etymological Dictionary of Astronomy and Astrophysics
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A member of a class of unevolved or slightly evolved → Population I disk stars with an → effective temperature, degree of evolution, → metallicity, and kinematic property not very different from those of the Sun. See also → solar-like star; → solar twin.

→ solar; → analogue.

The apparent direction (in the constellation → Columbia) away from which the Sun is moving in its orbit around the center of the Galaxy. → solar apex.

→ solar; → antapex.

The point on the celestial sphere toward which the Sun is apparently moving relative to the → local standard of rest. Its position, in the constellation → Hercules, is approximately R.A. 18h, Dec. +30°, close to the star → Vega. The velocity of this motion is estimated to be about 19.4 km/sec (about 4. AU/year). As a result of this motion, stars seem to be converging toward a point in the opposite direction, the → solar antapex.

→ solar; → apex.

The angle between the Sun's → rotation axis and perpendicular to the → ecliptic plane. In other words, the inclination of the Sun's → equator with respect to the → ecliptic plane. It is 7.25 degrees.

→ solar; → axial; → tilt.

A calendar based on the apparent yearly motion of the Sun on the → celestial sphere. The year is usually reckoned with respect to the → vernal equinox, approximately for example in the case of the → Gregorian calendar and accurately in the case of the → Iranian calendar.

→ solar; → calendar.

The amount of solar radiation in all wavelengths received per unit of time per unit of area on a theoretical surface perpendicular to the Sun's rays and at Earth's mean distance from the Sun. Its mean value is 1367.7 W m^-2 or 1.37 × 10⁶ erg sec^-1 cm^-2. In other words, the solar constant is the mean → solar irradiance on the outer atmosphere when the Sun and Earth are spaced at 1 → astronomical unit. See also: → solar luminosity.

→ solar; → constant.

The outermost atmosphere of the Sun immediately above the → chromosphere, which can be seen during a total solar eclipse. It consists of hot (1-2 × 10⁶ K), extremely tenuous gas (about 10^-16 g cm^-3) extending for millions of kilometer from the Sun's surface.

→ solar; → corona.

The periodic variation in frequency or number of solar active events (→ sunspots, → prominences, → flares, and other solar activity) occurring with an interval of about 11 years. The solar cycle was discovered in 1843 by Samuel Heinrich Schwabe (1789-1875), a German apothecary and amateur astronomer, who after 17 years of observations noticed a periodic variation in the average number of sunspots seen from year to year on the solar disk. Solar cycle numbering goes back to the 18-th century, when the Cycle 1 peak occurred in 1760. Cycle 23 peaked in 2000, and the following Cycle 24 will reach its maximum in 2013.

→ solar; → cycle.

The length of time between two successive transits of the Sun over the same meridian.

→ solar; → day.

The → angle between the → sea horizon, the → center of → Earth, and the center of the → solar disk.

→ solar; → depression.

The apparent shape of the → Sun's → photosphere.

→ solar; → disk.

A model for explaining the generation of the → solar magnetic field and the related observational features (mainly → solar cycle, → Sporer's law, → Hale's law, → Joy's law, → polarity reversal). The global frame of this model is the interaction between a → turbulent plasma in the → convective zone (reciprocal generation of magnetic and electric fields) and the solar differential rotation (mutual transformation of meridional magnetic field into azimuthal magnetic field). The idea that a dynamo is responsible for generating the solar magnetic field was first proposed by Larmor (1919) and further developed by Cowling (1933), Parker (1955) and others.

→ solar; → dynamo.

An eclipse in which the Earth passes through the shadow cast by the Moon. Solar eclipses only happen when the Moon is new and when the Moon lies close to the node of its orbit.

→ solar; → eclipse.

The greatest angular distance from a → lunar orbit node within which a → solar eclipse may occur when the Sun and Moon are in conjunction there. The solar ecliptic limit extends about 17° on each side of the node.

→ solar; → ecliptic; → limit.

In ancient astronomy, the difference between the Sun's mean and actual position. The ancients observed that, although the motion of the Sun in the ecliptic is almost uniform, it is subject to a small annual variation.

→ solar; → equation.

A bright eruption form the Sun's → chromosphere in the vicinity of a → sunspot. Solar flares are caused by tremendous explosions on the surface of the Sun. In a matter of just a few minutes they heat the material to many millions of degrees and release as much energy as a billion → megatons of → T.N.T..

→ solar; → flare.

An instrument especially designed for solar observations.

→ solar; → instrument.

The radiative power per unit area in all wavelengths from the Sun received by the Earth at its average distance from the Sun. Its mean value is called the → solar constant. The solar irradiance changes over a year by about 6.6% due to the variation in the Earth/Sun distance. Moreover, solar activity variations cause irradiance changes of up to 1%.

→ solar; → irradiance.

The edge of the → disk of the → Sun.

→ solar; → limb.

The ecliptic longitude of the Sun. It varies from 0° (at the vernal equinox) to 360° during the year. By Kepler's Second Law, the rate of change of the solar longitude is such that the Earth sweeps out equal areas on the ecliptic plane in equal times.

→ solar; → longitude.