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Glossary:

Time and Date

Date of validity of calculated output in local time and date, taking into account daylight saving time as well (see the current time zone on the left of the Earth icon on top right of almost all pages). The time is given as hours:minutes:seconds, or 00h00m00s. The time may also be rounded and given in decimal form: e.g., 10.1h means that the event will take place at about 5 minutes past 10 o'clock. This may also happen for days: 4.3d corresponds to the fourth day at around 7 o'clock. The start time is taken as selected by you, i.e., this is not necessarily at midnight. For intervals shorter than one day, decimal days are given. Times are given in 24 hour format (0h00m is midnight, 12h: noon, 18h: 6 pm.)

ET-UT1 / deltaT

Frictional forces from the Moon and the Sun, etc slow down the rotation of the Earth over time. Our civil clock (UTC) is kept in sync with the rotation of the Earth by inserting leap seconds, and thus UTC varies unpredictably from the time kept by atomic clocks and the motions of the solar system (ET or TDT, Terrestrial Dynamic Time). The difference between ET and UTC (or, more properly, UT1) is deltaT, currently a little over one minute.
For eclipses and transits, the events take place with calculations based on TDT. Relating them to fixed points on the Earth (latitude/longitude) is done by estimating the time difference deltaT. When the estimate is off, the predicted site on Earth is not at the place where the eclipse is taking place. Values from several millenniums ago can be reconstructed by evaluating historical records of e.g., solar eclipses. CalSky uses for the period from 1630 to now measured values published in the Astronomical Almanac and by IERS. Back to the year -500 points from F. R. Stephenson ('Historical Eclipses and Earth's Rotation' 1997) are used (table 14.1, Spline interpolated). Prior to -500 and for the future, approximation functions for the length of day are integrated. Those functions are based on Figure 14.7 from Stephenson (1997). Calculated values outside the telescopic era can be off by several percents.

Sidereal Time

The sidereal time is the right ascension of the points crossing the local meridian. For a certain time, the sidereal time is for all observers with the same geographical longitude the same. A sidereal day is slightly shorter (about 1/365.25, 4 minutes) than a mean solar day. After a sidereal day, the same star passes the meridian again.

Barycentric

Coordinates relative to the centre of mass, the barycenter, of the solar system. The barycenter does not coincide with the centre of the Sun. It is displaced to a point near its surface more or less in the direction of Jupiter. These coordinates can be used to start solving the N-body iteration for the solar system.

Heliocentric

Data is referring to the center of the Sun, which is slightly off the center of mass of the solar system.

Geocentric

Data is referring to the mass center of the Earth. Coordinates do not take into account perspective effects from the parallax.

Topocentric

Data is referring to your selected observing site.

Physical data

Data taking into account the physical dimensions and rotational characteristics of the celestial body.

Radius

Distance of the celestial body from main central body (Earth for the Moon, the Sun otherwise). For the Moon the unit is Earth radii (ER), otherwise Astronomical Unit (AU), the mean distance between the Sun and Earth.

Delta

Distance of the celestial body from Earth in Astronomical Units (AU). For the Moon, Delta is the topocentric distance of the Moons mass center from the observer in Earth radii (ER). It is also the fourth letter in Greek alphabet.

Horizontal Parallax

Difference between the topocentric and geocentric positions of an object, when the object is on the astronomical horizon. Seen from the object, it is the angle of the Earth radius.

Light-time

Time it takes for the light to travel from the celestial body to the observer.

Elongation

The elongation is the angular separation of the (ecliptic) longitudes of a celestial body and the central body (Sun, for moons: Jupiter or Saturn), as seen from the Earth mass center.

R.A., right ascension, RA

One coordinate used to indicate the position on the sphere. It is the angular distance of the object from the spring equinox measured along the celestial equator, expressed in hours of arc.

Dec., declination, DE

One coordinate used to indicate the position on the sky. It is the angular distance of the object from the celestial equator. North pole, close to Polaris, is 90° north.

Geometric coordinates

Celestial coordinates referring to the center of the Earth without correction of planetary aberration.

Astrometric coordinates

Position of an object corrected in such a way, that it can directly be plotted into a star chart for a given epoch (usually J2000). This is the geometric position, corrected for light-time.

Apparent coordinates

Celestial coordinates which are directly observable. Corrected for various effects: i.e., light-time, light deflection due to effects of relativity, planetary aberration, and corresponding to the true equator and equinox. Topocentric apparent coordinates include effects of refraction (if not assumed airless) and diurnal aberration (perspective displacement from the Earth mass center).

Ecliptic coordinates

Position of celestial body referred to the mean plane of Earth's orbit around the Sun and the spring equinox, expressed in ecliptic longitude (°) and latitude.

J2000, precession, nutation

The plains of ecliptic and equator shift with time by perturbations from the Sun, Moon and planets. The long-term shift is called precession; the short periodic variations are called nutation. The given celestial coordinates are referred to the true direction of the vernal equinox and the true obliquity of the ecliptic to the standard reference time 1 January 2000. For this date many star charts and coordinate tables are printed.

mean date / mean equator and equinox

Celestial coordinates that are corrected only for the long-term movement of the vernal equinox and obliquity of the ecliptic (precession).

true date / true equator and equinox

The celestial coordinates refer to the current (of the 'true' date of the coordinates) direction of the vernal equinox and obliquity of the ecliptic. Both the long-term precession and the short-term, periodic variations of nutation are corrected for.

In Zenith at

Geographical coordinates of the point on Earth, from which the object is seen directly in zenith (the geodetic coordinates refer to the WGS84 ellipsoid).

Parallactic Angle

The apparent direction of motion of the object relative to the local horizon. At transit - when the object passes the meridian - the parallactic angle is zero. When the object rises, the parallactic angle is the angle at which the object's path intersects the horizon. At the equator natural objects rise at a parallactic angle of 90°.

Rise, Transit, Culmination, Set

Rise and set times are for a mathematical horizon. Transit is the moment when the celestial object crosses the south meridian (for the northern hemisphere, north otherwise), i.e., it stands exactly in south direction. There it reaches (for objects other than stars: almost) its highest point on its diurnal journey. Culmination is the event of the highest point.

Constellation

The object is within the border of the given constellation of stars. The borders of the actual constellations have been defined by the International Astronomical Union (IAU).

Zodiac Sign

The zodiac is a band of about 8° width on both sides of the ecliptic, within which the Sun, the Moon and the planets reside. The zodiac is divided into 12 equally spaced, 30° long sections, called signs that are used in astrology. The signs corresponded to the actual constellations about two millenniums ago, but drifted by about one sign since then due to precession.

Diameter

Diameter is the geocentric apparent angular diameter of a celestial object (topocentric for artificial satellites). The value is given in seconds of arc for planets and satellites, and in minutes of arc for Sun and Moon.

Magnitude/Mag

Brightness of an object considered as a point source of light, on a logarithmic scale. Visual limiting magnitude is about 6mag, whereas the brightest star Sirius reaches -1.4mag. The Hubble Space Telescope can image objects as dim as 29mag.

Illuminance

The ground illumination from the natural source on a horizontal surface on Earth in clear conditions. The values for the Sun include both direct and indirect sunlight. The total illuminance of stars contributes only 0.00024 lux, less than the Sun at astronomical twilight. The stars and airglow illuminate the sky to 0.002 lux, a limit for ground based telescopes.

Surface Brightness

The visual magnitude of an average square arc-second area of the illuminated portion of the apparent disk (of a planet).

Phase

Ratio of the illuminated fraction of the apparent planetary or lunar disk to its entire area.

Phase angle

Angle between the Sun and the observer as seen from the center of the celestial body.

Sub-earth Phi/Lambda

Coordinates (lambda/phi) of the point on the celestial body, which sees the Earth directly overhead. The value of Lambda is equal to the central meridian of the body. Planetocentric coordinates refer to a sphere, whereas planetographic coordinates take the flattening of the planet into account (Earth in zenith). For Jupiter several rotational periods are known. System I: mean atmospheric equatorial rotation. System II: mean atmospheric rotation north of the south component of the north equatorial belt and south of the north component of the south equatorial belt. System III: rotation of the magnetic field. (For the gaseous planets Saturn, Uranus and Neptune the longitude is given in system III, the rotation of the magnetic field.)

Sub-solar Phi/Lambda

Coordinates of the body-graphic point with the Sun directly overhead (in local zenith). The latitude phi is taking into account the flattening of the physical body, and hence is not identical to the body-centered latitude.

Planetocentric orbital longitude of the Sun, Ls

The solar longitude Ls is the Sun angle as seen from the planet, measured from the northern hemisphere spring equinox where Ls=0°. Ls=90° marks to summer solstice (relative to the northern hemisphere), Ls=180° the autumn equinox and Ls=270° the winter solstice.