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Glossary:

Solar Eclipse

The disk of the moon occults the disk of the sun. These rare, but spectacular events should be observed with precautions (use eclipse viewers)! You may damage your eyes.

Total Solar Eclipse

This may be the most striking event, that the celestial bodies can offer us. The moon hides the disk of the sun completely and allows to see the much dimmer corona of the sun. The longest total solar eclipse can last about 7 minutes.

Annular Solar Eclipse

The distance of the moon from the earth is not constant. Since the apparend diameter of the moon is in the mean very close to the one of the sun, not every central eclipse is total. In case of an annular eclipse, the diameter of the moon is too small to cover the whole sun. However, the ring displayed is a great view as well.

Partial eclipse begins/ends

First and forth (last) contact. The moon starts/ends 'touching' the disk of the sun. This is a partial solar eclipse.

Umbra eclipse begins/ends

Second and third contact. This is the time of begin/end of the phase of totality or annularity. Without these two contacts, the eclipse is a partial eclipse.

Maximum eclipse

Time and phase of maximum eclipse, i.e. possible phases are partial (0-100%), annular or total eclipse (>100%).

Eclipse at rise/set

Eclipse starts/ends locally at rise/set of the occulted bodies. Other phases of the eclipse are hence not visible.

Magnitude

Magnitude of the eclipse for the time and position given. Partial phase: 0-100%, annular or total: >100%.

Obscuration

Fraction of the surface of the solar disk obscured by the moon during solar eclipse for the time and position given. Partial phase: 0-100%, total: 100%, annular: relative moon radius squared (i.e. <100%).

Separation

Angular distance between the center of disks of two object. For eclipses: the Sun and the Moon, for occultations: Moon/satellite and Star/Planet. For binary stars: Star/Star

Position Angle / PA

Angle, defining a position on an apparent disk or the position of a dimmer star with regard of the main star. It is counted around the reference points (center of disk/brighter star) from celestial north direction 0° to east (left) 90°, south 180° to west (right) 270° in counter clockwise direction.

Position Angle rel. Vertex

Angle, defining a position on an apparent disk. It is counted around the reference points (center of disk) from local up, zenith direction 0° to east (left) 90°, south 180° to west (right) 270° in counter clockwise direction.

Altitude/alt/h

Angular separation of the object from the local mathematical horizon. This accounts for refraction as well.

ET-UT1 / deltaT

Frictional forces from the moon, 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 milleniums 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 funcations for the length of day are integrated. Those function are based on Figure 14.7 from Stephenson (1997). Calculated values outside the telescopic era can be off by several percents.

Standard Values

The calculations are based on the authoritative values for calculating solar eclipses from the International Astronomical Union, IAU. The national agency NASA adopts different values, hence giving slighly deviating values. This, and the figure used for deltaT is important to remember when comparing the outputs from different sources.
Constants: k_Moon=0.2725076, both for umbra and penumbra [IAU 1982, Astronomical Almanac since 1986]. The center of mass and figure of the moon are slightly offset. Following corrections have been applied to ecliptical coordinates of the moon: dLambda=+0.5" and dBeta=-0.25". The diameter of the sun at unit distance is 15'59.63". Earth ellipsoidal parameters: f=1./298.257 [IAU 1976], a_Earth=6378137m. Online calculations based on third degree Besselian elements. The output is not adjusted for the lunar limb profile.