Local Time and Time Zone. When the program starts it gets the current date, time and zone from an operating system due to user settings. In this way date and time are the local time and the time zone is a difference between the Greenwich meridian and the local meridian as an integer number.
All these parameters can be edited later, changing the output astronomical situation, location of celestial bodies and their characteristics. The celestial events are displaied in local time for convinience. It is important to ensure that the time zone is correct, otherwise you may get an error calculating celestial events.
Universal Time (UT). It is the basis for the 24 hour time currently in use. It is based on the rotation of the Earth relative to the stars. Also, it is the local mean solar time at a longitude of 0° in this software.
Terrestrial Dynamic Time (TDT). The astronomical time standard, designed for astronomical observations from the Earth's surface. Formerly, the ephemeris time (ET), used to eliminate irregularities in the Earth's rotation. Currently used term Terrestrial Time (TT), equal to the TDT
Local Apparent Sidereal Time, LAST. Angle, measured in time units of the vernal equinox for the location. It has the same value as the right ascension of any celestial body, crossing the local meridian at this time. At a time when the vernal equinox crosses the local meridian, the local apparent sidereal time is 00:00.
Astronomical Meridian. The great circle passing through the celestial pole, the zenith and the nadir in the area defined by geographical coordinates. It has southern and northern points and perpendicular to the celestial equator and the celestial horizon.
Julian day JD. The astronomical time scale, measured from January 1, 4713 BC according to the Julian calendar. A new day begins at midnight UT. It is a real number. For example, January 1, 2000, 00:00 = 2451544.5. The fractional part of 1 hour equals 1/24, 1 second is approximately 1,157407e-5.
Delta T. The difference between terrestrial time (TT or TDT) and Universal Time (UT), ΔT = TDT - UT. Delta T vary from year to year, thus in 1900 ΔT = -2,72 sec, and in 2000, ΔT = 63,83 sec.
The geographic coordinates - latitude and longitude, can be set by user at a known numeric values, as well as from a Google map. The values of latitude and longitude can be stored in the program settings so that they are relevant in a subsequent program start. To retrieve geographical coordinates from Google Maps one should make a long touch on a point of interest.
Ecliptic longitude. Measures the angular distance of an object along the ecliptic from the primary direction. The ecliptic longitude is measured in the direction of motion of the sun along the ecliptic from 0° to 360°.
Ecliptic latitude. Measures the angular distance of an object from the ecliptic towards the north or south ecliptic pole. The north ecliptic pole has a celestial latitude of +90°.
Radius Vector. An imaginary straight line connecting the centers of celestial bodies. In the program is measured in astronomical units (AU).
Elongation. The angle between the Sun and the celestial body, with Earth as the reference point.
Apparent longitude of a celestial body. The longitude, adjusted for nutation and aberration.
Right ascension (RA). The angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the celestial body.
Declination. The angle between the celestial equator and the celestial body. Positive to the north of the celestial equator, negative to the south.
Constellation. A part of the celestial sphere, defined by the International Astronomical Union (IAU), where a celestial body can be found. Usually used together with the celestial coordinates. At present, the 88 official constellations are used.
Topocentric altitude. The angle between the celestial body and the horizon, where the observer is located. For the visible objects changes from 0° to 90°.
Topocentric azimuth. The angle of the object around the horizon, usually measured from the north increasing towards the east.
Aberration. The astronomical phenomenon that leads to the apparent position of celestial bodies, different from their true position. Caused by the finite speed of the light and rotation of the Earth.
Nutation. The effect of deviations celestial body axes while rotating.
Parallax. Change in the apparent position of the object relative to the remote background, depending on the observer's position.
Diurnal parallax. The difference in the directions on the same celestial body from the center of the Earth (geocentric direction) and from the point on the surface of Earth (topocentric direction). The angle depends on the height of a celestial body above the horizon. The maximum value is achieved at zero height (when the celestial body is right there on the horizon). This value is called the horizontal parallax.
Atmospheric refraction. The deviation of light from celestial bodies in the atmosphere. As a result of refraction, positions of celestial bodies that emit the light seem a little higher than their true location. Also as a result of refraction, the Sun and the Moon appear to be flattening a little.
Semi-diameter. Angle, denoting the actual radius of the celestial body from a point of the Earth. Semi-diameter decreases with increasing distance from the observer. Semi-diameter of the Sun and the Moon is about 16'. Semi-diameter of the planets does not exceed 20 seconds. Stars of the large distances from the observer does not have a semi-diameter at all.
Illuminated fraction. The notion refers mainly to the Moon. It ranges from 0 to 1.0 and shows the degree of illumination of a celestial body.