The Calendar-Sky

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Glossary:

Altitude/alt/h

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

Astronomical Twilight

The times are the moments of beginning/end of the astronomical twilight, i.e. the moments the sun reaches a depression of 18° below the horizon. If the sun is below this angle, no brightening of the sky can be observed.

Azimuth/az

Azimuth direction of the object is given in degrees counting from geographic North (0°) clockwise to the East direction. East is 90°, South 180°, and West 270°. The three-character direction code is given as well. For example, NNW stands for north-north-west.

Best from / LCT

The best visibility time interval of the object. The calculation takes into account the magnitude of the object (required elevation above horizon), and the elevation of the sun. The time is given in local civil time (LCT), i.e. the time zone and definitions as selected by you.

Center

This figures gives the distance and direction to the flare center line projected on the earth surface. For distances of less than 1km, there is a very good chance for a spectacular bright Iridium flare. You can still travel to the listed point (indicated distance and azimuth) to observe the flare from the center line. However, the attitude control and steering of the satellite must not be perfect: being within 5km of the listed center line will be sufficient.

Civil Twilight

The times are the moments of beginning/end of the civil twilight, i.e. the moments the sun reaches a depression of 6° below the horizon. On clear weather, no significant dimout can be distinguished compared to the time of sunset/sunrise.

Comet

Interplanetary celestial body, usually consisting of ice and rock. Due to heating up when approaching the sun, the ice starts evaporating and produces a nebulous coma and tail around the nucleus.

Date and Time

Date of validity of calculated output in local time and date, taking into account daylight saving time as well (see the current timezone 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 arond 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)

Dawn and Dusk: nautical Twilight

In CalSky, is taken as the moments of nautical twilight, i.e. the moments the sun reaches a depression of 12° below the horizon. Not astronomically trained people will recognize the brightening of the horizon at these times.

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.

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

Diameter

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

dRA, dDec

Apparent angular movement of the object. The value for right ascension is reduced to the movement at the celestial equator

Eastern/ western Elongation

Most extreme points on the orbit of the moons around the planet.

Elongation

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

Flare angle

The angle between the direction of the mirrored image of the sun and the observer. For bright flares, this angle must be as small as possible (i.e. the observer should be as close to the center line as possible).

Flare

The communication antennas and the solar panels reflect the sunlight almost as a perfect mirror. In case the observer lays within this reflected beam, the satellite suddenly appears very bright, as bright as the moon in the first quarter; the light is even strong enough to cast shadows. Since the sunlight is bundled, the duration of the whole event is short, and lasts about 10 seconds. The indicated time is the center of the flare event; hence the satellite can be spotted some seconds earlier. Due to the shortness of the event, it is important to look in the right direction at the right time.

Iridium

Wireless worldwide communication system, which consists of 66 satellites, that are in low earth orbits. The user who has a rather small phone directly contacts one of the satellites, i.e. one of the three Main Mission Antennas MMA (the three panels in the bottom of the image with a size of about 1x2m²). The satellites constellation consists of 6 planes with 11 satellites each (and some spares). Hence, another Iridium satellite passes at about the same place in the sky every 8 minutes.

J2000, precession, nutation

The plans of ecliptic and equator shift with time by perturbations from sun, moon and planets. The long-term shift is called precession, the short periodic variations are called nutation. The given celestial coordinates are refer 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.

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.

Moon sign

The position of the moon within a part of the zodiac, that is subdivided into 30° wide segments. Besides this sign of the zodiac the angle within the sign is given as well.

NGC

New General Catalogue of Nebulae. The British astronomers Herschel published in 1864 a catalogue of 2000 deep sky objects, in 1888 the Danish astronomer J. Dreyer supplemented it to 7840 entries. Until 1907 he added 5386 objects in a supplement called Index Catalogue (IC).

Opposition

An outer planet (orbit outside the orbit of earth around the sun) apparently stands opposite the sun. Hence, it can be observed throughout the night.

Phase

Ratio of the illuminated fraction of a planetary disk to its apparent diameter.

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.

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.

Rise, Transit, Set

Rise and set times are for a mathematical horizon. Transit is the moment the celestial object crosses the south meridian, i.e. it stands exactly in south direction and reaches its highest point on its diurnal journey. Times are listed only if they fall within the chosen interval, starting at the start time. Missing values indicate that the event does not take place at the underlying interval.

Sat above

Geographic coordinates of the sub-satellite point (in WGS84 coordinates). This is the point on Earth, from which the satellite is in the zenith at the indicated time. The altitude of the satellite from this point is given as "alt".

Spare satellite or unknown status

Not all Iridium satellites are operational. Some of them are spare satellites and are in a fuel save mode. Hence the attitude of the satellite is not as strictly stabilized as for operational ones. Predictions of the flare's brightness are not that accurate in this case, a no-show is also possible.

Sun sign

The position of the sun within a part of the zodiac, that is subdivided into 30° wide segments. Besides this sign of the zodiac the angle within the sign is given as well.

Supernova (SN)

WGS84 / Geographical Coordinates

Geographical coordinates are given by the angles longitude (Lon), latitude (Lat), and altitude in meters (Alt). A place nort