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Planet round-up

The main display is a graphical representation of the planets. South is up, and the scale is shown in the upper right corner. The white line represents 10 seconds of arc. For comparison the diameter of the Moon is about 1892 arc seconds. The planets are shown at their correct relative size as seen from Earth and at their correct phase and orientation.

You can zoom up the view of Mars by selecting Mars from the drop-down list. This is useful when you want to observe surface details on the Mars. Step time forward one hour at a time and watch the Red planet rotate.

The table below the main display lists the physical ephemerids of the planets. The following information is displayed:

Magnitude: The brightness of the planet. Lower numbers refer to objects of greater brightness. The dimmest star visible to the unaided eye is about 6. magnitude. The brightest star in the sky Sirius in Canis Major is mag -1.46. The planet Venus can reach mag -4.7, the full Moon is mag -12.7 and the Sun is mag -26.7. A difference in 5 magnitudes corresponds to a brightness difference of one hundred times.

Diameter: The equatorial diameter of the planet in arc seconds. To measure sizes in the sky astronomers use degrees, hours and seconds. The entire sky is divided into 360 degrees. Each degree is divided into 60 arc minutes and each arc minute is divided into 60 arc seconds. For comparison the Moon covers about 1/2 degree in the sky and Venus at is largest covers about one arc minute.

Phase: The percent of the planet illuminated as seen from Earth. A phase of 0% means that the planet is not illuminated, 50% means that half the planet is illuminated and 100% that the whole planet is illuminated. Only the inner planets Mercury and Venus shows all phases, the outer planets is always close to full phase (100%).

PA(S): Position Angle of the Sun. The Position angle of the Sun describes the orientation of the illuminated phase relative to the observer's north. The position angle is measured in the sense north-east-south-west on the scale from 0° to 360°.

PA(A): Position Angle of the Axis. The Position angle of the Axis describes the orientation of the rotational axis of a planet relative to the observer's north. The position angle is measured in the sense north-east-south-west on the scale from 0° to 360°.

L(I), L(II) and L(III): Longitude of the planets central median. Just like on Earth we can for the other planets use a system of spherical coordinates fixed with reference to the surface. The central Median is the median passing through the center of the apparent disc of a planet as seen from Earth. For the giant gas planets Jupiter, Saturn, Uranus and Neptune which do not rotate like solid bodies rotational systems with constant angular velocities have been introduced. For Jupiter System I applies to regions near the equator +/- 9 degrees and System II for all higher latitudes. System III describes rotation relative to the planets magnetic field.

B: Latitude of the Earth.