Mutual positions of Earth and internal planets
POSITION AND VISIBILITY
OF INTERNAL PLANETS
Internal planets (or inferior planets) are referred
to as to the ones that are nearer to the Sun in comparison with the
Earth.
We refer to Mercury and Venus.
The mutual positions of the Earth and these planets in comparison with
the Sun determine their conditions of visibility.
Mutual positions of Earth and internal planets
Position 1: it is called superior conjunction . The planet shows its hemisphere completely illuminated towards the Earth, but it results invisible because it is perspectively lined up with the Sun. The planet is at its maximum distance from the Earth.
From position 1 to 2: in this period the planet begins to be visible after the sunset of the Sun , for longer and longer periods.
Position 2: it is said maximum oriental elongation. The planet is in the optimal position to be observed in the evening. Its perspective separation from the Sun is maximum and therefore sunset occurs with the maximum possible delay in comparison with the Sun. On this occasion the planet shows half hemisphere being illuminated.
From position 2 to 3: in this period the sunset of the planet comes nearer to the one of the Sun, as the perspective distance between the two bodies is reducing. The transition between position 2 and 3 occurs more quickly in comparison with that between position 1 and 2. The crescent shown by the planet gets thinner and thinner itself.
Position 3: it is said inferior conjunction. The planet shows its not illuminated hemisphere to the Earth and it results invisible because again lined up with the Sun. The position of the planet is the one of shorter distance from the Earth.
From position 3 to 4: the planet is visible for longer and longer periods before the rising of the Sun. Its crescent starts to grow.
Position 4: it is said of maximum western elongation. The planet is in the optimal position to be observed in the morning. Its perspective separation from the Sun is maximum it therefore t rises with its maximum possible advance in comparison with the Sun. In this position the planet shows half hemisphere being illuminated.
From position 4 to 1: in this period the
rising of the planet recomes nearer to that of the Sun, even if the
transition from position 4 to 1 is slower in comparison with the
one from 3 to 4. The planet starts to show a fuller and fuller
disk.
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Position of external planets
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