Movement
Movement of objects in the sky can be very confusing for the beginner. The following points may help:
- Because the earth rotates from west to east on its axis most objects in the sky appear to rise in the east and set in the west. Because the earth is a sphere, objects appear to follow different east west arcs in the sky, depending on how far north or south they are. An object very far south in the sky can rotate in almost a circle around what we call the South Celestial Pole, whereas an object very far north rises on eastern horizon, appears briefly to form a low arc across the sky and then sets. Examples of both extremes are The Southern Cross to the South and the constellation Leo to the north.
- As the earth journeys around the sun all fixed objects in the sky rise four minutes earlier each night. Simple calculations reveal that this means any star or group rises two hours earlier each month and so on for a full calendar year when objects are again found in the same place.
- Planets beyond earth’s orbit have yet another movement – again west to east -so that each year they appear to rise later (this delay caused by their own rotation around the sun each year). Mars is a special case as its orbit is next to our own and it gains a half rotation on us each year. For this reason Mars only comes close to our planet every two years.
- Inner planets, Mercury and Venus do not move outside our orbit and so we follow their movements, which take them east and west of the sun (as seen from earth) during the course of their orbits. For this reason those planets always appear as morning or evening ‘stars’ rising before the sun or setting after it.
- Comets can arrive in any part of the sky, unlike planets, which follow a fixed east/west path, through the stars called the ecliptic.
- The Moon is a special case and little needs be said as its journey is easily followed from month to month. It journey through the sky depends on its current position on the ecliptic.