The diagrams on this page show the positions of the major planets, and the two brightest asteroids, in their orbits. The orbits are in correct relative scale. Planetary orbits are elliptical with the Sun at one focus of the ellipse. On the diagrams the position on the orbit where a planet is closest to the Sun, that is at Perihelion, is marked with a P The position of Aphelion, when the planet is furthest from the Sun is marked A.
The diagram for the inner planets, as far out as Mars, is for the period December 2010 to April 2011, April to August 2011 August to November 2011 The diagram covers four months, rather more than the time Mercury takes to complete one orbit round the Sun. The positions of the planets are marked every 10 days.
The last diagram shows the orbits of the outer planets as far as Saturn for 2011. The orbits of the Earth and Mars are also indicated as are the orbits of the two brightest Asteroids, (1) Ceres and (4) Vesta. The diagrams cover a year, with the positions of the Asteroids, Jupiter and Saturn marked at the start of every other month. The positions of the Earth and Mars are shown at the start of each month.
Also indicated on this diagram are the directions to Uranus, Neptune and Pluto at the beginning and end of the year. The distance of Uranus from the Sun is over twice that of Saturn, whilst the distances of Neptune and Pluto are over three times the distance of Saturn.
For the date you are interested in, imagine a line from the Earth to the Sun. If the planet lies to the left of this line, that is clockwise from the line, then the planet is in the evening sky. On the other hand, if the planet is to the right of the line, that is anti-clockwise from it, then the planet is in the morning sky.
Mercury and Venus, the inner planets, are at inferior conjunction when they are between the Earth and Sun and at superior conjunction when the Sun is between the Earth and the planet. In either case they will be too close to the Sun to observe. See positions of Mercury at inferior conjunction 20 December 2010 and at superior conjunction 25 February 2011.
The inner planets are at their best position for observing when the line from the Earth to the planet is at right angles to the line from the planet to the Sun. They are then at greatest elongation from the Sun. Examples occur for Venus on 8 January 2011 and Mercury the following day when they are west of the Sun. Mercury is at an easterly elongation on 1 December 2010 and on 22 March 2011.
The outer planets, Mars, Jupiter, Saturn etc, can never be at inferior conjunction with the Sun, only at superior conjunction (just called "conjunction"). On the other hand they can be on the opposite side of the Earth to the Earth. This is known as "opposition" and is the time when the planet is in the sky all night, rising close to sunset and setting close to sunrise. An opposition of Saturn occurs on 3 April 2011.
Two planets will appear close together in the sky when a line from the Earth passes close to both of them. An example occurs on 16 December 2010 when Mercury and Mars will be just over a degree apart in the sky. A straight line drawn on the diagram from the Earth's position at that date to Mars will pass through the position of Mercury. This conjunction occurs in the evening sky, but the planets will be too low in the twilight to observe.
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