In April to May 2002 all 5 planets known before the invention of the telescope were clustered in one part of the sky, and made a fine and rare spectacle after sunset - the next such grouping will not occur until July 2060.
In reality the planets are not in any danger of collision when they pass close to one another in the sky, for it is purely a line of sight effect from our vantage point on Earth. When the vantage point changes, as the diagrams below show, the true positions of the planets become more obvious.
The pictures are taken from a program that runs in a window on the NASA Lift-off site, though unfortunately it no longer seems to be available. It is ideal for a quick and easy view of the realtime positions of the planets. A similar program is also available at Fourmilab, from where you can also download a more sophisticated version (for free) for running on your PC.
In the pictures below the red lines mark the orbits of the planets while the green lines are the orbits of 2 comets and an asteroid. The planets travel anti-clockwise, which means that they move from right to left behind the Sun and left to right in front of it. The Earth is the 3rd from the Sun - its own orbital motion makes the Sun appear to move anticlockwise too, which explains why the Sun appears to catch up and overtake the outer planets at intervals of just over a year.
All the visible planets...
The vantage point of this picture is about 40 degrees above (north of) the plane of the Solar System, from a position such that the Earth lies between us and the Sun.
This picture shows how Venus, Mars, Jupiter and Saturn all lie to the left of the Sun. Therefore, when seen from the Earth these planets all appear in the same part of the sky, but this view shows how they actually lie at different distances.
Note how Venus lies almost directly between Earth and Jupiter. As seen from the Earth Venus will pass less than 2 degrees from Jupiter on 2 and 3 June.
See below for a close-up of the inner Solar System.
The inner Solar System...
This zoomed in view, from the same vantage point of the picture above, shows the inner Solar System more clearly.
Mercury currently lies between the Earth and the Sun, so is not visible from Earth. In late April through to early May it lay to the left of the Sun, nearer Venus, so all 5 planets (the 3 here, plus Jupiter and Saturn beyond, could be seen together.
The Solar System seen nearly edge-on...
By shifting the vantage point down towards the plane of the Solar System the planets fall into the almost straight line (referred to as the "Ecliptic") that marks the track of the planets across the Earth's sky. The two outermost red lines, both furthest from and nearest to us, are the orbits of Uranus and Neptune - the planets themselves lie below and to the right of this field of view and played no part in the 2002 planetary alignment.
A diagram such as this makes sense of what you see in the real photographs, such as the one of 4 May 2002. Note though that the order of the planets is different, because of the different date and our position near the edge of the Solar System rather than on Earth itself. I have, however, orientated the view so that it corresponds to the real angle of the ecliptic to the horizon after sunset in April / May from the UK.
Anybody on the ball will notice that the date on the pictures says 25th April, but there is no doubt that the planets are shown as on 25th May!
|Online Solar System view at NASA Lift-off (unfortunately no longer seems to be available)|
|Online & downloadable Solar System & Earth views at Fourmilab|