Sky Matters: Get ready for a rare planetary sight in our skies

We're weeks away from an exciting alliance between the planets Venus and Mars

This NASA image shows the planet Venus made with data from the Magellan spacecraft and Pioneer Venus Orbiter. 

We have already reached the turning point in the year when the nights begin to get longer, but it won’t be until September that day and night will be equal. So we can look forward to days which are longer than the nights for a while yet. We can also look forward to a rare event in July when the planets Mars and Venus will seem to almost become one to the unaided eye, so close will they approach to each other. Of course this is an illusion of perspective and in reality they are many millions of miles apart. And yet perspective matters, and on 12th July the two planets will be indistinguishable without binoculars or a telescope. To find them, just look towards the W-NW horizon around 10pm and you should easily be able to spot Venus in the twilight. Use it as your marker. Mars is more difficult to spot, being much fainter, but it will appear to the top left of Venus. On this occasion it’s worth getting out the binoculars, or telescope, to see both planets in the field of view at the same time.

You could also track the movement of the two planets relative to each other for a couple of weeks before and after 12th. I must admit, it never ceases to amaze me that such motions are easily visible. We know that our ancestors would have viewed similar events and we can predict such events for millions of years into the future thanks largely (but not exclusively) to Isaac Newton and the Laws of Motion he discovered as far back as the 17th and 18th centuries. And yet this “stability” of the planets in our Solar System (or any Solar System for that matter) is far from a given. It relies on a complex interplay involving the gravitational attractions between the planets, and given a long enough time it is possible for such interactions to conspire to fling an entire planet out into the cold depths of space. We may have even spied such objects wandering around our galactic neighbourhood. Recent calculations suggest they may be common at the time of the formation of a solar system. Did our solar system have more planets when it formed, some 4.5 billion years ago? Perhaps!