Massing or Missing?

If you look up in the May 2011 night sky, you might start to wonder where all the planets have gone. Saturn's still there of course, heading ever closer to the star at the base of the 'Bowl of Virgo' known as Gamma Virginis or Porrima, but the other's are notably absent. 

The reason for this is that there's something of a planetary party taking place in the pre-dawn twilight where you'll find no less than 5 of the major planets all in a very small region of sky. The planets in question are Mercury, Venus, Mars, Jupiter and Uranus. Actually Uranus is a bit out on a limb, sitting roughly mid-way between the others and dim and distant Neptune which is significantly further to the west.

So the main conjunctions are between Mercury, Venus, Mars and Jupiter. The correct term to describe a get together of three or more planets like this is a massing. If you have tried to see them just before sunrise though, you might be forgiven for wondering if the massing has gone missing!

In the middle of May, Mars and Mercury aren't at their best and are quite dim to pick out in the bright pre-dawn sky. However, Venus at around magnitude -4 and Jupiter at around magnitude -2 should, in theory anyway, be quite straightforward to see; bear in mind that Venus can be seen in broad daylight. If you can find Venus in a bright blue daylight sky, it's often quite shocking to realise just how bright it appears.

In practice though, even Venus and Jupiter are a bit of a struggle at the moment and there are several reasons why this is the case. Despite their brilliance, both planets are low down just before sunrise and their light has to do battle through quite a thick layer of atmosphere. Then there's the problem of changing light levels in the dawn sky. When the reverse situation occurs in the evening twilight, it's often possible to watch brilliant Venus drop right down to the horizon. In the dawn sky though, if Venus rises at a shallow angle to the horizon (as is currently the case), spotting it can be a struggle.

In the evening situation, the light levels are dropping so the sky gets darker making Venus stand out more. Couple this with the fact that your dark adaption is on the increase too, and Venus remains visible for ages. In the dawn sky however, the sky is getting brighter with time. Consequently, Venus gets harder to see as sunrise approaches. To make matters worse, your sensitivity to low light is also eroded by increasing light as dawn approaches. 

The Massing in May

If you've been frustrated trying to locate Venus and Jupiter low down in the eastern part of the sky this month, here are a few tips which may help you succeed.

Choose a viewing sight which gives you a reasonably unobstructed view from the east round to the northeast. If you know your stars, in the middle of the month, head off to the site around 02:30 BST and locate the point just above the Circlet in Pisces close to the horizon - this is where the planets will be rising in about 2.5 hours time. If you don't know your stars that well but have access to a compass, you can have a couple of extra hours in bed and miss out that last part. You'll need to keep an eye on the region between azimuth 70 degrees and 90 degrees; that's between east-northeast and east.

Use a pair of binoculars to scan the sky along this region. The binoculars should be pre-focussed at infinity before your attempt. Do this on the Moon (if visible) or the stars the night before your try.

Keep look out from approximately half an hour before sunrise - for me on the south coast in the middle of May, that's from about 04:40 BST onwards. The planets can be seen right up to sunrise and into daytime but you should take great care when using binoculars with the Sun up. In fact, unless you're an experienced binocular user, I'd recommend stopping as soon as the Sun pops up for safety.

Expect to see something faint. Actually, through my 15x70 binoculars, Venus looks quite bright - a lovely pinpoint of white against the bright pale blue twilight sky. Jupiter on the other hand looks quite ghostly and rather lost. This is do with the fact that Jupiter currently has an apparent diameter 3x that of Venus.

The other planets will be a bonus if you can catch them with Mercury getting brighter toward the end of the month. As the timing of dawn changes quickly at this time of year, you'll have to venture out approximately half an hour earlier by the end of May. As a reward, see if you can catch the thin crescent Moon low down above Venus on May 31st.

The show's definitely not over yet - in fact for us in the UK, it's only just beginning!

As ever, full details can be found in this month's magazine Skyguide.  

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