Astronomy Know How
Helping you See the Night Sky - Newsletter No. 19 May 2008
In this issue:
This month sees the return of the annual Eta Aqaurid Meteor Shower. These meteors are part of the dust from the tail of Halley's comet, probably the most talked about comet in history! THe shower reaches its peak on the 5th of the month, but these meteors can be detected from the 24th of April until the 20th of May. The ZHR (that's the Zenithal Hourly Rate) is usually around 40 but can be as much as 70. The ZHR iof a meteor shower is figured to be the expected number of meteors that can be seen under perfect conditions and with the radiant point directly overhead. So you can see that it may not be a good indicator of how many meteors that you can really see in the sky. But don't forget that this ZHR number is only a guide and in reality you can sometimes see more!
The radiant point of a meteor shower is where in the sky the meteors seem to appear from. So if during May you can trace the meteor(s) that you see back to a point in the sky in the constellation of Aquarius, then you have almost certainly seen a Eta Aquarid.
Mercury is well on show this month, but may be difficult to find - See Below.
Mars is still visible, although it is now quite a long way from Earth, meaning that the disc is now really quite small and so it is difficult to make out any features on the planet even with a large telescope.
Saturn is still a lovely sight in the constellation of Leo the Lion not far from the bright star Regulus at the bottom of the 'sickle' asterism marking the head of the Lion. The rings are now closing up from our perspective here on Earth and so it is harder to see such things as the Cassini division in the ring system, but not impossible. The shadow of the planet on the rings is still notable and at high power through a telescope it should be possible to make out a belt of subtle colour that is a cloud belt around the planet itself, not dissimilar to the cloud belts of Jupiter, but much less obvious.
Jupiter is now visible in the early hours of the morning but very low in the south for observers in mid-northern latitudes. We are going to have to wait a while before the king of planets puts on a good show for us.
May will see Mercury at its greatest eastern elongation, that is its furthest away point from the Sun from our point of view in the evening from mid-northern latitudes. It is at this point on the 14th of the month, although it is at its brightest earlier on. However it will be to close to the Sun to be properly see (NB Caution Below!), so although it will be dimmer at mid-month it will be easier to find.
It is best to use binoculars to try and find the planet but ALWAYS wait for the Sun to set fully before sweeping for Mercury with ANY optical aid. Once you have found it in binoculars you should then be able to pick it up with your naked eyes and if you have a telescope, do take a look as you should see a distinct phase, a little like looking at a distant Moon... In fact on the 6th it will be just to the west of a very thin crescent Moon, so this might help you track down this most elusive of the planets of our solar system.
What's the 'Buzz' on Mars?
Due to the relationship of the orbit of Mars to the orbit of our Earth, Mars appears to be moving eastward against the background stars form our point of view. This month it passes through the loose cluster of stars known as the 'Beehive' or M44 in Messier's catalogue. The best time to observe this is at around 10pm local time from the 21st to the 24th of the month. The Beehive cluster is in the constellation of Cancer the Crab, which is sinking low in the west now by late evening, but it should not be difficult to find Mars and of course the cluster in binoculars.
If you would like to try some day time astronomy, then you'll get a chance to see Mars on the 10th along with a waxing crescent Moon. What makes this really interesting is that Mars will be occulted by the Moon around this time and will be seen to disappear and re-appear at different times depending on your latitude. Try taking a look due east at around 12:00UT (same as GMT) although you will definitely need a telescope to observe this event. You'll find the Moon (and Mars!) about 25-degrees above the eastern horizon.
Deep Sky Delights
If you have a telescope and like looking at faint far away 'fuzzy' objects, then the spring is certainly good time of year for you. Mid evening finds the constellation of Virgo well placed in the south and therefore the Virgo group of galaxies. In fact you can find galaxies galore in a broad band up from the eastern side of Virgo through the constellations of Coma Berenices and Leo and on up to Canes Venatici and Ursa Major.
Other deep-sky wonders include the 'Cat's Eye nebula that can be found almost overhead in the constellation of Draco the Dragon, about a third of the way between the 'circlet' asterism in Draco and Polaris the Pole Star. You'll need a 6-inch or 150mm telescope to see it properly and at low power it will look like a small greenish patch of light. At higher powers it will elongate and a larger telescope will start to show a central 'core'.
I don't usually talk about variable stars, but if you are interested in seeing a fascinating example, then now is a good time to try to find R Coronae Borealis. It lies more or less in a mid point of the beautiful semi- circle of stars that is the constellation. This star will shine quite happily at magnitude +5.7 and then rapidly fade from view without warning. It can dim down to magnitude +14.8 a hard star to spot even in a large aperture telescope. It can sty this dim for many months at a time. The reason we think, is that the star pushes out lots of carbon into it vicinity which condenses and blocks the star's light. After a while the cloud of carbon thins out and the star resumes its normal brightness.
If you like finding globular clusters, then there are quite a few on show this month although some of them are not particularly bright and can give you an observing challenge. There are a couple of interesting ones in Bootes the Herdsman, the 'kite' shaped constellation to the east of the Plough asterism in the great bear. M3 or NGC5272 lies between the edge of Coma Berenices and Bootes and is a 6th magnitude cluster so it should be available in a pair of binoculars as a misty patch of stars and is well defined in a small telescope. 5-degrees to the east of M3 is NGC5466 a much fainter (magnitude 9) globular cluster that is much more of a challenge in binoculars. In a telescope, it will appear as a faint, small and compact cluster, an interesting contrast to its brighter neighbour.
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