The Night Sky in May, 2017

Transcription

1 The Night Sky in May, 2017 The dominating object in the sky this month is the planet Jupiter. It was at opposition on April 7 th when it was on the opposite side of the Earth from the Sun and was at its brightest. Although it is now slowly declining in brightness, it will continue to look stunning throughout May. The star chart below shows the sky in the south over Oxfordshire at 21:30 BST on May 7 th (note that during the Summer months all times will be given in BST). You can see that Jupiter has a close encounter with the Moon in the constellation of Virgo. You will get a good view of this event in binoculars. I have a pair of Celestron Skymaster 15x70 binoculars and they are superb for looking at the Moon. They give 15 times magnification and the diameter of the aperture is 70mm so they let in quite a lot of light. You can buy these from Amazon for about 65. Jupiter s moons will also be interesting to watch or image this month. The image below shows Jupiter at 23:00 on May 15 th. You can see that all four of the Galilean moons are aligned on the right of the planet in order of increasing distance from it. Nearest to Jupiter is Io, then Europa, Ganymede and Callisto. These moons were first observed by Galileo Galilei in January 1610! By March that year, he had realised that they were in orbit around Jupiter. This showed that not everything in the universe orbited the Earth and thus set him into conflict with the Church.

2 Later in May, Ganymede and Io will pass across the disc of the Jupiter. The transit of Ganymede starts at around 23:20 BST on May 27 th. The image on the left below shows Jupiter at 23:50 and you can see the shadow of Ganymede on the disc. The image on the right shows Jupiter at 01:20 BST on May 28 th. Now you can see the shadow of Io as well. The great red spot is also visible. You really need a small telescope to see these shadows but, if you have one and you enjoy watching these transits then there is another one on May 11 th and 12 th when Io and Europa will be passing across the disc. That event will start at 23:05 on the 11 th when Europa begins its transit. The shadows will be together on the disc between 02:58 and 03:04 BST on the 12 th. The moons of Jupiter are of great interest to NASA and ESA and there will no doubt be more missions to them. They are particularly interested in Europa which they believe has an ocean below its surface and they expect to find some form of life there. Saturn will make an appearance in the night sky this month. Unfortunately, it is very low on the south-eastern horizon as can be seen in the star chart below. This shows the sky at 23:45 on May 13 th. You can see that Saturn is just rising with the Moon in the constellation of Sagittarius. The image below shows what Saturn will look like at that time.

3 If you do have a flat horizon in the south-east and you have a pair of 15x70 binoculars then you may just be able to make out Saturn s rings (assuming clear, dark skies). If you have a telescope, then you will be able to get a beautiful view of the planet and its rings. Note that the rings are nicely tilted towards us. You may also be able to spot some of Saturn s moons. Our largest telescope is 12 inches in diameter and we have been able to see 7 of the moons with that. Saturn s moon Enceladus is of great interest also since there is thought to be an ocean beneath its surface in which life has evolved. This moon throws out huge jets of water thousands of kilometres into space. NASA has analysed this water using instruments on the Cassini spacecraft and has found the essential ingredients for life. Another event this month is the appearance of the Eta Aquariid meteor shower which occurs when the Earth passes through the dust left by Halley s comet. This will peak on the morning of May 6 th. But, as you can see below, the origin of these meteors (the radiant) is very low in the east. The shower is best viewed from the southern hemisphere. You can read more about it in the next section. Late spring is a good time to go hunting for globular clusters and so I would like to focus in on these this month. The star chart below shows the sky in the south-east at 23:00 on May 15 th. On the left you can see the constellation of Hercules which lies between the bright star Vega on the left and

4 Arcturus in the top right. To the right of Hercules is the small constellation of Serpens Caput which I have highlighted. In the star chart below I have zoomed in on this part of the sky. In the top left you can see the constellation of Hercules. The lopsided square in the centre of Hercules is known as the keystone and is a famous asterism. On the Arcturus side of the keystone you can see the Hercules cluster M13. In the bottom right you can see the globular cluster M5 in the constellation of Serpens Caput. M13 and M5 are both globular clusters listed in the catalogue published by the 18 th Century French astronomer Charles Messier. They are spectacular clusters of hundreds of thousands of stars tightly bound together by gravity This gives them their globular shape. This term is derived from the Latin globulus which means small sphere. The image below shows that globular clusters are situated in the halo of our galaxy outside the central disc and spiral arms. There are approximately 150 of them surrounding our galaxy. The units shown in the image are parsecs which is the unit used for very large distances. One parsec is approximately 3.26 light years. Globular clusters differ from open clusters where all the stars are born out of the same cloud of gas at the same time. Open

5 clusters are not so tightly bound together and the stars will slowly drift apart. These clusters are situated in the disc of our galaxy and not in the halo as globular clusters are.. M13 is seen in the image on the left below. It is the brightest and most famous globular in the northern hemisphere and is considered the best target for imagers or for a large telescope. It is 23,500 light years away and 150 light years in diameter. The cluster is thought to be about 13 billion years old, in other words almost as old as the universe! It contains approximately 300,000 stars many of which have evolved over time and become red giants. Although M13 is the most famous globular, many astronomers think that M5 is without question the finest globular cluster in the northern sky for small telescopes. A close-up image of this, taken by the Hubble telescope, is shown on the right below. It is about 25,000 light years away and 165 light years in diameter. It is 13 billion years old and contains about 100,000 to 500,000 stars If you have dark skies, M13 and M5 can just be seen with the naked eye. You can see them in binoculars as fuzzy blurs. But you really need a telescope to see them in their full glory. We have seen M13 in our 5inch telescope and it looked stunning. But you need a larger telescope to bring out the detail. A magnification of x is recommended. Now to the Southern Hemisphere!

6 What's Up in the Southern Hemisphere? The star chart below shows the sky over Sydney looking to the north at 23:00 on May 7 th. There you can see Jupiter close to the Moon. At the bottom right of the chart, in the north-east, you can see the constellation of Hercules with the globular cluster, M13, to the left of it. Above Hercules is the constellation of Serpens Caput which I have highlighted. To the left of this you can see the M5 globular cluster. Below this and to the left is the bright star Arcturus in the constellation of Boötes. This month is the time to see the Eta Aquariid meteor shower which occurs when the Earth passes through the dust left by Halley s comet. The shower will peak on the morning of May 6 th. The best time to see it is around 4am. It is best seen from the southern hemisphere since the meteors appear to come from a point in the sky called the radiant and this is in the constellation of Aquarius. This is low on the horizon for northern hemisphere observers. The star chart below shows the sky over Sydney in the east at 05:00 on May 6 th. Here you can see the constellation of Aquarius at the top of the chart well above the horizon. Below it you have a wonderful view of Venus. Just above that to the left is the head of Pisces the Fish. This is an asterism known as the circlet. To the left and just below this you can see the Great Square of Pegasus rising. You should get a good view of the meteor shower in the early morning if you have clear skies.

7 However, the sight that most northern hemisphere observers will envy occurs in the middle of May once the Moon is out of the way. If you look in the east around 22:00 on May 20 th you will enjoy the sight of Saturn rising in the magnificent constellation of Sagittarius with all it Messier objects. This is shown in the star chart below. But, as if that s not enough, if you look in the south at the same time, you will see Jupiter with the Southern Cross to the left of it. Jupiter can be seen on the right of the chart below with the southern cross on the left. Just off the right of the chart you can see some of the Messier objects which can be found near the constellation of Coma Berenices often referred to as the Realm of Galaxies. That is all for now. Until next month, happy stargazing and dark skies! Valerie Calderbank FRAS