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## The sky in June (2016)

The highlights in the sky this June are an opposition of Saturn on the 3 June and Mercury visible in the morning sky.

## The Sun

As most of you know, June is the month when we have the longest day of the year, the Summer Solstice. This is when the Sun reaches as far north in the sky that it can go, and on this day it is overhead as seen from the tropic of Cancer. This year the solstice falls on the 20 June, and in Wales the days are quite long. On 20 June the Sun rises in Cardiff at about 4:55am and sets at about 9:30pm. This means that there is about sixteen and a half hours between sunrise and sunset. Compare this to the shortest day (the winter solstice), when there is only about eight hours between sunrise and sunset; the days are more than twice as long in late June compared to late December!

Although this means that the nights are at their shortest, June is still a good month to look at the night-time sky as there are several interesting things to see.

## The Moon

In June the new Moon is on the 5th of the month, and the full Moon on the 20th. So, seeing the night-time sky in the days around the full Moon can be challenging, particularly trying to see any faint objects. On Saturday 11 June there is a conjunction between the Moon and Jupiter. A conjunction is when two bodies appear close together in the sky, and on this night they will be separated by 1.25 degrees from each other. For comparison, a full moon is 0.5 degrees across. They will become visible in south Wales by about 10pm when it is dark enough, over towards the west as they head towards setting at just after 1:30am.

## Mercury

On 5 June Mercury will appear as far to the west of the Sun as it can get. Some of you may have been aware that on 9 May Mercury passed across the disk of the Sun, known as a transit. Less than a month after that, Mercury has moved in the sky as it orbits the Sun so that it is now visible before sunrise. On 5 June it will rise at 4:13am in Cardiff, with the Sun rising at 4:57, giving you just over half an hour to catch a glimpse of this elusive planet. On 19 June Mercury will rise at 3:58 in Cardiff, and the Sun at 4:54, giving you nearly an hour to see it. Although Mercury is fairly bright in June, it is still very tough to see it as it will be low near the eastern horizon and the dawn light will quickly make the sky too light to see it. But, it is worth a go!

Mercury is visible before dawn in June. It reaches greatest western elongation on 5 June. By 19 June it will rise just under an hour before the Sun, so it is possible to catch a glimpse of it before the pre-dawn light gets too strong.

## Saturn

On 3 June Saturn will be at opposition, this means we will be passing it as we and it orbit the Sun. On 22 May Mars was at opposition, and has become considerably brighter in the sky over the last several weeks. This does not happen with Saturn, it is so much further away than Mars that it doesn’t really get any brighter as we pass it. But, it is certainly easily visible, and you can use Mars to find it. If you imagine a clock face, Saturn is about about 8 o’clock from Mars on 3 June. Saturn will be visible over the next few months. If you get the chance to look through a small telescope you will easily be able to see Saturn’s rings; this is one of the highlights of the Solar System so if you get the chance it is well worth it.

When you look at Saturn through a small telescope you should see something like this. You may also spot a dot of light near Saturn, this is probably its largest moon Titan, the only moon we know of in the Solar System with a substantial atmosphere. The rings are tiny particles of icy rock, tens of thousands of them in orbit about the planet. The origin of the rings remains a topic of scientific debate, but what you may not know is that Jupiter, Uranus and Neptune also have ring systems, although they are much fainter than Saturn’s.

Saturn as it might appear through a small telescope. The rings are easily visible through even a very modest telescope.

Saturn as imaged by the Voyager 2 space probe which flew past it in January 1981.

## Deep Sky Objects

One of the best deep sky objects (objects outside of our Solar System) to look for in June is Messier 13, one of the best globular clusters in the sky. A globular cluster is a huge collection of hundreds of thousands of stars which orbit the centre of our Milky Way galaxy. One of the most intriguing things about globular clusters is that they only contain old stars, and we believe that they were amongst the first structures to form when our Galaxy formed.

The globular cluster Messier 13 in the constellation Hercules (image credit Rainer Zmaritsch)

Messier 13 is in the constellation Hercules, and can be found by using the bright star Vega. If you imagine a clock face, Messier 13 (shown as “Hercules Cluster” in this image) is at about 2 o’clock from Vega. The constellation Hercules is also relatively easy to see, it has six bright stars which actually form a letter “H”, very appropriate that it should be called Hercules!

Where to find Messier 13 (the Hercules Cluster), a visually stunning globular cluster in the constellation Hercules. Hercules is an easy to find constellation, it has 6 relatively bright stars, and lies at about 2 o’clock from Vega. The 6 stars actually make the shape of a letter ‘H’.

## The planets this winter (2015/16)

With winter (in the Northern Hemisphere) approaching, I thought it was about time I gave a summary of which planets are visible over the next few months. The longer nights, enabling easier viewing of the night-time sky, is one of the few pluses about this time of year as far as I am concerned. So, which planets are visible this winter (2015/16)?

The times I will give for various planets rising or setting are for Cardiff, where I live. So, if you are living elsewhere the times will almost certainly be different. Obviously, if you are living in the southern hemisphere you are about to move into summer not winter. But, although times may vary depending on your location; whether a planet is visible or not, and whether it is visible in the evening after sunset or in the morning before sunrise will not be different.

Of the 5 naked-eye planets, all but Saturn are visible this winter. Here is more detail about each.

## Mercury

Mercury is currently in Sagittarius, rising before the Sun and thus setting after the Sun. So, it is currently an evening object. It reaches maximum elongation on the 29 December when it will be $25^{\circ}$ to the East of the Sun, and on this day it will set in Cardiff at 17:43. The Sun sets on this day at 16:11 in Cardiff, giving some 1.5 hours after sunset to see Mercury. Although these setting times will vary depending on your location, what will not vary is the time between sunset and Mercury setting, which will be about 1.5 hours no matter where you live.

1.5 hours between sunset and Mercury setting it very good. Mercury is rarely this far from the Sun; so for those of you who have never seen Mercury, this month of December provides a very good chance. Find a view to the western horizon which is uninterrupted and away from city lights, and use the chart below to find Mercury. It will be reasonably bright, at a magnitude of -0.5.

Mercury just after sunset as seen from Cardiff on 29 December 2015. This month is a good month to see Mercury, as its maximum eastern elongation (the maximum angle between it and the Sun) is nearly as large as it can ever be. There are no bright stars near Mercury at the end of December.

Mercury will reach inferior conjunction on 14 January, whereupon it will reappear as a morning object later in January and February.

## Venus

Venus is currently in Libra. It is a morning object, very bright before sunrise. At a magnitude of -4.1 you cannot fail to see it. It will reach maximum western elongation on 12 January. You can see it in the diagram below of the sky before sunrise, which also shows where to find Mars and Jupiter. Venus will be visible as a morning object throughout this winter and into the spring.

## Mars

Mars is currently in Virgo. It is rising at the end of December just after 2am, so is a morning object. In fact, it can be seen in the morning sky along with Venus and Jupiter throughout much of the winter, as the diagram below shows. At the end of December it has a magnitude of +1.3, fainter than nearby Spica, which is at +1.05. Mars will reach opposition on 22 May, by which time it will have brightened to -2.1, so some 23 times brighter; making April, May and June by far the best time to see this planet.

The morning sky at the end of December as seen from Cardiff. Venus, Mars and Jupiter are all visible in the morning sky this winter. Jupiter and Venus are easy to find as they are so bright. Mars is a little trickier, but will brighten as it approaches opposition in the spring

## Jupiter

Jupiter is in Leo, and is also currently a morning object. At the end of December it rises just before 11pm. It will be at opposition in early March (8 March), and so in late winter and spring it will be an evening object, but for most of this winter it is better seen in the morning before sunrise.

I like it when one can see Jupiter and Venus at the same time, as it allows one to see how much brighter Venus is than Jupiter. Normally Jupiter is the brightest point-like object in the sky, but when Venus is visible it outshines Jupiter by a factor of 6 or so.

## Saturn

Saturn is currently in Ophiuchus, and this winter is not the time to see Saturn. It will reach opposition in early June (3 June), so spring and summer are the best times to see Saturn in 2016 and over the next few years. It will not become a winter object again for another 14 years or so.

## The sky in June (2015)

In two days’ time, Venus will be as far to the East of the Sun in the sky as it can be (what is called “maximum Eastern elongation”), so I thought I would take that event as an excuse to summarise what is happening in the sky this month of June.

For anyone who has been paying even the most cursory attention to the evening sky over the last several months, Venus has been almost impossible to miss as it has dominated the Western sky after sunset. The only objects which can outshine Venus in the sky are the Moon and the Sun; so when Venus is visible it is the first object to appear as the sky darkens after sunset, or the last object to disappear as the sky lightens before dawn. For the last several months, Venus has been shining with a magnitude of about $-4$ (it varies because its distance from the Earth is changing and also its phase is changing). This is some eleven times brighter than Sirius, the brightest star in the sky (which has a magnitude of $-1.44$). It has even outshone Jupiter, the object which has dominated the later evening sky throughout most of the last several months, by a factor of about six.

This Saturday (the 6th), it will get as far to the East of the Sun as it can get as seen from Earth, and when it is at greatest Eastern elongation, the angle between looking towards the Sun and towards Venus is about $45^{\circ}$. This means that Venus will set some three hours after the Sun. For those who wish to be precise, this particular maximum eastern elongation will occur at 19:13 UT on the 6th, and the angle between the Sun and Venus will be $45^{\circ} 24^{\prime}$.

The figure below shows the orbit of Venus (in red) and that of the Earth (in blue). Both planets orbit the Sun in an anti-clockwise direction if one were to look down on the Solar System from above the Earth’s North pole (which is the convention used, sorry Southern Hemisphere people!). Venus is currently heading towards inferior conjunction (the time when Venus and the Sun lie in a straight line as seen from Earth, and so it is not visible). When it is heading towards inferior conjuction it is to the East of the Sun, and hence sets after the Sun and is seen as an “evening star”. The upcoming inferior conjunction happens on the 15th of August, so just a couple of months away.

I diagram of Venus’ and Earth’s orbits. Venus’ orbit is shown in red, the Earth’s orbit in blue. Currently, Venus is to the East of the Sun, and will reach maximum Eastern elongation on Saturday (the 6th), at 19:23 (UT)

After passing inferior conjunction, Venus will lie to the West of the Sun as seen from Earth, and so will slowly re-appear as a “morning star”, but you will have to wait for a few weeks after inferior conjunction for this, as initially it will be too close to the Sun and so lost in the glow of dawn.

However, long before it reaches inferior conjunction, there is a celestial highlight to look out for, which happens towards the end of June. As June progresses, Venus and Jupiter will appear to get closer and closer together in the sky, and by month’s end there will be a spectacular conjunction of the two brightest planets, something not to be missed. The two diagrams below show Venus and Jupiter on the evening of maximum Eastern elongation (the 6th), and then again at the end of the month.

Venus and Jupiter at 21:30 BST (20:30 UT) as seen from London on the 6th of June. On this evening, Venus will be at “maximum Eastern elongation”.

Venus and Jupiter as seen at 21:30 BST (20:30 UT) from London on the 28th of June. The two planets will get even closer over the following few nights, producing a spectacular conjunction of the two brightest planets.

The other planet worth looking out for this month is Saturn. Saturn is currently in Libra, but moving into Scorpio. In the middle of June it will be rising at just before 7pm and transiting at just before 11:30pm, so this month is a good time to see it.

Saturn in the middle of June, at 21:30 BST (20:30 UT) as seen from London. Saturn is currently in Libra, heading into Scorpio.

Saturn is not particularly bright at the moment, but you can use the bright stars Antares (in Scorpio) and Spica (in Virgo) to find it; just look at the diagram above.

The other highlight of June is, of course, the summer solstice (or winter solstice to people in the Southern Hemisphere). This is, of course, the longest day of the year for people living in the Northern Hemisphere, the moment when the Sun reaches its most northernly point in the sky. This year’s solstice will happen on the 21st of June at 16:38 UT, so at that moment the sky will be directly overhead for a person at the correct longitude on the Tropic of Cancer (for anyone on the Tropic of Cancer the Sun will effectively cast no shadow at midday on that day). Here is south Wales, the days around the Summer Solstice are really long, with the Sun rising at about 4:30am and not setting until nearly 9:30pm. It is my favourite time of the year!

## An exoplanet with a ring system found

Last week I came across this interesting story on the BBC website – an exoplanet which has been found to have a large ring system – the first such discovery to date. Saturn, of course, has the most famous ring system of any of the planets in our Solar System. They were visible to Galileo in 1610 when he first looked at Saturn, but he was not able to discern them as rings, this was first done by Dutch astronomer Christian Huygens, who also discovered Saturn’s largest moon Titan.

However, in fact Jupiter, Uranus and Neptune (the other three “gas giants”) also have ring systems, although they are much fainter and less extensive than Saturn’s. Uranus’ ring system was discovered by SOFIA’s predecessor, the Kuiper Airborne Observatory, in 1977. Jupiter’s ring system was the next to be discovered, by the Voyager 1 space probe in 1979. Neptune’s ring system was discovered in 1984 using a ground-based telescope at La Silla Observatory (the European Southern Observatory), and were later imaged by Voyager 2 in 1989.

The ring system discovered about this exoplanet has been found by a survey known as SuperWASP, which is a ground-based survey looking for exoplanets using the transit method (which I discussed here). The exoplanet is orbiting a star by the name of J1407, which lies about 420 light years from Earth. This star had been found to have a peculiar light curve, not fitting the light curve one sees when a “normal planet” transits it. Further analysis by the authors of this work, led by Dr. Kenworthy at Leiden Observatory in the Netherlands, suggests that the complexity in the light curve is due to the transiting planet having a ring system. Here is a link to the abstract of the team’s paper on the arXiv preprint website.

This is the first ever discovery of a ring system about an exoplanet.

The ring system is measured to be about 200 times larger than that around Saturn. Such a ring system would appear to be so large that, if Saturn’s ring system (which is impressive in its own right) were replaced by this one then the rings would be clearly visible from Earth, as this artist’s sketch attempts to show.

An artist’s impression of how the ring system would look if Saturn had such large rings. It would be many times larger than the Moon in angular size, and easily visible from Earth.

## Birth of a new Saturn moon?

This story caught my attention back in April, the possible creation of a new Saturn moon on the outskirts of its famous ring system (I’m not sure why it has taken me so long to blog about it, probably because in April I was working frantically finishing up my book on the Cosmic Microwave Background). The particles in Saturn’s rings are basically small lumps of rocky water-ice, and so are pretty good at sticking together if they collide. What we think is happening is that this little moon is being created by the collisions of the particles in the ring system, which is slowly building up a new moon. Quite spectacular really, to see the possible birth of a new moon!

We don’t fully know the origin of Saturn’s rings, there are two competing theories. One is that a larger moon which would have formed at the same time as Saturn (i.e. 4.6 billion years ago) and at some time in the past came too close to Saturn and was torn apart by Saturn’s gravitational tidal forces. The point at which an extended body will be torn apart by tidal forces is called the Roche limit, and once an object is closer than this the gravitational forces on e.g. the near and the far side are too different and the object gets torn apart. The size of Roche limit depends on the planet (so would be different for the Earth and Saturn), but also on the size of the object. So, the Roche limit for a larger moon will be different (at a larger distance) than for a smaller object.

The other theory is that they formed from debris left over from Saturn’s formation, just as the asteroid belt is debris left over from the formation of the Solar System. The problem with this theory is that the rings are thought to be just a few hundred million years old, whereas if they are left over debris we would expect them to be as old as Saturn. As of writing this, we are not sure which of these two competing theories is correct. Or, maybe there is another explanation entirely!

Galileo looked at Saturn through his telescope back in 1610, but was not able to make out the rings as such. He saw that Saturn had what looked like “ears”, but it was the Dutch astronomer Christian Huygens who was the first to describe them as rings. They are a complex system, and it would take several blogs to describe them, but suffice it to say that in the time that we have been sending space probes past or to Saturn (e.g. Voyager, Pioneer, and now Cassini), we have been learning more and more about how complex a system the rings are.

Saturn is not the only planet with a ring system, it may surprise you to hear that Jupiter, Uranus and Neptune also have ring systems, but none is as spectacular as Saturn’s. Here is an amazing photograph taken by Cassini of Saturn and her ring system, looking back towards the Sun. It also shows several of Saturn’s moons, Mars, Venus and even the Earth!!

This amazing photograph, taken by NASA’s Cassini space probe, shows the rings back-lit by the Sun. The photograph also shows Mars, Venus and the Earth!

The Cassini probe has been an overwhelming success. Launched in 1997, it arrived at Saturn in 2004, and some of you may remember it sent a little probe called Huygens which landed on the surface of Titan in January 2005. For the last ten years it has been orbiting Saturn and some of its moons, and has made numerous discoveries in doing so. It is scheduled to remain in operation until 2017. Here are some remarkable images from the probe, together with a model of Enceladus based on the observations that Cassini has made of it.

A Cassini image of Enceladus, showing plumes of water coming from its pole.

One of the surprises of the Cassini mission has been the moon Enceladus, which shows geysers of water shooting out from its poles. Because of the presence of water, Enceladus has become a prime candidate to look for life beyond our Earth.

A Cassini image showing the rings nearly edge-on, with the moon Dione in the background.

## The planets this summer (2013)

There are not too many planets visible this summer. Venus will be visible very low in the Western sky in the evening in early summer. Jupiter will be visible as the Sun is setting, also over towards the West. But really the only planet visible when night has fallen this summer is Saturn, which is currently in the constellation Libra.

Where to find Saturn this summer (2013). It is in the constellation Libra; to the South of Arcturus, and to the East of Spica.

The image above shows where Saturn is to be found at about 21:45. The easiest way to find it is to find the bright star Arcturus (the second brightest star in the summer sky after Vega), and to the South of it is Saturn. You can also check that Spica, the brightest star in the constellation Virgo, is to the West of it.

Throughout the summer, Saturn will have a magnitude of about +1.1, which makes it roughly the same brightness as Spica and about 2.5 times fainter than Arcturus. Even with the naked eye you should be able to see the colour difference between Spica and Saturn. Whereas Spica is a bluish-white, Saturn is a distinctly brown colour.

In mid-July, Saturn will be rising at about 14:45, transiting at about 19:50 and setting at about 01:00. So it will be at its highest in the night-time sky a little before sunset in northern latitudes.

Although Saturn is easily visible to the naked eye, it is well worth looking at through a small telescope, as when one does so the famous rings become visible. If the atmosphere is particularly stable, you should be able to make out the Cassini division in the rings. You may also be able to spot Titan, Saturn’s largest moon, which is indicated in the photograph below.

Saturn as seen through a small telescope, with its largest moon Titan visible to the right.

Titan is a fascinating moon, and one which has become quite well studied by the spacecraft Cassini and the space probe Huygens, which plunged through its atmosphere in January 2005 and landed on its surface. It is thought to be one of the most likely places beyond the Earth in our Solar System to harbour life. I will do a blog about Titan and what we so far know about it in the near future.

## Cardiff Castle Astronomical (and Astrological) Statues

On the clock tower of Cardiff Castle is a wonderful collection of Astronomical (or astrological) figures. Here is a little photo gallery of the figures. I will do a separate blog about Cardiff Castle and its history in the near future. Although the castle dates back to Roman times, most of what one sees these days was built by the Third Marquess of Bute in the late 1800s, with the clock tower itself being built in 1868.

Mars and the Sun on the clock tower of Cardiff Castle.

The clock tower shows statues of figures representing the Sun, the Moon, and all 5 “naked eye” planets: Mercury, Venus, Mars, Jupiter and Saturn. In a separate blog I will show photographs of the sumptuous interior of the castle, including a room in the clock tower which has a star-painted ceiling and many astronomical motifs.

## The planets this winter (2012/13)

As the nights are now getting longer (for those of us in the Northern Hemisphere) and the clocks are about to go back making sunset earlier, I thought it was about time I blogged about the planets which will be visible this winter. Of the 5 naked eye planets, Mercury, Venus, Mars, Jupiter and Saturn, 4 of them are visible this winter. These are Mercury, Venus, Jupiter and Saturn. Mars, unfortunately, will not be visible at all this winter.

Here is a table showing the rise, transit and setting times of the Sun and the 5 naked eye planets (the transit time is the time at which the object crosses the local meridian, an imaginary line going from due North to due South across the sky).

The Planets in mid-October
Planet Rise Time Transit Time Set Time Constellation
The Sun 07:44 12:58 18:11 Virgo
Mercury 10:11 14:24 18:40 Libra
Venus 04:09 10:41 17:12 Leo
Mars 11:50 15:47 19:44 Ophiuchus
Jupiter 20:12 04:20 12:25 Taurus
Saturn 08:08 13:20 18:33 Virgo

The Planets in mid-January
Planet Rise Time Transit Time Set Time Constellation
The Sun 08:12 12:22 16:34 Sagittarius
Mercury 08:22 12:16 16:11 Sagittarius
Venus 07:12 11:07 15:02 Sagittarius
Mars 09:15 13:48 18:22 Capricorn
Jupiter 12:54 20:51 04:52 Taurus
Saturn 02:11 07:09 12:06 Libra

The Planets in mid-March
Planet Rise Time Transit Time Set Time Constellation
The Sun 06:27 12:22 18:18 Pisces
Mercury 05:45 11:08 16:31 Aquarius
Venus 06:32 12:12 17:53 Aquarius
Mars 06:46 12:50 18:56 Pisces
Jupiter 09:09 17:10 01:15 Taurus
Saturn 22:17 03:19 08:17 Libra

## Mercury

Mercury, the closest planet to the Sun, is the most difficult to see. Because it is close to the Sun it can never be seen in the middle of the night, it is only visible near sunset or sunrise. Over the next 5 months there are two opportunities to see Mercury, the first one being in early December. As the figure below shows, Mercury will be visible in the morning sky before Sunrise in December, with early December being the best time to see it, as its elevation above the eastern horizon will be greatest, about 10 degrees. The Earth turns 15 degrees each hour, so 10 degrees represents about 40 minutes before the Sun rises.

This figure shows the position of the planet Mercury in the morning sky between mid-November and the end of December. The y-axis is the elevation above the horizon, the x-axis is azimuth, where due East is 90 degrees, and due South is 180 degrees. The azimuth angles in this figure show the planet will be visible in the South-East. As this figure shows, Mercury’s maximum elevation is in early December, when it is about 10 degrees. This means it will be visible some 40 minutes before Sunrise.

The next opportunity to see Mercury is in mid-February, in the evening sky. As the figure below shows, on 17th/18th of February, Mercury will be about 10 degrees above the horizon in the Western sky after sunset.

This figure shows the visibility of Mercury in the morning sky between the dates of early February and the end of February. The y-axis shows elevation in degrees above the horizon. The x-axis is azimuth. Due West is 270 degrees, and due South is 180 degrees. The azimuth angles in this figure show the planet will be visible in the South-West.

## Venus

Venus is currently (mid-October) easily visible in the Eastern sky before sunrise. The figure below shows where to find it. You cannot really fail to find Venus when it is visible, it is by far the brightest “star like” object in the sky, outshining any of the stars (and even Jupiter) by a factor of at least two. At the moment, with Venus and Sirius visible in the same part of the sky before sunrise, one can see quite clearly how much brighter Venus is than the brightest star in the sky. Over the next few months it will get lower and lower before Dawn, until by the end of December it will disapppear and will not be visible again this winter.

Where to find Venus in the pre-dawn sky. It is to the East of the constellation Orion and Sirius, the brightest star. Seeing Venus and Sirius at the same time shows clearly how much brighter Venus is than Sirius.

Venus in the morning sky from mid-October to mid-December. Over the next 2 months it will sink lower and lower, and by the end of December will not be observable for the rest of the winter. The y-axis shows elevation above the horizon in degrees. The x-axis shows azimuth, with due East being 90 degrees and due South being 180 degrees. Venus is visible in the South-East sky before sunrise at the moment.

## Mars

Mars is not visible over the next 5 months. It will next be at opposition in April 2014, so will begin to become visible towards the end of 2013.

## Jupiter

Jupiter is currently in the constellation Taurus. It is currently easily visible high in the early morning sky before sunrise. The figure below shows where to expect to see it at 1am in mid-October. It is currently near Aldebaran, but outshines this red giant star by a factor of over 30 (the visual magnitude of Jupiter at the moment is -2.7 and of Aldebaran is +1). It is the brightest object in that part of the sky, with only Venus in the pre-dawn sky outshining it. If you wait until January, Jupiter will be visible in the evening sky rather than after midnight. A small telescope will reveal Jupiter’s 4 Galilean satellites, Io, Europa, Ganymede and Callisto.

An image of Jupiter taken by the Cassini space probe.

A map showing Jupiter in the sky at 1am as seen in mid-October as seen from Cardiff.

## Saturn

Saturn is not currently visible, but will be as we move towards winter. By December it will be visible in the morning sky. It is currently in the constellation Libra, where it will remain for the next 2 years. By mid-January, Saturn will be quite visible in the early hours of the morning, but it will be even higher by mid-March. You might find it easier to wait until the Spring to see Saturn, as by April it will be quite high before midnight.

Saturn, the ringed planet. This image clearly shows the Cassini division in the rings.

## Humanity’s most distant travellers (part 1)

Next Thursday (27th of September), I am going to be on live TV talking about the Voyager space probes. These remarkable crafts left our small planet in 1977, and are now on the point of entering interstellar space. Voyager 1 is currently 18.2 billion km from the Sun, and Voyager 2 is 14.8 billion km away. They are, by a large margin, the most distant objects human beings have ever sent into space.

This cartoon, from the official NASA Voyager website, shows where the two spacecraft are, compared to the heliopause. The heliopause is a theoretical boundary where the Sun’s Solar wind is stopped by the stellar winds from other, nearby stars. The heliosphere is the sphere of space within which the Sun and the planets reside, as they are within the heliopause, the surface of this sphere (in reality it is not a sphere, it would only be a sphere if the radiation from nearby stars was perfectly uniformly incident on our Solar system).

This cartoon, from the NASA Voyager website, shows the position of the two Voyager space probes, and the heliosphere and heliopause.

Because these distances are so huge, it is sometimes easier to use larger units. The Astronomical Unit (AU) is defined as the average distance from the Earth to the Sun, so 150 million km. In these units, Voyager 1 is 122 AU from the Sun, and Voyager 2 is 99 AU away. The weak signals that we are still able to receive from the two space craft travel at the speed of light, and are currently taking about 30 hours (not minutes as I previously typed) to get to us.

The Voyager space probes were identical copies of each other, but were launched a few weeks apart and went on a different journey into the outer Solar System

## Voyager 1 and 2 at Jupiter

Voyager 1 arrived at Jupiter in January 1979. Voyager 2 reached the planet in July of the same year. Both space craft returned the most detailed pictures yet of the Solar System’s largest planet. In addition to making important studies of the great red spot, the two probes made the surprising discovery of volcanic activity on Io, Jupiter’s closest moon.

A Voyager 1 image of Jupiter’s great red spot

The plume of material on the left is a volcanic eruption on the moon Io, the nearest of the Galilean moons.

We now know that Io is the most volcanically active body in the Solar System. The source of its internal heating is the tidal forces from Jupiter. Because it doesn’t orbit Jupiter in a perfect circle, but rather in an ellipse, the Moon gets repeatedly deformed in different directions and this heats its interior up (in the same way that squeezing a tennis ball repeatedly will lead to its getting warm). With a warm molten interior, the conditions are just right for this to escape through the crust as volcanic eruptions.

## Voyager at Saturn

In November 1980, Voyager 1 flew past Saturn. By August 1981, Voyager 2 had arrived at the ringed planet. Voyager made important discoveries about Saturn and her moons, in particular about Saturn’s rings. It discovered new ring structures, and even “spokes” in the rings.

A backlit image of Saturn taken by Voyager 1 after its flyby in late 1980.

Voyager 2 discovered mysterious spokes in Saturn’s rings. It was many years before we understood what causes these.

Later this week I will write a part 2 to this blog, talking about Voyager 2’s encounters with Uranus and Neptune, the famous pale blue dot photograph, and the messages being carried on the probes as they head off into interstellar space.

Here are part 2 and part 3 of this post.

## The planets this summer (2012)

Back in November (2011), I wrote a blog on the planets which would be visible over the winter months. I thought it was about time, being over a week into the official summer, that I wrote a blog about the planets visible over the summer months this summer (2012). Unfortunately, there aren’t many planets visible this summer, Saturn and Mars is your lot.

This summer, Mars is to the Western side of the constellation Virgo, and is transiting at the moment (in early July) at 18:24. This means that, by the time it gets dark, which in Wales is not before 21:30 this time of year, Mars is quite far over to the West and on its way down in the sky. On the 3rd of March, Mars was at opposition, which means the Earth was at its closest to it. As a consequence, not only is Mars quite low (25 degrees above the horizon) by the time it gets dark, but it is also not very close to us. These two things combined mean Mars will be quite an unspectacular sight through a telescope.

Mars through a small telescope. If you are very lucky, you may see signs of the polar caps.

The other planet visible this summer is Saturn. Saturn is transiting at the moment (early July) at 19:55, so is reasonably high (30 degrees) in the sky after it has got dark. It is also to be found in the constellation Virgo, but over towards the constellation’s Eastern end, just to the North of the constellation’s brightest star Spica.

Saturn and Titan through a small telescope. Even with quite a small telescope, you should be able to see the rings and Titan quite easily.

Seeing Saturn for myself never ceases to excite me. Even through quite a small telescope one can clearly see the rings, and usually Saturn’s brightest moon Titan. If you want to see either Mars or Saturn this summer, then you really need to do so over the next few weeks, as by August they really will be setting too early to be able to see at all.

Although there aren’t too many planetary highlights this summer, there is still a lot to see in the Summer sky. One of the easiest things to find is the summer triangle, which is an asterism made up of Vega, Deneb and Altair (the brightest stars in the constellations Lyra, Cygnus and Acquila respectively).

The Summer Triangle, which is made up of the stars Vega, Deneb and Altair.

One of the other hightlights of the summer sky is the Ring Nebula, Messier 57. It is, in fact, what is called a Planetary Nebula. These are nothing to do with planets, but are in fact dying stars. Their name comes from the fact that, through 17th Century telescopes, they resembled the gas giant planets Jupiter and Saturn.

A planetary nebula is an object where the central star has thrown off its outer layers, and the remaining core (which we call a White Dwarf), is the remains of the once active star. The gases glow due to the electrons in the gas being excited by the energetic ultra violet light coming from the white dwarf. The white dwarf at the centre of the Ring Nebula is quite clearly visible through a medium-sized telescope.

The constellation Lyra (the harp), showing the location of Messier 57, the Ring Nebula

Messier 57, the Ring Nebula, one of the best planetary nebulae in the sky.

Our own Sun will end its life as a planetary nebula and white dwarf, as it is not massive enough to become e.g. a neutron star or a black hole. For a brief period (about 50,000 years), what hydrogen which the Sun will throw off during its asymptotic giant branch phase will glow in the sky, before fading from view as the white dwarf remains of the Sun slowly cools over time.

## Update

I am going to be on BBC radio this Friday (13th of July 2012) talking about the summer sky. In preparing for this interview I realised that Jupiter is, of course, visible in the morning sky. It is to be found in the constellation Taurus, which is itself an easy constellation to find with the bright star Aldebaran in it. Jupiter is currently (mid July) rising at 02:45, so over the next few months is actually the best planet to see, by mid-August it will be rising about 00:45 and my mid-September by about 22:45.

Jupiter is in Taurus at the moment, just to the north of the bright red star Aldebaran, and to the East of Capella, “the Shepherd’s star”, which is in the constellation Auriga.

Jupiter is well worth looking at in a telescope. As I commented in my blog about the 2011/12 Winter sky, one can nearly always see the Galilean moons of Jupiter through a small telescope, and if one is lucky one can also see the bands and the great red spot. So, if you are out looking at the sky over the summer, don’t forget to stay up late (or get up early) to catch a glimpse of Jupiter.

Venus is in the same constellation. It is only some 5 weeks ago that Venus transited the Sun, but already it has moved to the West of the Sun in the sky so that it is now rising before it. Venus will appear as a large crescent at the moment, as it is on the near side to us in its orbit.