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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.

## Crash landing on Mercury

A few weeks ago, NASA’s Mercury MESSENGER space probe crash landed on the surface of the planet. This was not a mistake, scientists had deliberately sent it hurtling towards the surface of mysterious Mercury. It brought to an end a highly successful mission to learn more about the smallest planet in the inner solar system.

NASA’s Mercury MESSENGER space probe crashed into the surface of the planet on the 30th of April

MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) was launched by NASA in August of 2004 and arrived at Mercury in April 2011. You might be wondering why it took so long to get to Mercury, which is much closer to us than e.g. Jupiter. The reason is that the space probe could not fly directly to Mercury, otherwise it would have just whizzed straight past. Instead it had to go on a circuitous route so that when it arrived at Mercury it was moving slowly enough to be able to go into orbit about the planet. During this flight it flew past Earth once and past Venus twice. These fly-bys, as well as being used to slow down a space probe (in this case, usually they are used to speed them up), are also used to test the instruments.

The path that Mercury MESSENGER took to get to the planet, and the dates

During the four years that MESSENGER has been orbiting Mercury it has obtained a wealth of data. It would take me too long to describe all of its findings, but some highlights are

Mercury has a magnetic field
Discovery of water in craters
Discovery of volcanism
Discovery of organic compounds
Discovery of unusually high concentrations of calcium and magnesium

As is often the case with gathering more information than we have ever previously gathered, we now have more questions about Mercury than we have answers. How can such a slowly rotating planet (it rotates once every 58.6 Earth days) produce a magnetic field? Scientists are now going to have to wait a while to find out more about Mercury, the European Space Agency (ESA) plan to launch BepiColombo in January 2017, it will arrive at Mercury in January 2024.

## 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.