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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 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!