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## What is that bright object in the evening sky?

A number of people have been asking me over the last two or three weeks what the (very) bright object is in the evening sky. It is Venus, the brightest of all the planets. If you look towards the west (the same part of the sky as where the Sun has set) on any clear(ish) evening over the next two months, within a few hours of sunset, you should easily be able to see Venus.

Here is a diagram showing the evening sky for this evening (12 January 2017) as seen from Cardiff, and I have set it up to show the sky at 6pm. In Cardiff today the Sun sets at 16:29. Venus will not set until 20:51, nearly 3.5 hours after the Sun has set. This is why it is visible for such a long time after sunset.

The western sky at 6pm as seen from Cardiff. Today the Sun will set in Cardiff at 4:29pm, with Venus not setting until 8:51pm. This is nearly 3.5 hours after sunset, and today is the day of maximum eastern elongation.

In fact, today (12 January) is the day when the time between the Sun setting and the time at which Venus sets is at its greatest. That is why I chose today to blog about Venus. This is called maximum eastern elongation, and it is shown in the diagram below.

When the angle between a line from Earth to Venus and Venus to the Sun is a right angle, we have maximum elongation. As Venus is currently to the East of the Sun (rising after and setting after the Sun), it is today at maximum eastern elongation.

Venus will dominate the evening sky for another 6 weeks or so, although it will start setting closer and closer to the time of sunset now that we have passed maximum eastern elongation. It will swing in front of the Sun (something called inferior conjunction) on 25 March, so will be lost in the glow of the Sun for a few weeks before that. A few weeks after inferior conjunction, it will reappear as a morning object, becoming increasingly visible before sunrise as opposed to after sunset.

So, enjoy the wonderful sight of Venus in the evening sky over the next 6 weeks or so. And, if you can get hold of a pair of binoculars or a small telescope, you will see that Venus exhibits phases. Currently it is a quarter phase (half of it is illuminated), but as it approaches inferior conjunction it will become more and more crescent, but also appear to get larger in your viewing device (this cannot be seen with the naked eye). It was observations like these which enabled Galileo to show in 1610/1611 that Venus could not be orbiting the Earth, but that both Earth and Venus must be orbiting the Sun.

## 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!

## The sky from Buenos Aires

One of the lectures I will be giving on my cruise from Buenos Aires to Santiago is how the sky as seen from the southern parts of South America will look considerably different to the skies that Europeans and people from North America are used to seeing.﻿ Let me explain some of the obvious differences. First of all, although the Sun rises in the East and sets in the West in both the northern and southern hemisphere, if you are as far south as the southern parts of South America you need to look north to see the Sun. This means that you are facing north, and the Sun will appear to move from right to left across the sky, not from left to right as we northerners are used to seeing it. This can be quite disorientating.

Jupiter, and all the other planets, are visible from the Southern Hemisphere but again, one needs to look north to see them, not south. Just as in the Northern Hemisphere, Jupiter will dominate the evening sky for the next several months, and is in the constellation Cancer, slowly moving eastwards into Leo over the next 6-12 months.

The evening sky (7:15pm) from Buenos Aires on the 5th of March 2014. Jupiter is clearly visible, and will dominate the evening sky for the next several months. Just as with the Sun, from this location you need to look north to see Jupiter, not south as in the Northern Hemisphere.

This next diagram below shows Orion and Sirius, two very well known things in the winter sky, but as you can see from the Southern Hemisphere everything looks upside down! We are used to seeing Orion with Betelgeuse in the top left and Rigel in the bottom right, but from Buenos Aires this is flipped; Betelgeuse is in the bottom right, and Rigel in the top left (just imagine looking at Orion from the Northern Hemisphere but standing on your head to do so!). Just as confusingly, we are used to seeing Sirius (the brightest star in the sky) below Orion, closer to the horizon, because it is to the south of Orion. But, from Buenos Aires, it is above it, further away from the horizon. Very confusing!

This shows how confusing the southern skies can be to someone from the Northern Hemisphere. Orion is upside down, and Sirius is above (further south) Orion, not below as we see it in the Northern Hemisphere.

During the cruise, the other very bright object that people cannot miss is Venus, which is dominating the early evening sky. Venus will be at greatest eastern elongation on﻿ the 6th of June, which means that between now and then it will be moving further and further to the east of the Sun as seen from Earth (remember both we and Venus are moving in orbit about the Sun as this is going on), and as it moves further and further east the time between sunset and Venus setting gets bigger and bigger. On the 5th of March the Sun sets at 7:25pm from Buenos Aires, and Venus will set at 8:46pm. This gives a good hour to see Venus after sunset.
By early June, from the same location, the Sun sets at 5:50pm and Venus will set at 9:04pm, giving about three hours.

Venus is the evening sky as seen from Buenos Aires on the 5th of March at 7:14pm. At the moment Venus and Mars are close, and Uranus is near them too.

## Venus and Mars in conjunction on February the 21st

On Monday (the 16th) I am going to be on live TV talking about the conjunction of Venus and Mars, which will happen on the 21st (Saturday). In the western sky, just after sunset, you will see Venus and Mars come very close together, and it is this chance alignment as seen from Earth that we call a conjunction. Venus, for anyone who has been looking at the evening sky of late, is very very bright at the moment just after sunset. Mars, on the other hand, is much fainter, but knowing it is close to Venus this next week to ten days will help you find it.

The diagram below, which is a screen capture using SkySafari on my iPad, shows the sky at 6pm (18:00) as seen from London on Monday the 16th. As you can see, Venus is very bright and Mars is much fainter, at about 11 o’ clock to Venus if you imagine the face of a clock.

The positions of Venus and Mars at 6pm (18:00) on the evening of Monday the 16th of February, as seen from London.

Both planets are currently in Pisces, and Venus is approaching the brightest it can be. It has a magnitude of $-4.0$ this week (for an explanation of the magnitude system, see my blog here). Mars, as is obvious from the diagrams and if you look yourself, is much fainter; currently $+1.3$, making it $10^{0.4(1.3+4.0} = 131.8$ times fainter! A magnitude of $+1.3$ makes Mars easily visible, but it doesn’t jump out at you like Venus does.

The positions of Venus and Mars at 6pm (18:00) on the evening of Thursday the 19th of February, as seen from London.

The positions of Venus and Mars at 6pm (18:00) on the evening of Friday the 20th of February, as seen from London.

The positions of Venus and Mars at 6pm (18:00) on the evening of Saturday the 21st of February, as seen from London.

If I zoom in a bit for a couple of these evenings, this is how things will look

The positions of Venus and Mars at 6pm (18:00) on the evening of Friday the 20th of February, as seen from London.

The positions of Venus and Mars at 6pm (18:00) on the evening of Saturday the 21st of February, as seen from London.

To the naked eye, Venus and Mars will not be separable, but through binoculars or a small telescope you will be able to see enough detail to be able to see the small angular distance between them.

One of the theories for what the star of Bethlehem was (if it existed at all, and assuming it was not supernatural), is that it was a triple conjunction of Mars, Jupiter and Saturn. Modern sky simulation software has allowed us to show that this happened in 7 B.C., but it shouldn’t come as any surprise to anyone who knows their history that Jesus could not have been born after 4 B.C. because that is the year King Herod died. A conjunction between Venus and Jupiter, the two brightest planets, can be very spectacular.

## Measuring the Solar System – part 1

In June 2012 I travelled to the Gobi Desert in Mongolia to observe the 2012 Transit of Venus, the last one until December 2117. But, my reason for wanting to see this event was not just because they are incredibly rare. It was also because of their historical importance. They provided the first reliable method astronomers had for measuring the distance from the Earth to the Sun.

Over the next several weeks I will blog the slides from a lecture I put together back in 2004 (when we also had a Transit), explaining how a Transit of Venus can be used to measure the distance from the Earth to the Sun. I also provide some of the historical background to early observations of transits, including the heroic efforts undertaken by scientists in the mid 1700s.

This is the first part of the lecture, taking us from early Geocentric models of the Solar System to Galileo’s evidence that the Sun (and not the Earth) was at the centre of the Solar System, and up to the first ever predicted Transit, which was in 1631, although as far as we know no-one observed it.

This is a lecture I gave in Mongolia the night before the June 2012 Transit of Venus, but it is based on a talk I gave to schools and the public in 2004.

## 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.