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Posts Tagged ‘Planetary Conjunctions’

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)

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 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 at the end  of June. The two planets will get even closer over the following few nights,  producing a spectacular conjunction of the two brightest planets.

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 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!

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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 xxx the xxx of February, as seen from London.

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 xxx the xxx of February, as seen from London.

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 xxx the xxx 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 xxx the xxx 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 xxx the xxx 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 xxx the xxx 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.

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