Posts Tagged ‘Transit of Venus’

Yesterday (Monday 9 May 2016) saw a transit of Mercury, a rare celestial event where the planet Mercury passes across the disk of the Sun as seen from Earth. The last one of these to happen was the 8th of November 2006, the next one will be on the 11th of November 2019. Here is a diagram showing where you needed to  be to see this transit; it was visible in its entirety from Wales and other parts of western Europe, as well as the eastern United States and western Africa and most of South America.


Where on Earth the 2016 Transit of Mercury was visible

Here is a diagram showing the path that Mercury took across the Sun, showing the timings (in Universal Time) of the beginning, mid-point and end of the transit.


The timings of the 2016 Transit of Mercury (Universal Time – UTC)

Unfortunately, it rained the whole afternoon here in Cardiff. I did not manage to catch single a glimpse of the Sun, let alone the transit itself. Here is a picture which was taken by Wolgang Ellsässer (who obviously had much clearer skies) at 13:52 UTC, about an hour before the transit reached its mid-point. A few sunspots are also visible, in fact some of the sunspots are appear bigger than the disk of Mercury.


A photograph of the 2016 Transit of Mercury taken by Wolgang Ellsässer at 15:52 (UTC)

If, like me, you failed to see this transit then the next one is on the 11th of November 2019. It will start at 12:35 UTC and end at 18:04 UTC, so roughly the same part of the Earth will see it as this 2016 transit. However, being in November those of us in western Europe will  not see the end of the transit as the Sun will have already set. After that there are transits in 2032 and 2039, again both in November. The next transit visible in its entirety from western Europe will not be until May 2049!

But, if you think this is a long time, it is an even longer wait until the next transit of Venus. Some of you may remember that I was lucky enough to go to the Gobi desert in Mongolia to see the June 2012 transit of Venus. I had also seen the June 2004 transit of Venus, which was visible in its entirety from Wales. The next transit of Venus will not occur until 2117!

It was seeing a transit of Mercury from the island of Helena in 1677 that led astronomer Edmond Halley to realise that one could use transits of either Mercury or Venus to measure the distance from the Earth to the Sun. He later realised that Mercury was too far from the Earth to be used, but the transits of Venus in 1761 and 1769 were successfully used to calculate the Sun-Earth distance for the first time.

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One year ago yesterday (10th of June 2012) I ended a week in one of one of the most extraordinary countries I have ever visited – Mongolia. I had gone there as part the International Space School Education Trust (ISSET)’s “Astronaut Leadership Experience” to observe the very rare Transit of Venus.

Chris Barber, founder and director of ISSET, and I had decided to try to organise a trip to somewhere in the World to observe the 2012 Transit of Venus. Only the last 30 minutes or so would be visible from the Disunited Kingdom, with most of it happening when the Sun was still below the horizon as seen from this part of the World.

This would be the last of these rare celestial events until December 2117, so of course represented our last chance to see one in our lifetimes. After a little bit of research of where would be a good place to see it in terms of (a) the Sun being high in the sky for the whole Transit, and (b) a place with a reasonable likelihood of clear(ish) skies, we decided upon the Gobi Desert in Mongolia.

Chris then set about putting together what turned into an extraordinary itinerary for our week there. In addition to the Transit, which fell on Wednesday the 6th of June, he arranged a memorable series of places to visit and savour. Anyone was welcome to sign up, and when we arrived in Mongolia’s capital of Ulaanbaator, our party consisted of people from the Disunited Kingdom, the USA, South Africa and Australia. The official team consisted of Chris, myself, and Michelle and Ken Ham from NASA.




Chinggis Khan (Genghis Khan) airport, Ulaanbaator

Chingis Khan (Genghis Khan) airport, Ulaanbaator


After some 24 hours in the capital, which included going to see a performance of traditional dancing and throat singing, we flew to Dalanzadgad in the Gobi Desert early on the Monday morning. From there we headed to our first Ger camp, one of many we would stay in during our week in the Gobi. During the week we did a camel ride, climbed the “whistling dunes”, saw well over half of the 6-hour long Transit, visited the “flaming cliffs”, went to several Buddhist Temples, and travelled across many miles of this extraordinary part of the World.

But, despite the most dramatic scenery, the best part of the week were the people. During the course of the week I got to know some truly lovely people, and thankfully FaceBook has allowed us to stay in touch since going our separate ways on the 10th of June at the Genghis Khan International Airport in Ulaanbaator. I hope it is not too long before we can cross paths again.







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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.


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.

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On Saturday 2nd of June I am leaving for a trip to the Gobi desert. The reason I am heading off there is to observe the Transit of Venus. The June Transit on the 6th will be the last one until Decemeber 2117 – in fact transits of Venus are the rarest predictable astronomical event we know of.

During a transit, Venus appears to pass across the disk of the Sun. Venus passes the Earth in its orbit every 584 days, something we call an inferior conjunction. But, because the planes of orbit of the two planets around the Sun are inclined to each other at 3 degrees, Venus will only appear to pass across the disk of the Sun on the rare occasion when an inferior conjunction happens when the two planets are on the line of nodes – the line where the two planes cross.

The planes of orbits of Venus and the Earth. Transits will only occur if Venus passes the Earth (an inferior conjunction) when the two planets happen to both be on the lines of nodes

Kepler was the first person to predict a transit of Venus. In 1629, after he had worked out his 3 laws of planetary motion, he calculated that in December 1631 Venus would appear to pass across the disk of the Sun. He also calculated that Mercury would transit in November of the same year. The November Mercury transit was observed by Pierre Gassendi in Paris, and Jesuit Father Cycat in Innsbruck and Johannes Remus in Alsace. The only surviving sketch we have is from Gassendi.

Gassendi also tried to observe the December transit of Venus, but failed. We now know that the Transit of December 1631 was not visible from Paris. Kepler had predicted that the next Transit of Venus would be in 1761, but in fact he got his calculations wrong. In 1639 the young English astronomer and mathematician Jeremiah Horrocks calculated that Venus would transit across the disk of the Sun in December of that year, 8 years after the transit of 1631. He wrote letters to his friend Crabtree in Manchester, and the two of them became the first human beings we know of to observe a Transit ot Venus. In fact, Horrocks kept a detailed journal of the observations.

Horrocks observing the 1639 Transit of Venus

At this point transits of Venus were just a curiosity. But this all changed in 1715 when Edmund Halley presented at paper at the Royal Society where he showed that transit of Venus could be used to measure the distance from the Earth the Sun, a distance which had eluded all attempts to be measured up to this time.

Edmund Halley – and the cover of his paper to the Royal Society describing the Transit of Mercury across the disk of the Sun

The paper presented by Edmund Halley to the Royal Society describing how a Transit of Venus could be used to determine the Earth’s distance from the Sun

The method Halley proposed depended on the effect of parallax. If two observers were to observe and time the Transit of Venus from different locations on Earth, separated by latitude, then the difference in the path length of Venus across the disk of the Sun, if one knew the distance between the two observing stations, could determine the distance from Venus to the Sun and hence the distance of Earth from the Sun.

Using the parallax of seeing a transit from two different locations to determine the Earth-Sun distance

Halley also knew that he would not live to see the next pair of transits in 1761 and 1769, but his admonishment was remembered, and in 1761 an international effort was made to observe the Transit of Venus in order to finally determine the distance from the Earth to the Sun. As (bad) luck would have it, altough nearly all the scientists were based in Europe, the Transit could only be seen in its entirety in Asia, southern Africa, and the northern parts of the Northern Hemisphere.

The visibility of the 1761 Transit of Venus, the unshaded areas are where the transit was visible in its entirety

Data for the 1761 were obtained from 60 different observing stations in 8 countries, making it at the time the largest international science project ever undertaken. There were a number of problems in the timings of the 1761 transit times from the various locations. This was due to the so-called “black drop” effect, which no one expected. It led to errors in the contact times between Venus and the disk of the Sun, rendering much of the data gathered useless. This led to the 1769 Transit gaining importance, as astronomers knew it was their last chance until 1874 to observe a transit, and with fore-knowledge of the black-drop effect, they hoped their data would be less error prone.

The visibility of the 1769 Transit. The shaded areas are where the Transit is visible in its entirety.

The 1769 Transit led to Captain Cooke going to Tahiti, where he and his team set up an Observatory. They made crucial observations near the Halleyan point, the point on the Earth where the Transit would appear to be the longest.

Captain, and the ship “The xx* which took him to Tahiti to observe the 1769 Transit of Venus

Due to the astronomers for the 1769 Transit knowing about the black-drop effect, the timings were far more accurate. About 160 scientists made observations from over 70 different observing stations. In 1771 the the data were complied by Thomas Hornsby, Professor of Astronomy at Oxford University, to finally determine the distance from the Earth to the Sun, a calculation which astronomers had been trying to do for thousands of years. He determined the value to be 93,726,900 miles (this compares extremely well with the currently accepted value determined by RADAR which is 92,957,133 miles).

There was a pair of transits in 1874 and 1882, and 8 years ago, in 2004, Europe was lucky enough to be well placed to see the entire Transit. I led the organisation of a public observing event in South Wales, and from tables published by NASA I was able to see that it was the first Transit visible in its entirety from Wales since 1283, and the next one visible in its entirety from Wales will be in 2247. Unfortunately for Europeans, the 2012 Transit requires another trip. As the map below shows, one has to be in the Pacific Ocean area of the World to see the 2012 Transit in its entirety. That is why I am going to the Gobi, in the hope that the only desert in the Northern Hemisphere that is in the region to be able to see the entire Transit will give me clear skies. I don’t expect to be around for the next one in 2117!

The visibility of the 2012 Transit

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