Posts Tagged ‘Sun’

On Friday the 20th of March there was a Solar eclipse. In Cardiff it was partial, reaching a maximum of 87% at 09:28 GMT.

The details of the 20th March 2015 eclipse from the NASA eclipse website

The details of the 20th March 2015 eclipse from the NASA eclipse website

This image is taken from the NASA eclipse website, which can be found here. On this amazing website, you can look up eclipses going back thousands of years and going thousands of years into the future.

An alternative graphic of the eclipse is this.

An alternative graphic of the 20th of March 2015 eclipse

An alternative graphic of the 20th of March 2015 eclipse

These are some of the pictures I took. I started taking photographs as 08:33 using a 300mm lens on my Nikon DSLR, with Baader paper taped over a spare lens shield which I have. The pictures below are about every 10 minutes (I was taking them more frequently so these are just a sub-set). The last picture shown below was taken at 10:33.

The eclipse at its maximum from Cardiff, 87% at 09:28.

The eclipse at its maximum from Cardiff, 87% at 09:28.

It didn’t actually get dark (anyone who has witnessed a total eclipse will know that even when the eclipse is 99.95% it is not dark, it requires totality for it to go dark). But, the light did take on a strange ethereal quality, a little like dusk but not quite the same either.




If you managed to get any photographs of this eclipse, feel free to share them below in the comments box.


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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.

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.

Which is your favourite castle?

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Today is the day of the Hindu festival Diwali, the festival of light. About 200m from my house is a Hindu temple, and it has been lit up for the last several days in celebration of this important Hindu festival. Here is a picture I took on Sunday evening.

The Hindu temple in Cardiff, lit up for Diwali

Like most religions, with the exception of Islam, the Hindu calendar is a luni-solar calendar, and so the date of Diwali changes. Last year (2011) it was on the 26th of October, and next year (2013) it is on the 3rd of November, but this year it is today, the 13th of November. How is the date of Diwali calculated?

Being a luni-solar calendar, this means it follows something both the Moon and the Sun are doing. Today is a new Moon. So, the question is, what is special about today’s new Moon? What recent important Solar event happened? The Autumn equinox of course, which was on the 22nd of September this year (2012). The firsrt new Moon after this year’s Autumn equinox was on the 15th of October, and today is the second new Moon after the Autumn equinox. So, Diwali always falls on the 2nd new Moon after the Autum equinox! It’s that simple……

I wish a very happy Diwali to all my Hindu friends and students (and readers) around the World.

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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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Today (23rd of January 2012) is Chinese New Year, so happy Chinese New Year to all my Chinese friends and students. Today, over 1 billion Chinese will be celebrating the start of the year of the Dragon (龍). From what I heard yesterday on the radio, many Chinese couples await to have children in the year of the Dragon, as this year is thought to be the most lucky of the cycle of 12 animals.

Chinese New Year

Candles being lit for Chinese New Year

Last year (2011), Chinese New Year was on the 3rd of February, and next year (2013) it will be on the 10th of February. The table below shows the dates of Chinese New Year from 2009 to 2014.

year date
2009 26th January
2010 14th February
2011 3rd February
2012 23rd January
2013 10th February
2014 31st January

Clearly, Chinese New Year does not fall each year on the same date in the civil calendar. So, how is it calculated?

The Chinese calendar is an example of a lunisolar calendar, which means it depends on both the Moon (Luna) and the Sun (Solar). The same is true of the traditional Jewish calendar, and the calendars of many other civilisations and religions including Hindu, Tibetan and Buddhist calendars.

The date of the Chinese New Year is determined by the following, very simple, formula.

The date of the Chinese New Year is the day of the 2nd New Moon after the Winter Solstice (the shortest day of the year in the Northern Hemisphere).

This fixes it between the 21st of January (the earliest it can be, which would occur if there were a New Moon on the day after the Winter Solstice), and the 20th of February, which would occur if there were a New Moon on the day of the Winter Solstice.

So, it is that simple. Today (23rd of January) is a New Moon, and the previous New Moon (the first after the Winter Solstice) was on the 24th of December, with the Winter Solstice itself falling on 22nd of December in 2011. Next year, 2013, the first New Moon after the Winter Solstice will be on the 11th of January, the 2nd one will be on the 10th of February, so this will be the date of the Chinese New Year in 2013.

How will you be celebrating Chinese New Year?

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