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Posts Tagged ‘Astronomy’

In late October 2017, astronomers announced the first ever discovery of an asteroid (or comet?) coming into our Solar System from another stellar system. The object was first spotted on 19 October by the University of Hawaii’s Pan-STARRS telescope, during its nightly search for near-earth objects. Based on its extreme orbit and its rapid speed, it was soon determined that the object has come into our Solar System from somewhere else, and this makes it the first ever asteroid/comet with an extra-solar origin to have been discovered. Originally given the designation A/2017 U1, the International Astronomical Union (IAU) have now renamed it 1I/2017 U1, with the I standing for “interstellar”. 

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The object, given the designation A/2017 U1, was deemed to be extra-solar in origin from an analysis of its motion.

In addition to its strange trajectory, observations suggest that the object also has quite an unusual shape. It is very elongated, being ten times longer than it is wide. It is thought to be at least 400 metres long but only about 40 metres wide. This was determined by the rapid and dramatic changes in its brightness, which can only be explained by an elongated object tumbling rapidly.

The object has also been given the name Oumuamua (pronounced oh MOO-uh MOO-uh), although this is not its official name (yet).  This means “a messenger from afar arriving first” in Hawaiian. In other respects, it seems to be very much like asteroids found in our own Solar System, and is the confirmation of what astronomers have long suspected, that small objects which formed around other stars can end up wandering through space, not attached to any particular stellar system.

To read more about this fascinating object, follow this link.

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A couple of weeks ago this fascinating version of the periodic table of the elements was the NASA Astronomy Picture of the Day (APOD). Most people have seen the periodic table of the elements, it is shown on the wall of most high school chemistry classrooms. But, what is totally fascinating to me about this version is it shows the origin of each element.

It has been a long process of several decades to understand the origin of the elements. In fact, we have not totally finished understanding the processes yet. But, we do know the story for most elements. All the hydrogen in the Universe was formed in the big bang. This is true for nearly all the helium too. A small amount of the 25% or so of helium in the Universe has been created within stars through the conversion of hydrogen into helium. But, not much has been created this way because most of that helium is further converted to carbon.

The only other element to be formed in the big bang is lithium. About 20% of the lithium in the Universe was formed in the big bang, the rest has been formed since,

Together, hydrogen and helium comprise 99% of the elements in the Universe by number (not by mass).

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Where Your Elements Came From – from the NASA Astronomy Picture Of the Day (APOD) 24 October 2017.

I have decided to use this fascinating table as the basis for a series of blogs over the next few weeks to explain each of the 6 processes in these six boxes

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My latest book, Astrophotography, is now available. You can order a copy by following this link. Astrophotography is a book of exquisite images of space, including some of the latest images such as New Horizons’ images of Pluto, Rosetta’s images of Comet 67P, and Hubble Space Telescope images of the most distant galaxies ever seen. Each stunning image, reproduced to the highest quality, is accompanied by text that I have written to explain the object, and any background science relating to the object.

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Astrophotography is now available. You can order your copy by following this link.

One unique aspect of Astrophotography is that it emphasises the multi-wavelength approach taken to understanding astronomical objects. For millennia we could only study the Universe in visible-light (the light to which are eyes are sensitive), but for the last few decades we have used every part of the electromagnetic spectrum from radio waves to gamma rays to better understand the Universe. This multi-wavelength approach has also enabled us to discover previously unknown aspects of the Universe such as the Cosmic Microwave Background, the true appearance of Venus’ surface which lies hidden below its thick atmosphere, and huge quantities of gas between galaxies (the intracluster medium) which emit no visible-light but prodigious amounts of X-rays.

Astrophotography is split into 5 sections, namely

  1. Exploring the Solar System
  2. Exploring the Milky Way
  3. Exploring the Local Group
  4. Beyond the Local Group
  5. At the Edge of the Universe

Below are examples of some of the beautiful images found in Astrophotography, along with examples of the accompanying text. At the beginning of each page’s text I caption which telescope or space probe has taken the main image, and at which wavelength (or wavelengths).

Exploring the Solar System

Two examples from the first section of Astrophotography, the section on the Solar System, are stunning images of Mercury and of Mars. The images of Mercury were taken by NASA’s MESSENGER spacecraft. There are several pages of images of Mars, the page shown below shows an image of the Martian surface taken by the Mars Curiosity Rover, and an image of Victoria Crater taken by the Mars Reconnaissance Orbiter.

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Images of Mercury taken by NASA’s MESSENGER spacecraft. The four main images are spectral scans, and show information on the chemical composition of Mercury’s surface.

The section on the Solar System also includes images of Pluto taken by New Horizons, images of Saturn and Titan taken by the Cassini space probe, images of Comet 67P taken by Rosetta, and images of Jupiter and her moons taken by the Galileo space craft.

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The surface of Mars as imaged by NASA’s Mars Curiosity Rover and, at right, Victoria Crater, as imaged by NASA’s Mars Reconnaissance Orbiter.

Exploring the Milky Way

The second section of Astrophotography includes images of the Orion Nebula (Messier 42), the reflection nebula Messier 78, the Horsehead Nebula, the Pillars of Creation (part of the Eagle Nebula), and the Crab Nebula, the remnant of a supernova which exploded in 1054.

The example I show below is of the reflection nebula Messier 78, and is a visible light image taken by the Max Planck Gerzellschaft Telescope, a 2.2 metre telescope located at the European Southern Observatory’s facility in La Silla, Chile. The text describes the history of observing Messier 78, and explains what produces a reflection nebula.

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The reflection nebula Messier 78 imaged in visible light by the Max Planck Gesellschaft Telescope. The text explains what reflection nebulae are, and the history of observing this particular object.

Exploring the Local Group

The third section of Astrophotography looks at the Local Group, our part of the Universe. The Local Group includes our Milky Way galaxy, the Large and Small Magellanic Clouds, and the Andromeda galaxy. Some of the images shown in this section include the Tarantula Nebula in the Large Magellanic Cloud, NGC 602 (in the Small Magellanic Cloud), the Andromeda galaxy, Supernova 1987A and the Seahorse Nebula.

The example I show here is the Seahorse Nebula, a dark cloud of gas and dust located in Large Magellanic Cloud. This Hubble Space Telescope image was taken in 2008, and the nebula is in the bottom right of the image.

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The Seahorse nebula is a dark cloud of gas and dust found in the Large Magellanic Cloud, an irregular galaxy visible to the naked eye and in orbit about our Milky Way galaxy. The seahorse nebula is in the bottom right of the image.

Beyond the Local Group

The fourth section of Astrophotography looks at the rich variety of galaxies found beyond our own neighbourhood. Examples are galaxies like Messier 82, which is undergoing a huge burst of star formation in its centre, Centaurus A, which shows huge lobes of radio radiation stretching far beyond the stars we see in visible light, colliding galaxies such as The Antennae galaxies, and evidence for dark matter such as the Bullet cluster.

The example I have shown here is the spread for Messier 81, a beautiful spiral galaxy found in Ursa Major. It is one of the best known galaxies in the sky, and is visible to northern hemisphere observers throughout the  year. The main image illustrates the multi-wavelength approach astronomers take to studying many objects. The image combines visible light, infrared light and ultraviolet light to teach us far more about the galaxy than we would learn if we only looked in visible light.

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Messier 81 is a beautiful spiral galaxy found in Ursa Major. Hence it is visible throughout the year to northern hemisphere observers. The main image shown here is a combination of of a visible light image (taken by the Hubble Space Telescope), an infrared image taken by the Spitzer Space Telescope, and an ultraviolet image taken by Galaxy Evolution Explorer (GALEX).

At the edge of the Universe

In the final section of Astrophotography, I show examples of some of the most distant objects known. Images include the Hubble Deep Field, the Cosmic Microwave Background, the most distant galaxy seen (GN-z11, lying about 13.4 billion light years away), gravitational lenses and the recent discovery of gravitational waves made by LIGO.

The example I show here is the spread about the gravitational lens SDP81, a galaxy lying about 12 billion light years away which is being lensed (and brightened) by an intervening cluster of galaxies which lie about 4 billion light years away. The top image was taken at millimetre wavelengths by the Atacama Large Millimetre Array (ALMA), the bottom image in visible light by the Hubble Space Telescope.

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Gravitational lenses enable us to see distant galaxies which would otherwise be too faint to see, but they also provide us with a way of tracing the distribution of dark matter in clusters.

I hope these few examples from Astrophotography have whetted your appetite to find out more. I really enjoyed putting the book together, and am very pleased with the quality of the images and their aesthetic beauty.

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Some of you may have noticed that I haven’t blogged much this last month. The reason is that I have been putting the finishing touches on a book – which has just been sent off to the publishers Springer. I am sure it will need some revision, but am also hopeful that it should be hitting the shelves / bookshops / electronic stores in the next few months.



The cover, even the title may change!

The cover, even the title may change!



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In 1619, Johannes Kepler published a relationship between how long a planet takes to orbit the Sun and the size of that orbit, something we now call his 3rd law of planetary motion, or just “Kepler’s 3rd law”. It states that


T^{2} \propto a^{3}


where T is the period of the orbit and a is the size of the orbit. Kepler also found that the planets orbit the Sun in elliptical orbits (his 1st law), and so the size of the orbit a that we refer to is actually something called the “semi-major axis”, half the length of the long axis of an ellipse.

Any proportionality can be written as an equality if we introduce a constant, so we can write


T^{2} = k a^{3} \text{  (equation 1)}


where k is our constant of proportionality.



Kepler found that the planets orbit the Sun in ellipses, with the Sun at one of the foci. The long axis of an ellipse is called its major axis.

Kepler found that the planets orbit the Sun in ellipses, with the Sun at one of the foci. The long axis of an ellipse is called its major axis. The a in Kepler’s 3rd law refers to the length of the semi-major axis of a planet’s ellipse.



Newton was able to show in his Principia, published in 1687, that this law comes about as a natural consequence of his laws of motion and his law of gravity. How can this be shown?

Why is Kepler’s law true?

To show how Kepler’s law comes from Newton’s laws of motion and his law of gravitation, we will first of all make two simplifying assumptions, to make the mathematics easier. First we will assume that the orbits are circular, rather than elliptical. Secondly, we will assume that the Sun is at the centre of a planet’s circular orbit. Neither of these assumptions is strictly true, but they will make the derivation much simpler.

Newton’s law of gravity states that the gravitational force between two bodies of masses M \text{ and } m is given by


F = \frac { G M m }{ r^{2} } \text{  (equation 2)}


where r is the distance between the two bodies and G is a constant, known as Newton’s universal gravitational constant, usually called “big G”. In the case we are considering here, r is of course the radius of a planet’s circular orbit about the Sun.

When an object moves in a circle, even at a constant speed, it experiences an acceleration. This is because the velocity is always changing, as the direction of the velocity vector is always changing, even if its size is constant. From Newton’s 2nd law, F=ma, which means if there is an acceleration there must be a force causing it, and for circular motion this force is known as the centripetal force. It is given by


F = \frac{ m v^{2} }{ r } \text{  (equation 3)}


where m is the mass of the moving body, v is its speed, and r is the radius of the circular orbit. This centripetal force in this case is provided by gravity, so we can say that


\frac{ G M m }{ r^{2} } = \frac{ m v^{2} }{ r }


With a little bit of cancelling out we get


\frac{ G M }{ r } = v^{2} \text{  (equation 4)}


But the speed v is given by the distance the body moves divided by the time it takes. For one full circle this is just


v = \frac{ 2 \pi r }{ T }


where 2 \pi r is the circumference of a circle and T is the time it takes to complete one full orbit, its period. Substituting this into equation (4) gives


\frac{ G M }{ r } = \frac{ 4 \pi^{2} r^{2} }{ T^{2} }


Doing some re-arranging this gives


\boxed{ T^{2} = \frac{4 \pi^{2} }{ GM } a^{3} } \text{  (equation 5)}


where we have substituted a for r. This, as you can see, is just Kepler’s 3rd law, with the constant of proportionality k found to be (4 \pi^{2})/(GM). So, Kepler’s 3rd law can be derived from Newton’s laws of motion and his law of gravity. The value of k above is true if we express a in metres and T in seconds. But, if we express a in Astronomical Units and T in Earth years, then k actually comes out to be 1!

Newton’s form of Kepler’s 3rd law

A web search for Newton’s form of Kepler’s 3rd law will turn up the following equation


(M + m) T^{2} = a^{3} \text{  (equation 6)}


How can we derive this? I will show how it is done in part 2 of this blog, as we will need to learn about something called “reduced mass”, and also the “centre of mass”.

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I mentioned in this blog here that I would be on TV talking about the calculation that the Milky Way galaxy contains some 17 billion Earth-like planets.

Here is a youtube video capture of my appearance on the TV show. My apologies that the subtitles lag behind what is being said, and for the subtitles only being a summary of what is said. But at least if will give you a vague idea of what I’m saying if you cannot understand Welsh.



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Next Thursday (27th of September), I am going to be on live TV talking about the Voyager space probes. These remarkable crafts left our small planet in 1977, and are now on the point of entering interstellar space. Voyager 1 is currently 18.2 billion km from the Sun, and Voyager 2 is 14.8 billion km away. They are, by a large margin, the most distant objects human beings have ever sent into space.

This cartoon, from the official NASA Voyager website, shows where the two spacecraft are, compared to the heliopause. The heliopause is a theoretical boundary where the Sun’s Solar wind is stopped by the stellar winds from other, nearby stars. The heliosphere is the sphere of space within which the Sun and the planets reside, as they are within the heliopause, the surface of this sphere (in reality it is not a sphere, it would only be a sphere if the radiation from nearby stars was perfectly uniformly incident on our Solar system).


Voyager 1

This cartoon, from the NASA Voyager website, shows the position of the two Voyager space probes, and the heliosphere and heliopause.


Because these distances are so huge, it is sometimes easier to use larger units. The Astronomical Unit (AU) is defined as the average distance from the Earth to the Sun, so 150 million km. In these units, Voyager 1 is 122 AU from the Sun, and Voyager 2 is 99 AU away. The weak signals that we are still able to receive from the two space craft travel at the speed of light, and are currently taking about 30 hours (not minutes as I previously typed) to get to us.

Voyager 1

The Voyager space probes were identical copies of each other, but were launched a few weeks apart and went on a different journey into the outer Solar System

Voyager 1 and 2 at Jupiter

Voyager 1 arrived at Jupiter in January 1979. Voyager 2 reached the planet in July of the same year. Both space craft returned the most detailed pictures yet of the Solar System’s largest planet. In addition to making important studies of the great red spot, the two probes made the surprising discovery of volcanic activity on Io, Jupiter’s closest moon.


Jupiter's great red spot

A Voyager 1 image of Jupiter’s great red spot


Volcanic activity on Io

The plume of material on the left is a volcanic eruption on the moon Io, the nearest of the Galilean moons.


We now know that Io is the most volcanically active body in the Solar System. The source of its internal heating is the tidal forces from Jupiter. Because it doesn’t orbit Jupiter in a perfect circle, but rather in an ellipse, the Moon gets repeatedly deformed in different directions and this heats its interior up (in the same way that squeezing a tennis ball repeatedly will lead to its getting warm). With a warm molten interior, the conditions are just right for this to escape through the crust as volcanic eruptions.

Voyager at Saturn

In November 1980, Voyager 1 flew past Saturn. By August 1981, Voyager 2 had arrived at the ringed planet. Voyager made important discoveries about Saturn and her moons, in particular about Saturn’s rings. It discovered new ring structures, and even “spokes” in the rings.


Saturn by Voyager 1

A backlit image of Saturn taken by Voyager 1 after its flyby in late 1980.


Spokes in Saturn's rings

Voyager 2 discovered mysterious spokes in Saturn’s rings. It was many years before we understood what causes these.


Later this week I will write a part 2 to this blog, talking about Voyager 2’s encounters with Uranus and Neptune, the famous pale blue dot photograph, and the messages being carried on the probes as they head off into interstellar space.



Here are part 2 and part 3 of this post.

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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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On Friday (20th of January) I was called by the BBC in Cardiff to ask me if I would talk about the success of the 2nd series of Stargazing Live (this is a link to the interview, I’m on about 25 minutes into it), and also to summarise the events going on in the Wales National Museum in Cardiff on the Saturday. Yesterday I became aware, through Nick Howes (@NickAstronomer on Twitter), of a twitter “storm” going on between fans of Stargazing Live and the Daily Mirror TV critic Kevin O’Sullivan (@TVKev on Twitter) due to this article, in which he criticised Stargazing Live. Then, this morning I got another call from the BBC, this time asking me to talk tomorrow about an article by Jim Shelley in yesterday’s (Monday 23rd January) Daily Mirror. In this article Jim Shelley lists the Top 25 things he learned from the series. The BBC asked me to choose 10 to talk about tomorrow, and to talk in more detail about 4 of them.

I am delighted that the BBC Stargazing Live series has been such a success. Although Brian Cox comes in for quite a bit of criticism these days, I think one should interpret that as a mark of his impact and success. I have never been a fan of Patrick Moore, even when I was a teenager I found him irritating, and now I find him more so. Brian Cox has an easy, down to earth, unpretentious way of explaining things. Some of his basic astronomy mistakes frustrate me, but then again he’s never done a course in astronomy or astrophysics, so considering this he does a very good job.

One shouldn’t underestimate the impact a good TV programme or series can make. My own lifelong passion with astronomy and cosmology was ignited by a BBC Horizon programme I saw in January 1977 called “The key to the Universe”.

The Horizon programme that got me hooked on astronomy

I vividly remember, as I watched this 50 or 60 minutes of TV, a realisation that I wanted to study this fascinating subject. 32 years on, the subject still keeps me awake at night with wonder and excitement, and I feel very lucky to be paid to do something I’d gladly do for free.

So why did Kevin O’Sullivan have such a go at BBC Stargazing live? My own interpretation of his article, which his subsequent comments on Twitter seem to confirm, is that he was being deliberately provocative. I am in no position to comment on whether this is good or bad journalism, but it is certainly something I do on occasion – make deliberately provocative or outrageous statements to elicit reactions. Judging by the flood of traffic on Twitter about his article, @TVKev certainly did provoke considerable reaction.

In an age of multi-channel TV, there should be room for both the most highbrow and lowbrow TV. I enjoy, from time to time, watching the most mindless TV like Celebrity Big Brother or Snog, Marry, Avoid (that one with my daughters), I don’t spend all my time solving the equations of General Relativity or watching science documentaries.

That the BBC Stargazing Live series got 4-5 million people tuning in on BBC2 is a remarkable achievement. The fact that some of them may have been teenage girls with a crush on Brian Cox is irrelevant, but yet another reason he’s preferable to Patrick Moore. Many academics make fun of those of us who try and engage the public in our subject. I do it because (a) I enjoy sharing something I find interesting with as wide an audience as possible and (b) I haven’t forgotten the TV programme that first got me interested in understanding the Universe.

The 4 “facts” from Jim Shelley’s list that I have chosen to go into more detail on are

  1. The age of the Universe and the age of the Earth
  2. That the Sun will swell up and become a red giant
  3. That we may one day return to the Moon to mine Helium-3 from its surface
  4. That we have been sending radio and TV signals out into space, announcing our presence to any intelligent civilisations out there

Which 4 would you have chosen?

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Yesterday (Wednesday) afternoon I was on live TV talking about which planets would be visible over the coming months. Here is the YouTube clip of my appearance:

There are 5 planets visible to the naked eye, those being Mercury, Venus, Mars, Jupiter and Saturn. Along with the Sun and the Moon, these seven objects formed the most important celestial bodies to ancient people, and is almost certainly the reason we have 7 days in our week. I will blog about this connection in the near future.

Of the 5 planets visible to the naked eye, only Mars, Jupiter and Saturn will be visible over the months of this winter. Whereas the stars we see each month are the same from year to year (so, for example, we will always see the constellations Aries, Orion, and Virgo in the winter sky, but never the constellations Sagittarius, Cancer or Libra); which planets are visible in any given month changes from year to year. This is because the planets wander amongst the stars. In fact, the word “planet” comes from the Greek for “wandering star“.

The planet which is dominating the evening sky at the moment is Jupiter. Jupiter is to be seen low in the East soon after sunset, in the constellation Aries. Currently it is rising at 16:28 and setting at 06:38, so is visible nearly all night long. It is very bright, but to the naked eye looks like a star. However, if you look closely, particularly when Jupiter is low in the sky, you will notice that Jupiter does not twinkle like a star. Again, I will explain why this is in a future blog, but it provides the easiest way to know whether one is looking at a star or a planet; if it twinkles when low in the sky then you are not looking at a planet.

If you have access to a pair of binoculars or a small telescope, Jupiter is a wonderful sight. The first thing you will notice when you look at it through binoculars or a telescope are 4 small dots, sometimes 3 on one side and 1 on another, sometimes all 4 on one side, and sometimes only 2 or 3 of them might be visible. These are the four Galilean moons of Jupiter, so named because Galileo was the first person to observe them in 1610. They have the names Io, Europa, Ganymede and Callisto.

A typical view of Jupiter and her moons through a small telescope

It is well worth getting a copy of his “Starry Messenger” book and seeing copies of his original sketches of these objects, and his description as he realised they were not background stars but in fact, moons orbiting this vast planet.

This was the first piece of evidence that not everything went around the Earth, which was the prevailing view of the time. It led to Galileo going against the teachings of the Church, which was that the Universe had the Earth at the centre.

Io, the closest of the four moons, only takes 2 days to go around Jupiter, so even in the course of a few hours one can see it has moved. This never fails to excite me. If you have a good pair of binoculars or a good telescope, and the observing conditions are good (meaning, primarily, that the atmosphere is nice and steady), you may be able to see the bands of Jupiter’s clouds, and even its famous great red spot. Although one can find much clearer pictures of these in books these days, it never ceases to excite me to see these things with my own eyes through a telescope.

Mars, the 2nd planet visible over these winter months, is not coming up until midnight at the moment. It is in the constellation Leo at the moment. If this is a little late for you, then wait a month as it will then be rising at 10pm, and if you wait until January it will be rising at 8pm. if you do observe Mars this month and again in January or February, you will see quite clearly that it has moved relative to the “fixed” stars in Leo.

I always find Mars to be a bit of a disappointment through a telescope, and certainly this month it will not look that great as we are currently 5 months away from it being at its closest to us. When it is at its closest (the next closest approach is March the 3rd 2012), one can sometimes see the white polar caps. These are frozen carbon dioxide rather than the frozen water we have in our polar caps.

Mars, showing the white polar caps

Mars is, of course, in the news a lot at the moment because of the numerous probes and robots we have studying it. Again, I will blog about these in the near future, but just this last week a news item came out about what life on Mars could be like, if there is any.

Finally, at 05:20 at the moment, Saturn rises. Again, in 2 months it will be rising 4 hours earlier, and in February by 11pm. Saturn is in the costellation Virgo at the moment, where it will be for the next couple of years. Saturn appears brownish to the naked eye, but is probably the greatest marvel to look at through a telescope or binoculars, as then one can see the rings.

Saturn as seen through a small telescope

Just like with Jupiter and her moons, I never tire of seeing the rings of Saturn with my own eyes, even though I have seen countless sharper pictures of them taken by space probes. In September 2009 the rings were edge-on, so they are just beginning to open up, and we are now seeing them with the northern side inclined towards us. Next winter (2012/13) Saturn will be visible pretty much over the same months, but the rings will be slightly more open than they are this winter. It takes just over 14 years between each edge-on view of the rings, so until 2016 they will be opening up, then they will begin to close again.

If anyone reading this is thinking of buying a telescope can I give a word of caution. The cheap telescopes that cost a few tens of £s or $s are not worth buying, in fact avoid them. The optics are dreadful, and you will quickly beocme disappointment and stick it in the cupboard. If you want to spend less than £100/$100, buy a pair of binoculars, something like 8×50 is perfectly fine for seeing the moons of Jupiter or the rings of Saturn.

If you want to spend a little more, then a few hundred £s or $s will get you a telescope with good optics which is more than adequate for obsering the planets. Reputable manufacturers are Meade, Celestron and Orion, but I would avoid bying from any company, as again the optics can be bad. If you are new to star and planet gazing, the best way to start is to join a local astronomy club. Then, you don’t even have to buy your own telescope, as you will be able to use the club’s on their star-gazing evenings.

So, with the clocks having gone back and the nights getting longer, let’s look on the positive side of things – there is more darkness for looking at the wonders of the heavens.

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