Blog Archives

Our nearest planet?

The BBC radio4 programme More or Less (broadcast weekly on Fridays; repeated on a Sunday) deals with claims involving numbers or statistics. Unusually, the episode broadcast on 11 January 2019 contained an astronomical question (‘Which is our nearest planet?). That arose from a remark by (Prof) Chris Lintott in a Sky at night TV programme, where he claimed that Mars is our nearest neighbour. That was odd since surely everyone knows that the orbit of Venus is nearer to us that the orbit of Mars.

Consequently the programme, presented by Tim Harford, went to work on the answer. Since the inner planets constantly change in their relationship to one another the answer is not obvious. The question had to be adjusted to ‘Which planet spends more time as our nearest neighbour for more time than any other planet?’

This task was given to a statistical expert (‘Oliver’) who wrote a computer program to calculate which planet on average is nearest to Earth than any other. This was then commented on by David Rothery, Professor of Planetary Geoscience at the Open University. He was as surprised as anyone that the answer turned out to be Mercury! The calculation showed that the nearest planet is Mars 18 per cent of the time, Venus 36 per cent of the time, but Mercury 46 per cent of the time. This may be because Mercury’s short orbital period means that it is more frequently on our side of the sun than the two other candidates, which often spend long periods on the far side.

Actually, at the time Chris Lintott made his claim, Mars was the nearest planet.

A podcast of the radio programme is available from the More of Less website, where the discussion starts 22 minutes in.

Steuart Campbell

This is a slightly expanded version of a presentation Steuart Campbell gave to the ASE on 1 February 2019. Steuart is a member of ASE and a science writer.

City light pollution could get worse

For the amateur astronomer in a city, the biggest obstacle is light pollution, mainly from street lighting. Until recently, most street lights were sodium discharge lamps, initially low pressure (LPS), but later high pressure (HPS). These lamps gave good and efficient lighting although the LPS lamps gave poor colour rendering (better with HPS).

For amateur astronomers, because sodium lamps produce virtually monochromatic light in a small wavelength band averaging about 593 nm, the light pollution (skyglow) could be removed by using optical glass filters although nowadays the filtering can be achieved via astronomy software for astrophotograhy.

For this discussion colour temperature is important; that is the temperature at which a black body would emit radiation of the same colour as a given object. The relationship between colour and temperature is familiar to astronomers because it is how stars are classified. The colour temperature of LPS is 1800 K while the SON type of HPS has a colour temperature of 2700 K (the higher the colour temperature, the bluer the light). HPS lamps give a wider colour spectrum, but can still be filtered.

Like many cities across the world, the City of Edinburgh Council (CEC) has begun to replace sodium street light luminaires with light-emitting diode (LED) clusters. The case for this change is that LEDs are more energy efficient (using less electricity) and give a nearly full spectrum of light. The colour temperature of CEC’s LEDs is between 3700 and 4300 K. (For comparison moonlight has a colour temperature of 4100-4150 K and for daylight it is about 6000 K.) As a result, LED street lights emit much more blue-rich light than HPS lamps. This can impede night sky observations and also affect the health of humans, plants and animals. There is even evidence that exposure to this blue light can increase the risk of breast and prostate cancer.

Unfortunately for amateur astronomers, blue light is far more polluting than red or yellow light as the amount of light scattered by small particles in the atmosphere is inversely proportional to the wavelength. Blue light also contributes to glare, a growing problem in brightly-lit cities [1].

This is a matter that greatly concerns the International Dark-Sky Association (IDA) [2]. Its Fixture Seal of Approval (FSA) provides objective, third-party certification for luminaires that minimize glare, reduce light trespass and do not pollute the night sky. In 2014 the FSA programme began requiring lighting that has a colour temperature of 3000 and lower (up to 3220 K actual measured value). However, with the rapid pace of technological advance in the lighting industry, the IDA is likely to reduce its recommendation to 2700 K or lower. The IDA seeks the best possible scenario for new LED installations and retrofits to replace old technology without increasing light output and minimizing short wavelength emissions while also decreasing operational costs and energy consumption.

In April this year I asked CEC if they would be following the IDA’s recommendations, but their response was to claim that their design and specifications are in line with current British Standards (these appear to require LEDs to have a minimum colour temperature of 4000 K). Understandably, CEC would be reluctant to change its specification for LEDs.

ASE is the only amateur organisation in Edinburgh that can legitimately express a view on this matter. Consequently ASE should press CEC to correct its present specification to reduce light pollution to reduce skyglow, if for no other reason of which there are many. Without this change, Edinburgh residents are unlikely to see much of the night sky and amateur astronomers will be frustrated.


  1. ‘Night skies get the blues’ by Gabriel Popkin in Physics World, March 2015.

Steuart Campbell

This article is based on a short presentation given to the ASE by Steuart Campbell on the 6th of April 2018. Steuart is a science writer, a member of the ASE and a regular contributor to the Journal. His website is

Are We Alone?

In 1961, Professor Frank Drake attempted to estimate the number of extra-terrestrial civilizations in the Milky Way with which we might come into contact by making several assumptions. The Drake equation [1] states that:

N = R* x Fp x Ne x Fl x Fi x Fc x L


N = the number of civilizations in our galaxy with which communication might be possible;


R* = the average rate of star formation per year in our galaxy

Fp = the fraction of those stars that have planets

Ne = the average number of planets that can potentially support life per star that has planets

Fl = the fraction of the above that actually go on to develop life at some point

Fi = the fraction of the above that actually go on to develop intelligent life

Fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space

L = the length of time such civilizations release detectable signals into space.

Drake gave each parameter the following values:

R* = 10/year (10 stars formed per year, on average over the life of the galaxy)

Fp = 0.5 (half of all stars formed will have planets)

Ne= 2 (stars with planets will have 2 planets capable of supporting life)

Fl = 1 (100% of these planets will develop life)

Fi = 0.01 (1% of which will be intelligent life)

Fc = 0.01 (1% of which will be able to communicate)

L = 10,000 years (which will last 10,000 years).

So that N = 10 × 0.5 × 2 × 1 × 0.01 × 0.01 × 10,000 = 10.

Recently, Professor Paul Davies has made a different estimate with a range of different values in the Equation [2]. His N is between 1 and a billion!

I find Drake’s approach strange. A more logical approach might be to ask how many stars there are in the Galaxy. If there are between 100 and 400 billion stars, if half of all stars have planets, if there is life on only one planet in each system, but if only one in a million of those planets develops intelligent life, then there are between 50,000 and 200,000 planets with intelligent life.

Of course the values chosen for the Equation are highly questionable; they are merely wild guesses. However, one can question some more than others. The guess that, where stars have planets, two of them will harbour life is hardly justified from the example of the Solar System, where, as far as we know, only one planet (Earth) carries life. Even that change could halve Drake’s estimate to five. More importantly, these estimates seem to overlook the circumstances in which intelligent life has emerged on Earth. In particular, the value given to Fi (that intelligent life emerges on only one in a hundred planets where life has developed) is questionable.

It is easy to assume that because we exist, intelligent life is common (see the popular belief in aliens). However, we should consider the peculiar circumstances that have allowed us to evolve. Although life appeared very early on Earth (at least only 500 million years after the planet’s birth), multicellular life did not emerge until about 600 million years ago (MYA), fish only 500 MYA, reptiles only 300 MYA and our species only about 500,000 years ago. So it may be that modern humans have existed for only about 0.1 per cent of the life of the planet and it is certain that our modern technological civilization has existed for only about 200 years (~0.00004% of the life of planet Earth). That is a chance of only 1 in 2.5 billion that anyone looking for an advanced technological civilization (ATC) on Earth between the planet’s birth and now would be successful. What does that say for our chance of finding another ATC now?

Then consider the possibility that such a civilization will destroy itself. Nuclear war could have destroyed our civilization in 1962, before we even began looking for signals from another Galactic civilization (although not before our radio, TV and radar signals leaked out). This could lead to the conclusion that the chance of finding another ATC at this time is vanishingly small (Paul Davies allows for fi to be zero).

The Equation does not appear to have made allowance for the fact that we owe our existence to the demise of the dinosaurs 65 MYA. It should not be assumed that such destruction does not threaten other planets, or that it does. Without that event, the dinosaurs, who had ruled for 180 million years would probably still rule the Earth. If life on other planets follows such a path, do we have to assume some equivalent calamity before intelligent life can emerge? If so, what odds do we put on it?

Another important factor is our Moon, which is unusual in being so large and influential. We already believe that the Moon’s birth was the result of a catastrophic collision been the proto-Earth and another planetismal the size of Mars. How typical would such a collision be and what odds do we put on it occurring in a planetary system? If the result is a moon such as ours and such a large moon is unusual, then perhaps such collisions themselves are unusual. But does that mean that we owe our existence, inter alia, to the Moon?

Professor Neil F. Comins asked himself what the implications would be if the Moon did not exist [3]. There would have been many differences, including a shorter rotation period and a different chemical composition, but those that might influence the development of life include the possibility of a different tilt axis and instability of that axis. The Moon, besides gradually slowing Earth’s rotation, also stabilizes Earth’s axis. The lack of the Moon would mean smaller ocean tides, perhaps making the transfer of life from the oceans to land more difficult. It may also have meant more bombardment of Earth by asteroids and/or comets (the Moon has shielded Earth to some extent). This may have interfered with the development of life. Comins also thought that a Moon-less Earth (he called it ‘Solon’) would have a different atmosphere, with such a large amount of carbon dioxide that ‘life as we know it may never have been feasible’.

It has already been observed that our civilization has developed in a balmy interglacial, but Professor James Hansen has recently drawn attention to the fact that (unusually) sea levels have been remarkable stable for the last 7000 years (the climate kept an ice sheet from forming in Canada but kept stable ice sheets in Greenland and Antarctica). He pointed out that, because our major civilizations have mostly developed on coasts, especially on river deltas, this may have contributed to the development of civilization. Repeated changes in sea level would have inhibited the development of civilization [4].

Most anthropologists agree that bipedal hairless apes (humans) evolved out of many other varieties of hominins due to fortuitous climatic changes. Some believe that these forced our ancestors out of the trees onto the African savannah (the ‘Tarzan hypothesis’) and some believe that we evolved our special characteristics, not least of all our large brains, in an aquatic environmental excursion (hardly a normal evolutionary experience) [5]. Either way, we appear to owe our emergence to random climatic fluctuations. How typical would that be of life on other planets?

Some point to the explosion of the super-volcano Toba (Indonesia) about 70,000 years ago, which may have led to the extinction of many rival hominins and severely reduced our own numbers and created a bottle neck in our evolution. This catastrophe may also have been the trigger for our migration out of Africa, which itself may have led to the development of civilization. It is fortunate for us that no other super-volcano has erupted since (the next one to do so may be the end of civilization).

Does it not seem that we have been lucky [6]? Or rather that we owe our existence to a series of fortuitous chance events that must be rare in themselves never mind in combination? If that is true, then we probably are a very rare phenomenon: an intelligent species that has developed advanced technology, even now venturing into space. My guess is that the chance of another such species emerging elsewhere in our Galaxy is almost nil and we may indeed be alone, even in the whole universe.


  1. See
  2. The Eerie Silence: Are We Alone in the Universe? by Paul Davies (2010, Allen Lane).
  3. ‘The Earth Without the Moon’, Astronomy 19:2 (Feb 1991); later in What if the Moon didn’t exist? by Neil F. Comins (1993, Harper Collins, New York).
  4. Storms of My Grandchildren by James E. Hansen (Bloomsbury, 2009).
  5. The Aquatic Ape Hypothesis by Elaine Morgan (1997, Souvenir Press).
  6. Lucky Planet – Why Earth is Exceptional – and What that Means for Life in the Universe by David Waltham (Icon Books, 2014).

Steuart Campbell

Steuart is a science writer, a member of the ASE and a regular contributor to the Journal.

The Big Bang: who first suggested it?

The phrase ‘Big Bang’ was coined in 1949 by astronomer Fred Hoyle as a label for a cosmological model of the universe, although one with which he happened to disagree. However, the theory itself had an earlier origin.

Many think that George Lemaitre, a Belgian Roman Catholic priest, astronomer and professor of physics at the Université Catholique de Louvain was the first to suggest cosmic expansion. In his 1927 report, ‘A homogeneous universe of constant mass and growing radius accounting for the radial velocity of extragalactic nebulae’, he proposed that the universe expanded from the finite static state imagined by Einstein. But only in 1931, at a meeting of the British Association on the relation between the physical universe and spirituality (sic), did he propose that the universe originated in a ’primeval atom’ (but this was 2 years after Edwin Hubble had demonstrated cosmic expansion).

Many think it was mathematician Alexander Friedmann who, unknown to Lemaitre, proposed a similar solution to Einstein’s equations in 1922.

However, what seems to be little known is the fact that both Friedmann and Leamaitre were forestalled by the American writer and poet Edgar Allan Poe.

Edgar Allan Poe

In 1848 (79 years before Lemaitre and 74 years before Friedmann), he wrote Eureka: A Prose Poem, also subtitled ‘An Essay on the Material and Spiritual Universe’. It was his last major work and his longest non-fiction work at nearly 40,000 words. It was based on a lecture he gave on the 3rd of February 1848 in the Society Library in New York entitled ‘On The Cosmography of the Universe’. He died the following year.

Poe dedicated the work to Alexander von Humboldt, whose book Kosmos he must have read, at least the first two volumes. It was Humboldt who coined the word ‘cosmos’ (from the Greek kosmos) in the sense that modern cosmology uses it, to describe everything that exists in the universe, or the universe itself. In the volumes Poe must have read, he examined what was then known of the Milky Way, cosmic nebulae, and planets. The first volume was so popular that it sold out in two months.

Eureka describes Poe’s intuitive conception of the nature of the universe with no reference to any scientific work done to reach his conclusions (well there were none). His general proposition was ‘Because Nothing was, therefore All Things are’.

That is a bit vague, but it seems to suggest that the universe came out of nothing! Hasn’t modern science come to that conclusion? Indeed, he proposed that it had an origin: Poe contended that the universe filled with matter after a single, high-energy particle exploded and that, since the energy of the explosion is pushing matter outward, the universe must be expanding.

A reviewer in the New York Review of Books in February last year observed that [1]:

‘This by itself would be a startling anticipation of modern cosmology, if Poe had not also drawn striking conclusions from it, for example that space and ‘duration’ [i.e. ‘time’] are one thing, that there might be stars that emit no light, that there is a repulsive force that in some degree counteracts the force of gravity, that there could be any number of universes with different laws simultaneous with ours, that our universe might collapse to its original state and another universe erupt from the particle it would have become, and that our present universe may be one in a series.’

Apart from suggesting a Big Crunch, Poe was the first to explain Olbers’ Paradox (the night sky is dark despite the vast number of stars in the universe); I wrote about this in the Journal 8 years ago [2]. Poe claimed, as many do now, that the universe is not old enough to fill the sky with light. The universe may be infinite in size, he thought, (we think that now don’t we?) but there hasn’t been enough time since the universe began for starlight, travelling at the speed of light, to reach us from the farthest reaches of space. A Wikipedia page on the Paradox recognises Poe’s priority in this matter.

Response to Eureka was overwhelmingly unfavourable and the lecture on which it was based received negative reviews such as ‘hyperbolic nonsense’, but one newspaper called in ‘a noble effort’. Many were bored by the lecture which evidently was too long and rambling. However, Poe considered Eureka to be his masterpiece. He believed that the work would immortalize him because it would be proven to be true. Indeed, much of what he claimed has been verified and some, like Arthur Eddington, praised it. Albert Einstein called it ‘a beautiful achievement of an unusually independent mind’. 

Eureka was published in a small hardcover edition in March 1848 by Wiley & Putnam priced at 75 cents. Poe persuaded George Putnam, to publish Eureka after claiming the work was more important than Isaac Newton’s discovery of gravity (Newton did not discover gravity, but he did explain it)! Putnam paid Poe $14 (3-4 hundred dollars today) for the work. Poe suggested an initial printing of at least one million copies, but Putnam settled on 750, of which 500 were sold that year.

The book can still be bought in various editions and it can also be read online [3]. The National Library of Scotland has two copies, one of them the original 1848 edition, apparently once owned by the poet Dante Gabriel Rossetti.

What Poe suggested in this inspired work, with no antecedents, except perhaps Humboldt, is astonishing in its prescience. He deserves more recognition for his insights.

Finally, Poe has a Scottish connection. He was briefly at school in Irvine in 1815 when the Allans, his foster family, visited Britain. Let’s celebrate him.


      1. The New York Review of Books, February 5, 2015 – “On Edgar Allan Poe” by Marilynne Robinson
      2. ASE Journal No. 57, September 2008 – “Why is it dark at night?” by Steuart Campbell
      3. Eureka by Edgar Allan Poe, 1848. For an analysis of the work, see Eureka, an annotated edition by Stuart and Susan F Levine, University of Illinois Press, 2004.

Steuart Campbell

This article is based on an illustrated talk given to the ASE by Steuart Campbell on 4 November 2016. Steuart is a member of the ASE and a regular contributor to the Journal.

Why the Star of Bethlehem did not exist

Almost every Christmas an astronomer attempts to explain the Star of Bethlehem. The Sky at Night team did so in their programme broadcast on 2015 Dec 30. They concluded that it was most likely to have been a comet and showed Giotto’s painting of the Nativity with a comet in the sky (see below). Other artists portraying the Nativity usually just showed a distant star.


The Adoration of the Magi, Giotto di Bondone ca. 1305

I am a member of the ASE because I am interested in astronomy and cosmology. But I also have an abiding interest in the origin of Christianity and the life of Jesus. This is the result of a youth misspent as a Christian, a religion I abandoned a long time ago. This interest deepened until I found that I could write a book on the subject, which covers all aspects of the gospel story (see The Rise and Fall of Jesus, by Steuart Campbell). Necessarily the book examines Jesus’ birth and the story of the Star of Bethlehem.

The story comes only from Matthew’s Gospel, chapter 2, as follows (Authorised version):

Now when Jesus was born in Bethlehem of Judæa in the days of Herod the king, behold, there came wise men from the east to Jerusalem, saying, Where is he that is born King of the Jews? for we have seen his star in the east, and are come to worship him. When Herod the king had heard these things, he was troubled, and all Jerusalem with him.  And when he had gathered all the chief priests and scribes of the people together, he demanded of them where Christ should be born.  And they said unto him, In Bethlehem of Judæa: for thus it is written by the prophet,…Then Herod, when he had privily [secretly] called the wise men, enquired of them diligently what time the star appeared.  And he sent them to Bethlehem, and said, Go and search diligently for the young child; and when ye have found him, bring me word again, that I may come and worship him also.  When they had heard the king, they departed; and, lo, the star, which they saw in the east, went before them, till it came and stood over where the young child was.

Several things about this account should trouble astronomers. Does the account mean that the magi saw the ‘star’ in the east, i.e. rising, and followed it during a night as it travelled west? Or does it just mean that, being in the east themselves (Arabia?), they saw the ‘star’ in the west, over Palestine. If the latter, the ‘star’ would have set before they even came to Jerusalem. It is not clear.

The question of the time of the appearance of the ‘star’ is also obscure. No answer is given to this question and one wonders how it could be answered. Herod seems to have thought it significant, but we are not told why.

Most puzzling of all is the idea that the magi could follow the ‘star’ to identify a particular building in Bethlehem. Astronomers especially know that a celestial object or phenomenon cannot be identified with a particular location on the surface of the earth. Perhaps they are ignorant of this account or choose to ignore it as they search for any celestial phenomenon that might explain it. The entire confused account should alert astronomers to the possibility that it is unreliable and that they might not be looking for a real ‘star’.

It is important to understand that the two accounts of Jesus birth, one here in Matthew and another incompatible one in Luke’s Gospel are additions to the first Gospel, that of Mark. Both Matthew and Luke, took Mark as their basis and made additions to give Jesus an origin and background commensurate with his later deification and to elevate him the status of a Saviour God at least equal to contemporary such gods. The obvious comparison is with Mithras, the god of the Roman Army. Indeed, Matthew may have borrowed from the Mithraic books, which, it is reported, tell how, when Mithras was born, a star fell from the sky and was followed by Zoroastrian priests called ‘Magi’ on the way to worship him (by the way, Mithras birthday was Dec 25!).

Neither Mark’s nor John’s Gospel know anything about Jesus’ origin. Biblical scholars believe that the entire Birth Narratives of Matthew and Luke are inventions, for the purpose explained above.

Matthew in particular, writing for the Jewish community in Alexandria, was at pains to show fulfilment of Jewish prophecy, or at least to show links between Jesus’ origin and the Jewish Scriptures. Consequently he may have borrowed from a Jewish apocryphal book like The Testament of Levi (one of the Jewish patriarchs). In that book, in a description of the last days (18:3), one finds the statement that ‘his star shall arise in heaven as of a king. Lighting up the light of knowledge as the sun the day’. Also, in 24:1, the statement that ‘shall a star arise to you from Jacob in peace’. One can even see forecast of a star in Numbers 24:17 (‘There shall come a star out of Jacob’). In the Old Testament, the word ‘star’ often stood for the Messiah.

Jewish readers would easily see the connection and be persuaded that Jesus really was the Messiah, the point Matthew was trying to convey. I understand that The Talmud, a central text of Rabbinic Judaism, contains a statement that ‘when the Messiah is to be revealed a star will rise in the east…and seven other stars round it will fight on every side’.

It is common knowledge that ancient peoples saw celestial phenomena as signifying or celebrating some important event, such as the birth of a king, on Earth. It is not so obvious, but equally logical, that an important historical Earthly event must somehow have been reflected in the sky. Consequently, even though nothing appeared at the time, such an event was easily invented to convince people that the event described had great significance. Miraculous events were often invented to accompany the births or deaths of Roman Emperors. Such was the case here. Believing that Jesus was the expected Messiah, Matthew invented a celestial event to convince his readers of Jesus’ importance.

Astronomers even make a mistake about the date. Our year dating system was invented in 525 by a Scythian monk called Dionysius Exiguus. He based it on the assumed age of Jesus, by then thought to be in Heaven (we still keep to this system which was adopted by Bede in the 8th century). However, astronomers and many others are misled by the reference in Matthew’s account to king Herod. They assume that it must be Herod the Great, known for his cruelty and who died in 4 BCE. Consequently, they look for a celestial phenomenon prior to that date, perhaps 5 or 6 BCE and sometime they find one. However, ‘Herod’ was a family name and all of Herod the Great’s sons also carried the name. So merely calling a king ‘Herod’ was not sufficient identification and Dionysius’ calendar should not be accused of making a mistake. He almost certainly worked from Luke’s account of when John the Baptist began to preach (chapter 3). Note the reference to ‘Herod the Tetrarch’, whose name was actually ‘Antipas’:

1 Now in the fifteenth year of the reign of Tiberius Cæsar, Pontius Pilate being governor of Judæa, and Herod being tetrarch of Galilee, and his brother Philip tetrarch of Ituræa and of the region of Trachonitis, and Lysanias the tetrarch of Abilene

2 Annas and Caiaphas being the high priests, the word of God came unto John the son of Zacharias in the wilderness.

Also a statement about the age of Jesus:

23 And Jesus himself began to be about thirty years of age

Tiberias’ 15th year was the year we call, using Dionysius’ system, 28 CE. Making allowances for the period between the appearance of John and Jesus’ mission, his birth must be place in the year 1 BCE (there was no year zero). There is no reason to abandon Dionysius’ calendar and every reason to eschew the idea that he made a mistake. Consequently, even if there had been a celestial phenomenon at the time of Jesus’ birth, astronomers have been looking in the wrong time.

The mistake made by astronomers is a classic example of ‘the law of the instrument’ or over-reliance on a familiar tool. It means that astronomers have been looking at the biblical record only from their own point of view, ignorant of the fact that the record does not lie within their competence. There are other examples of experts in one discipline believing that they can explain something that lies in another discipline. In this case, astronomers have seen what appears to be an astronomical record and assumed that they would be able to explain it. But the star is imaginary. It never really existed.

Please remember this when you next hear, as you will, of an astronomer trying to explain The Star of Bethlehem.

Steuart Campbell

This article is based on a talk given to the ASE by Steuart Campbell on the 8th of January 2016.  Steuart has been a member of the ASE for many years and our thanks go to him for sharing with us his theory on the Star of Bethlehem.