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Perihelion Again

Posted in The Universe and Stuff with tags perihelion on January 4, 2023 by telescoper

Earth’s elliptical orbit viewed at an angle (which makes it look more eccentric than it is – in reality is very nearly circular).

According to my new RAS Diary, today (Wednesday 4th January 2023) at approximately 16.17 GMT the Earth reaches at the point on its orbit which which it is at its closest to the Sun, i.e. its perihelion. At this time the distance from the Sun’s centre to Earth’s centre will be 147,098,925 km. This year, aphelion (the furthest distance from the Sun) is at 21.06 GMT on July 6th 2023 at which point the centre of the Earth will be 152,093,251 km from the centre of the Sun. You can find a list of times and dates of perihelion and aphelion for future years here.

At perihelion the speed of the Earth in its orbit around the Sun is greater than at aphelion (about 30.287 km/s versus 29.291 km/s). This difference, caused by the Earth’s orbital eccentricity, contributes to the difference between mean time and solar time I blogged about when discussing the Winter Solstice a couple of weeks ago.

It surprises me how many people think that the existence of the seasons has something to do with the variation of the Earth’s distance from the Sun as it moves in its orbit in that the closer to the Sun we get the warmer the weather will be. The fact that perihelion occurs in the depth of winter should convince anyone living in the Northern hemisphere that this just can’t be the case, as should the fact that it’s summer in the Southern hemisphere while it is winter in the North.

The real reason for the existence of seasons is the tilt of the Earth’s axis of rotation. I used to do a little demonstration with a torch (flashlight to American readers) to illustrate this when I taught first-year astrophysics. If you shine a torch horizontally at a piece of card it will illuminate a patch of the card. Keep the torch at the same distance but tilt the card and you will see the illuminated patch increase in size. The torch is radiating the same amount of energy but in the second case that energy is spread over a larger area than in the first. This means that the energy per unit area incident on the card is decreases when the card is tilted. It is that which is responsible for winter being colder than summer. In the summer the sun is higher in the sky (on average) than in winter. From this argument you can infer that the winter solstice not the perihelion, is the relevant astronomical indicator of winter.

That is not to say that the shape of the Earth’s orbit has no effect on temperatures. It may, for example, contribute to the summer in the Southern hemisphere being hotter than in the North, although it is not the only effect. The Earth’s surface possesses a significant North-South asymmetry: there is a much larger fraction of ocean in the Southern hemisphere, for example, which could be responsible for moderating any differences in temperature due to insolation. The climate is a non-linear system that involves circulating air and ocean currents that respond in complicated ways and on different timescales not just to insolation but to many other parameters, including atmospheric composition (especially the amount of water vapour).

The dates when Earth reaches the extreme points on its orbit (apsides) are not fixed because of the variations in its orbital eccentricity so, in the short-term, the dates can vary up to 2 days from one year to another. The perihelion distance varies slightly from year to year too; it’s slightly smaller this year than last year, for example.

There is however a long-term trend for perihelion to occur later in the year. For example, in 1246, the December Solstice (Winter Solstice for the Northern Hemisphere) was on the same day as the Earth’s perihelion. Since then, the perihelion and aphelion dates have drifted by an average of one day every 58 years and this trend will continue. This means that by the year 6430 the timing of the perihelion and the March Equinox will coincide, although I will probably have retired by then…

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Ireland and CERN

Posted in Maynooth, Politics, Science Politics, The Universe and Stuff with tags CERN, CERN Membership, Cormac O'Raifeartaigh, ireland on December 30, 2022 by telescoper

Not long ago I posted an item about Ireland’s potential membership of CERN. There seems to have been some progress at political levels in this direction. In Mid-December, the Seanad called for a detailed proposal for CERN membership to be drawn up. More recently still, Minister Simon Harris has indicated that he will bring such a proposal to Cabinet on the matter.

There’s an article in yesterday’s Irish Times by Cormac O’Raifeartaigh reviewing the situation.

As I understand things, if the Irish Government were to decide to take Ireland into CERN then it would first have to become an Associate Member, which would cost around €1.5 million per year. That’s a modest contribution, and the financial returns to Irish industry and universities are likely to far exceed that. This Associate member stage would last up to 5 years, and then to acquire full membership a joining fee of around €16.8 million would have to be paid, though that could be spread out over ten years, along with an annual contribution of around €13.5m.

While I support the idea of Ireland joining CERN I feel obliged to stress my concerns. The most important of these is that there seems to me to be a real danger that the Government would simply appropriate funding for CERN membership from within existing programmes leaving even less for other forms of scientific research. In order to reap the scientific reward of CERN membership the Government will have to invest the additional resources needed to exploit the access to facilities membership would provide. Without a related increase in research grant funding for basic science, the opportunity to raise the level of scientific activity in Ireland would be lost and science overall may end up worse off.

Ireland recently joined the European Southern Observatory (ESO), a decision which gave Irish astronomers access to some amazing telescopes. However, there is no sign at all of Irish funding agencies responding to this opportunity by increasing funding for academic time, postdocs and graduate students needed to do the actual science. In one respect ESO is very like CERN: the facilities do not themselves do the science. We need people to do that. CERN membership could turn out to be like a very expensive Christmas gift that looks very exciting until you open the box and find that the batteries are not included.

P.S. At least Cormac’s employers in Waterford have been quick off the mark in exploiting the potential of CERN by renaming their entire institution after it…

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The Winter Solstice 2022

Posted in The Universe and Stuff with tags Analemma, Earliest Sunset, Equation of Time, Latest Sunrise, Solar noon, Winter Solstice on December 21, 2022 by telescoper

The Winter Solstice in the Northern hemisphere happens later today, Wednesday 21st December, at 21:48 Irish Time.

People often think that the Winter Solstice is defined to be the “shortest day” or the “longest night” of the year. The Solstice does indeed happen on the shortest day, but it is defined in astronomical terms much more precisely than that. It happens when the axial tilt of the Earth away from the Sun is greatest, so that the Sun appears in the sky with its lowest maximum elevation. The timing of this event can be calculated with great precision.

Anyway, today is the shortest day of the year in the Northern hemisphere. Days will get steadily longer from then until the Summer Solstice next June. The shortest day – defined by the interval between sunrise and sunset – is today, although not by much. Today in Dublin is shorter than yesterday by about six seconds, but tomorrow will be longer than today by less than a second.

This does not, however, mean that sunrise will happen earlier tomorrow than it did this morning. Actually, sunrise will carry on getting later until the new year, the length of the day nevertheless increasing because sunset occurs even later. Sunrise yesterday morning (20th December) was at 08.42 Dublin Time while today it was 08.43; the latest sunrise will be on 30th December (09.05). Sunset last night was at 16.49 and tonight it will be at 16.50. The earliest sunset this year was actually on 13th December (16:48).

These complications arise because there is a difference between mean solar time (measured by clocks) and apparent solar time (defined by the position of the Sun in the sky, i.e. what you would measure on a sundial), so that a solar day does not always last exactly 24 hours as measured by a clock. A description of apparent and mean time was given by Nevil Maskelyne in the Nautical Almanac for 1767:

Apparent Time is that deduced immediately from the Sun, whether from the Observation of his passing the Meridian, or from his observed Rising or Setting. This Time is different from that shewn by Clocks and Watches well regulated at Land, which is called equated or mean Time.

The discrepancy between mean time and apparent time arises because of the Earth’s axial tilt and the fact that it travels around the Sun in an elliptical orbit in which its orbital speed varies with time of year (being faster at perihelion than at aphelion). The upshot of this is that solar noon – when the Sun is at its highest point in the sky on a given day – is not always at 12 noon local mean time. Solar noon today in Ireland is actually at 12.30 Irish time. Around the time of the Winter Solstice, solar noon is getting later in the day and this will continue to happen until well into the New Year; solar noon on New Year’s Eve is at 12.34. While the interval between sunrise and sunset shrinks towards the solstice, the mid-point of this interval is drifting later in the day, making both sunrise and sunset occur later despite the gap between the two getting smaller.

The discrepancy between latest sunrise (or earliest) and the solstice varies with latitude, although if you go far enough North into the Arctic Circle, there is neither sunrise or sunset around the Winter Solstice, and if you go far enough South to the Equator the length of the day does not vary at all with time of year. The behaviour is illustrated for North America in this graphic produced by the United States Naval Observatory

If you plot the position of the Sun in the sky at a fixed time each day from a fixed location on the Earth you get a thing called an analemma, which is a sort of figure-of-eight shape whose shape depends on the observer’s latitude. Here’s a photographic version taken in Edmonton, with photographs of the Sun’s position taken from the same position at the same time on different days over the course of a year:

The winter solstice is at the lowermost point on this curve and the summer solstice is at the top. These two turning points define the time of the solstices much more precisely than the “shortest day” or “longest night”. The Winter Solstice is takes place at a very specific time, when the angle of tilt of the Earth’s axis relative to the Sun is maximum.

Anyway, the north–south component of the analemma is the Sun’s declination, and the east–west component arises from the equation of time which quantifies the difference between mean solar time and apparent solar time. This curve is used to calculate the earliest and/or latest sunrise and/or sunset. Looking at a table of the local mean times of sunrise and sunset for Dublin around the 2022 winter solstice shows that today is indeed the shortest day (with a time between sunrise and sunset of 7 hours 33 minutes and 49 seconds).

P.S. As usual, crowds gathered today at the spectacular neolithic monument at Newgrange in County Meath to observe the sunrise at the Solstice.

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SpaceX launch confirmed for Euclid

Posted in Biographical, Euclid, The Universe and Stuff with tags Euclid, Euclid Consortium, Falcon 9, SpaceX on December 15, 2022 by telescoper

I’m a bit fragile today. I don’t know why, but it may be connected with our Departmental Christmas partylast night. I’m glad I didn’t have too much to drink. Ahem.

Anyway, in my current condition I only have time for a short post to pass on the news that I today saw official confirmation that, negotiations having been successfully completed, the European Space Agency’s Euclid mission will indeed be launched by SpaceX, on a Falcon 9 rocket.

Various working meetings have been scheduled to start next week between ESA, SpaceX and Thales Alenia Space – Italia (TAS-I, who constructed the vehicle). The launch period is confirmed to be somewhere in the 3rd Quarter of 2023 and may even be as early as 1st July 2023. The actual launch window of one month will be agreed on the 1st of February.

Now there will be intense activity preparing the Euclid vehicle for launch as well as readying the Ground Segment – the bit that collects and processes the data.

The 2023 Annual Euclid Consortium Meeting, scheduled to be in Copenhagen in from 19th to 23rd June will be the last such meeting before the launch. I am very much looking forward to attending it.

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Latest Results from the South Pole Telescope

Posted in The Universe and Stuff with tags arXiv:2212.05642, Cosmic Microwave Background, Cosmology, South Pole Telescope, SPT on December 13, 2022 by telescoper

Just time for a quick post to point out the latest results from the South Pole Telescope (SPT) have now hit the arXiv. The measurements were made in 2018 but the outcome of a full analysis of temperature and polarization has only just appeared.

Here’s a grab of the abstract:

The key figures showing the constraints on the Hubble Constant H₀and the parameter S₈ are shown here:

As you can see, the results from SPT-3G are consistent with the standard cosmological model and agree on H₀ with Planck rather than the higher value obtained from local measurements. If you thought there was Hubble tension before this measurement, then you will still think so now!

Save the Holmdel Antenna!

Posted in History, The Universe and Stuff with tags Arno Penzias, Bell Labs, Cosmic Microwave Background, Holmdel, Robert Wilson on December 11, 2022 by telescoper

I’ve used the above image hundreds of times in popular talks. It shows Robert W. Wilson (left) and Arno A. Penzias (right) standing in front of the famous horn antenna that (accidentally) discovered what we now know to be the cosmic microwave background radiation left over after the Big Bang. Penzias and Wilson made their historic measurements in 1964, published their results in 1965, and received the Nobel Prize for Physics in 1978.

At the time of their historic experiment, the scientists were working at Bell Telephone Laboratories at Holmdel, New Jersey, on Project Echo. The antenna was built to receive radio signals bounced off a passive satellite in a low Earth orbit to check the feasibility of satellite radio communication. They found excess noise in their receiver, which was eventually identified as a relic of a time when the Universe was extremely hot. Coincidentally, the theory of this yet undiscovered radiation was being worked on by Bob Dicke and his group in Princeton at about the same time (and also in New Jersey). Discussions ensued, and the discovery paper by Penzias & Wilson appeared in the Astrophysical Journal in 1965 beside a paper by Dicke et al. giving the theoretical interpretation.

Anyway, in case you were wondering whatever happened to the Holmdel Antenna, it is still there in Holmdel (at the top of Crawford Hill) and in 1988 was declared a National Historic Landmark:

Bell Labs (as it was usually known) was acquired by Nokia in 2016 and subsequently called Nokia Bell Labs. In 2019, however, Nokia put the entire Holmdel site up for sale and redevelopment of the entire site is currently being considered. This would not only bring to an end the connection between Holmdel and the telecommunications industry but also places a big question mark over the famous antenna. A petition has been raised to secure the future of this extremely important piece of scientific history. I encourage you to read more about the situation here and consider signing the petition.

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PhD Opportunity in Theoretical Astrophysics at Maynooth!

Posted in Maynooth, The Universe and Stuff with tags black holes, John Regan, Maynooth University, theoretical physics on December 8, 2022 by telescoper

The Department of Theoretical Physics at Maynooth University invites applications for a PhD in Theoretical Astrophysics starting in September 2023. The successful applicant will work in the group led by Dr. John Regan on a project examining the formation processes of massive black holes in the early Universe. Massive black holes populate the centres of all massive galaxies and are now also observed in both the centres and in off-centre locations in less massive dwarf galaxies.

For more details and instructions on how to apply, see here.

The Special Beards of Relativity

Posted in Beards, History, The Universe and Stuff with tags Albert Einstein, Gedanken, George Francis Fitzgerald, Hendrik Lorentz, special relativity, Thought Experiment on December 7, 2022 by telescoper

I’ve recently moved on to the part about Special Relativity in my module on Mechanics and Special Relativity and this afternoon I’m going to talk about the Lorentz-Fitzgerald contraction or, as it’s properly called here in Ireland, the Fitzgerald-Lorentz contraction.

The first thing to point out is that the physicists George Francis Fitzgerald and Hendrik Lorentz, though of different nationality (the former Irish, the latter Dutch), both had fine beards:

One of the interesting things you find if you read about the history of physics just before Albert Einstein introduced his theory of special relativity in 1905 was how many people seemed to be on the verge of getting the idea around about the same time. Fitzgerald and Lorentz were two who were almost there; Poincaré was another. It was as if special relativity was `in the air’ at the time. It did, however, take a special genius like Einstein to crystallize all that thinking into a definite theory.

Special relativity is fun to teach, not least because it throws up interesting yet informative paradoxes (i.e. apparent logical contradictions) arising from that you can use to start a discussion. They’re not really logical contradictions, of course. They just challenge `common sense’ notions, which is a good thing to do to get people thinking.

Anyway, I thought I’d mention one of my favorite such paradoxes arising from a simple Gedankenerfahrung (thought experiment) here.

Imagine you are in a railway carriage moving along a track at constant speed relative to the track. The carriage is dark, but at the centre of the carriage is a flash bulb. At one end (say the front) of the carriage is a portrait of Lorentz and at the other (say the back) a portrait of Fitzgerald; the pictures are equidistant from the bulb and next to each portrait is a clock.The two clocks are synchronized in the rest frame of the carriage.

At a particular time the flash bulb goes off, illuminating both portraits and both clocks for an instant.

It is an essential postulate of special relativity that the speed of light is the same to observers in any inertial frame, so that an observer at rest in the centre of the carriage sees both portraits illuminated simultaneously as indicated by the adjacent clocks. This is because the symmetry of the situation means that light has to travel the same distance to each portrait and back.

Now suppose we view the action from the point of view of a different inertial observer, at rest by the trackside rather than on the train, who is positioned right next to the centre of the carriage as the flash goes off. The light flash travels with the same speed in the second observer’s frame, but this observer sees* the back of the carriage moving towards the light signal and the front moving away. The result is therefore that this observer sees the two portraits light up at different times. In this case the portrait of Fitzgerald is lit up before the portrait of Lorentz.

Had the train been going in the opposite direction, Lorentz would have appeared before Fitzgerald. That just shows that whether its Lorentz-Fitzgerald contraction or Fitzgerald-Lorentz contraction is just a matter of your frame of reference…

But that’s not the paradoxical thing. The paradox is although the two portraits appear at different times to the trackside observer, the clocks nevertheless display the same time….

*You have to use your imagination a bit here, as the train has to be travelling at a decent fraction of the speed of light. It’s certainly not an Irish train.

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Guest Post – by A.I. Addio

Posted in The Universe and Stuff with tags AI, Artificial Intelligence, astro-ph, ChatGPT, Cosmic Web on December 6, 2022 by telescoper

Quite a few people have been playing around with a new-fangled AI tool called ChatGPT the developers of which say this:

We’ve trained a model called ChatGPT which interacts in a conversational way. The dialogue format makes it possible for ChatGPT to answer followup questions, admit its mistakes, challenge incorrect premises, and reject inappropriate requests. ChatGPT is a sibling model to InstructGPT, which is trained to follow an instruction in a prompt and provide a detailed response.

Here is an example (stolen from here) wherein this “model” creates the abstract of a scientific paper on a suggested topic:

This makes me wonder how many abstracts on astro-ph are actually written this way!

Please note that no papers of mine involved the use of any form of Artificial Insemination. I hope this clarifies the situation.

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String Theory – Dead Again?

Posted in Biographical, mathematics, The Universe and Stuff on December 5, 2022 by telescoper

The other day I came across an old clipping from the December 2005 issue of Physics World. It’s from an article called What will they think in 2105? looking forward from 2005 at likely developments in the next 100 years of physics, given the context of the centenary of Einstein’s “year of miracles” (1905) in which he came up with, among other things, Special Relativity which I start teaching today.

The article asks what present-day discoveries would be remembered in a hundred years. Many of those asked the question said string theory. My response was somewhat less enthusiastic:

I got quite a lot of stick at the time from senior physicists for this statement! My use of the phrase “dead again” was based on the observation that the popularity of string theory has waxed and waned several times over the years. It may not have died in 2015 as I predicted, but it does seem to me to be in a moribund state, in terms of its impact (or lack thereof) on physics.

I’m mindful of the fact that many mathematicians think string theory is great. I’ve had it pointed out to me that it has a really big influence on for example geometry, especially non-commutative geometry, and even some number theory research in the past few decades. It has even inspired work that has led to Fields medals. That’s all very well and good, but it’s not physics. It’s mathematics.

Of course physicists have long relied on mathematics for the formulation of theoretical ideas. Riemannian geometry was `just’ mathematics before its ideas began to be used in the formulation of the general theory of relativity, a theory that has since been subjected to numerous experimental tests. It may be the case that string theory will at some point provide us with predictions that enable it to be tested in the way that general relativity did. But it hasn’t done that yet and until it does it is not a scientifically valid physical theory.

I remember a quote from Alfred North Whitehead that I put in my ~~PhD~~ DPhil thesis many years ago. I wasn’t thinking of string theory at the time, but it seems relevant:

There is no more common error that to assume that, because prolonged and accurate mathematical calculations have been made, the application of the result to some fact of nature is absolutely certain.

My problem is not with string theory itself but with the fact that so many string theorists have become so attached to it that it has become a universe in its own right, with very little to do with the natural universe which is – or at least used to be – the subject of theoretical physics. I find it quite alarming, actually, that in the world outside academia you will find many people who think theoretical physics and string theory are more-or-less synonymous.

The most disturbing manifestation of this tendency is the lack of interest shown by some exponents of string theory in the issue of whether or not it is testable. By this I don’t mean whether we have the technology at the moment to test it (which we clearly don’t). After all, many predictions of the standard model of particle physics had to wait decades before accelerators got big enough to reach the required energies. The question is whether string theory can be testable in principle, and surely this is something any physicist worthy of the name should consider to be of fundamental importance?