Feeds:
Posts

## What is that bright object in the evening sky?

A number of people have been asking me over the last two or three weeks what the (very) bright object is in the evening sky. It is Venus, the brightest of all the planets. If you look towards the west (the same part of the sky as where the Sun has set) on any clear(ish) evening over the next two months, within a few hours of sunset, you should easily be able to see Venus.

Here is a diagram showing the evening sky for this evening (12 January 2017) as seen from Cardiff, and I have set it up to show the sky at 6pm. In Cardiff today the Sun sets at 16:29. Venus will not set until 20:51, nearly 3.5 hours after the Sun has set. This is why it is visible for such a long time after sunset.

The western sky at 6pm as seen from Cardiff. Today the Sun will set in Cardiff at 4:29pm, with Venus not setting until 8:51pm. This is nearly 3.5 hours after sunset, and today is the day of maximum eastern elongation.

In fact, today (12 January) is the day when the time between the Sun setting and the time at which Venus sets is at its greatest. That is why I chose today to blog about Venus. This is called maximum eastern elongation, and it is shown in the diagram below.

When the angle between a line from Earth to Venus and Venus to the Sun is a right angle, we have maximum elongation. As Venus is currently to the East of the Sun (rising after and setting after the Sun), it is today at maximum eastern elongation.

Venus will dominate the evening sky for another 6 weeks or so, although it will start setting closer and closer to the time of sunset now that we have passed maximum eastern elongation. It will swing in front of the Sun (something called inferior conjunction) on 25 March, so will be lost in the glow of the Sun for a few weeks before that. A few weeks after inferior conjunction, it will reappear as a morning object, becoming increasingly visible before sunrise as opposed to after sunset.

So, enjoy the wonderful sight of Venus in the evening sky over the next 6 weeks or so. And, if you can get hold of a pair of binoculars or a small telescope, you will see that Venus exhibits phases. Currently it is a quarter phase (half of it is illuminated), but as it approaches inferior conjunction it will become more and more crescent, but also appear to get larger in your viewing device (this cannot be seen with the naked eye). It was observations like these which enabled Galileo to show in 1610/1611 that Venus could not be orbiting the Earth, but that both Earth and Venus must be orbiting the Sun.

## Is there life on Mars? (part 1)

As those of you following my blog will know, I am currently on a cruise around New Zealand, giving astronomy talks. One of my six talks is about our current understanding of whether there is (or was) life on Mars. I try to only talk about objects which are visible during the cruise, and Mars is currently visible in the evening sky, albeit a lot fainter than it was in May when it was at opposition.

One of the talks I am giving on this cruise is our current understanding of whether there is (or was) life on Mars.

The question of whether there is life on Mars, or whether there ever has been in its history, is a fascinating one. I thought I would do a series of blogs to explore the question. But, I have to begin by saying that ANY search for life beyond Earth is predicated by our understanding of life on Earth. The only thing, it would seem, required by all forms of life which we have found on earth is water. Extremophiles show that life can exist without oxygen, without light, at high pressure, in radioactive environments; in fact in all sorts of environments which humans would find impossible. But, none of the life so far found on Earth can exist without water.

As a consequence, all searches for life in our Solar System tend to begin with the search for water. Now, it may be that life beyond Earth could have evolved to exist without the need for water. I am no chemist, but I don’t think there is anything particularly unique about water in its chemistry which makes it impossible for living cells to use some other substance. Water is the only substance on Earth which can exist in all three forms naturally (solid, liquid and gas), so it does occupy an unique place in the environment found on Earth. But, on Titan for example, methane seems to exist in all three forms. Maybe life has evolved on Titan to metabolise using methane in the same way that life on Earth has evolved to metabolise using water. We don’t know.

So, I thought I would start this series of blogs with the big news in the 1890s, that Martians had built canals on the red planet!

## Schiaparelli and Martian ‘canali’

The Schiaparelli space probe which ESA sadly failed to land on Mars recently was named after Italian astronomer Giovanni Schiaparelli. In the late 1880s he reported seeing ‘canali’ on the surface of Mars. Although this means ‘channels’, it got mis-translated to ‘canals’, and led to a flurry of excitement that this was evidence of an intelligent civilisation on Mars.

The idea grew that Martians had built canals to transport water from the “wet” regions near the poles to the arid equatorial regions. The ice caps of Mars are easily visible through a small telescope, so astronomers had known for decades that Mars had ice caps which they assumed were similar to the ice caps on Earth.

Giovanni Schiaparelli’s map of ‘canali’ on Mars, from 1888.

One person who became particularly taken with this idea of canals on Mars was American Percival Lowell. Lowell came from a rich Bostonian family, and had enough personal wealth to build an observatory in Flagstaff, Arizona. He set about proving the existence of life on Mars, writing several books on the subject. He published Mars (1895), Mars and Its Canals (1906), and Mars As the Abode of Life (1908). But, by 1909 the 60-inch telescope at Mount Wilson Observatory had shown that the ‘canali’ were natural features, and Lowell was forced to abandon his ideas that intelligent life existed on Mars.

However, his Flagstaff Observatory was to go on and make important contributions to astronomy. In the 1910s Vesto Slipher was the first person to show that nearly all spiral nebulae (spiral galaxies as we now call them) showed a redshift, the first bit of observational evidence that the Universe is expanding. And, in 1930 Clyde Tombaugh discovered Pluto at Flagstaff Observatory.

In part 2 of this blog, next week, I will talk about the first space probes sent to Mars, and the first images taken of Mars by a space probe which successfully orbited the planet, Mariner 4.

## Off to New Zealand

Tomorrow (Friday 25 November) I am boarding a plane which will eventually get me to Brisbane (Australia), via Seoul. Yes, I’m aware that Brisbane is not New Zealand, but in Brisbane I am joining a cruise which is going around New Zealand. The cruise will last for 14 nights, and I will give about 6 talks during the two weeks.

The Princess Cruise leaves Brisbane on 27 November and returns on 11 December. I will be giving astronomy talks on the 14-night cruise.

This will be the 5th cruise which I’ve done with Princess, and the 6th in total. The last time I did a cruise in the southern hemisphere was in February, when I cruised from Buenos Aires to Santiago around Cape Horn. Unfortunately, during that 14-night cruise, we had only one clear night! I am hoping for better weather this time, as in addition to my talks I run star parties to show the guests what is visible in the night-time sky.

Many of the guests will probably be from Europe or the United States, and so will be very keen to see the Southern Cross. I will also show them the Magellanic Clouds if weather permits. The New Moon is on the 29 November, so the first week of the cruise will be ideal to see the Magellanic Clouds if the skies are clear. After that, the brightening moon will render them all by invisible. So, fingers crossed we get some clear skies during the first week!

## Beagle 2 “close to Mars success”

New images of the European Space Agency’s Beagle 2 have emerged recently, suggesting that it came closer to success than has long been thought. These new images have been analysed more thoroughly and carefully than previous images of Beagle 2, and with the help of a computer simulation it is being suggested that Beagle 2 did not crash land. Instead, this team led by Professor Mark Sims of Leicester University are arguing that Beagle 2 deployed, but not completely correctly. They suggest that, due to not deploying correctly, that it may well have done science for a period of about 100 days, before shutting down due to lack of power. They even suggest that there is a very slim possibility that it is still working.

I do have to take issue, however, with the way this story is worded on the BBC website. It implies that we now know, with certainty, that Beagle 2 operated for some period on the surface of Mars. This is not true. One study has argued that it did. One swallow does not make a summer. This particular team’s analysis and study will need to be looked at by others before we can say with any reasonable certainty that Beagle 2 survived its landing.

New images of Beagle 2 taken by NASA’s Mars Reconnaissance Orbiter have been analysed by a computer model, suggesting it may have actually worked for a short period of time.

As with any suggestion which flies in the face of conventional wisdom, this claim will need to be checked and followed up by others. But, if the evidence is sufficiently strong that Beagle 2 did not crash, then it will come as a relief to those who worked on it and have long felt that it failed in a crash. Sadly, even if it did work, we have not received any data back from it; and that is not going to change.

## Do GPS satellites move in the sky?

Following on from my blog “Is Tim Peake getting younger or older?” , a bit of fun to work out whether time was passing more slowly or more quickly for Tim Peake in the ISS than it is for us on the ground, it got me thinking about the global positioning system (GPS) that so many of us use on a daily basis. Whether it is using a SATNAV in our car, or a GPS-enabled watch to measure how far and fast we have run, or using maps on a smartphone, GPS must be one of the most-used satellite developments of the last few decades.

As I blogged about here, communication satellites need to be at a particular height above the Earth’s surface so that they orbit the Earth in the same time that it takes the Earth to rotate. In addition to their altitude, communication satellites can only orbit the Earth about the equator, no other orientation will allow the satellite to hover in the same place relative to a location on Earth.

But what about the satellites used in GPS? What kind of an orbit are they in?

## The GPS satellites’ orbits

It turns out that the GPS satellites are not in a geo-stationary orbit, but are in fact in an orbit which leads to their orbiting the Earth exactly twice in each sidereal day (for a definition of sidereal day see my blog here).

The GPS system consists of 31 satellites in orbit around the Earth

We can work out what radius from the Earth’s centre this needs to be by remembering that the speed of orbit is given by

$v = \sqrt { \frac{ GM }{ r } } \text{ (1) }$
where $v$ is the speed of orbit, $G$ is the universal gravitational constant, $M$ is the mass of the Earth and $r$ is the radius of orbit from the centre of the Earth (not from its surface).

A sideral day is 23 hours and 56 minutes, which in seconds is $8.6160 \times 10^{4}$ seconds. So, half a sidereal day is $4.308 \times 10^{4}$ seconds. We will call this the period $T$. The speed of orbit, $v$ is related to the period via the equation
$v = \frac{ 2 \pi r }{ T }$
where $r$ is the radius of the orbit, the same $r$ as in equation (1), and $2 \pi r$ is just the circumference of a circle. So, squaring Equation (1), we can write
$v^{2} = \frac{ GM }{ r } = \left( \frac{ 2 \pi r }{ T } \right)^{2}$
So, in terms of $r$ we can write
$r^{3} = \frac{ G M T^{2} }{ 4 \pi^{2} } \rightarrow r = \sqrt[3]{ \frac{ G M T^{2} }{ 4 \pi^{2} } }, \; \text{ so } r = 26.555 \times 10^{6} \text{ m}$
In terms of height above the Earth’s surface, we need to subtract off the radius of the Earth, so the altitude, which I will call $a_{gps}$, is going to be
$a_{gps} = 26.555 \times 10^{6} - 6.371 \times 10^{6} = 20.184 \times 10^{6} \text{ m } \text{ or } \boxed{ 20.2 \text{ thousand kilometeres} }$

## Why are GPS satellites in this kind of an orbit?

As I didn’t know what kind of an orbit GPS satellites were in before I wrote this blog, the next obvious question is – why are they in an orbit which is exactly half a sidereal day? It is clearly not coincidental! To answer this question, we need to first of all discuss how GPS works.

GPS locates your position by measuring the time a signal takes to get to your GPS device from at least four satellites. Your device can identify from which satellites it gets a signal, and the system knows precisely the position of these satellites. By measuring the time the signals take to you reach you from each of the satellites, it is able to calculate how far each one is from you, and then by using triangulation it can work our your location. There are currently 31 satellites in the system, so often there are more than four visible to your GPS device. The current 31 satellites have all been launched since 1997, the original suite of 38 satellites launched between 1978 and 1997 are no longer in operation.

As I mentioned in my blog about geostationary satellites, a satellite in a geostationary orbit can only orbit above the Earth’s equator. This would clearly be no good for a GPS system, as all the satellites would lie to the south of someone in e.g. Europe or North America. As I said above, there are currently 31 operational satellites; the 31 are divided into 6 orbital planes. If there were 30 satellites this would be 5 in each orbit. The orbits are inclined at $55^{\circ}$ to the Earth’s equator. Each orbit is separated from the other one by 4 hours (equivalent to $60^{\circ}$) in longitude.

As one can see approximately 6 hours in right ascension to both the east and west of one’s location, this means that there will be at least 3 of the orbits above the horizon, and sometimes more. If there were 5 satellites in each orbit this would mean that each one would pass a particular latitude 4 hours before the next one. So, at any particular time there should be some satellites further north than one’s location and some further south, as well as some further east and some further west. This configuration allows for the necessary triangulation to obtain one’s location.

The orbits are inclined at $55^{\circ}$ to the equator and separated by 4 hours (equivalent to $60^{\circ}$) in right ascension, as this diagram attempts to show

## Is the time-dilation effect due to SR or GR more important for these satellites?

We already showed in this blog that, for the International Space Station, the time-dilation due to Special Relativity (SR) has a greater effect on the passage of time than the time-dilation due to General Relativity (GR). What about for the GPS satellites?
The speed of orbit for the GPS satellites at a radius of $26.555 \times 10^{6}$ from the Earth’s centre is, using Equation (1),
$v = \sqrt{ \frac{ GM }{ r } } = 3.873 \times 10^{3} \text{ m/s}$
As we showed in my blog about Tim Peake, the speed of someone on the Earth’s surface relative to the centre of the Earth is $v_{se} = 463.35 \text{ m/s}$, so the relative speed between a GPS satellite and someone on the Earth’s surface is given by
$v = 3.873 \times 10^{3} - 463.35 = 3.410 \times 10^{3} \text{ m/s}$
Compare this to the value for the ISS, which was $7.4437 \times 10^{3}$, it is less than half the speed.

This value of $v$ leads to a time dilation factor $\gamma$ in SR of
$\gamma = \frac{ 1 }{ \sqrt{ 0.9999999999} } \approx 1$
which means that the time dilation due to SR is negligible.
The time dilation due to GR is given by (see my blog here on how to calculate this)
$\left( 1 - \frac{ gh }{ c^{2} } \right) = (1 - 2.2 \times 10{-9}) = 0.9999999978$, or 22 parts in $10^{10}$. Compare this to the ISS, where it was about 1 part in $10^{11}$. Clearly the GR effect for GPS satellites is greater, by about a factor of 5, than it was for the ISS. But, conversely, the SR time-dilation effect has become negligible.

To conclude, the time dilation for GPS satellites is nearly entirely due to General Relativity, and not due to Special Relativity. Time is passing more quickly for the clocks on the GPS satellites than it is for us on Earth, the converse of what we found for the ISS, which is in a much lower orbit.

Because the timings required for GPS to work are so precise, the time dilation effect due to GR needs to be taken into account, and is one of the best pieces of evidence we have that time dilation in GR actually does happen.

## Has dark energy had its day?

There have been a number of rumours of late that the evidence for dark energy is suspect, and that maybe, after all, it doesn’t exist. The stories are due to a paper which was recently published in Nature Scientific Reports, for a link to the original paper follow this link. Unlike most papers published in Nature, which are behind a paywall, this paper is available in its entirety for free. The authors argue that a larger data set of Type Ia supernovae, which were used in the 1990s as evidence for an accelerating Universe, now calls into question that whole interpretation of the data.

This paper – “Marginal evidence for cosmic acceleration from Type Ia supernovae”, published in Nature Scientific Reports on 21 October 2016, calls into question the evidence for the existence of dark energy.

The 2011 Nobel prize for physics was awarded to Saul Perlmutter, Brian Schmidt and Adam Riess for their original “discovery” of cosmic acceleration, so if new data now call into question that whole idea it is, clearly, big news.

However, Adam Riess has co-authored an interesting guest blogpost in Scientific American, entitled “No, Astronomers Haven’t Decided Dark Energy Is Nonexistent”, here is a link to that article. Riess and his co-author Dan Scolnic (a cosmologist based at the University of Chicago’s Kavli Institute for Cosmological Physics) point out in this blogpost that the re-analysis by Nielsen etal. reduces the confidence that the Universe is accelerating to a 3-sigma result, which is still at a confidence level of 99.7%! So, it hardly does away with the need for acceleration, at a confidence of 99.7% it is still pretty likely. True, it now falls short of the usual 5-sigma result that scientists usually require for a “definite result”; but they also take issue with the way that Nielsen etal. have analysed their data.

Also, as Scolnic and Riess point out, evidence for cosmic acceleration is not just based on the results from surveys of Type Ia supernovae. Studies of the details of the anisotropies in the cosmic microwave background also require dark energy (thought to be responsible for cosmic acceleration), and so do surveys of the large scale structure of the Universe done by surveys such as the Sloan Digital Sky Survey and the 2 Degree Field Galaxy Redshift Survey.

This model, often called the concordance model, as it is supported by these 3 separate lines of evidence, is summarised in this figure. In this diagram, “BAO” are the results from the large scale structure surveys (the acronym stands for Baryonic Acoustic Oscillations). As the figure shows, the percentage of dark energy required to explain the results of SN, CMB and BAO is about 70% (0.7 on the y-axis).

This figure shows the so-called “concordance model”, three separate lines of evidence which support the existence of dark energy at about the 70-80% level. The figure is from Scolnic and Riess’s blogpost “No, Astronomers Haven’t Decided Dark Energy Is Nonexistent” which can be read by following this link.

You can read more about the three separate lines of evidence for dark matter in my book The Cosmic Microwave Background, How It Changed Our Understanding Of The Universe (follow this link to find out more about the book, including reviews).

My book “The Cosmic Microwave Background – how it changed our understanding of the Universe” is published by Springer and can be found by following this link.

It seems to me that this is a lot of fuss about nothing, and that the case for cosmic acceleration is as strong as ever. What do you think?

## Why do we move our clocks back in autumn?

This last weekend the clocks went back 1 hour in the Disunited Kingdom. It is done on the night of the last Saturday/Sunday in October. We have switched from being on British Summer Time (BST) to being on Greenwich Mean Time (GMT). We will move them forward again by 1 hour on the night of the last Saturday/Sunday in March. In the United States, the clocks go back this coming weekend (the night of the 5/6 November), changing from e.g. Eastern Daylight Time (EDT) to Eastern Standard Time (EST), and they will go forward again on the night of the second Saturday/Sunday in March.

Conversely, in e.g. Sydney (Australia), they moved their clocks forward (as they are in spring) in early October, and will move them back in early April.

Whenever we change our clocks, I tend to get people asking me why we do this? This is asked by many people who have grown up here, but also by people who have come from countries like e.g. Nigeria or India or Saudi Arabia where they don’t change their clocks in spring and autumn. And, for the first time, my youngest daughter, who is now 15, asked me why we did it. So, here is my best attempt to explain it.

It has to do with the varying length of the time between sunrise and sunset during the summer months and the winter months. And, in addition to this variation, trying to shift the time of sunset to a later time during the longer days of summer. The difference between what I will call “the length of the day” (what I really mean is the time between sunrise and sunset) varies most for places far from the Earth’s equator, and varies very little for places near the equator.

## The variation in the length of the day

To illustrate this, I have chosen four cities, Reykjavik (in Iceland), London (England), Lagos (Nigeria) and Cape Town (South Africa) which have differing latitudes. As you can see from the map below, Reykjavik is a long way from the equator (which, in this map, goes through e.g. Gabon, Congo Kinshasa, and Kenya), at a latitude of 64 degrees North. London is at a latitude of 53.5 degrees North. Lagos is close to the equator, at only 6.5 degrees North of it, and Cape Town is just outside of the tropics, at a latitude of 34 degrees South of the equator.

The four cities marked are Reykjavik (Iceland), London (England), Lagos (Nigeria) and Cape Town (South Africa).

Of these four cities, Reykjavik will have the biggest variation between the length of the day in late June and late December (the summer and winter solstices), and Lagos will have the smallest difference. Here is a table showing the length of time between sunrise and sunset on the summer and winter solstices for these four cities (to the nearest quarter of an hour).

## Length of longest and shortest day for 4 cities

The length of the longest and shortest days in Reykjavik, London, Lagos and Cape Town
City Latitude Longest day Shortest day
Reykjavik (Iceland) 64 degrees N 21h 4h 15m
London (England) 53.5 degrees N 16h 30m 8h
Lagos (Nigeria) 6.5 degrees N 12h 30m 11h 30m
Cape Town (South Africa) 34 degrees S 13h 30m 10h

## Changing our clocks in spring/autumn

So, this shows how the length of the day varies between June and December, but why do we move our clocks in e.g. the Disunited Kingdom (or the USA), but not in e.g. Nigeria? Well, for countries nearer the equator, the variation in the length of the day is pretty small. For Lagos, the longest day is only 1 hour longer than the shortest day (12 hours 30 minutes compared to 11 hours 30 minutes). For London, the longest day is 16 hours and 30 minutes, the shortest only 8 hours, a difference of 8 and a half hours between the length of the day in June and the length of the day in December. In Reykjavik the difference is even more extreme. In late June the length of the day is 21 hours, whereas in late December it is only 4 hours and 15 minutes, a massive variation.

Let us now look at what time the sun rises and sets in London on the longest day. It rises at 03:45 GMT, and sets at 20:20 GMT (8:20 PM). Most people are not in bed before about 10pm, and very few people are awake at 3:45 AM. Therefore, by shifting the clocks one hour forward we make the sunrise one hour later (4:45 AM), which is still before most people get up, and gain an hour of extra daylight in the evening instead, with the sun not setting until 9:20 PM, when most people are still awake to take advantage of it.

There has been talk for as long as I can remember of having the Disunited Kingdom be on “double” British Summer Time during the summer months, and be on British Summer Time (GMT + 1 hour) during the winter months. This would mean that the sunrise in late June in London would be at 5:45 AM (still before most people get up), and sunset would be at 10:20 PM. That would be agreeable to a lot of people, but if we were on GMT + 1 hour in the winter months sunrise in late December would not be until after 9 AM, which I think most people would not like at all!

Of the four cities I have used in this illustration, only London changes its clocks. Reykjavik does not, and neither does Lagos nor Cape Town. I guess with Reykjavik, the days are so long in the summer months that it is light before anyone gets up and it is still light when most people go to bed. With Lagos, the change between longest day and shortest day is so short that it is pointless to change the clocks, but I was surprised to see that Cape Town does not utilise daylight saving.

In fact, South Africa does not observe daylight saving. Surprisingly, Namibia, which is closer to the equator than South Africa, does observe daylight saving between early April and early September. Namibia seems to be the only country in southern Africa which observes daylight saving; Botswana, Zimbabwe, Mozambique, Malawi, Angola, Zambia and South Africa do not observe daylight saving. If you want to check whether a particular city observes daylight saving, and when the changes happen, you can follow this link.

Does anyone know of any countries in Europe which do not move their clocks forward and back in spring/autumn? I know in the United States there are states which do not go on to Daylight Savings Time, e.g. Arizona and parts of Indiana. And, of course, Hawaii.

## The World’s Time Zones

Here is a map of the time zones in the world, centred on Greenwich Mean Time (GMT), as Greenwich is where zero of longitude is set. The country with the most time zones is, not surprisingly, Russia, as it extends from Europe all the way to eastern Asia, even further to the east than any part of Australia or Japan.

Surprisingly for two countries which are so extended in an east-west direction, both China and India only have 1 time zone respectively. This means that if you live in e.g. Beijing your local sunrise will be about two hours earlier than if you live in a city in the west of China such as Aksu. And, there are parts of China which lie to the west of most of India, but these western parts of China are 2.5 hours ahead on the time used in the two countries (GMT+5.5 in India, GMT+8 in China).

A map of the world’s time zones. The time is centred around Greenwich Mean Time (GMT), as Greenwich is the zero line of longitude on the Earth.