What happens when a car travelling near the speed of light turns on its headlights?

This car looks fast, but could it travel near the speed of light? Picture by Biczzz, from Flickr.

What happens when a car travelling near the speed of light turns on its headlights? That is an interesting question that you would not expect to find in the sports pages of a major newspaper. However, this question was posed by journalist Peter FitzSimons in his sports column, The Fitz Files, in the Sydney Morning Herald. TFF specifically requested scientists to provide answers.

Two of my Observatory colleagues, Dr Henry Woodruff and Allan Kreuiter, both prepared good answers and I edited them together, adding a little bit of extra information. Allan then sent off the combined answer to TFF under my name so embarassingly I was the only person from the Observatory acknowledged in the final piece in TFF. In the column Peter FitzSimons did a great job collating the answers from the Observatory, from Alan Kennett and from Professor Tim Bedding of the School of Physics at Sydney University.

Here I just reiterate and slightly expand on the answer given in the column. The car is travelling, say, at 99% of the speed of light (c). To an observer on the side of the road the light from the car headlights is travelling at c and pulling ahead of the car at .01c.

The puzzle is that to the occupants of the car the light is leaving the car at c. That is, whatever the speed at which you are travelling you always measure the speed of light as the same value of c, approximately 300,000 km per second. This constancy of the speed of light is the main plank of Albert Einstein’s Special Theory of Relativity published in 1905.

How can the occupants of the car measure the same value for the speed of light? It is because for them time passes much more slowly relative to the stationary observer at the side of the road. A most interesting set of UNSW lecture notes on time dilation mentions that the most energetic cosmic rays measured to hit the Earth’s atmosphere can cross the visible Universe in a matter of months on the particles’ time scale.

As mentioned in TFF, if the car is travelling near the speed of light an observer in front of the car would see the beam from the headlights not as visible light, but as energetic gamma rays since the light is shifted towards shorter wavelengths by the Doppler effect. Another complication is that of relativistic beaming, which means that at such “relativistic” speeds all light emitted from the car is in a narrow concentrated beam in the forward direction.

Think about the brave and foolhardy physicist who wants to check out the theory from in front of the high-speed car. Even if she is hundreds of thousands of kilometres away, within seconds she would be fried by the intense beam of gamma rays and then a blink of an eye later flattened by the car itself. Bravery in the name of science!

Harry saw a great prominence erupt on 20 June 2010

The erupting prominence of 20 June 2010, drawing and copyright Harry Roberts ©, all rights reserved

While the sunspots of Cycle 24 are currently puny and often short-lived – the prominences (sunspots’ distant cousins) are increasingly larger and brighter. It is such a contrast to see a few small spots in white light – then in Hα to find a huge prominence wrapped around the sun’s limb for almost 20 degrees. Such was the case on June 20 00:09UT (Fig).

Earlier on June 19 00:00UT a bright arch prominence had been logged at the NE limb between 32ºN and 24ºN, with faint wisps to 10ºN – a first glimpse of the giant that would come into view next day.

The 20th revealed a huge prominence between latitudes 13ºN and 30ºN, and almost 60,000km (60Mm) high, (or about 5 Earth diameters). It was a spectacular sight, and while recording it I saw signs of motion particularly in places where the prominence had broken free of the solar disc. It was clear from 00:29 onwards that the southern half of the prominence was slowly erupting as the three following records show.

“Once an arch breaks loose or rises beyond 30,000 km it will soon erupt”, writes Harold Zirin (“Astrophysics of the Sun”, p269) in referring to a large quiescent prominence – and close monitoring of the arches is a good sign of an impending eruption. In the drawing above two foot-points are labelled ‘a’ and ‘b’, see how ‘a’ breaks free at 00:33 while ‘b’ lasts until 00:47. Note that events in the figure proceed from right to left.

Another sign of an imminent eruption was the rapid ascent of the arch just north of (above) point ‘a’ near latitude 20º– that quickly rose from 20 Mm high at 00:29 to over 60 Mm at 00:47. What happened next we might ask?

The coronal mass ejection of 20 June 2010 as recorded by the LASCO C2 camera of the SOHO spacecraft. Click on image for animation. Courtesy SOHO/NASA/ESA

In my case the sun went behind nearby trees and the session ended – but the LASCO C2 instrument on the SOHO satellite recorded a spectacular coronal mass ejection (CME) from the prominence site. The LASCO camera is protected by an occulting disc roughly twice the apparent solar diameter – and it took the ejecta another hour (~01:50 UT) to travel that distance (about 600 Mm) and be recorded– an ejection velocity of ~180km/s. A strangely distorted analogue of the prominence was then seen moving across LASCO’s field of view as a CME.

Can amateurs with Hα ‘scopes observe CME’s? Sadly no – but you can see some precursor events, particularly if you know the location of such events on the sun, and log what is seen in universal time (UT).

Presumably the earlier view of the 19th showed just the top parts of the large prominence that would come into full view next day due to solar rotation – placing it nicely for us to witness the huge eruption.

Harry Roberts is a frequent contributor to this blog and is a member of the Sydney City Skywatchers

Looking for Venus or how to find planets in the night sky

A drawing of the solar system. As in any illustration of it liberties had to be taken by the artist such as showing the planets closer together and making them relatively larger than they are in reality. Image courtesy of NASA/JPL

Whenever Venus is prominent in the evenings, there are numerous enquiries to Sydney Observatory either by ‘phone, email or on the Lights in the sky page on the Observatory blog about the strange bright light in the western sky. To help with identifying Venus and other planets let us discuss how to find planets.

The first thing to consider is that we look for planets from a moving platform, the Earth. Hence the relative positions of the planets in the sky changes not just because the planets are moving around the Sun, but because our view point is continually shifting as the Earth itself is circling the Sun.

What does simplify the situation is that the planets all circle the Sun in the same plane, or close to it. This plane, called the ecliptic, forms a curved line across the sky and the planets are always found on or close to this line. How the line stretches across the sky varies during the year – you can see it marked on the monthly sky maps available from this blog and on the maps published in the Australian Sky Guide.

Bright Venus and other planets are easy to find when they are bunched together as on the evening of Sunday 8 August 2010. There is another bunching of four planets in May 2011 while the much anticipated close bunching of all five naked eye planets takes place on 8 September 2040. The picture is the sum of three 4-second exposures and the planets are indicated as well as two stars in the constellation of Virgo the Maiden. Picture Nick Lomb

How do we find Venus and Mercury? These are the two inner planets closer to the Sun than the Earth as illustrated on the top diagram. Hence as seen from Earth they can never stray too far from the Sun. If Venus, for example, happens to be east of the Sun it will still be above the horizon when the Sun sets and it will be an evening object in the western sky. If it happens to be west of the Sun then it will be visible in the mornings before sunrise and so it will be a morning object in the western sky. It is this property that it can be seen either in the mornings before sunrise or in the evenings after sunset that gives Venus the common designations as the Morning Star or the Evening Star.

Venus is the brightest of all the planets for a number of reasons. It is perpetually covered by clouds that efficiently reflect the light it receives from the Sun. It is also close to the Sun and so receives more sunlight than the Earth. Lastly, Venus is relatively close to the Earth so that when it reaches us its light is less dissipated than light from more distant planets. To the unaided eye Venus is always just a pin-point of light, but a telescope shows that Venus shows phases like the Moon. This leads to changes in brightness that are mainly compensated by changes in angular size: Venus shows a small full disc when on the other side of the Sun and a larger crescent when closer to the Earth.

The situation with Mercury is similar except it is not as bright as Venus and is more constrained to be visible close to the horizon after sunset or before sunrise.

Do planets twinkle ? One indication that a bright object is a planet and not a star is that is that it twinkles less. Twinkling, known to astronomers as scintillation, is caused by starlight being bent towards or away from the eye by moving, turbulent air masses in the atmosphere. There is less twinkling from a planet as the situation is different. To the unaided eye the light from as planet appears to be a pin-point of light same as from a star, but that is not the case. Planets have visible discs that are revealed through a telescope so that their light comes from a range of angles and the twinkling averages out. This is not to say that planets do not twinkle at all as there maybe noticeable twinkling when a planet appears close to the horizon.

Summary To find planets note 1. they do not appear all over the sky, but only along a curved line called the ecliptic. 2. Venus and Mercury can only be seen in the west after sunset or in the mornings in the east before sunrise 3. planets twinkle less than stars do. When all else fails, look up the rise and set times of the planets in the newspapers or preferably in a copy of the Australian Sky Guide.

Using the Southern Cross to find the date or the time

The position of the Southern Cross in the southern sky at 8 pm standard time during each month of the year. As on the diagram the Southern Cross is circumpolar, that is it never sets, for most places in Australia. Sketch Nick Lomb

The Southern Cross has a special place in Australia as illustrated by its appearance on the national flag and its celebration in the National Anthem by the words, “Beneath our radiant Southern Cross”. As well, it appears on the logos of numerous companies such as a major bank and on the liveries of airlines.

The smallest of the 88 constellations or star pictures named by astronomers, it is prominent in the southern sky for most of the year apart from the summer months. Even in December and January it does not sink beneath the horizon as seen from most places in Australia. To find it look for the two bright stars known as the Pointers that always point to the Cross. The two stars are Alpha Centauri and Beta Centauri, which is the one closer to the Cross in the sky. Confusedly, there are a number of other stars in the shape of a cross in the vicinity such as the ones making up the False Cross, but they are are less bright than the stars of the Southern Cross, have a less compact grouping and do not have the Pointer stars.

This prominent constellation is not only interesting to look at, but has a variety of uses. Sometime ago we discussed on this blog here how to use the Southern Cross to find direction. Here we discuss how the changing position of the Cross during each night and during the year can be used to find time and the date.

The diagram at the top shows the position of the Southern Cross at 8 pm standard time (or 9 pm summer time) for each month of the year. If you do not know the date you can go outside at 8 pm, find the orientation of the Cross and use the diagram to deduce the month. More realistically, the diagram shows where to look for the Southern Cross at a convenient time in the evening on any date during the year. As can be seen March to August is the best period to view the Cross in the evenings as it low in the sky for the remaining months, especially during summer.

The Southern Cross not only spins around the southern sky once a year, it also does a complete circle each 24 hours. Actually it makes the complete circle in 23 hours 56 minutes and 4 seconds or one sidereal day. The approximately four minutes difference between an ordinary day and a sidereal day is significant for it makes the Southern Cross rise four minutes earlier each day, which is 30 minutes a week or two hours per month. And that two hours a month leads to the monthly change in orientation illustrated at the top.

The Southern Cross at various times in June. As discussed in the text it can indicate the time during other months as well. Sketch Nick Lomb

As the Southern Cross spins around the sky once a day we can use it to find the time as long as we know the date. The diagram above illustrates how the Cross changes its orientation during a night in mid June. A similar scheme can be used during other months. For example, for August subtract four hours from the times shown on the diagram so that then the Cross is in the same orientation at 8 pm as it is at midnight in June.

Keep watching the sky.

Nick visits the Melbourne Solar System trail, finds six planets, loses Neptune and takes a surprise trip to Proxima Centauri

The Sun model with its plaque visible in the background. In accordance with the one to one billion scale of the Solar System trail, the Sun model is 1.39 metres across representing the 1.39 million km diameter of the real Sun. Photo Nick Lomb

On a glorious sunny winter afternoon I visited the Melbourne Solar System trail on the Port Phillip foreshore. Initially not being fully certain of its location, I was pleased that my parking spot close to the St Kilda Marina Reserve at the south end of St Kilda beach was right near the Sun model. After inspecting the model I walked along towards the beach and soon found the Mercury model on a stone pedestal 60 m further. Then came Venus and at 150 m from the Sun, the double model of the Earth and the Moon. After these, at progressively larger separations I found the other planets out to Saturn. Pictures of three of the planet models are below:

The Earth and Moon model is 150 m from the Sun. A little unfortunately, it is on the other side of the bicycle track from the footpath making it a little tricky to reach for a close-up inspection and to read the plaques. Photo Nick Lomb

The impressive Jupiter model is detailed enough to show the planet’s cloud-belts. The model is 14.3 cm across representing the 143,000 km width of the giant planet. Photo Nick Lomb

Saturn at 1.4 km distance from the Sun model is also impressive with the rings clearly shown. Photo Nick Lomb

After admiring Saturn I found that I had to race back to the Sun and my nearby parking spot since the one hour on the parking meter ($4/hour) was going to expire. On reaching the Sun, I had a surprise as there was another pedestal and a model just behind. Vulcan, the legendary planet that astronomers of the past had thought was circling the Sun inside the orbit of Mercury? No, it was a model of the nearest star to the Earth, Proxima Centauri. I was confused at first thinking that the scale of the trail must have changed enormously to be able to show Proxima so close to the Sun.

On reading the plaque, however, all was solved and very cleverly too. Proxima Centauri is about 40 thousand billion km (40 trillion km) from the Sun, which distance at the scale of one to a billion is the 40,0000 km circumference of the Earth. So during the short walk from the Sun model to Proxima you need to imagine travelling around the globe like Puck in Midsummer Night’s Dream who says, “I’ll put a girdle round about the Earth in forty minutes.”

The model of Alpha Centauri is much smaller than that of the Sun as it is a small red dwarf star. Photo Nick Lomb

At this point I decided to skip Uranus, forget about the distant model of the dwarf planet Pluto and search for Neptune. This time I parked near historic Station Pier and walked back on the Beach Road footpath looking for the Neptune model. If I interpret the maps on the pedestals correctly, Neptune is near the Port Melbourne Yacht Club at the intersection of Beach and Bay streets. However, it was nowhere to be found. Has it gone AWOL or was it hiding? Admittedly, by this stage it was getting cool and your blogger was in need of sustenance so the search may not have been as thorough as it could have been. On a future occasion the search may continue.

Station pier with Tasmanian ferry berthed. Picture Nick Lomb

Is the Melbourne Solar System trail successful? Should there be a similar one in Sydney? The trail is a great idea and beautifully illustrates the great distances in our solar system and further to the nearest star to the Sun. On the day I visited though only the Sun model was popular with children and their parents while the rest of the models seem to be ignored. Maybe there are too many distractions at the beach location even on a winter afternoon. In Sydney, as has long been discussed, a solar system trail leading up to Sydney Observatory could be perfect.

August 2010 night sky guide and podcast

To help you learn about the southern night sky, Sydney Observatory provides a monthly audio guide/podcast, transcript of that audio, and a sky map or chart (links below). This month’s audio sky guide is presented by Melissa Hulbert, a Sydney Observatory Astronomy Educator. You can listen online, or download the audio onto your ipod or mp3 player.

The free monthly night sky map PDF (below) shows the stars, constellations and planets visible in the night sky from anywhere in Australia. To view PDF star charts you will need to download and install Adobe Acrobat Reader if it’s not on your computer already.

August 2010 night sky map

For a year’s night sky maps and much more information, you can buy ‘The Australian sky guide’ book by Dr Nick Lomb at Sydney Observatory, Powerhouse Museum, good bookshops or through Powerhouse Publishing.

Read the transcript of the audio podcast. Hear the audio podcast:

 August 2010 night sky guide – 17 mins 10 secs: Play Now | Play in Popup | Download

Harry observes the strange morphing sunspot group AR11089

The sunspot group AR11089 as it first appeared on the east limb or edge of the Sun. Drawing. Drawing and copyright Harry Roberts ©, all rights reserved

AR11089 is not the name of an underground rock band, but the label applied by the US NOAA organisation, charged with keeping track of sunspots – as counting spots is still the key way of comparing solar activity across the centuries. Was the sun really spotless during the 17th century? Was its activity in the mid 20th century a four hundred year peak? Such questions are answered mainly by looking at the sunspot count.

The second half of the 20th century was the age of satellites; they imaged the sun in a range of wavebands particularly EUV (extreme ultraviolet). But satellites have short lives (one was even ‘shot down’ by the USAF) and normalising their data with the classic spot count isn’t easy. In fact, the best view of any sunspot is still the one in your trusty 3” or 4” amateur ‘scope with an aperture filter!

While this is titled AR11089 I want to briefly recall AR11085 – the spot group that “nobody saw” (except NOAA and the lucky writer). This small bipolar group perhaps lasted less than ten hours, and then disappeared – but when its locality next returned to the sun’s east limb, there was a large active-looking spot group at the exact site. And now everyone could see it, renumbered AR11089.

There are several odd things about this new group, apart from its Lazarus like reappearance. It’s roughly triangular in layout while most spot groups are bipolar and align E-W on the sun. And AR11089 sits in a large area of faculae (WL) and plage (Hα) that has another such activity patch only a few degrees west of it; this latter area is void of spots. Are they related in some way?

Solar Dynamics Observatory views of sunspot region AR11089, annotated by Harry Roberts

They are; the new SDO satellite views show magnificent arches (field transition arches) connecting both plage regions – the whole being one huge magnetic entity (Fig 3, main spots arrowed). And yet while fields within AR11089 briefly reached a ‘strong-ish’ 2500G the group has produced no flares >GOES C1. Curiously it also has few of the usual dark active region filaments (arf) that attend even the smallest sunspots.

The writer first saw the group at the east limb early in its development amid brilliant faculae – having six spots, the largest sited at –21/204 (Fig 1). The Fig shows changes from 19th to 21st (UT) – the group developing substantial penumbrae and ten or more umbrae, still in a triangular layout.

Sunspot group AR11089 on 23 July 2010. Drawing and copyright Harry Roberts ©, all rights reserved

Twenty-four hours later saw an increase to 20+ spots, with many tiny ones in short chains. The preceding spot (p) had a field of R25G (red 2500 gauss) and the larger following (f) spot to the SE had V24. Both are strong fields; but the trailing spot of the ‘triangle’ was now fading, with a few small spots in a chain. Overall the group seems to be ‘morphing’ into a more conventional E-W arrangement, and the remaining (p) and (f) components with 6º N-S separation may soon feel the pull of the Hale-Nicholson force and undergo some dramatic rearranging.

Harry Roberts is a regular contributor to this blog and a member of the Sydney City Skywatchers.

Mars is coming……….in 2018!

A map of Mars centred on the giant volcano Olympus Mons. Taken from Microsoft World Wide Telescope, credit NASA/USGS/MalinSpace Science Systems/JPL.

An email, often based on a PowerPoint presentation, has been circulating for the last six years or so and tells people that Mars is approaching in August and will be seen as big as the full Moon. To the disappointment of many of the recipients this email is a hoax. It recounts the events of August 2003 when Mars was exceptionally close to the Earth, but in a highly exaggerated fashion. Mars was not near Earth in August 2004, 2005, 2006, 2007, 2008 or 2009 and will not be near in August 2010. The next time we will have a good opportunity to view Mars will be in late July 2018.

Oppositions of Mars from 2003 to 2018. The central yellow disc represents the Sun, the next circle the path of the Earth and the outer circle or oval represents the path of Mars. Drawing Nick Lomb.

Why is Mars sometimes relatively near the Earth? Mars and the Earth both circle the Sun with Mars taking 687 days and the Earth 365 days to do so. That means Earth circling inside the path of Mars, will periodically “lap” the slower planet on the outside. On average this will happen every two years and two months with Mars being at “opposition”, that is on the opposite side of the Earth to the Sun, on those occasions. At oppositions Mars is relatively close the Earth.

To complicate matters, Mars has a much more oval-shaped path around the Sun than the Earth. As can be seen from the diagram above that means that some oppositions of Mars are more favourable, that is closer, than other ones. The one in August 2003, which triggered off the hoax email, was a favourable opposition while the next favourable opposition is the one on 31 July 2018.

Even at favourable oppositions Mars only appears like a bright red pinpoint of light to the unaided eye and certainly does not appear like the wonderful image at the top of this post. Through a telescope, like the the ones at Sydney Observatory, some detail can be discerned on the surface of the planet at those favourable oppositions. And even then the amount of surface detail is subject to the vagaries of atmospheric conditions above the telescope and on Mars itself. There have been disappointing Mars oppositions when planet-wide duststorms on Mars completely hid its surface from view.

While we wait for the opportunity that the favourable opposition of 2018 will provide to view the planet, we have unprecedented opportunities to study and become familiar with the surface of Mars on any personal computer. Numerous spacecraft have circled Mars sending back images and many are continuing to do so. Much of the mapping of Mars has been collected and is available through the Microsoft World Wide Telescope/Mars. With this web experience you can see Mars as you could during a favourable opposition or in much greater detail or in ways that are impossible from Earth such as looking down on the north or south poles of the planet.

As well as having fun with Mars on your computer, you can often see Mars in the sky with your unaided eye without having to wait for an opposition, favourable or otherwise. For example, in August 2010 Mars will be one of four planets giving a sky show by bunching up in the western sky after sunset. From 17 to 21 August it will appear close to Venus, the brightest planet.

Neptune almost back to the discovery position – in 2010 the outermost planet has nearly completed a circuit of the sky

The positions of the planet Neptune on 23 September 1846, 2010 and 2011 in relation to selected stars in the constellations of Capricornus and Aquarius. Drawing Nick Lomb

The planet Neptune is now regarded as the outermost planet of the solar system. It was discovered in somewhat controversial circumstances by the German astronomer Johann Gottfried Galle on 23 September 1846 at Berlin Observatory.

What was the controversy about? Galle did nothing controversial – he just searched the sky for a predicted new planet at a position sent to him by the director of Paris Observatory – a position calculated by the French scientist Urbain Le Verrier. This was all brilliant work, but the problem that arose was that an Englishman John Couch Adams had also made a similar prediction for the location of the new planet and the English scientists wanted Adams to share some of the credit for the discovery.

According to the story of Adams’ supporters he wanted to give the position that he had calculated to the Astronomer Royal at Greenwich Observatory, but due to the indifference of the Astronomer Royal George Airy no search was made for the planet. Recent historical research, helped by the rediscovery of files “borrowed” from the archives by an ex-director of Mt Stromlo Observatory Olin Eggen, suggests that Adams is due little credit as, unlike Le Verrier, he had not calculated the path of the predicted planet around the Sun.

How did Le Verrier and Adams know that there was an unseen planet in the sky? They knew that the planet Uranus discovered by William Herschel in 1781 was not quite following the path predicted by Newton’s Law of Gravity. Sometimes the planet was moving a little too fast and sometimes a little too slowly. A simple and correct explanation was provided by the gravitational pull of an unseen planet.

The planet takes 164.79 years to circle the Sun. How do we know the period so accurately? After the discovery of the planet, researchers found a number of pre-discovery observations made by people who had observed Neptune without realising that they were looking at a planet. The great Italian scientist Galileo observed Neptune at least twice, on 28 December 1612 and on 27 January 1613, without realising that he was seeing a planet, probably because his tiny telescope did not allow him to make out the disc of the planet.

A full period from the discovery date of 23 September 1846 takes us to the middle of 2011. However, as can be seen from the diagram above Neptune is already close to where it as first observed, a position in the vicinity of the star Deneb Algedi.