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Perihelion 2016

January 1, 2016 2 comments

At 2249 GMT on 2 January 2016 the Earth reaches perihelion, its closest approach to the Sun this year.

If that sounds confusing to you, and has you wondering why it’s so cold given that the Earth is at its closest to the Sun, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s summer right now), and (b) a misunderstanding of what causes the seasons.

The Earth orbits the sun in a nearly circular orbit called an ellipse. The degree by which an orbit differs from a perfect circle is called the eccentricity, e. If e = 0 then the orbit is circular; if e = 1 then the orbit is parabolic, and therefore not gravitationally bound to the Sun. 

The Earth’s orbital eccentricity is 0.0167, meaning that it is very nearly circular, with the short axis of the ellipse being around 96% the length of the long axis. 
Thus, during perihelion Earth is 0.983AU from the Sun, while during aphelion (its furthest distance from the Sun, occurring this year on 4 July) Earth is 1.017AU from the Sun. (1AU = 1 astronomical unit = the average distance between the Earth and the Sun = 150 million km).

The seasons on Earth have really nothing to do with how close the Earth is to the Sun at different times of year. Indeed how could they, given that the difference in distance between closest and furthest approach is only a few per cent? The seasonal differences we experience are of course caused by the tilt of the Earth’s axis, which is inclined by 23.5 degrees from the vertical.

This tilt means that, as Earth orbits the Sun, for six months of the year one hemisphere tips towards the Sun, so that it experiences longer days than nights, becoming most pronounced at midsummer, at which point the Sun reaches its highest in the sky at noon. Simultaneously the other hemisphere tips away from the Sun, and experiences shorter days than nights, becoming most pronounced at midwinter, on which day the Sun is at its lowest noontime altitude.

The further you are from the equator the more pronounced the seasonal effects. In fact equatorial countries don’t experience seasonal variations, while the poles experience extremes with six-month-long winters and summers. The timing of perihelion and aphelion relative to our seasons is entirely random. The fact the southern hemisphere midsummer (21 Dec) almost coincides with perihelion (2 Jan) is simply that; a coincidence. Given that fact, there is no reason to be surprised that perihelion occurs so close to northern hemisphere midwinter: it has to happen some time and it’s a coincidence that it happens to occur within two weeks of midwinter / midsummer.

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Kepler-186f – Earth’s Exoplanet Twin

April 18, 2014 1 comment

Astronomers yesterday announced the discovery of the first Earth-sized planet found in the habitable zone of its star. Revelling in the name of Kepler-186f this “twin Earth” was discovered by the Kepler telescope, adding to the 1800 or so exoplanets we’ve already detected.

Artists Impression of Kepler-186f

Artists Impression of Kepler-186f – we’ve no idea what it actually looks like…

The Kepler telescope surveys many stars at one time looking at whether the light we receive from those stars dims temporarily. If it does then that could mean its being blocked out by a planet passing across the face of the star. The dip in star light is tiny, a fraction of one percent of the star’s light, but nonetheless we can get a lot of information about the planet and its orbit from this dimming of its parent star.

By measuring how long the star’s light dims for we can work out how fast the planet is going, and therefore how far from the star it is. By the amount of the star’s light that is blocked out we can tell how big the planet is. In fact we can use mathematical techniques to strip out information from a complicated dimming pattern to work out these factors for a family of planets.

And indeed the parent star in this case, Kepler-186, has five planets going round it, named, from closest to furthest, Kepler-186b, -c, -d, -e, and -f. Only the last of these though is orbiting far enough from the parent star to be in the Goldilocks Zone, the region around a star where it is not too hot, not too cold, but just right for liquid water – a prerequisite for life on Earth at least – to exist. And not only that, but the amount of starlight that Kepler-186f blocks out tells us that it’s very similar in size to the Earth, which means it must be a rocky planet like our own rather than a gas planet, as gas planets are much bigger than the Earth.

Kepler-186 system compared to the inner solar system

Kepler-186 system compared to the inner solar system, showing the HZ in green

The parent star Kepler-186 is much smaller than the Sun; it’s a red dwarf star with a mass of 0.48 M(solar masses), a radius of 0.47 R(solar radii), and a temperature of around 4000°C compared to the Sun’s 6000°C. This means that Kepler-186’s Goldilocks Zone (also known as the habitable zone, or HZ, green above) is much nearer the star than is the case in our solar system. In fact all five of Kepler-186’s planets orbit their star closer than Mercury orbits the Sun, with the most distant Kepler-186f orbiting at a distance of 0.356AU compared to Mercury’s 0.387AU, going round its star every 130 days.

Might there be life?

No one would have believed in the first years of the twenty-first century that this world was being watched keenly and closely by intelligences greater than man’s and yet as mortal as his own; that as men busied themselves about their various concerns they were scrutinised and studied, perhaps almost as narrowly as a man with a microscope might scrutinise the transient creatures that swarm and multiply in a drop of water. With infinite complacency men went to and fro over this globe about their little affairs, serene in their assurance of their empire over matter… Yet across the gulf of space… intellects vast and cool and unsympathetic, regarded this earth with envious eyes, and slowly and surely drew their plans against us.

– an unlikely scenario, borrowed from H.G. Wells’ War of the Worlds

As soon as this planet was discovered (yesterday!) the Search for Extra Terrestrial Intelligence (SETI) trained their Allen Telescope Array on the star in the hope of hearing a message from an intelligent civilisation. So far: nothing. However in order to be detectable to us here on Earth the Keploids would have to be transmitting at 10x the power we do when beaming signals at potential alien civilisations.

Another route to detecting life – any kind of life, not just the intelligent kind – is to use powerful telescopes to study the planet’s atmosphere. If there’s oxygen there then it must be being produced by plant life; if there are industrial pollutants there (like CFCs that don’t occur naturally) then something would have to be making them. However our scopes are not powerful enough to see the atmosphere of Kepler-186f yet, partly because it’s so far away: 490 light years from us.

E.T. Phone Kepler-186f

Even if we did find evidence of intelligent life on this twin Earth, it’s so far away that communicating with it would be terribly slow. Limited as we are in this universe to sending signals at the speed of light, this planet is 490 light years away, and so the conversation would go something like this:

US: “Hello, how are you guys?
[wait 490 years for them to get the signal]
[wait for them to translate the message]
[wait 490 years for their reply to reach us]
THEM: “Fine thanks, how are you?” [980+ years later…]

Visiting Kepler-186f

As you can imagine if it takes light that long to get there, it would take our spaceships even longer. The furthest we’ve ever sent a spacecraft out into space (Voyager 1) is 19 billion km, which sound pretty far, but is only 35 light minutes away. And Voyager 1 has been traveling for 37 years. 37 years for 35 light minutes. That means it would take Voyager 1 around 270 million years to get to Kepler-186f…

Finding Kepler-186 in the sky

Where can you find Kepler-186 in the sky? The short answer is: you can’t. It’s far too distant and faint to be seen with anything other than the most powerful of telescopes, but you can see roughly where it is by looking in the constellation of Cygnus the Swan.

Cygnus rising in the NE at midnight, created using Stellarium

Cygnus rising in the NE at midnight, created using Stellarium

Cygnus is low in the north-east as the sky darkens, rising to high in the east by dawn, and looks like a large cross, with the long leg of the cross representing the swan’s neck, the short leg of the cross being its tail, and the two arms of the cross being its wings. The bright star in the “right wing” (the higher one) is called δ Cygni and Kepler-186 is near this star, towards the tail of the swan.

Other Kepler-186fs

The discovery if this twin Earth is very exciting, but it’s just the very start of our exploration of exoplanets (planets beyond our solar system). The star that Kepler-186f orbits is a red dwarf, a very typical star. approximately 70% of the 300 hundred billion stars in our galaxy are of this type (called M-type). If only one in a thousand of these stars has a planet like Kepler-186f that still leaves 200 million Earth twins in our galaxy, and some of them might be closer to us, making them easier to study, and perhaps to talk with…

 

 

Perihelion 2013

January 2, 2013 2 comments

At around 0500 GMT on 2 January 2012 the Earth was at perihelion, its closest approach to the Sun this year.

If that sounds confusing to you, and has you wondering why it’s so cold given that the Earth is at its closest to the Sun, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s summer right now), and (b) a misunderstanding of what causes the seasons.

The Earth orbits the sun in a nearly circular orbit called an ellipse. The degree by which an orbit differs from a perfect circle is called the eccentricity, e. If e = 0 then the orbit is circular; if e = 1 then the orbit is parabolic, and therefore not gravitationally bound to the Sun. The Earth’s orbital eccentricity is 0.0167, meaning that it is very nearly circular, with the short axis of the ellipse being around 96% the length of the long axis. Thus, during perihelion Earth is 0.983AU from the Sun, while during aphelion (its furthest distance from the Sun, occurring this year on 4 July) Earth is 1.017AU from the Sun. (1AU = 1 astronomical unit = the average distance between the Earth and the Sun = 150 million km).

The seasons on Earth have really nothing to do with how close the Earth is to the Sun at different times of year. Indeed how could they, given that the difference in distance between closest and furthest approach is only a few per cent? The seasonal differences we experience are of course caused by the tilt of the Earth’s axis, which is inclined by 23.5 degrees from the vertical.

This tilt means that, as Earth orbits the Sun, for six months of the year one hemisphere tips towards the Sun, so that it experiences longer days than nights, becoming most pronounced at midsummer, at which point the Sun reaches its highest in the sky at noon. Simultaneously the other hemisphere tips away from the Sun, and experiences shorter days than nights, becoming most pronounced at midwinter, on which day the Sun is at its lowest noontime altitude.

The further you are from the equator the more pronounced the seasonal effects. In fact equatorial countries don’t experience seasonal variations, while the poles experience extremes with six-month-long winters and summers. The timing of perihelion and aphelion relative to our seasons is entirely random. The fact the southern hemisphere midsummer (21 Dec) almost coincides with perihelion (2 Jan) is simply that; a coincidence. Given that fact, there is no reason to be surprised that perihelion occurs so close to northern hemisphere midwinter: it has to happen some time and it’s a coincidence that it happens to occur within two weeks of midwinter / midsummer.

Leap Seconds

January 19, 2012 Leave a comment

Today the International Telecommunications Union are voting on whether to abolish the leap second. This miniscule measure of time is added in to our time-keeping systems every so often to make sure they align more accurately with the time as measured by the spin of the Earth.

The original definition of the second was 1/86400 of a mean solar day which is related to the speed of the Earth’s spin about its axis. We might call this the “Earth second”. However the Earth’s spin in not regular. To begin with the Earth is slowing down by a couple of milliseconds per century due to tidal breaking. This breaking action is as a result of the drag of the Earth spinning beneath the tides created by the Moon. In effect the Moon is “stealing” energy from the Earth, increasing in its orbit about us while our spin slows.

In addition to this discrepancy the Earth is occasionally wobbled off course by major geological events, such as earthquakes. The 2004 Pacific earthquake which resulted in the Boxing Day Tsunami actually caused the Earth to speed up by over 2 milliseconds.

To avoid the problem of an irregular length of day – and therefore an irregular length of second – scientists adopted the much more regular SI second, which is the length of time it takes for 9,192,631,770 cycles of vibration of atomic caesium. This “atomic clock second” is accurate to one part in ten billion, and since 1972 this has been the international standard in timekeeping.

But time kept using the the SI second doesn’t match exactly with time kept based on the spin of the Earth, which after all is the time we experience every day. In order to make these two time signals match leap seconds are added every so often. Since 1972 25 leap seconds have been added. The last leap second was added at 23:59:59 on 31 December 2008, and the next one is due to be added at 23:59:59 on 30 June 2012. But leap seconds themselves are irregular, and are decided on by the ITU whenever the two time signals drift by more than 0.9 seconds.

The argument for abolishing these additional leap seconds is that it creates problems for modern computing and navigation systems that use the atomic clock second. Every time one of these irregular leap seconds is added the world’s hi-tech time keeping devices need to check and adjust by one second. It would be far simpler for us to use only “atomic clock seconds”.

However if we were to ditch the leap second then our civil time keeping would begin to drift with respect to “real” Earth time, so that in thousands of years time our clocks might read 8am just as the Sun is setting. This might seem like a minor concern right now – after all a millennium is a long time – but it’s something that astronomers and scientists do need to consider to avoid future problems. One alternative would be to introduce a “leap hour” to be introduced every few hundred years to keep the clock aligned with the real world.

Perihelion 2012

January 4, 2012 Leave a comment

At around 0100 GMT on 5 January 2012 the Earth will be at perihelion, its closest approach to the Sun this year.

If that sounds confusing to you, and has you wondering why it’s so cold given that the Earth is at its closest to the Sun, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s summer right now), and (b) a misunderstanding of what causes the seasons.

The Earth orbits the sun in a nearly circular orbit called an ellipse. The degree by which an orbit differs from a perfect circle is called the eccentricity, e. If e = 0 then the orbit is circular; if e = 1 then the orbit is parabolic, and therefore not gravitationally bound to the Sun. The Earth’s orbital eccentricity is 0.0167, meaning that it is very nearly circular, with the short axis of the ellipse being around 96% the length of the long axis. Thus, during perihelion Earth is 0.983AU from the Sun, while during aphelion (its furthest distance from the Sun, occurring this year on 4 July) Earth is 1.017AU from the Sun. (1AU = 1 astronomical unit = the average distance between the Earth and the Sun = 150 million km).

The seasons on Earth have really nothing to do with how close the Earth is to the Sun at different times of year. Indeed how could they, given that the difference in distance between closest and furthest approach is only a few per cent? The seasonal differences we experience are of course caused by the tilt of the Earth’s axis, which is inclined by 23.5 degrees from the vertical.

This tilt means that, as Earth orbits the Sun, for six months of the year one hemisphere tips towards the Sun, so that it experiences longer days than nights, becoming most pronounced at midsummer, at which point the Sun reaches its highest in the sky at noon. Simultaneously the other hemisphere tips away from the Sun, and experiences shorter days than nights, becoming most pronounced at midwinter, on which day the Sun is at its lowest noontime altitude.

The further you are from the equator the more pronounced the seasonal effects. In fact equatorial countries don’t experience seasonal variations, while the poles experience extremes with six-month-long winters and summers. The timing of perihelion and aphelion relative to our seasons is entirely random. The fact the southern hemisphere midsummer (21 Dec) almost coincides with perihelion (5 Jan) is simply that; a coincidence. Given that fact, there is no reason to be surprised that perihelion occurs so close to northern hemisphere midwinter: it has to happen some time and it’s a coincidence that it happens to occur within two weeks of midwinter / midsummer.

Close Encounters with Asteroid 2005 YU55

November 8, 2011 Leave a comment

This evening (8 November 2011) at 2328 GMT a 400m diameter asteroid will hurtle past the Earth, missing us by an astronomical whisker, less than 200,000 miles. The chunk of space debris in question is snappily titled 2005 YU55.

Radar image of 2005 YU55 taken at 1945UT on 7 November 2011, when the asteroid was 1.38 million km away

This kind of asteroid fly-by is rather rare. The last time that something this size passed so close to us was in 1976, and the next time it’s due to happen (that we know of) is 2028. Still, tonight’s pass poses absolutely no risk to the Earth.

Asteroid 2005 YU55 was discovered, as its name suggests, is 2005. This designation method is used by the Minor Planet Center, and designates minor planets until a proper name is given (if ever). Upon discovery it became clear that this asteroid was one of the Apollo asteroids, near-Earth asteroids named after 1862 Apollo, the first of the group to be discovered. The Apollo asteroids are all Earth-crossing asteroids, and so warrant special attention. The fact that their orbits cross that of the Earth does not automatically mean they pose a threat of impact, but does mean that we need to very carefully monitor their orbits in case they are on a collision course with us in the future.

Astronomers rank near-Earth asteroids relative to the risk they pose to us using the Torino Impact Hazard Scale. This ten point scale runs from 0, meaning that “the likelihood of a collision is zero, or is so low as to be effectively zero”, to 10, meaning “a collision is certain, capable of causing global climatic catastrophe that may threaten the future of civilization as we know it”. 2005 YU55 is currently ranked at 0 in this scale. There are no asteroids currently known that are ranked higher than 1. The highest ranking ever given was 4, given briefly to asteroid Apophis, meaning “a close encounter, meriting attention by astronomers…[with] a 1% or greater chance of collision capable of regional devastation”, but this has since been downgraded to a 0 based on new observations refining its orbit.

Torino Scale

Asteroid 2005 YU55 will not be visible to the naked eye, but amateur astronomers with good telescopes, and knowledge of how to use them, might locate it. See Robin Scagell’s excellent description of how to find it over at the Society for Popular Astronomy.

If 2005 YU55 did hit the Earth (it won’t) it could certainly destroy a large city and cause significant loss of life (to find out what would happen head over to the Down 2 Earth Impact Simulator) which is just one of the reasons that asteroid observation projects are so important.

Aphelion 2011

July 4, 2011 2 comments

At 1600 BST (1500 GMT) on 4 July 2011 the Earth will be at aphelion, its furthest from the Sun this year.

The Sun

If that sounds confusing to you, and has you wondering why the Earth is at its furthest from the Sun in Summer, then this belies (a) a northern-hemisphere-centric attitude (in the Southern Hemisphere it’s winter right now), and (b) a misunderstanding of what causes the seasons.

The Earth orbits the sun in a nearly circular orbit called an ellipse. The degree by which an orbit differs from a perfect circle is called the eccentricity, e. If e = 0 then the orbit is circular; if e = 1 then the orbit is parabolic, and therefore not gravitationally bound to the Sun. The Earth’s orbital eccentricity is 0.0167, meaning that it is very nearly circular, with the short axis of the ellipse being around 96% the length of the long axis. Thus, during perihelion (its closest approach to the Sun, which occurred on 3 January 2011) Earth is 0.983AU from the Sun, while during aphelion Earth is 1.017AU from the Sun. (1AU = 1 astronomical unit = the average distance between the Earth and the Sun = 150 million km).

The seasons on Earth have really nothing to do with how close the Earth is to the Sun at different times of year. Indeed how could they, given that the difference in distance between closest and furthest approach is only a few per cent? The seasonal differences we experience are of course caused by the tilt of the Earth’s axis, which is inclined by 23.5 degrees from the vertical.

This tilt means that, as Earth orbits the Sun, for six months of the year one hemisphere tips towards the Sun, so that it experiences longer days than nights, becoming most pronounced at midsummer, at which point the Sun reaches its highest in the sky at noon. Simultaneously the other hemisphere tips away from the Sun, and experiences shorter days than nights, becoming most pronounced at midwinter, on which day the Sun is at its lowest noontime altitude.

Earth

The further you are from the equator the more pronounced the seasonal effects. In fact equatorial countries don’t experience seasonal variations, while the poles experience extremes with six-month-long winters and summers. The timing of perihelion and aphelion relative to our seasons is entirely random. The fact the northern hemisphere midsummer (21 Jun) almost coincides with aphelion (4 Jul) is simply that; a coincidence. Given that fact, there is no reason to be surprised that aphelion occurs so close to northern hemisphere midsummer: it has to happen some time and it’s a coincidence that it happens to occur within two weeks of midwinter / midsummer.

To take this explanation even further, we can calculate how much variation in incident sunlight there would be in two scenarios: 1. an imaginary scenario where the seasonal variations in temperature are due to the tilt of the Earth’s axis but where the Earth goes round the Sun in a perfectly circular orbit and 2. an imaginary scenario where the Earth’s axis isn’t tilted, but where its orbit is elliptical in the same degree as ours actually is.

1. The Sun appears at its highest point in our sky each day at noon. The highest it ever gets is at noon on midsummer. The lowest noontime altitude occurs at noon on midwinter. In Scotland the Sun is around 55 degrees above the horizon at noon on midsummer, and only 10 degrees above it at noon on midwinter. The amount of energy from the Sun radiant on a fixed area is proportional to the sine of the altitude, so the ratio of the solar energy radiant on a square metre of Scotland between midsummer and midwinter is

sin(55) / sin(10) = 4.72

So here in Scotland we get 372% more energy from the Sun in summer than we do in winter, due to the tilt of the Earth’s axis.

2. If the Earth’s axis was not tilted, then we would only experience temperature differences from the Sun depending on how far or near we are from it. In this case, the amount of energy from the Sun radiant of a fixed area is proportional to the square of the distance from the Sun, so the ratio of the solar energy radiant on a square metre of Scotland between perihelion and aphelion is

(1.017/0.983)^2 = 1.07

So we get 7% more energy from the Sun at perihelion than we do at aphelion, due to the differing distances to the Sun. From this you can see that, while the distance to the Sun has some effect on how much heat we receive, it is a very small effect compared to that produced by our axial tilt.

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