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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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Winter Solstice 2013

December 21, 2013 Leave a comment

The northern hemisphere winter solstice occurs on 21 December 2013, at 1711 GMT. At this point the Earth’s north pole will be tipped away from the Sun. As seen from Earth, the Sun will stop its slow daily decent south in our sky – over the past six months the Sun’s mid-day height above the horizon has been decreasing steadily – and once again turn north, getting higher in the sky at noon each day, until it gets to its highest point in midsummer 2013.

The actual day of the winter solstice – in this case 21 December 2013 – is commonly known as midwinter, the shortest day, and is the day when the Sun spends least time above the horizon. The further north of the equator you are, the more profound the effect. Indeed if you live within the arctic circle the Sun won’t actually rise today.

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.

Transit of Venus

May 23, 2012 2 comments

This year, on 5 and 6 June 2012, there is a very rare astronomical occurrence: a transit of Venus across the face of the Sun. There have only been six of these transits ever observed before – in 1639, 1761, 1769, 1874, 1882, and in 2004 – and this year’s transit is the last for 105 years!

So what exactly will you see, if you’re lucky enough to catch this last-chance-to-see event? If you’re able to look at the Sun safely you’ll see a tiny black dot moving slowly across the surface – that dot is the planet Venus! NASA has the exact times of the transit from major cities. Importantly, this transit is best seen from the Pacific. Observers in north and central America will see only the start of the transit before the Sun sets, while those of us in Europe will only catch the end of it if we’re up at sunrise.

UK observers: set your alarms! You’ll see the transit between sunrise and 0536 BST, at which point Venus begins leaving the Sun’s disk, taking about 18 minutes to do so.

Venus is 6000km across – just a little smaller than the Earth – and at transit it will be around 43 million km away, directly between us and the Sun. The Sun is 1.4 million km across and around 150 million km away. This means that, seen from Earth, Venus is only about 58 arcseconds in diameter, while the Sun is 1891 arcseconds across, about 33 times the apparent diameter of Venus. So: Venus small dot; Sun big bright ball.

Also, we know how far from the Sun Venus is (107 million km), and how long it takes to orbit the Sun (225 days), so we can work out how long it should take to pass across the Sun’s disk (around 6.5 hours). However the start and end times for the transit vary depending on where on Earth you’re observing, with observers in eastern Canada seeing Venus start to cross the Sun’s disk a whole thirteen minutes earlier than observers in Australia! This is because Canadians are looking at the transit from a slightly different angle than Australians.

Why transits of Venus are (were) important

If you have observations from two widely spaced points on the Earth’s surface, and if you time the start and end of transit accurately at each, you can work out the solar parallax, that is, the difference in position of the Sun when viewed from two different points on Earth, the two points being one Earth radius apart. (Hold your thumb up, close one eye, and obscure a distant object; now switch eyes, and your thumb appears to move with respect to the distant object. That’s parallax).

From the solar parallax, if you know the Earth’s radius, you can work out the Earth-Sun distance (known as the astronomical unit) using high-school trigonometry. This was important to astronomers in the 18th century, as up until then all we knew were the relative distances between all the planets in our solar system, not the actual distances. Once we had one measurement within the solar system – the astronomical unit, say – we could work out how far away everything else was.

The technique of using transits of Venus to work out the solar parallax was first suggested by Edmund Halley in 1716, after he had observed a much more common (although still only 13 times per century) transit of Mercury from the island of Saint Helena. Halley knew that Venus would give much more accurate measurements than Mercury, since it was closer to the Earth and so the angles would be easier to measure. He also knew that the next transit of Venus would happen in 1761, and urged future astronomers to make observations world-wide and thereby calculate the solar parallax, and from that the astronomical unit.

This was duly done, and a value for the astronomical unit of 153 million km was calculated. Later transits in the 19th century yielded a value of 149.59 million km. The current accepted value, calculated from telemetry from space craft is 149.60 million km, so the transit method worked pretty well.

Solar Storm Incoming

March 8, 2012 5 comments

At 0028GMT on 7 March a giant X-class solar flare blasted material off the Sun and sent it hurtling towards the Earth. To be specific it was a X5.4-class flare, the most powerful we’ve seen in 5 years. That material is due to hit some time this morning (UK time) 8 March and could result in increased aurora activity as well as potentially disrupting communications systems and power transmission. A similar storm in 1989 knocked out the power grid in northern Canada, resulting in a black out for over 6 million people.

Here’s a graphic of the storm as it heads our way, courtesy of NASA (click on graphic for animation):

Keep up-to-date with activity via Spaceweather.com

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.

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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