Archive for the ‘Sun’ Category
Beginners Guide To Aurora
Here is a very quick beginners guide to explaining aurora
Aurora = The Northern (or Southern) lights/ Aurora Borealis/ Australis
Usually seen near the poles of the Earth, but can be seen further South in the UK or USA.
So how and where does it come from?
“Coronal Mass Ejection” = A load of solar material hurled out of the Sun. A big one can contain billions of tons of “plasma”.
Plasma hits Earth’s “magnetosphere” causing “geomagnetic storms” = Aurora, also known as the Northern or Southern lights.
Geomagnetic storms are measured using a scale called the “Planetary Kp index” ranging from 1 to 9. 1 being low and 9 being a very heavy storm.
The higher the Kp index the higher the likelihood of Aurora and the further South it can be seen. 5 = Scotland 8+ Southern England.
Geomagnetic storms and aurora are very unpredictable and forecasts can be very vague, we don’t know the intensity or where the aurora can be seen from until it hits.
Here is a link to NOAA Space Weather Scales
A highly active region on the Sun threatens to deliver powerful geomagnetic storms over the week ahead. Highly energetic solar eruptions are likely heading in our direction to give Earth’s magnetic field a significant glancing blow!
The massive sunspot, many times larger than the Earth (see images below) is expected to increase in size and energy, and is expected to release powerful solar flares, sparking strong geomagnetic storms. Read the rest of this entry »
A severe geomagnetic storm (Kp=7-8) that began yesterday when a CME hit Earth’s magnetic field is subsiding. At the peak of the disturbance, auroras were sighted around both poles and in more than five US states and Northern Europe.
Sky watchers at the highest latitudes should remain alert for auroras as Earth’s magnetic field continues to reverberate from the CME impact.
More solar activity is expected, so stay posted for more Aurora news.
AURORA UPDATE! New Auroral oval predictions for the UK and North America! We are definitely going to see Aurora tonight
A strong-to-severe geomagnetic storm is in progress following the impact of a coronal mass ejection (CME) at approximately 12:15 UT on Sept. 26th. The Goddard Space Weather Lab reports a “strong compression of Earth’s magnetosphere. Simulations indicate that solar wind plasma [has penetrated] close to geosynchronous orbit starting at 13:00UT.” Geosynchronous satellites could therefore be directly exposed to solar wind plasma and magnetic fields. High-latitude sky watchers should be alert for aurors after nightfall. (Credit: Spaceweather.com)
The best time to try and spot Aurora (The Northern lights) is around midnight, but this could be soon er or later.
You don’t need a telescope or binoculars to see the show (if it happens from your location) just your eyes.
Find a dark spot away from street lights and other light sources and look North. You should see Aurora very close to the horizon or higher, depending on your location, current conditions and intensity of the geomagnetic storm.
Today, 13 June, is one of only four days in the year when the time as read on a sundial will be exactly correct.
Sundials usually tell the time using the shadow of the gnomon as cast by the Sun. This is possible as the Sun appears to move across the sky at an approximately constant speed, and so the shadow of the gnomon also moves at an approximately constant speed. The inconstancy of the Sun’s apparent motion in the sky – and therefore of the gnomon’s shadow on a sundial – is the subject of this article, and is calculated using the Equation of Time.
If you look at the shadow of a sundial’s gnomon it will fall onto a curve of numbers, along hour lines indicating local solar time. This is not equal to the official clock time until three important corrections are made:
Please read the rest of this article on Dark Sky Diary
At 1900 GMT on 3 January 2011 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 (3 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 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 (called the flux) there would be in two scenarios:
1. an imaginary scenario where the seasonal varioations 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
2. an imaginary scenario where the Earth’s axis isn’t tilted, but where it’s 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 Glasgow between midsummer and midwinter is
sin(55) / sin(10) = 1.84
So here in Scotland we get 84% 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 radian of a fixed area is proportional to the square of the distance from the Sun, so the ration of the solar energy radiant on a square metre of Glasgow 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.