Tag Archives: sun

Analemma

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If you took a picture of the Sun every day at noon and then compared the position of the Sun in each of the photographs you’d find that it was in a different place every day. If you joined the positions of the Sun together you’d form an analemma.

analemma-trinity

Source

An analemma of the Sun’s position as measured from London is shown above. Elevation, on the y-axis is the angle between the horizon and the Sun, and azimuth, on the x-axis is the compass bearing of the Sun (for example, 90° is due east and 180° is due south).

The shape and size of a solar analemma will vary depending on your position on Earth.

analemma-tokyo-newyork-sydney

In some cases, such as in Longyearbyen, the world’s northernmost town, the Sun does not rise above the horizon on some days.

analemma-longyearbyen

At locations near the Tropics of Cancer and Capricorn, for example Muscat in Oman, the analemma has a very “lopsided” shape.

As you can see, at some points in the year (around the Summer Solstice) the Sun is almost due east, even at noon. This is because the Tropics of Cancer and Capricorn are the northernmost and southernmost points respectively at which the Sun can appear directly overhead, and here the Sun rises around east-northeast and sets around west-northwest, rather than east and west. Suncalc allows you to play with location and the time of year to visualise the position of the Sun during the day.

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The position of the Sun during the summer solstice as seen from Muscat.

Solar eclipse types

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The word “eclipse” is a general astronomical term that applies to any situation in which the view of one object is blocked by another, caused either by the second object’s shadow falling over the first, or by the second object coming in between the first object and the observer.

In general use the term eclipse usually applies to one of two situations: a solar eclipse, in which the Moon obscures our view of the Sun; or a lunar eclipse in which the Earth’s shadow falls over the Moon. The planes in which the Earth orbits the Sun and which the Moon orbits the Earth are at an angle to each other, which is why there is not a total solar eclipse once every month.

There are three types of solar eclipse:

  • A total eclipse, in which the Moon appears the same size as the Sun and blocks light from it completely.
  • An annular eclipse, in which the Moon passes between the Sun and the Earth but because of the relative distances of each it appears slightly smaller than the Sun, causing a ring of light to appear.
  • A partial eclipse, in which the orbits of the Moon and Sun are such that the Moon only covers only part of the Sun.

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A comparison of a total solar eclipse (left) and an annular eclipse (right). Only in the case of a total solar eclipse is the Sun’s corona (the white “cloud”) visible.

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A diagram showing how the three different types of eclipse are formed.

There are also hybrid eclipses, which appear as total eclipses to some parts of the Earth and an annular eclipse to other parts. These are very rare, with the most recent in April 2005 and visible mainly from the Pacific Ocean and also Costa Rica, Panama, Colombia and Venezuela; and the next to occur in November 2013, visible from Central Africa (Gabon, Congo, Democratic Republic of the Congo, Uganda, Kenya, Ethiopia and Somalia).

 

Solar wind and real wind

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“Solar wind” is a colloquial term for the stream of charged particles ejected from the surface of the Sun by heat and strong magnetic fields. It is the interaction between the solar wind and Earth’s magnetic field that is responsible for the aurorae.

During solar storms, when larger amounts of material are ejected at higher speeds (a coronal mass ejection) the solar wind can cause damage to satellites and even to power grids on Earth.

But a new paper from South Korean scientists suggests that the solar wind affects the real wind; that the solar wind hitting the atmosphere can affect the pressure at sea level. Their suggestion is that changes in clouds are caused by currents flowing from the ionosphere (a charged upper layer of the atmosphere) to the land/ocean; and that the solar wind affects these currents. They measured a increase in pressure of about 2500 pascals (over a standard pressure of 101325 pascals) in the days following solar winds above 800km/s.

Il-Hyun Cho et al. 2011. “Changes in Sea-Level Pressure over South Korea Associated with High-Speed Solar Wind Events.” arXiv: 1107.1841v1 [astro-ph.EP]

The Northern Lights over the UK

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A sunspot is an area of intense magnetic activity on the surface of the Sun (the photosphere) that causes it to decrease in temperature and darken.

One particular sunspot (imaginatively named “Active Region 1158”, shown above) has been growing in size over the past week and is now wider than the planet Jupiter. At 0156 on Tuesday morning an enormous X-class solar flare, the largest and most powerful type, erupted from AR1158. This was the Sun’s first X-class flare for more than four years.

The X-class flare shown in an image taken by NASA’s Solar Dynamics Observatory at 193Å.

The flare was accompanied by a coronal mass ejection (CME), a plasma of highly energetic electrons and protons propelled outwards at great speed. This CME travelled towards Earth at about 900 kilometres per second and will strike the atmosphere at about 0100 on Friday morning.

Update (2011‑02‑18): the “blow” from the CME seems to have been fairly glancing and will therefore not impact as severely as thought. The solar wind speed on impact has been about 500-600 km/s, peaking at around 700 km/s for brief periods of time.

There is still the risk of further, less powerful, M-class flares from AR1158 in the future.

A CME is essentially a more powerful version of the solar wind. It is the collision of the solar wind and the Earth’s magnetic field that produces aurora (aurora borealis in the northern hemisphere and aurora australis in the southern hemisphere). It is possible, though unlikely, that the increased strength of the solar wind due to the CME from AR1158 will mean that the aurora borealis can be seen from the UK, especially at higher latitudes (e.g. the Hebrides or Shetland Islands) where the Earth’s magnetic field is stronger.

Update (2011‑02‑18): the current NOAA POES auroral oval clearly shows (top-right quadrant) that there is almost no chance of visible aurora anywhere in the UK.

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