This page answers questions about the Sun. The questions are:
The Sun is a star, but is so much closer to us than the other stars that we see the Sun as a very bright disk and the other stars only as weak points of light. Sunlight reaches all planets and asteroids and comets and other members of our Solar System, and also reaches into space beyond the Solar System. One could see our Sun from planets around other stars, but then the Sun would appear as a weak point of light, just like we see other stars only as weak points of light from here. Also see question 334.
The distance between the Earth and the Sun is about 150 million km (see question 347). 150 million km divided by 100 km/h is (150/100) million hours = 1.5 million hours = 1,500,000 hours. One day is 24 hours, so 1,500,000 hours = 1,500,000/24 = 62,500 days. One year is about 365 days, so 62,500 days is 62,500/365 = about 171 years. So, if you could drive to the Sun at 100 km/h without stopping, then it would take you 171 years to get there.
The gravity between the Sun and a planet makes the planet and the Sun orbit around their common center of gravity. Because the Sun has enormously more mass than the planet does, the Sun is much closer to the common center of gravity than the planet is, so the orbit of the Sun is much smaller than the orbit of the planet, and the Sun moves much less than the planet does. Yet, the gravity from the planets does make the Sun wobble a little bit.
The planets make the Sun wobble (as seen from distant stars), by at most about 1.5 million kilometers or 1.0 million miles, which is about equal to the diameter of the Sun. Of this wobble, about half is due to the planet Jupiter alone, and almost all of the rest is due to the planets Saturn, Uranus, and Neptune combined.
It is not difficult to see that the Sun rotates around its axis. Just look at a number of pictures of the Sun that are taken at different dates, for example at http://sohowww.nascom.nasa.gov/data/realtime/realtime-mdi_igr.html and http://sohowww.nascom.nasa.gov/data/realtime/realtime-eit_171.html. You can tell that things like sunspots move across the face of the Sun. It takes about a month for something on the Sun to make a complete rotation around the Sun. The simplest explanation for this is that the Sun rotates in about a month.
Some people think that the Sun appears brighter on average when it rises than when it sets, but I have not noticed any such difference myself (though I must admit that I do not witness many sunrises). I expect that any such difference is very small, if it can be reliably detected at all.
If the Sun does appear brighter on average when it rises than when it sets, then the difference may be one of perception. People cannot judge very accurately how bright the Sun is at any time, and cannot remember well at sunset how bright the Sun appeared at sunrise, so any such comparison is very subjective. At sunrise, the level of light increases rapidly from previously very low levels, whereas at sunset it decreases rapidly from previously very high levels, so it seems reasonable to assume that the rising, brightening Sun is judged to be brighter than the setting, dimming Sun, even if they are in fact equally bright.
If the Sun is really brighter at sunrise than at sunset (as measured with some kind of scientific tool), then that must be due to changes in the atmosphere that are associated directly or indirectly with sunlight. The Sun appears dimmer if the atmosphere absorbs or scatters more sunlight. Such absorption and scattering is associated with dust and water vapor and other small particles in the atmosphere. It is conceivable that there is a net increase of the number of such particles in the atmosphere during the day, and a net decrease at night, and that might yield an average difference between the brightness of the rising Sun and the setting Sun.
There is gravity on the Sun. If there were not, then the hot gases that the Sun is made of would fly away into space, and then there would be no Sun anymore. If you could stand on the surface of the Sun, then you would feel 28 times heavier than you feel on Earth.
The Sun is so bright that it will hurt your eyes if you look directly at it, and the ultraviolet rays from the Sun that can cause sunburn can also destroy cells in the back of your eyes (in your retinas) that allow you to see.
This does not mean that you will go blind immediately if you look at the Sun briefly by accident, but if you do it often or for a long time, then your eyesight will suffer.
You can use sunglasses to block out much of the sunlight, but you cannot be sure that those sunglasses also block out the harmful ultraviolet rays that can destroy the cells in your retinas. You cannot see ultraviolet rays, so you can't tell if your sunglasses block them or not, even if they say that they have "UV protection". And perhaps the glasses are scratched or perhaps the protective layer disappeared or got too old, so that harmful rays get through after all. The only way to be sure that the glasses offer enough protection is by testing them before each use, and that would be inconvenient and expensive.
If you burn your fingers, then you feel pain, so you notice it quickly and can pull your fingers away. Unfortunately, you do not feel any pain when the cells in your retinas are destroyed by ultraviolet rays, so you don't even notice it when it happens. That's why you have to be extra careful with your eyes.
If you really do want to look at the Sun directly, then there are two ways in which you can do that safely:
In any case, the best advice is not to look directly at the Sun at all. Your eyesight is valuable; why risk hurting it? There is rarely anything interesting to see on the Sun anyway that you can see with your unaided eyes. It is much better to let camera's at observatories or in satellites look at the Sun for you. They produce pictures that show much more detail than you could ever see by looking at the Sun directly. If you want to see some of those pictures, then you can take a look at, for example, http://umbra.nascom.nasa.gov/images/latest.html.
The Sun does not have the same temperature everywhere. The visible surface of the Sun has a temperature of on average 6000 K (6000 ℃ or 10,800 ℉), but in large sunspots the temperature can drop to "only" 3700 K (4000 ℃ or 7200 ℉). It gets hotter the further you go below the surface of the Sun. Astronomers think that the temperature in the center of the Sun is about 16 million K (16 million ℃ or 29 million ℉).
We can deduce the temperature of the surface of the Sun from the division of sunlight across the different colors or wavelengths. That division is called the spectrum of the sunlight.
Every object (including yourself) emits thermal radiation that depends on the temperature of that object. Hot objects such as the Sun and the stars emit most of their thermal radiation as visible light. Cooler objects such as people or planets emit most of that thermal radiation as infrared light. From the division of the radiation across the different colors or wavelengths one can deduce what the temperature must be of the surface of the object that emits the radiation. For the Sun we find a surface temperature of about 5500℃. See http://nl.wikipedia.org/wiki/Thermal_radiation.
We cannot measure temperatures inside the Sun directly, but we can yet say something about them, because the temperatures inside the Sun influence things at the surface that we can see. To do this, a scientists makes a model of the inside of the Sun in his computer. That model must follow all of the natural laws that we have discovered (such as the Law of Gravity and the gas laws and the Law of Conservation of Energy). The scientist then adjusts the model (while keeping it follow the laws of nature) until the surface of the model looks enough like the real Sun, so it has exactly the same size, the same amount of mass, the same temperature at the surface, and the same distribution of sunlight across the different colors. And then the scientist can see in his model what the temperatures must be inside the Sun. It follows from such models that the temperature in the center of the Sun must be about 15 million ℃.
We use the same general method also to learn things about other places where we cannot go, like the inside of the Earth. I continue to be amazed at how we can use smart tricks to find out things about places we cannot reach.
If you use the right equipment, then you can see many details on the Sun. Some of them are described below.
A prominences or filament is an often elongated or thread-like cloud of solar gas that sits up to about 30,000 miles (50,000 km) above the surface of the Sun. Such a cloud is held in place by the magnetic field of the Sun. Prominences and filaments are not visible in white light (the continuum) but only in light from the center of strong spectral lines. When such a cloud is visible beyond the limb of the Sun, then it appears bright and is called a prominence. When the cloud is seen against the background of the solar disk, then it appears dark and is called a filament. Filaments and prominences can stay around for up to about two months, though some of them disappear much faster. Some seem to appear as a result of a solar flares.
Energy gets from the center to the surface of the Sun by convection (through the convection zone), and leaves the surface in the form of radiation.
The average distance between the Earth and the Sun is 149,590,787 km or 92,951,640 mi. This distance is called an Astronomical Unit (AU). The orbit of the Earth deviates a bit from a circle, so the distance between the Earth and the Sun isn't always the same. The Earth is closest to the Sun (in the perihelion of the Earth's orbit) around 4 January, when it is winter in the northern hemisphere of the Earth and summer in the southern hemisphere, and is furthest from the Sun (in the aphelion of the Earth's orbit) around 4 July, when it is summer in the north and winter in the south.
The difference between the distances on 4 January and 4 July is about 3 percent, which corresponds to about 5 million km or 3 million mi.
The annual variation in the distance between the Earth and the Sun is not the most important cause of temperature change on Earth, or else it would be summer in January across the whole world (when the Earth is closest to the Sun), and winter in July (when the Earth is furthest from the Sun), but this is just the opposite of what happens in the northern hemisphere. The tilt of the rotation axis of the Earth compared to the plane of the orbit of the Earth (the ecliptic) is much more important for the seasons and temperature on Earth.
The diameter of the Sun is 1,391,980 km or 864,938 mi and that of the Earth is 12,756 km or 7926 mi (at the equator). The ratio of those two is 109 (rounded) so the diameter of the Sun is 109 times greater than the diameter of the Earth.
But perhaps you meant the surface area? The surface area of the Sun is about 109*109 = 12,000 (rounded) times the surface area of the Earth (land and sea combined).
Or perhaps you wanted to compare volumes? The volume of the Sun is about 109*109*109 = 1,300,000 (rounded) times the volume of the Earth.
The Sun is very large.
  
The corona is the name of the very tenuous outer layers of the Sun, far above the visible surface of the Sun. Those layers get less dense the further you get from the Sun, but they don't really stop. You could say that the Earth moves through the corona of the Sun.
The corona is made up of particles (mostly protons and electrons) that have escaped from the Sun and that blow through the Solar System as some sort of solar wind.
The corona is so tenuous that you can see right through it, and it emits so little light that ordinarily you cannot see it next to the very bright disk of the Sun. You can see the corona during a total solar eclipse, or from space if you cover the solar disk with your hand or something else, and then the corona looks like some sort of luminous smoke around the Sun. You can see an example at http://antwrp.gsfc.nasa.gov/apod/ap060407.html, which was taken during a total solar eclipse when the Moon covered the Sun, but that picture was enhanced to make the details better visible: in real life you won't be able to see the streamers that far from the Sun.
Just like in the atmosphere of the Earth, the density of the gases decreases as you go higher above the Sun. And if you get further from the Sun, then the sunlight is spread over more space, so you'd expect that it would be colder the further you get from the Sun, but that turns out not to be the case for the corona. In the corona, it gets hotter as you go higher above the Sun, so the corona must get its heat (also) from something other than sunlight. How this works in detail is one of the great unsolved mysteries of solar physics. It is clear that it has something to do with the magnetic field of the Sun, but how this works exactly we don't know yet. That magnetic field sticks through the surface of the Sun and all the way through the corona, till far beyond the Earth, and energy can escape from the Sun along that magnetic field. Apparently part of that ends up in the corona, where it heats the gas to high temperatures.
The corona has a temperature of about a million degrees Celsius, much higher than the temperature of the surface of the Sun, but because the corona is so tenuous there isn't actually much energy in it. The temperature in space near the Earth is also very high, but this will not burn an astronaut in space, because there is hardly any gas in space and the total amount of heat in that gas is far too small to hurt the astronaut. See the answer to question 501.
At first sight, the Sun always appears the same, as a bright, unmarked sphere. However, if you study the Sun using a telescope (Warning: it is dangerous to look at the Sun through binoculars or a telescope!) then you can sometimes see sunspots, and there are also many other signs of what astronomers call solar activity. The number of sunspots, and also the other measures of solar activity, vary from day to day and from year to year.
Figure 1 shows the monthly average sunspot number from 1749 through 2006, as found at ftp://ftp.ngdc.noaa.gov/STP/SOLAR_DATA/SUNSPOT_NUMBERS/MONTHLY. The sunspot number is a measure for the number of sunspots and sunspot groups that are visible on the near side of the Sun. It is easy to see that the sunspot number periodically decreases to almost zero and then rises again. This cycle is called the solar cycle and lasts about 11 years.
That the activity of the Sun has a period of about 11 years has been known since the 19th century. There is much debate about whether there are yet other periods in the solar activity and about their length and importance. There are several problems that make answering that question difficult:
All in all it turns out to be easy to find periods, but very difficult to figure out what the periods that you found really mean for the phenomenon that you are investigating.
You can find more information about solar activity and periodicity at
The calculation page about frequency determination.
The full Moon high in the sky gives about as much light on the ground as the Sun does when the Sun is about 8 degrees below the horizon. A half Moon (about one week before or after full Moon) high in the sky gives about as much light on the ground as the Sun does when the Sun is about 10 degrees below the horizon, which is about 1/9th as much light as the full Moon gives. If the Moon is low in the sky, then it gives much less light on the ground (per square meter of ground).
Here is a table that shows how bright the Moon is on different numbers of days before or after full Moon, compared to the brightness of full Moon, assuming that the Moon stays at the same distance from Earth the whole time:
The energy in sunlight can cause some chemical reactions, such as the chemical reaction that produces vitamin D₃ from another type of molecule in the human skin (see http://en.wikipedia.org/wiki/Vitamin_d). If the product of the chemical reaction is unstable, then it can spontaneously break down, perhaps returning to its original state. What happens to the product depends on many things, so there is no single answer to the question how long it takes for the effects of sunlight to disappear after sunset. The vitamin D in our bodies does not break down immediately but gets used up slowly, so we need to keep creating more vitamin D, otherwise we get the disease called rickets.
Sunlight produces many changes on Earth and in the atmosphere. The changes in the atmosphere depend not just on sunlight but also on what kind of gases are nearby, and on things like the temperature and gas pressure. Some effects of the sunlight disappear quickly, others disappear more slowly, and those effects may interact with each other, so that it is impossible to give a duration for each effect separately. For more information about the atmosphere, see http://en.wikipedia.org/wiki/Earth%27s_atmosphere.
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Last updated: 2012-01-13