Astronomy Answers
AstronomyAnswerBook: the Sun

This page answers questions about the Sun. The questions are:

• [537] How long would it take to travel to the Sun if you could fly there in a passenger jet going 1000 kilometers per hour?
• [536] Can the Sun rise in the West?
• [518] Why is the corona of the Sun so hot?
• [517] What is the corona of the Sun?
• [516] How can we measure the temperature on and in the Sun, when we've never been there?
• [495] When does the Sun rises or set the earliest or the latest in the day?
• [490] With which periods does the solar activity vary?
• [470] Is the Full Moon at midnight on 21 December as high in the sky as the Sun is at noon on 21 June?
• [464] It appears that it gets cooler right at sun rise. Is this fact?
• [460] Is there some formula to determine the time it takes a planet to rotate around the Sun?
• [459] Why do the planets take so long to revolve around the Sun?
• [456] Astronomers discover planets near another star because those planets make the star wobble a bit. Does Jupiter make the Sun wobble like that, too?
• [444] Does any heat from the Sun get reflected to the Earth by the Moon, and does that correlate with the phase of the Moon?
• [439] How was the distance between the Sun and the Earth measured?
• [419] If I travelled at 100 kmph, how many days would it take to reach the Sun?
• [410] Does the Sun shine on all nine planets?
• [403] If I stand on the North Pole on March 21st, in what direction do I then see the Sun rise? And the same for sunset on 23 September?
• [399] Who first proposed that the planets revolve around the Sun?
• [393] How often does the Sun rise and set?
• [391] When are sunrise and sunset at the poles?
• [386] Does the Sun rise in about the east everywhere on Earth, or is it in the east in one hemisphere but in the west in the other hemisphere?
• [383] When is the Sun in a certain direction at sunrise or sunset, as seen from a certain location?
• [375] I read in a book that a rocket on its way to a planet cannot escape from an orbit around the Sun if it misses the planet. Why can't you just turn on the rocket's engines and return to the planet?
• [371] How can you tell that the Sun rotates around its axis?
• [368] What time is it if the Sun is on the exact opposite side of the Earth from me?
• [366] How far does the Sun move along the sky in an hour?
• [364] Why does the ecliptic carry the Sun north and south of the celestial equator?
• [358] Is it a coincidence that modern star atlases show the center line of the Milky Way passing through the solstitial points of the Sun's path?
• [357] You mention that day and night are always 12 hours long at the equator and everywhere at the equinoxes, but your sunrise and sunset tables give the length of the day as greater than 12 hours at those dates and locations. How come?
• [347] What is the distance between the Earth and the Sun called?
• [346] How does one detect the meridian transit of the Sun?
• [343] How can the top of the Moon ever be illuminated when the Moon is high in the sky and the Sun is low in the sky?
• [340] Can the atmosphere bend light so that there is some sunlight behind the Earth after all?
• [335] How long does sunlight take to get to each planet?
• [334] How much sunshine does each planet get?
• [331] Why can't they invent glasses that are guaranteed to protect your eyes when you look at the Sun?
• [325] What would happen (for example to Earth) if the Sun became a red giant?
• [313] Why isn't the transit of the Sun at the beginning of spring at exactly 12 o'clock?
• [312] If you stand on a very tall building, can you then see the Sun at night?
• [308] How high can the Sun get in the sky, and when is that?
• [304] What is the difference between the ways in which comets and planets move around the Sun?
• [301] Is the Sun brighter when it rises than when it sets?
• [295] Which formulas predict when Venus passes in front of the Sun?
• [294] If you divide the year (orbital period) of a planet such as Mars into 12 months where each month corresponds to 30 degrees of motion around the Sun, and the year begins at the ascending equinox, then what is the length of each month?
• [293] Where does the energy of a star such as the Sun come from?
• [267] What is the origin of the name "Sun"?
• [266] Is there gravity on the Sun?
• [265] Why do constellations and the Sun and the Moon appear larger when they are close to the horizon and smaller when they are overhead?
• [246] Why should you not look directly at the Sun?
• [243] Can the Sun be seen from all countries?
• [240] How long does it take the Sun to rise or set?
• [231] What is the maximum temperature of the Sun?
• [230] How large is the surface area of the Sun?
• [229] How many moons are there around the Sun?
• [217] Who first claimed that the Earth orbits around the Sun?
• [208] What is a prominence?
• [206] On which two days of the year is the Sun straight above the equator?
• [202] How would the Earth move if you could turn off the gravity between the Earth and the Sun?
• [199] How long does it take the Earth to rotate around the Sun?
• [186] In what years does February have five Sundays? When will this occur again?
• [184] Is the rotation of the Earth around its axis or around the Sun slowing down, and is that a problem for the calendar?
• [183] What is the formula for calculating the range of an equatorial sundial, depending on your geographical latitude?
• [180] Is it true that the Sun is not exactly at its highest point when it goes through the meridian (at local high noon)?
• [173] How is energy transported out of the Sun?
• [165] Could there be a second Earth-like planet in the same orbit as the Earth but hidden exactly on the other side of the Sun?
• [164] Where can one buy a booklet that shows times of sunrise and sunset?
• [162] Why is it that shortly after 21 December the Sun rises and sets later every day? And also after 21 June?
• [146] How did the Sun and the planets form?
• [145] What are the least and greatest distances of the planets from the Sun?
• [143] How do I calculate at what time the Sun rises for me if I fly at cruising altitude in an arbitrary direction?
• [141] When is the Sun in which constellations?
• [136] Do all planets travel around the Sun in the same direction?
• [127] What country or part of the world has the most sunlight annually?
• [111] Why does it not get colder but hotter when you go up in the chromosphere of the Sun?
• [104] At what place on Earth does the Sun never shine?
• [78] How many days, hours, and minutes does the Earth take to rotate around the Sun?
• [74] Why can you look at the Sun when it is setting but not when it is high in the sky?
• [73] Can the Earth, the Sun, and the center of the Galaxy be on one line?
• [72] Why doesn't one ever see the Sun towards the north if one is in the northern hemisphere?
• [57] Are there solar systems other than our Solar System that contains the Earth, and is the sun in those other solar systems similar to our Sun?
• [55] What is the name of the second-closest singular or multiple star system (other than the Sun)?
• [49] How can you determine your location using the Sun and the stars?
• [44] What is the maximum temperature on Earth in full sunshine, not in the shade?
• [38] Exactly how many times does the Earth revolve around the Sun?
• [36] Is the distance between the Earth and the Sun in the winter greater or smaller than in the summer?
• [35] How big is the Sun compared to the Earth?
• [31] Does the Sun always set exactly towards the west?
• [25] Why do comets go faster when they get closer to the Sun?
• [7] Which planet is closest to the Sun?
• [5] How does the Sun turn relative to the Earth?
• [3] At what time does the Sun set on Wednesday 16 April 2003?

[410]

1. The Sun

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.

[419]

2. The Sun is Far Away

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.

[456]

3. The Sun Wobbles

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.

[371]

4. Rotation of the Sun

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.

[301]

5. Brightness of the Sun

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.

[266]

6. Gravity on the 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.

[246] [331]

7. Looking at the Sun

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:

1. Use an undamaged professional welder's mask. Welding also produces ultraviolet light, so welders have to protect their eyes from it, too. (And that's why you should not look directly at welding, either.)
2. Look at the Sun when it is rising or setting, when your eyes can bear it without using sunglasses. When the Sun is very low in the sky, then it is much less bright than when it is high in the sky, because all of the air that the sunlight passed through on its way to you has scattered or absorbed much of the sunlight. Ultraviolet light is scattered or absorbed even more than visible light, so when there is only a little visible light left, then there is hardly any ultraviolet light left.

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.

[231]

8. The Temperature of the Sun

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 ℉).

[516]

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.

9. The Face of the Sun

If you use the right equipment, then you can see many details on the Sun. Some of them are described below.

[208]

9.1. Prominence, Filament

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.

[173]

10. Energy Transport through the Sun

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.

[36]

11. The Distance of the Sun

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.

[35] [230]

12. The Size of the Sun

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.

[111] [517] [518]

13. The Corona of the Sun

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.

[490]

14. The Solar Cycle

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.

Fig. 1: Solar Cycles 1749-2006

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:

1. There are many different measures for the activity of the Sun, and those generally do not show the same periods with the same amplitudes. Which one should we use? The best-known measure is the sunspot number, and even measuring that number is ambiguous (for example, because a larger telescope allows detection of a greater number of sunspots than a smaller telescope). There are also many proxy measures, such as the concentration of carbon-14 or beryllium-10, and old reports about northern lights. These measures do not yield the same results (apart from the main period of about 11 years).
2. The measurements must be calibrated, and this is difficult to do for old measurements. For example, sunspot counts must be corrected for differences in the used telescopes (which may not exist anymore), in the observing methods and the sharpness of eye of the observers (who may have died already), and perhaps for air pollution at the observation sites (which may be very different today from when the observations were made). You must correct for the natural decay of carbon-14, which reduces its concentration as time goes by. Also, there may be other influences on the measurements besides just the solar activity, such as from the weather. Differences in the calibration lead to differences in the periods that are found in the calibrated measurements.
3. The oldest measurements are very important for determining precise periods, but the oldest measurements also tend to be the least accurate. The accuracy must be taken into account when looking for periods.
4. All measurements can be split into a combination of certain periods with certain amplitudes (for example using the Fourier transform). Even completely random series of measurements can be decomposed and re-assembled in that way, and those must show some main periods, too, but those are then themselves random. How do we find out whether the periods that we detect are "real" and not merely coincidence?
5. The investigated phenomenon need not be strictly periodical at all. The length of a solar cycle (from one minimum to the next) is not always 11 years long. Some cycles were only 9 years long, and others lasted for 14 years. The amplitudes of the phenomenon can vary as well (and does so for sunspot numbers), and there can be non-periodical components as well. If you only look for periodical components in such a phenomenon that has a variable period and/or amplitude and/or a non-periodical component, then you will find periodical components (see above at #4), but those periodical components won't describe the phenomenon very well.

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

http://en.wikipedia.org/wiki/Solar_activity
http://en.wikipedia.org/wiki/Fourier_analysis
The calculation page about frequency determination.

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Last updated: 2007-09-16