$$\def\|{&}\DeclareMathOperator{\D}{\bigtriangleup\!} \DeclareMathOperator{\d}{\text{d}\!}$$

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## 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.

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## 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.

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## 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.

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## 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 //sohowww.nascom.nasa.gov/data/realtime/realtime-mdi_igr.html and //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.

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## 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.

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## 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.

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## 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, //umbra.nascom.nasa.gov/images/latest.html.

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## 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 ℉).

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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 //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.

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### 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.

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## 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.

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## 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.

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## 12. Distance to the Sun at High Noon Compared to Sunrise

How much closer are you to the Sun at noon than you were at sunrise? That depends mostly on how long it is since the last time that the Earth as a whole was closest to the Sun. After that, the most important influence is how high in the sky the Sun is at noon (which depends on your geographical latitude and on the season). And there are many lesser influences as well, such as the Moon pulling the Earth a bit further away or closer to the Sun, and the Earth not being perfectly spherical.

If the Sun passes straight overhead (through the zenith) at noon, then the rotation of the Earth has brought you about $$r$$ = 3960 miles (6380 km) closer to the Sun than you were at sunrise. If the Sun doesn't pass straight overhead, then the distance change is less. If the Sun gets $$h$$ degrees above the horizon at noon, then the rotation of the Earth brings you $$r \sin(h)$$ closer to the Sun than you were at sunrise. If your latitude (omitting any minus sign) is $$φ$$, then $$h$$ varies between about $$66.6° − φ$$ at midwinter and about $$113.4° − φ$$ at midsummer, with an average of $$90° − φ$$. The further you get from the equator, the less the distance change becomes.

Fig. 1: Distance Difference Noon - Sunrise

However, the distance between Earth and Sun is not fixed. The Earth follows an elliptical orbit around the Sun, in which its distance from the Sun varies between about 91.4 and 94.5 million miles (147.1 and 152.1 million km). Earth is closest to the Sun (in perihelion) near January 3rd of each year and furthest from the Sun (in aphelion) near July 4th. (These dates change slowly with the centuries.) So, between about January 3rd and about July 4th, the Earth generally gets further away from the Sun all the time, and between July 4th and January 3rd the Earth generally gets closer to the Sun all the time.

The maximum rate of change of the distance between the Earth and the Sun is about 1120 mph (1800 km/h), and this rate is reached about two weeks after the March equinox (when the distance is increasing) and about two weeks after the September equinox (when the distance is decreasing).

Between the times of sunrise and noon, the Earth as a whole moves closer or further away from the Sun by a distance equal to that rate multiplied by the time between sunrise and noon. That time is about 6 hours on average, and cannot be greater than 12 hours. It approaches 12 hours for locations where the polar day is just beginning or ending, where the Sun only barely sets during a 24-hour period. There the maximum change in distance due to the Earth's orbit around the Sun is about 12 × 1120 = 13440 miles (21600 km), which is greater than the distance change that the rotation of the Earth can cause. Those locations are in the polar regions.

The distance change due to the rotation of the Earth is always negative: All other things being equal, you're distance to the Sun is less at noon than it was at sunrise by an amount of at most 3960 miles (6380 km). The distance change due to the Earth's elliptical orbit can be negative (between July 4th and January 3rd) or positive (between January 3rd and July 4th), and is at most about 13440 miles (21600 km). These two effects add up, but the greatest distance change due to the elliptical orbit only occurs for certain locations in the polar regions, and there the distance change due to the rotation of the Earth is smallest, so the greatest distance change due to the two effects combined is less than the sum of the greatest distance change of each effect separately.

All in all, the greatest distance changes are: about 13200 mi (21300 km) closer to the Sun at noon than at sunrise (in October near the South Pole), and about 11100 miles (1799 km) further from the Sun at noon than at sunrise (in March - April near the North Pole).

Outside of the polar regions the greatest distance changes are about 10800 mi (17400 km) closer to and about 5400 mi (8700 km) further from the Sun at noon than at sunrise.

Figure 1 shows the distance difference for various geographical latitudes during the year. The horizontal axis shows the beginning of months $$M$$; for example, the 2 indicates the beginning of the second month, February. The vertical axis shows the distance difference measured in units of 1 Mm = 621.4 mi. Positive values indicate that the Sun is further away at noon than it was at sunrise, and negative values indicate that the Sun was closer at noon than it was at sunrise. Data for the years 2015 - 2018 are plotted on top of each other, which makes the lines look fat. This shows the influence of the Moon, which pulls the Earth a bit closer to or further away from the Sun depending on the lunar phase, which is different each year on the same date.

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## 13. 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.

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## 14. 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 //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.

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## 15. 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. 2: Solar Cycles 1749-2006

Figure 2 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.

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

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## 16. Sky Brightness after Sunset

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:

days brightness
0 1
1 0.78
2 0.61
3 0.48
4 0.37
5 0.28
6 0.20
7 0.14
8 0.090
9 0.054
10 0.029
11 0.013
12 0.0053
13 0.0017
14 0.00045

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## 17. Loss of Influence of the Sun after Sunset

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 //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 //en.wikipedia.org/wiki/Earth%27s_atmosphere.

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## 18. The Sun is not a Pleiad

The Solar System (the Sun and all that orbits around it, including the planets) are not part of the Pleiades. The Pleiades is a group of stars at about 440 lightyears from the Sun. The group contains hundreds of stars and is about 12 lightyears in size, which is much less than its distance from us.

The Pleiades are only about 100 million years old, while the Solar System is already about 5 thousand million years old. The stars from the Pleiades are spreading out slowly, and in a few hundred million years' time it won't be possible to tell anymore that they once formed a group. The Solar System was probably born in a similar group of stars, but we don't know which stars were the sister stars to the Sun.

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## 19. The EclipticalLatitude of the Sun

A loose definition of the ecliptic is that that is the average path of the Sun along the sky in between the stars. There is a coordinate system tied to the ecliptic, in which the ecliptic plays the role of the equator. The ecliptical latitude indicates how many degrees the position on the sky is to the north or the south of the ecliptic, just like on Earth your geographical latitude says how many degrees you are to the north or the south of the equator. The ecliptical longitude indicates how many degrees the position is to the east or west of the chosen prime meridian, just like for the geographical longitude on Earth.

If the Earth and the Sun always moved through space in the exact same fixed plane, then as seen from Earth the Sun would always move along the same path through the sky against the background of stars, and then we'd call that path the ecliptic, and then the ecliptic latitude (the perpendicular angular distance between the ecliptic and the celestial object) of the Sun would always be exactly equal to 0.

However, in practice the ecliptic latitude of the Sun is usually nearly but not exactly equal to zero degrees. In the period from the beginning of 2011 through the end of 2021, the ecliptic latitude of the Sun varies between −0.00028 en +0.00031 degrees.

This deviation from ecliptic latitude 0 has several causes:

1. The Earth does not always move through space in the exact same plane. The Earth, like the Moon, moves around the common center of gravity of the Earth and the Moon, and that motion is not always in the same plane, and isn't in the same plane as the motion of that center of gravity around the Sun, so the Earth is sometimes a bit above and sometimes a bit below the plane of the ecliptic, which makes the Sun appear to stand sometimes a bit below and sometimes a bit above the ecliptic − even if the Sun itself did not move.
2. In addition, the combined gravity of the other planets pulls the common center of gravity of the Earth and the Moon sometimes a bit up and sometimes a bit down, so that it isn't always in the same fixed plane in space.
3. Also, the Sun does not always move through space in exactly the same plane. The Sun moves in a complicated way around the center of gravity of the Solar System, because of the force of gravity of the planets (especially Jupiter) on the Sun, and those planets are nearly always far above or below the plane of the ecliptic, so they pull the Sun below or above the ecliptic.

The variation of the ecliptic latitude of the Sun in the mentioned interval of time shows main periods of 27.211 and about 420 days. The former period is the draconitic month (between two passages of the Moon through the same node of its orbit), and is the result of cause #1 mentioned above.

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Last updated: 2021-07-19