To those of united state who live ~ above earth, the many important expensive object by far is the sun.It offers light and also warmth. Its motions through our sky cause day and also night, the passage of the seasons, and earth"s varied climates.

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The Sun"s daily Motion


Multiple-exposure photo of the setting sun, reflecting that it follows the samediagonal course that a star would, as viewed from a mid-northern latitude. This photowas make on June 21, as soon as the sun collection considerably phibìc of early west.

On any type of given day, the sunlight moves v our skies in the same means as a star.It rises somewhere follow me the eastern horizon and sets what in the west. If you liveat a mid-northern latitude (most of north America, Europe, Asia, and northern Africa), you always see thenoon sunlight somewhere in the southern sky.

But as the weeks and also months pass, you"ll an alert that the sun"s activity isn"t fairly thesame as that of any star. For one thing, the sun takes a complete 24 hrs to make a completecircle approximately the celestial sphere, rather of just 23 hours, 56 minutes. For obviousreasons, we specify our day based on the motion of the sun, no the stars.

Moreover, the place of the sun"s path across the skies varies v the seasons,as displayed in the computer-generated photo below, which shows the east sky, regarded from a mid-northernlatitude.


This simulated multiple-exposure imageshows the path of the rising sun through the eastern sky on the morning of the 21stof every month, from December in ~ the appropriate through June in ~ the left. The latitude wasset to 41° north. (The spreading of the trails together they go increase is adistortion caused by stretching the domed skies onto a level semicircle.)

The sun"s route through the remainder of the sky is likewise farther north in June and also farther south in December. In summary:


The sun shows up to move together with the celestial sphere on any given day, butfollows different circles at various times the the year: many northerly in ~ the Junesolstice and most southerly at the December solstice. At the equinoxes, the sun"spath adheres to the celestial equator.

In late March and also late September (at the "equinoxes"), the sun"s path followsthe celestial equator. It then rises straight east and sets straight west. The exactdates the the equinoxes differ from year come year, but are constantly near in march 20 andSeptember 22.After the march equinox, the sun"s path slowly drifts northward. By the Junesolstice (usually June 21), the sunlight rises substantially north of due east and also setsconsiderably phibìc of due west. Because that mid-northern observers, the noon sunlight is stilltoward the south, but much higher in the sky than in ~ the equinoxes.

The illustration reflects three that the sun"s everyday paths about the celestial sphere, again as watched byan observer at latitude 41° north. At the equinoxes, exactly half of the sun"s circularpath lies above the horizon. But notice that in June, considerably an ext than fifty percent ofthe one is over the horizon, if in December, much less than fifty percent the circleis visible. This is why, if friend live in the north, friend have much more hours the daylight in June(during her summer) than in December (during her winter).

Question: If her latitude is 41° north, what is the angle(in degrees)between the noon sun and also your southern horizon at the march or September equinox?

Check your answer: 49Good! (180 minus 90 minus 41)No, usage the diagram above, and also remember her geometry.

The Seasons

The included hours that daylight room one reason why summer is warmer 보다 winter. However there"sanother factor that"s even much more important: the angle that the mid-day sun. Notification fromthe illustrations over that the noon sunlight is much higher in June 보다 in December. This meansthat the sun"s light ray strike the ground much more directly in June. In December, ~ above the other hand,the exact same amount of energy is diluted over a larger area that ground:


The strongness of sunlight striking the ground depends on the sun"s anglein the sky. When the sunlight is at a lower angle, the very same amount of power isspread over a bigger area the ground, therefore the ground is heated less. The anglesshown here are because that the noon sunlight at latitude 41° north.

There is a common misconception the summer is warmer than winter due to the fact that the sun iscloser to us in the summer. In reality the sun"s distance hardly changes at all—and in fact, the sunlight happens to be closest to united state in January. Again, the seasonal changesin climate are resulted in by the varying angle of the sun"s rays, together withthe differing amount of time that the sunlight is over our horizon.

The sunlight on the Celestial Sphere

Although we never ever see the sun and the stars at the very same time, it"s not particularly hard to number out i m sorry stars and also constellations the sun is lined up v on anygiven day: just look at the constellations in the east a tiny before sunrise, or the constellationsin the west a little after sunset, and enable for the edge of the sun listed below your horizon.


The ecliptic is a great circle top top the celestial sphere, tipped 23.5° withrespect to the celestial equator. Its orientation through respect to our horizon changesas the round spins around us every day. It has the orientation presented here in ~ noon inDecember and also at midnight in June.

If friend plot the sun"s everyday location ~ above a star chart or celestial globe, you"ll find thatit progressively traces the end a an excellent circle, dubbed the ecliptic. For this reason the ecliptic is an imagine circlearound the celestial sphere, centered on us, the marks all the possible locations that the sunwith respect to the constellations. Every day, together the sun takes 4 minutes much longer than the constellations to spin approximately us, that creeps roughly one degree eastward along theecliptic. The completes the circle in precisely one complete year (365.24 days).

The ecliptic intersects the celestial equator at 2 opposite points, the sun"s locationsat the equinoxes. But the ecliptic is tipped at a 23.5° angle with respect to the celestialequator, so half of it is in the celestial sphere"s north hemisphere and half is in the south.The sun reaches the ecliptic"s northernmost suggest at the June solstice, and also reaches that is southernmost allude at the December solstice.

The constellations that the zodiac are merely those that happen to lie along the ecliptic.Traditionally there space 12 that them: Pisces, Ares, Taurus, Gemini, Cancer, Leo, Virgo,Libra, Scorpius, Saggitarius, Capricornus, and Aquarius. According to the modern officialconstellation boundaries, however, most of the Scorpius part of the ecliptic actuallylies in the nearby constellation Ophiuchus.


In this 360-degree map that the entire celestial sphere, the phibìc celestial pole is stretchedacross the peak edge and the south celestial pole across the bottom edge. The celestial equatoris marked in blue, and the 12 constellations that the zodiac space outlined. The ecliptic, shownin yellow, marks the sun"s yearly path amongst the stars. In ~ the in march equinox the sunlight is in ~ thefar right, in Pisces. The sunlight drifts leftward by about one degree per day, moving very first into thenorthern fifty percent of the sky and then, after ~ the September equinox, right into the southern half.

The sun from various Latitudes

The sun"s location with respect to the stars doesn"t depend on her observing location on earth, therefore you currently know enough to figure out how the sun appears tomove through the skies from various other locations.

If girlfriend travel eastern or west, you"ll watch the sun increase and set earlier or later, respectively,just like a star would. Again, we partly compensate because that this by setup our clocks to various time zones.

If you take trip north or south, the sun"s day-to-day motion is quiet the same as that of a starseen from your latitude.So in ~ the equinoxes, for example, the sunlight still complies with the celestial equator, when atthe solstices, the sun adheres to a circle the lies 23.5° north (in June) or south(in December) that the celestial equator. If you can visualize the routes of stars on theseparts the the celestial sphere, then you deserve to visualize the daily path of the sun.

So, because that example, together you travel northward from Utah, you"ll watch the noon sun acquire lowerand lower in the southerly sky. At some point you"ll involved a latitude whereby the noon sunat the December solstice lies on your southern horizon; this latitude, 23.5° belowthe phibìc Pole, is dubbed theArctic Circle. North of the Arctic circle there will certainly be daysaround the December solstice once the sun never ever rises.What"s a small less evident is the at the Arctic circle on the Junesolstice, the sun never sets—it simply grazes the north horizon at midnight (see the illustration below).Still farther north there will be much more and more days the darkness in winter and also continuoussunlight in summer. In ~ the north Pole, the sun is over the horizon for 6 straight months (March with September), spinning around in horizontal circles, reaching a maximumheight that 23.5° over the horizon in ~ the June solstice.

As you travel southward in the northern hemisphere, the noon sunlight gets greater andhigher. The an initial qualitative change occurs at 23.5° latitude, whereby the noon sunon the June solstice passes straight overhead. This latitude is called the Tropic the Cancer.Farther south, in the so-called tropics, the noon sunlight will show up in the northern skyfor a period of time approximately the June solstice. In ~ the equator, the noon sunlight is straightoverhead top top the equinoxes. And also after you happen 23.5° southern latitude (the Tropic that Capricorn),the noon sunlight is constantly in the north. Much farther south is the Antarctic Circle, wherethe sun never ever quite rises top top the June solstice and never rather sets ~ above the December solstice. Researchersat the southern Pole have continuous daylight from September v March, and also continuousnight (including twilight) native March through September.


The Arctic and Antarctic Circles note the maximum with of the sun"s rays atthe solstices. The Tropics of Cancer and Capricorn mark the locationswhere the rays of the noon sun room perpendicular come the ground at the solstices.(Earth image adjusted from NASA data using John Walker"s Earth and also Moon Viewer.)

These geography variations in the sun"s angle over the horizon additionally account forthe significant geographical sports in earth"s climates. The arctic and antarctic areas are practically alwayscold—even in the summer once they get 24 hours of sunlight a day—because the sun"s angle above the horizon is never an extremely high. And also the tropics are almost always warm—eventhough lock never acquire much an ext than 12 hrs of sunlight in a day—because the mid-day sunis always so high in the sky. The intermediate latitudes, which normally have warm summers and coolor cold winters, are called the pleasant zones. The north warm zone lies betweenthe Tropic of Cancer and also the Arctic Circle, if the southern temperate ar (where the seasonsare reversed) lies between the Tropic of Capricorn and also the Antarctic Circle.

Question: If friend live ~ above the Arctic Circle, what is the maximumangle of the sun above your horizon (in degrees)?

Check her answer: 47Right! (at noon top top the June solstice)No, shot to figure out the edge from the diagram above.

Size and Color that the Sun

Besides the sun"s location in the sky, we can likewise easily measure its apparentsize and the color of that light. The results could surprise you.

A straightforward but unsafe method to calculation the sun"s apparent size is to organize up your tiny fingertoward it. The trouble is the the sunlight is for this reason bright, looking straight at that can damages your eyes.Still, if friend wait till the sunlight is greatly dimmed by clouds or haze, girlfriend can acquire away through a very quick glance. You"ll then discover that the sun"s angular width is just about half that the yourlittle finger hosted at arm"s length—that is, only about half of a degree!

A lot safer means to measure up the sun"s apparent size is with a homemade pinhole projector.

Because the sunlight is for this reason bright, most human being are surprised to learn that that is angular broad isonly half a degree. A full circle is 360 degrees, therefore it would certainly take around 720 suns, inside wall up side-to-side,to surround you in a complete circle.

The sun"s angular dimension doesn"t depend on whereby in the sky we see it. A typical optical illusion,however, provides the sun appear larger once it is close to our horizon. This is because we"re thencomparing its size to the of other remote objects top top the horizon. When the sun is high in the sky,on the various other hand, we usually compare its size to the of the entire sky. In any case, it"s basic to check for yourself that the sun"s measured angular dimension is always the same.The sun"s color also seems to readjust with its ar in the sky, ending up being yellow-orange,or occasionally even red, when it is close come the horizon. As soon as the sun is high in the skies it appearsessentially white—although this is difficult to see due to the fact that it"s much harder to (safely) lookat the sun at these times. Yet as you could guess, the variations in the sun"s obvious color have nothing to execute with the sun itself; the reddening near our horizon is actually led to by earth"s atmosphere.Most of the waiting in our atmosphere is confined to a really thin shell, just a couple of miles thick.When the sun is high in the sky, its light therefore travels through only a couple of miles of wait beforereaching our eyes. As soon as the sunlight is top top the horizon, however, we see its irradiate filtered through tensof miles of air. The sun"s white irradiate is actually a mixture of all the color of the rainbow, fromviolet and also blue to orange and also red. The air tends to scatter the bluer colors, do the sky appearblue. The red colors, on the various other hand, are scattered much less and also therefore have the right to penetrate muchfarther v the atmosphere—making sunsets appear yellow-orange.

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Sunlight is a mixture of every the colour of the rainbow. Air tends to scatter thebluer colors, making the sky appear blue. The red colors deserve to penetrate through manymiles the air, bring about sunsets to appear red.