The Sun rises at the South Pole on September 22 and Doesnt Set Again Until March 21.

Understanding Astronomy

The Dominicus and the Seasons

To those of us who live on world, the almost important astronomical object by far is the sun. Information technology provides light and warmth. Its motions through our heaven cause twenty-four hours and dark, the passage of the seasons, and earth'due south varied climates.

The Sun's Daily Motion

Multiple-exposure photograph of the setting sun, showing that it follows the same diagonal path that a star would, as seen from a mid-northern latitude. This photo was made on June 21, when the sun set up considerably northward of due due west.

On whatever given 24-hour interval, the sun moves through our sky in the same mode as a star. It rises somewhere along the eastern horizon and sets somewhere in the westward. If you live at a mid-northern latitude (nearly of Northward America, Europe, Asia, and northern Africa), you always encounter the noon sun somewhere in the southern sky.

But as the weeks and months laissez passer, you'll observe that the sun's motility isn't quite the same as that of any star. For one matter, the lord's day takes a total 24 hours to make a complete circumvolve around the celestial sphere, instead of just 23 hours, 56 minutes. For obvious reasons, we ascertain our day based on the movement of the sun, non the stars.

Moreover, the location of the sun's path across the sky varies with the seasons, as shown in the estimator-generated epitome below, which shows the eastern heaven, viewed from a mid-northern latitude.

This simulated multiple-exposure prototype shows the path of the rising sun through the eastern sky on the morn of the 21st of each month, from Dec at the right through June at the left. The latitude was fix to 41° north. (The spreading of the trails as they become upward is a baloney acquired by stretching the domed sky onto a flat semicircle.)

The dominicus's path through the rest of the sky is similarly farther north in June and farther south in December. In summary:

The sunday appears to motility along with the celestial sphere on any given mean solar day, but follows different circles at different times of the yr: most northerly at the June solstice and most southerly at the December solstice. At the equinoxes, the sun's path follows the celestial equator.

• In late March and belatedly September (at the "equinoxes"), the sun's path follows the celestial equator. It then rises direct east and sets directly west. The exact dates of the equinoxes vary from twelvemonth to year, simply are always near March 20 and September 22.
• After the March equinox, the lord's day's path gradually drifts northward. By the June solstice (usually June 21), the dominicus rises considerably north of due east and sets considerably north of westward. For mid-northern observers, the noon sun is still toward the due south, merely much higher in the sky than at the equinoxes.
• After the June solstice, the dominicus'southward path gradually drifts southward. By the September equinox, its path is again along the celestial equator. The due south drift and so continues until the Dec solstice (usually December 21), when the lord's day rises considerably southward of due east and sets considerably s of due w. For mid-northern observers, the apex sun is quite low in the southern heaven. After the Dec solstice, the sun's path drifts north once more, returning to the celestial equator past the March equinox.

The illustration shows three of the sun's daily paths around the celestial sphere, once again every bit seen by an observer at latitude 41° northward. At the equinoxes, exactly half of the sun's circular path lies above the horizon. But notice that in June, considerably more than than half of the circle is to a higher place the horizon, while in December, much less than half the circle is visible. This is why, if you live in the due north, you have more hours of daylight in June (during your summer) than in December (during your winter).

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

The Seasons

The added hours of daylight are ane reason why summertime is warmer than winter. But there's another reason that'southward even more of import: the angle of the mid-day sun. Notice from the illustrations in a higher place that the noon sun is much higher in June than in December. This ways that the lord's day's rays strike the ground more directly in June. In December, on the other hand, the same amount of energy is diluted over a larger area of basis:

The intensity of sunlight striking the ground depends on the sun'southward bending in the heaven. When the sun is at a lower bending, the same amount of energy is spread over a larger area of basis, so the ground is heated less. The angles shown here are for the noon sun at breadth 41° north.

At that place is a common misconception that summer is warmer than winter considering the sun is closer to us in the summer. Actually the sun's distance hardly changes at all—and in fact, the sun happens to be closest to united states in January. Again, the seasonal changes in climate are acquired past the varying bending of the sun's rays, together with the varying corporeality of time that the sun is above our horizon.

The Sun on the Celestial Sphere

Although we never encounter the lord's day and the stars at the same time, it's not peculiarly difficult to figure out which stars and constellations the sunday is lined upward with on any given mean solar day: Just look at the constellations in the east a picayune before sunrise, or the constellations in the w a piffling after dusk, and allow for the angle of the sun below your horizon.

The ecliptic is a groovy circumvolve on the angelic sphere, tipped 23.v° with respect to the celestial equator. Its orientation with respect to our horizon changes every bit the sphere spins around united states of america each day. It has the orientation shown here at noon in December and at midnight in June.

If you plot the sun's daily location on a star nautical chart or celestial globe, you lot'll find that information technology gradually traces out a great circle, chosen the ecliptic. So the ecliptic is an imaginary circle around the angelic sphere, centered on the states, that marks all the possible locations of the sun with respect to the constellations. Each mean solar day, equally the sunday takes four minutes longer than the constellations to spin around us, information technology creeps approximately one degree east forth the ecliptic. It completes the circumvolve in exactly i full twelvemonth (365.24 days).

The ecliptic intersects the celestial equator at ii contrary points, the sunday's locations at the equinoxes. But the ecliptic is tipped at a 23.5° angle with respect to the celestial equator, and then half of it is in the celestial sphere's northern hemisphere and half is in the s. The sun reaches the ecliptic's northernmost indicate at the June solstice, and reaches its southernmost point at the December solstice.

The constellations of the zodiac are just those that happen to lie along the ecliptic. Traditionally there are 12 of them: Pisces, Ares, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Saggitarius, Capricornus, and Aquarius. Co-ordinate to the modern official constellation boundaries, however, most of the Scorpius portion of the ecliptic actually lies in the adjacent constellation Ophiuchus.

In this 360-caste map of the entire celestial sphere, the n celestial pole is stretched across the top edge and the south celestial pole across the bottom border. The celestial equator is marked in blue, and the 12 constellations of the zodiac are outlined. The ecliptic, shown in yellow, marks the sun'due south annual path amid the stars. At the March equinox the sunday is at the far right, in Pisces. The sun drifts leftward by about ane degree per twenty-four hour period, moving first into the northern one-half of the heaven then, afterwards the September equinox, into the southern one-half.

The Lord's day from Different Latitudes

The sun'southward location with respect to the stars doesn't depend on your observing location on earth, so y'all now know enough to figure out how the sun appears to motility through the sky from other locations.

If you travel e or west, you'll see the sun ascent and set up before or later, respectively, just like a star would. Again, we partially recoup for this past setting our clocks to different time zones.

If you travel north or due south, the dominicus's daily motility is still the aforementioned as that of a star seen from your latitude. So at the equinoxes, for example, the sunday still follows the angelic equator, while at the solstices, the sun follows a circle that lies 23.5° north (in June) or due south (in December) of the angelic equator. If you lot can visualize the paths of stars on these parts of the celestial sphere, then you can visualize the daily path of the lord's day.

And so, for example, as you travel northward from Utah, you'll run across the apex sun become lower and lower in the southern sky. Eventually you'll come up to a latitude where the noon sun at the December solstice lies on your southern horizon; this breadth, 23.five° beneath the N Pole, is called the Chill Circle. North of the Arctic Circle at that place will exist days around the December solstice when the sun never rises. What's a little less obvious is that at the Arctic Circle on the June solstice, the sun never sets—it merely grazes the northern horizon at midnight (come across the illustration below). Still farther north there will be more and more days of darkness in winter and continuous sunlight in summer. At the North Pole, the sun is above the horizon for six straight months (March through September), spinning around in horizontal circles, reaching a maximum summit of 23.5° above the horizon at the June solstice.

As you travel south in the northern hemisphere, the noon dominicus gets college and higher. The first qualitative alter occurs at 23.5° breadth, where the noon sun on the June solstice passes direct overhead. This breadth is called the Tropic of Cancer. Farther south, in the so-called tropics, the noon sun will appear in the northern heaven for a catamenia of time around the June solstice. At the equator, the noon sun is straight overhead on the equinoxes. And afterwards you pass 23.5° south latitude (the Tropic of Capricorn), the apex sunday is always in the n. Much farther south is the Antarctic Circle, where the sun never quite rises on the June solstice and never quite sets on the December solstice. Researchers at the South Pole have continuous daylight from September through March, and continuous night (including twilight) from March through September.

The Chill and Antarctic Circles mark the maximum reach of the sun'southward rays at the solstices. The Torrid zone of Cancer and Capricorn marking the locations where the rays of the noon sunday are perpendicular to the basis at the solstices. (Globe image adapted from NASA data using John Walker's Earth and Moon Viewer.)

These geographical variations in the lord's day's angle above the horizon also business relationship for the major geographical variations in earth's climates. The chill and antarctic regions are nearly always common cold—fifty-fifty in the summer when they get 24 hours of sunlight a 24-hour interval—because the sunday'south angle above the horizon is never very loftier. And the torrid zone are almost ever warm—even though they never get much more than 12 hours of sunlight in a twenty-four hour period—because the mid-day sun is ever and so high in the sky. The intermediate latitudes, which mostly take hot summers and cool or cold winters, are chosen the temperate zones. The north temperate zone lies betwixt the Tropic of Cancer and the Arctic Circle, while the south temperate zone (where the seasons are reversed) lies between the Tropic of Capricorn and the Antarctic Circumvolve.

Question: If you lot live on the Arctic Circumvolve, what is the maximum bending of the sun in a higher place your horizon (in degrees)?

Size and Color of the Sunday

Besides the sun's location in the sky, we can also easily measure its apparent size and the color of its light. The results might surprise y'all.

A simple only dangerous manner to estimate the lord's day'due south apparent size is to concur up your little finger toward it. The problem is that the lord's day is so bright, looking directly at information technology can damage your eyes. Withal, if you wait until the sun is greatly dimmed by clouds or brume, you lot tin can go abroad with a very quick glance. You'll so detect that the sun'southward angular width is only well-nigh half that of your little finger held at arm's length—that is, only nearly half of a degree!

A much safer way to measure the sun's apparent size is with a homemade pinhole projector.

Considering the sun is so bright, most people are surprised to larn that its angular width is only one-half a degree. A full circle is 360 degrees, so it would take near 720 suns, lined up side-to-side, to environs you in a full circumvolve.

The sun's angular size doesn't depend on where in the sky nosotros come across it. A common optical illusion, however, makes the lord's day announced larger when it is close to our horizon. This is because we're then comparing its size to that of other distant objects on the horizon. When the sun is loftier in the heaven, on the other paw, nosotros normally compare its size to that of the entire sky. In any example, it's easy to check for yourself that the sunday's measured angular size is always the same.

The sun's colour too seems to modify with its location in the sky, condign yellow-orange, or occasionally fifty-fifty reddish, when information technology is close to the horizon. When the sun is high in the sky it appears substantially white—although this is hard to see considering it'southward much harder to (safely) look at the sun at these times. Only as yous might guess, the variations in the sunday's apparent colour have nothing to exercise with the sun itself; the reddening near our horizon is actually caused by earth'south temper. Most of the air in our atmosphere is confined to a very sparse vanquish, only a few miles thick. When the lord's day is high in the heaven, its lite therefore travels through only a few miles of air before reaching our eyes. When the lord's day is on the horizon, yet, we see its light filtered through tens of miles of air. The lord's day's white lite is actually a mixture of all the colors of the rainbow, from violet and bluish to orange and carmine. The air tends to scatter the bluer colors, making the sky announced blue. The redder colors, on the other hand, are scattered much less and therefore tin can penetrate much farther through the temper—making sunsets appear xanthous-orange.

Sunlight is a mixture of all the colors of the rainbow. Air tends to scatter the bluer colors, making the sky appear blue. The redder colors can penetrate through many miles of air, causing sunsets to appear ruby-red.

Copyright ©2010-2011 Daniel 5. Schroeder. Some rights reserved.

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Source: https://physics.weber.edu/schroeder/ua/sunandseasons.html

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