We start through the stars. Even today, as soon as we’re distracted by indoor living andartificial lights, a lot of of us uncover the stars beautiful and fascinating. But imagineexactly how they need to have captivated our ancestors, who invested much more time under thestarry night sky!

For thousands of years, civilization have wondered about the stars. What are they?How far away are they? What is the meaning of their setup in the sky?How execute their areas in the skies adjust over time, and why?

This last question turns out to be the easiest to answer, so it’s a organic placeto begin our research of astronomy. As you’ll see, the answer ends up telling us just alittle bit about the stars, however a great deal around the earth.

You are watching: No matter where you are on earth, stars appear to rotate about a point called the

The Stars from Utah

Here’s a time-exposure photo that vividly illustprices the movement of the stars via a part of our sky:


Stars rising in the east, as viewed from Ogden, Utah. The cam was mounted on atripod and also the shutter was left openfor ten minutes, to display the stars’ evident motion. The foreground mountainswere illuminated by city lights.

As time passes, the stars rise in the eastern (simply favor the sun). But notice thatthey rise diagonally, not straight up. The diagonal goes from north (left) tosouthern (right). After a couple of hrs, these very same stars will certainly show up high in the southerly sky.

Turning to challenge south, we watch that the stars tbelow are moving from left (east) to right(west):

Ten-minute time exposure dealing with southern and slightly west, from the exact same area as theprevious photo. The stars are relocating from left (east) to right (west) acrossthe area of check out. In spite of the annoying light pollution, you can barely make outpart of the Milky Way, right of facility.

By currently you can most likely guess that stars set in the western sky, again alonga diagonal:


Ten-minute time exposure dealing with west, from the very same area as theprevious two photos. The stars are setting alengthy a diagonal, from south (left)to north (right). The bbest star at the lower-best is Arcturus.

And in the north, the activity is the majority of exciting. Stars increase in the northeastern andset in the northwest, relocating in counter-clockwise circles around a suggest that"s high above the north horizon:


Half-hour time expocertain facing north and slightly west, from the same area as theprevious three photos. The stars are tracing counter-clockwise circles, centeredon a point close to the prominent North Star (Polaris). Notice the Big Dipper at thelower-left.

The magestic activities of the night sky were intimately acquainted to primitive people. Today thisfamiliarity has been lost (except by astronomy geeks), so you"ll must make a special effort to remember and also visualize the patterns. It helps to stand under the night skies and suggest via your hands, tracing out the courses of differentstars. In summary:

Other stars climb in the southeast and follow shorter, reduced arcs throughout thesouthern prior to setting in the southwest.

Besides direct monitoring, you have the right to obtain accustomed to these activities by playing through theSky Motion Appletthat I"ve developed for this objective. A selection of other useful sources are listed atthe bottom of this web page.



Orion the Hunter is among the brightest and also a lot of familiar constellationsof the night sky. The row of three stars close to the middle is called Orion"s Belt.

Notice likewise that as the stars move with the sky, they remain in the exact same trends.That is, the evident “distance” in between any type of two stars never before transforms. A offered patternof stars might relocate across the sky and also revolve sideways or also upside-down, but it won"tprosper larger or smaller, or adjust its shape in any kind of various other means.

The permanence of the stellar trends encourages us to mentally attach the dotsto make photos, called constellations. Different cultures have actually done this in differentmeans, and you can reap comprising your own constellations when you"re out underthe stars. To much better communicate, but, skilled astronomers have agreed on a setof 88 official constellations, many type of of which originated with the ancient Greeks.Several of the official constellations are straightforward to recognize, while others are obscureand also tough. Learning the constellations is useful if you want to navigate or telltime by the stars, or determine wright here to look in the sky for a details star or otherexciting object.

If you desire to learn the constellations, you have the right to begin through the Sky Motion Appletand then relocate on to some of the sources listed at the bottom of this web page.

Measuring Angles

When we talk about the evident "distance" in between two points in the sky, we"re reallytalking around an angle, measured in between the two imaginary lines runningfrom your your eye out to those points:


The angle in between two points in the sky is defined as the angle in between 2 imaginary lines running from you out to those points. For the two stars shown, theangle is about 16 levels.

The bigger the angle, the farther apart the two points show up to be in the skies.The actual distance between 2 stars is much harder to identify, as we"ll laterview.

Here"s a photo of the Big Dipper, through some of the angles between stars labeled:


The bowl of the Big Dipper is about 10° wide and 5° deep. The whole take care of is16° lengthy, and the entirety Dipper is 26° lengthy.

Similarly, the width of Orion"s Belt is a small under 3°, andthe 4 star-trail photos above each expectations a width of around 60° from side to side.

To meacertain the angles between stars and various other points in the sky, astronomers useprotractors and also similar tools, frequently attached to a telescope for exact pointing. To get an approximate measurement, yet, youcan usage tools that are constantly through you: your hands. The width of your fist, heldat arm"s size, is about 10°, while the width of your bit finger guideline, alsoorganized at arm"s size, isaround 1°. These angles don"t depfinish a lot on your dimension, because human being via biggerhands also tend to have much longer arms. Next time you watch the Big Dipper, hold out yourfist and inspect that the Dipper"s bowl is about one fist wide. To estimate largerangles you have the right to usage both hands to count multiple fists. It"s additionally useful to rememberthat the angle in between the horizon and also zenith (right overhead) is 90°, as isthe angle in between 2 adjacent cardinal directions such as north and eastern, or eastand south.


Angle estimates making use of fist and also fingers, via arm outstretched.

Question: How many fists, stacked one on height of an additional, would certainly it taketo reach from the horizon to zenith?

Check your answer: 9Right. Now usage your actual fists to check this!No, remember that a appropriate angle is 90 degrees.

The Rate of Rotation

Now look ago at the east- and also west-dealing with star trail photosat the peak of this web page. The stars in these photos are complying with circular arcs thatstart in the east, pass high throughout the southern sky, and also finish in the west.You, the observer, are at the approximate facility of these circular arcs, so you canstraight measure the angle with which these stars move, by holding up your hands(to the genuine skies, not the photo!). If you make this measurement very closely, you"ll findthat in 10 minutes, each of these stars moves via an angle of 2.5°. The rate ofmotion is stable, so simple multiplication or division enables us to calculate theangle for any kind of other time period: 5° in 20 minutes, 7.5° in 30 minutes,15° in an hour. Over a full 24-hour day, the angle of rotation would certainly be

15° × 24 = 360°.

The stars therefore finish a full circle (360°) in 24 hours.(Of course, you usually can"t view the stars during daylight, however they"re still thereand also still adhering to their circular courses,as you can confirm with a telescope or by getting above earth"s setting.)

Question: How many minutes would certainly it take for a star to relocate just one degree?(Calculate the answer carefully—don"t simply guess.)

Check your answer: 4That"s correct!No, remember that the stars move 15° in 60 minutes.

The rate of angular activity is the same in other components of the skies, although you can"t just measurethe angles via your hands because you"re not at the facility of the circles. In the northernskies, however, you deserve to meacertain the angles straight by laying a protractor dvery own on a photograph. Here"s a much longer time exposure of star trails near the North Star:

In the northern sky, all stars move at the same price roughly the common centerof their circles. Throughout this 75-minute time expocertain, the stars rotated by around 19°.

If you use the information in this photo to calculate the timefor a one-level rotation (or 15° or any type of other angle), you must getroughly the exact same answer as over.

Question: How would certainly you use the data from the coming before photo to calculatethe moment required for a one-degree rotation?

Multiply 19 by 75. No, even a 19° rotation takes just 75 minutes. Divide 19 by 75. No, remember which is the angle and also which is the moment. Divide 75 by 19. Yep! And round the answer to the nearemainder minute.

This computer-simulated multiple-exposure photo (made via Sky Motion Applet)reflects Orion in the southern skies at the very same time on salso succeeding nights. Each night,after completing a complete circle, the stars have shifted rightward by about one degree.

To be precise, though, I should tell you that all of the angles quoted above are onlyapproximate. In fact, it takes a tiny less than an hour for the stars torelocate by 15°, and therefore it takes a little less than 24 hours for thestars to complete a whole circle. In reality, it takes just 23 hrs and 56 minutes,or four minutes much less than a full day. During those last four minutes the stars willrelocate by a secondary degree, so in exactly 24 hrs, the stars actually move by361°, not 360.

These added 1° rotations add up over the weeks and months, so that after a fullyear, at any type of provided time of night, you"ll check out the stars in the very same positions as before.(Due to the fact that a year is 365 days, not 360, you can probably guess that the extra per-day rotationis actually slightly less than 1°. If you really desire to be specific aboutthese things, you additionally must take right into account leap years—but let"s not bvarious other.)

So, as the periods pass, we check out various groups of stars in a offered direction, atany type of offered time of night. In January you can watch Orion climbing in the east just after sunset,yet by March, Orion will certainly be high in the southern, heading westward, by the time the skies isdark. At the same time the bideal star Arcturus will be rising in the eastern, a sign that spring is coming. If you learn to identify the influential stars and also constellations, theywill give you a strong feeling of the passage of the periods. Night owls and also early on risersdeserve to likewise reap a preview of the stars that evening observers will view in thecoming months.

The Celestial Sphere

To simplify their knowledge of the activities of the sky, primitive civilization invented amechanical design to describe these motions. We still usage this design this day because it"s soconvenient—even though it"s wrong. If you have the right to visualize the design, you won"t have actually tomemorize a entirety bunch of separate facts around exactly how the stars relocate.


The stars show up to be attached to a large celestial spbelow, spinning aboutthe celestial poles, and approximately us, when eexceptionally 23 hours and also 56 minutes.

The version is simply that the stars are all attached to the inside of a huge rigid celestial spherethat surrounds the earth and spins about us when eexceptionally 23 hrs, 56 minutes. The spinning carrieseach star roughly in its oboffered circular path, while a special point in the north sky,at the facility of the circles, stays addressed. The sphere"s rigidity accounts for just how the shapesof the constellations never before adjust, and also its enormous dimension accounts for howthe constellations never before flourish or shrink, as they would certainly if a specific point on earth weresignificantly closer to one side of the spright here than the other.

To better define places in the skies, we provide names to the various components of the celestialspright here. The addressed suggest in the northern sky is referred to as the north celestial pole, andis situated just about a level amethod from the renowned North Star (which provides tiny circlesabout it). Ninety levels from the pole is the celestial equator, a great circle thatruns from straight east to straight west, passing high above our southerly horizon. Mintaka,the rightmost star in Orion"s Belt, happens to lie practically precisely on the celestial equator, so youcan think of the celestial equator as tracing the course of this star. Another crucial greatcircle is the meridian, which runs from straight north to straight southern, passingdirectly overhead. As the sphere transforms, the meridian continues to be solved in the sky. The pointstraight overhead is referred to as zenith.

The Stars from Other Locations

I"ve explained the stellar movements as they appear from my residence in Ogden,Utah, at a latitude of 41° north of the equator. What about various other locations?Moving eastern or west makes no difference, other than to identify when you see points.If you live farther east, you"ll watch any type of offered star increase and collection sooner; if you live fartherwest, each star rises and sets later on. We compensate for these differences, in an approximate means, by establishing our clocks according to various time areas.

Moving north or south is even more amazing. The farther north you go, the better in theskies you"ll view the north celestial pole and also the stars roughly it—and the lower all the stars will certainly appear in the southern.In fact, the angle between your north horizon and also the north celestial pole is preciselyequal to your latitude.

For example, in Ogden the north celestial pole is 41° above my north horizon, however if you"re in Anchorage, Alaska, the angle is 61°. At the earth"s north pole,you would certainly see the north celestial pole straight overhead, and also the celestial equator wouldlie along yourhorizon, so you would certainly never watch any kind of stars rise or set; they would just relocate in counter-clockwisecircles if you"re facing upward, or horizontally to the ideal if you"re encountering the horizon.Stars below your horizon (that is, southern of the celestial equator) would constantly be hidden from your check out.

On the various other hand also, if you travel south to Mexico City, you"ll view the north celestial pole only19° over your northern horizon. The Big Dipper will certainly no longer always be visible, settingin the northwest and also rising in the northeast rather. But in the southern skies, you"ll check out stars thatare never visible in Utah, consisting of the famed Southern Cross.

Farther south, at earth"s equator, the north celestial pole lies on the northernhorizon, and also the celestial equator passes directly overhead. From right here, as the constellations risein the east, they show up to head straight up, quite than alengthy a diagonal.In the west, they head directly down as they collection. Even morestars are visible in the southerly sky, making clockwise half-circles about a allude on thesoutherly horizon, the southern celestial pole.

From the southern hemisphere, you can not see the north celestial pole at all. The south celestialpole, however, will appear above your southerly horizon, by an angle equal to your southerly latitude.Stars rising in the eastern will head upward and to the left, toward the north sky.The celestial equator will also pass with the northern skies, lower and lower as you headfarther south.


This several-hour-lengthy time exposure, taken from tropical north Australia,shows the clockwise motion of the southernstars around the southern celestial pole. The trails of the Southern Cross begin at the peak of the photo, through the optimal of the cross initially over the edge. Photograph by David Miller/DMI.


The primitive Greeks conceived the universe as a large spbelow of stars,neighboring the much smaller sized spherical earth. In this modern-day plasticdesign, but, the dimension of the earth is significantly exaggerated in comparison to the celestialspbelow.

Finally, if you visit earth"s southern pole, you"ll check out the south celestial pole right overhead,via the stars making clockwise circles around it. The celestial equator will lie on your horizon,through the stars moving parallel to it, from appropriate to left. You always check out the same fifty percent of the celestialsphere, totally unique from the half that you would certainly watch from earth"s north pole.

The explacountry for all these impacts is simply that the earth"s surconfront is curved. So whenyou travel to a different place, your horizon tilts through respect to the stars. Today eincredibly institution kid is taught that the earth is (approximately) a sphere. Even in prehistoric times, however,astute travelers realized that the changes in the stars as you take a trip north or south should beresulted in by the curvature of the earth. The primitive Greeks also reasoned that the earthhave to be a spbelow, and for this reason pictured the cosmos as a pair of spheres: an substantial celestialspbelow, delivering the stars about us when a day, and the much smaller spherical earth, fixedat the center of the universe.


The photo at ideal reflects Orion close to the western horizon. The photo was nottaken from Utah.

Inquiry 1: Is Orion climbing or setting?

Rising No, you"re dealing with west. Setting Stars almost always collection in the west. Depends on where photo was taken No it doesn"t!

Concern 2: From what latitude was this photo taken?

40° north Compare to the Utah photo over. 20° north Yep! Trails are 20° from vertical. 20° south The stars are setting from the southern. 40° south The stars are establishing from the southern.

Inquiry 3: About just how lengthy was the cam shutter open?

1 minute How lengthy are the trails? 2 minutes How wide is Orion"s Belt? 5 minutes Good! 10 minutes Stars relocate 2.5° in 10 min.

The Size of the Earth

Once you understand how the earth"s curvature renders the stars change as you take a trip,you have the right to conveniently recognize the earth"s circumference. All you have to carry out is travel straight north or southfor some measured distance, and also measure the angular shift of stars near the meridian. Because the North Staris always exceptionally close to the meridian (and simple to learn to recognize), it"s more than likely the a lot of convenient referral star.

The only genuine obstacle through this measurement is that you need to take a trip pretty much beforethe transition becomes noticeable. If you"re mindful, though, you can measure an angular transition of onelevel whenever you travel directly north or south by about 70 miles. So, for circumstances, if youtake a trip from Ogden to Provo (70 miles south), you"ll see the North Stargain reduced in the skies by 1°. By the same token, stars in the southerly skies will certainly obtain higherby 1°. (Of course, it"s hard to meacertain a shift of only 1°, so it"s ideal to travel fartherthan 70 miles and also measure the proportionally bigger angles.)

Now imagine proceeding your journey southward. For eextremely 70 miles traveled, the stars shiftby 1°. After 700 miles, the stars would certainly shift by 10°. As you cross the equator, the NorthStar would certainly disappear listed below your horizon, yet you could proceed to measure the shifts in the new starsthat you check out in the southern. Eventually you would certainly come to the south pole, then continue previous it,currently traveling northward, back to the equator, then to the north pole, and also finally about to yourfounding suggest. The stars have now shifted by a full 360-level circle, ago to their originalpositions. And to accomplish this, you would need to take a trip a complete distance of approximately

The earth"s circumference is therefore around 25,000 miles. The circumferencearound the equator is additionally about 25,000 miles, though this is a tiny harder to measure.You may discover it straightforward to remember that at the equator, each of the 24 (idealized) time zones is about1000 miles wide.

Question: Did you examine my calculation of the earth"s circumference? If not, examine it now! Whichstatement best explains my accuracy?

The calculation is exactly appropriate. Nope. Do the calculation! The calculation is wrong bereason I made a mistake. On purpose? Not most likely. I rounded off the answer because "70 miles" isn"t exact to begin with. Indeed.

Exercise: Use this worth of the circumference to calculate the diameterof the earth in miles, rounded to the nearemainder thousand.

Check your answer: 8000Yep!No, remember the meaning of "pi", which is around 3.

The first perboy known to have done this calculation was the prehistoric Greek astronomerEratosthenes. Instead of making use of the North Star for recommendation he offered the noon sunlight, which isadditionally on the meridian. There"s some problem over exactly how accurate his calculations actually were,however the important thing is that he used a valid technique to calculate earth"s size, more than 2100 years earlier. (If you weretaught as a boy that everyone before Columbus thought the earth was flat, you were lied to.Columbus was ridiculed not for believing the earth to be a spright here, but for grossly underestimatingits dimension, reasoning he might sail all the method to Asia prior to running out of provisions. He wasincredibly fortunate to hit land also long prior to he would certainly have reached eastern Asia.)

Which is Really Moving?

Throughout this conversation I"ve described the activities of the starsvia respect to our horizon. It"s organic to assume that our horizon, and for this reason the earthlisted below it, is truly resolved, and therefore that the stars truly move roughly in huge circlesas soon as each day.

But if you think around it, we have the right to account for all the exact same monitorings if we assume thatthe stars are resolved in area, and also the earth spins roughly when each day (in the oppowebsite direction).The initially person recognized to have actually suggested this opportunity was another primitive Greek,Heraclides. But the idea didn"t catch on for one more 1900 years, because it appeared so obviousto everyone that the earth does not move. (After all, just look at it! Does the earth look favor it"smoving? So there!)

Today, of course, we know that Heraclides was ideal. It really is the earth that spins incircles, not the stars. I"ll define later how we understand this, andjust how it"s possible for the earth to spin attremendous speed without our feeling it. On the other hand, you have actually my permission to say that the stars "rise" and also "set" and also "move" via the sky—as long as you save "through respectto our horizon" in the earlier of your mind.

See more: What Is The Direction Of The Magnetic Field That Produces The Magnetic Force On A Positive Charge

More Resources

Tright here are loads of good resources out tright here that have the right to help you learn about stellarmotions, constellations, and the celestial sphere:

Here"s an impressive photo reflecting star trails (from Australia) in eincredibly direction at as soon as.You can uncover even more star trail photos right here,right here,right here,below,and below.If you"re interested in the history of ancient Greek astronomy and the two-sphere version of theworld, there"s no much better therapy than Thomas Kuhn"s book, The Copernideserve to Revolution.