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Turning to face south, we see that the stars there are moving from left east to right west : Ten-minute time exposure facing south and slightly west, from the same location as the previous photo. The stars are moving from left east to right west across the field of view. Despite the annoying light pollution, you can barely make out part of the Milky Way, right of center. By now you can probably guess that stars set in the western sky, again along a diagonal:.

Ten-minute time exposure facing west, from the same location as the previous two photos. The stars are setting along a diagonal, from south left to north right. The bright star at the lower-right is Arcturus. And in the north, the motion is most interesting.

How Stars Work

Stars rise in the northeast and set in the northwest, moving in counter-clockwise circles around a point that's high above the northern horizon:. Half-hour time exposure facing north and slightly west, from the same location as the previous three photos. The stars are tracing counter-clockwise circles, centered on a point near the prominent North Star Polaris. Notice the Big Dipper at the lower-left. The magestic motions of the night sky were intimately familiar to ancient people.


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Today this familiarity has been lost except by astronomy geeks , so you'll need to make a special effort to remember and visualize the patterns. It helps to stand under the night sky and point with your hands, tracing out the paths of different stars. In summary:.

One Reply to “Do Stars Move?”

Besides direct observation, you can get accustomed to these motions by playing with the Sky Motion Applet that I've created for this purpose. A variety of other useful resources are listed at the bottom of this page. Orion the Hunter is one of the brightest and most familiar constellations of the night sky. The row of three stars near the middle is called Orion's Belt. Notice also that as the stars move through the sky, they stay in the same patterns.

Summary and reviews of The Movement of Stars by Amy Brill

A given pattern of stars may move across the sky and turn sideways or even upside-down, but it won't grow larger or smaller, or change its shape in any other way. The permanence of the stellar patterns encourages us to mentally connect the dots to make pictures , called constellations. Different cultures have done this in different ways, and you might enjoy making up your own constellations when you're out under the stars. To better communicate, however, professional astronomers have agreed on a set of 88 official constellations , many of which originated with the ancient Greeks.

Some of the official constellations are easy to recognize, while others are obscure and difficult. Learning the constellations is helpful if you want to navigate or tell time by the stars, or determine where to look in the sky for a particular star or other interesting object. If you want to learn the constellations, you can start with the Sky Motion Applet and then move on to some of the resources listed at the bottom of this page.

When we talk about the apparent "distance" between two points in the sky, we're really talking about an angle , measured between the two imaginary lines running from your your eye out to those points:. The angle between two points in the sky is defined as the angle between two imaginary lines running from you out to those points. For the two stars shown, the angle is about 16 degrees. The bigger the angle, the farther apart the two points appear to be in the sky.

The actual distance between two stars is much harder to determine, as we'll later see. To measure the angles between stars and other points in the sky, astronomers use protractors and similar instruments, often attached to a telescope for accurate pointing. To get an approximate measurement, however, you can use instruments that are always with you: your hands. These angles don't depend much on your size, because people with bigger hands also tend to have longer arms.

Next time you see the Big Dipper, hold out your fist and check that the Dipper's bowl is about one fist wide.

Motion of the Stars

To estimate larger angles you can use both hands to count multiple fists. Question: How many fists, stacked one on top of another, would it take to reach from the horizon to zenith? Now look back at the east- and west-facing star trail photos at the top of this page. The stars in these photos are following circular arcs that begin in the east, pass high across the southern sky, and end in the west.

You, the observer, are at the approximate center of these circular arcs, so you can directly measure the angle through which these stars move, by holding up your hands to the real sky, not the photo! If you make this measurement carefully, you'll find that in 10 minutes, each of these stars moves through an angle of 2.

Over a full hour day, the angle of rotation would be. Of course, you normally can't see the stars during daylight, but they're still there and still following their circular paths, as you can confirm with a telescope or by getting above earth's atmosphere. Question: How many minutes would it take for a star to move just one degree? Calculate the answer carefully—don't just guess. The rate of angular motion is the same in other parts of the sky, although you can't just measure the angles with your hands because you're not at the center of the circles.

In the northern sky, however, you can measure the angles directly by laying a protractor down on a photograph. Here's a longer time exposure of star trails near the North Star: In the northern sky, all stars move at the same rate around the common center of their circles. Question: How would you use the data from the preceding photo to calculate the time required for a one-degree rotation?

This computer-simulated multiple-exposure image made with Sky Motion Applet shows Orion in the southern sky at the same time on seven successive nights.


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Each night, after completing a full circle, the stars have shifted rightward by about one degree. To be precise, though, I need to tell you that all of the angles quoted above are only approximate. In fact, it takes just 23 hours and 56 minutes, or four minutes less than a full day. If you really want to be precise about these things, you also need to take into account leap years—but let's not bother. So, as the seasons pass, we see different groups of stars in a given direction, at any given time of night.

In January you can watch Orion rising in the east just after sunset, but by March, Orion will be high in the south, heading westward, by the time the sky is dark. Meanwhile the bright star Arcturus will be rising in the east , a sign that spring is coming. If you learn to identify the prominent stars and constellations, they will give you a strong sense of the passage of the seasons.

Night owls and early risers can also enjoy a preview of the stars that evening observers will see in the coming months. If you make this measurement carefully, you'll find that in 10 minutes, each of these stars moves through an angle of 2. Over a full hour day, the angle of rotation would be. Of course, you normally can't see the stars during daylight, but they're still there and still following their circular paths, as you can confirm with a telescope or by getting above earth's atmosphere. Question: How many minutes would it take for a star to move just one degree?

Calculate the answer carefully—don't just guess. The rate of angular motion is the same in other parts of the sky, although you can't just measure the angles with your hands because you're not at the center of the circles. In the northern sky, however, you can measure the angles directly by laying a protractor down on a photograph. Here's a longer time exposure of star trails near the North Star: In the northern sky, all stars move at the same rate around the common center of their circles.

Question: How would you use the data from the preceding photo to calculate the time required for a one-degree rotation? This computer-simulated multiple-exposure image made with Sky Motion Applet shows Orion in the southern sky at the same time on seven successive nights. Each night, after completing a full circle, the stars have shifted rightward by about one degree. To be precise, though, I need to tell you that all of the angles quoted above are only approximate.

In fact, it takes just 23 hours and 56 minutes, or four minutes less than a full day. If you really want to be precise about these things, you also need to take into account leap years—but let's not bother. So, as the seasons pass, we see different groups of stars in a given direction, at any given time of night. In January you can watch Orion rising in the east just after sunset, but by March, Orion will be high in the south, heading westward, by the time the sky is dark.

Meanwhile the bright star Arcturus will be rising in the east , a sign that spring is coming. If you learn to identify the prominent stars and constellations, they will give you a strong sense of the passage of the seasons. Night owls and early risers can also enjoy a preview of the stars that evening observers will see in the coming months. To simplify their understanding of the motions of the sky, ancient people invented a mechanical model to explain these motions. We still use this model today because it's so convenient—even though it's wrong.

If you can visualize the model, you won't have to memorize a whole bunch of separate facts about how the stars move. The stars appear to be attached to a giant celestial sphere, spinning about the celestial poles, and around us, once every 23 hours and 56 minutes. The model is simply that the stars are all attached to the inside of a giant rigid celestial sphere that surrounds the earth and spins around us once every 23 hours, 56 minutes. The spinning carries each star around in its observed circular path, while a special point in the northern sky, at the center of the circles, remains fixed.

The sphere's rigidity accounts for how the shapes of the constellations never change, and its enormous size accounts for how the constellations never grow or shrink, as they would if a particular point on earth were significantly closer to one side of the sphere than the other. To better describe locations in the sky, we give names to the various parts of the celestial sphere.

The fixed point in the northern sky is called the north celestial pole , and is located only about a degree away from the famous North Star which makes tiny circles around it. Ninety degrees from the pole is the celestial equator , a great circle that runs from directly east to directly west, passing high above our southern horizon. Mintaka , the rightmost star in Orion's Belt, happens to lie almost exactly on the celestial equator, so you can think of the celestial equator as tracing the path of this star.

Another important great circle is the meridian , which runs from directly north to directly south, passing straight overhead. As the sphere turns, the meridian remains fixed in the sky. The point straight overhead is called zenith. What about other locations? Moving east or west makes no difference, except to determine when you see things.

If you live farther east, you'll see any given star rise and set sooner; if you live farther west, each star rises and sets later. We compensate for these differences, in an approximate way, by setting our clocks according to different time zones. Moving north or south is more interesting. The farther north you go, the higher in the sky you'll see the north celestial pole and the stars around it—and the lower all the stars will appear in the south. In fact, the angle between your northern horizon and the north celestial pole is precisely equal to your latitude.

At the earth's north pole, you would see the north celestial pole straight overhead, and the celestial equator would lie along your horizon, so you would never see any stars rise or set; they would just move in counter-clockwise circles if you're facing upward, or horizontally to the right if you're facing the horizon. Stars below your horizon that is, south of the celestial equator would always be hidden from your view.

The Big Dipper will no longer always be visible, setting in the northwest and rising in the northeast instead. But in the southern sky, you'll see stars that are never visible in Utah, including the famous Southern Cross. Farther south, at earth's equator , the north celestial pole lies on the northern horizon, and the celestial equator passes straight overhead.

From here , as the constellations rise in the east, they appear to head straight up, rather than along a diagonal. In the west, they head straight down as they set. Even more stars are visible in the southern sky, making clockwise half-circles about a point on the southern horizon, the south celestial pole. From the southern hemisphere, you can't see the north celestial pole at all. The south celestial pole, however, will appear above your southern horizon, by an angle equal to your southern latitude.

Stars rising in the east will head upward and to the left , toward the northern sky. The celestial equator will also pass through the northern sky, lower and lower as you head farther south. This several-hour-long time exposure, taken from tropical northern Australia, shows the clockwise motion of the southern stars around the south celestial pole.

The trails of the Southern Cross start at the top of the image, with the top of the cross initially above the edge. The ancient Greeks conceived the universe as a giant sphere of stars, surrounding the much smaller spherical earth. In this modern plastic model, however, the size of the earth is greatly exaggerated in comparison to the celestial sphere. Finally, if you visit earth's south pole, you'll see the south celestial pole straight overhead , with the stars making clockwise circles around it. The celestial equator will lie on your horizon, with the stars moving parallel to it, from right to left.

You always see the same half of the celestial sphere, completely distinct from the half that you would see from earth's north pole. The explanation for all these effects is simply that the earth's surface is curved. So when you travel to a different location, your horizon tilts with respect to the stars. Coupled with measurements of the stars' radial velocities , proper motions can be used to compute the distance to the cluster. Stellar proper motions have been used to infer the presence of a super-massive black hole at the center of the Milky Way.

The Moving Stars of the Southern Hemisphere

Proper motion was suspected by early astronomers according to Macrobius , AD but a proof was not provided until by Edmund Halley , who noticed that Sirius , Arcturus and Aldebaran were over half a degree away from the positions charted by the ancient Greek astronomer Hipparchus roughly years earlier. The term "proper motion" derives from the historical use of "proper" to mean "belonging to" cf, propre in French and the common English word property. The following are the stars with highest proper motion from the Hipparcos catalog. There are a number of software products that allow a person to view the proper motion of stars over differing time scales.

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Free ones include:. From Wikipedia, the free encyclopedia. Not to be confused with proper velocity. Kuhn In Quest of the Universe. Observational Astronomy. Green; Mark H. Jones An Introduction to the Sun and Stars. Cambridge University Press. Retrieved 7 December Smith RR Lyrae Stars. In F Combes; Keiichi Wada eds. Mapping the Galaxy and Nearby Galaxies. Annual Review of Astronomy and Astrophysics. Reid; Andreas Brunthaler; Heino Falcke The Astrophysical Journal.

Bibcode : ApJ Textbook on Spherical Astronomy. The Stars: A study of the Universe. Archived from the original PDF on Mar 3, Retrieved