3) You Note That a Particular Star Is Directly Overhead. It Will Be Directly Overhead Again in:

Learning Objectives

By the end of this section, yous will be able to:

• Define the primary features of the celestial sphere
• Explicate the system astronomers utilize to describe the sky
• Describe how motions of the stars appear to us on Earth
• Describe how motions of the Sun, Moon, and planets appear to united states of america on World
• Sympathise the modern meaning of the term constellation

Our senses propose to united states of america that Globe is the center of the universe—the hub around which the heavens turn. This geocentric (Earth-centered) view was what almost everyone believed until the European Renaissance. Afterwards all, information technology is elementary, logical, and seemingly self-evident. Furthermore, the geocentric perspective reinforced those philosophical and religious systems that taught the unique role of homo beings as the cardinal focus of the cosmos. However, the geocentric view happens to be wrong. I of the great themes of our intellectual history is the overthrow of the geocentric perspective. Permit us, therefore, take a wait at the steps by which we reevaluated the place of our world in the catholic order.

The Angelic Sphere

If y'all go on a camping ground trip or live far from urban center lights, your view of the heaven on a clear night is pretty much identical to that seen by people all over the globe before the invention of the telescope. Gazing up, you go the impression that the sky is a great hollow dome with you at the center (Effigy 1), and all the stars are an equal distance from you on the surface of the dome. The acme of that dome, the signal directly to a higher place your caput, is called the zenith, and where the dome meets Earth is chosen the horizon. From the sea or a flat prairie, it is piece of cake to encounter the horizon as a circle around you, but from most places where people live today, the horizon is at least partially subconscious by mountains, trees, buildings, or smog.

Figure 1: The Heaven effectually United states. The horizon is where the sky meets the ground; an observer's zenith is the point directly overhead.

If you prevarication dorsum in an open field and notice the night sky for hours, equally ancient shepherds and travelers regularly did, y'all will run across stars rising on the eastern horizon (just as the Sun and Moon do), moving across the dome of the heaven in the class of the nighttime, and setting on the western horizon. Watching the sky turn like this dark after night, yous might somewhen get the idea that the dome of the heaven is really part of a great sphere that is turning around y'all, bringing different stars into view equally information technology turns. The early on Greeks regarded the heaven equally simply such a angelic sphere (Figure 2). Some thought of it as an actual sphere of transparent crystalline textile, with the stars embedded in information technology like tiny jewels.

Effigy 2: Circles on the Celestial Sphere. Here we show the (imaginary) celestial sphere around Earth, on which objects are fixed, and which rotates around Globe on an axis. In reality, information technology is Earth that turns around this centrality, creating the illusion that the sky revolves effectually us. Annotation that Earth in this pic has been tilted and so that your location is at the elevation and the North Pole is where the Northward is. The apparent motion of celestial objects in the sky around the pole is shown by the circular arrow.

Today, we know that it is not the celestial sphere that turns as nighttime and twenty-four hour period go on, merely rather the planet on which we alive. We tin can put an imaginary stick through Earth's N and Due south Poles, representing our planet's axis. It is because Earth turns on this axis every 24 hours that nosotros see the Sunday, Moon, and stars rise and fix with clockwork regularity. Today, we know that these angelic objects are non actually on a dome, simply at profoundly varying distances from usa in infinite. However, it is sometimes still convenient to talk about the celestial dome or sphere to aid usa keep runway of objects in the sky. At that place is even a special theater, called a planetarium, in which we project a simulation of the stars and planets onto a white dome.

As the celestial sphere rotates, the objects on it maintain their positions with respect to ane another. A grouping of stars such as the Big Dipper has the same shape during the course of the night, although it turns with the sky. During a single night, even objects we know to have meaning motions of their ain, such every bit the nearby planets, seem fixed relative to the stars. Merely meteors—cursory "shooting stars" that flash into view for just a few seconds—move appreciably with respect to other objects on the celestial sphere. (This is considering they are not stars at all. Rather, they are small pieces of cosmic dust, called-for upward as they hit Earth'due south atmosphere.) We tin can employ the fact that the unabridged celestial sphere seems to turn together to help the states set up systems for keeping track of what things are visible in the sky and where they happen to be at a given fourth dimension.

Celestial Poles and Celestial Equator

Figure three: Circling the Southward Celestial Pole. This long-exposure photo shows trails left by stars equally a result of the apparent rotation of the celestial sphere around the south celestial pole. (In reality, it is World that rotates.) (Credit: ESO/Iztok Bončina)

To help orient usa in the turning sky, astronomers use a system that extends Earth's centrality points into the sky. Imagine a line going through Earth, connecting the N and S Poles. This is Earth'south centrality, and Globe rotates well-nigh this line. If we extend this imaginary line outward from Globe, the points where this line intersects the celestial sphere are called the north celestial pole and the south celestial pole. As Earth rotates about its axis, the heaven appears to plough in the opposite direction effectually those celestial poles (Figure 3). Nosotros as well (in our imagination) throw Earth'south equator onto the sky and call this the celestial equator. It lies halfway betwixt the angelic poles, simply every bit Earth's equator lies halfway between our planet's poles.

Now let's imagine how riding on unlike parts of our spinning Earth affects our view of the sky. The apparent motion of the angelic sphere depends on your latitude (position due north or south of the equator). First of all, discover that Globe's axis is pointing at the angelic poles, then these two points in the sky do not announced to turn.

If you stood at the North Pole of Earth, for example, yous would run into the north celestial pole overhead, at your zenith. The angelic equator, 90° from the celestial poles, would lie along your horizon. As you lot watched the stars during the course of the dark, they would all circle effectually the celestial pole, with none rising or setting. Just that one-half of the sky north of the celestial equator is ever visible to an observer at the North Pole. Similarly, an observer at the South Pole would run into simply the southern one-half of the sky.

If you were at Earth'southward equator, on the other mitt, you see the celestial equator (which, after all, is just an "extension" of World's equator) pass overhead through your zenith. The angelic poles, beingness 90° from the angelic equator, must and so be at the north and due south points on your horizon. As the sky turns, all stars ascent and set up; they move directly up from the eastward side of the horizon and ready straight downwardly on the west side. During a 24-hour menstruation, all stars are in a higher place the horizon exactly one-half the fourth dimension. (Of course, during some of those hours, the Sun is too bright for us to see them.)

What would an observer in the latitudes of the United states of america or Europe see? Remember, we are neither at Earth'southward pole nor at the equator, merely in between them. For those in the continental United states and Europe, the north angelic pole is neither overhead nor on the horizon, but in between. It appears above the northern horizon at an angular height, or altitude, equal to the observer's breadth. In San Francisco, for case, where the breadth is 38° Northward, the n celestial pole is 38° above the northern horizon.

For an observer at 38° N latitude, the south celestial pole is 38° below the southern horizon and, thus, never visible. As Earth turns, the whole sky seems to pivot about the north celestial pole. For this observer, stars inside 38° of the Northward Pole can never set. They are always to a higher place the horizon, mean solar day and night. This function of the sky is called the due north circumpolar zone. For observers in the continental Usa, the Large Dipper, Footling Dipper, and Cassiopeia are examples of star groups in the north circumpolar zone. On the other hand, stars inside 38° of the s celestial pole never rise. That part of the sky is the south circumpolar zone. To most U.Southward. observers, the Southern Cross is in that zone. (Don't worry if you are not familiar with the star groups just mentioned; we volition introduce them more formally later.)

The Rotating Sky Lab created past the University of Nebraska–Lincoln provides an interactive sit-in that introduces the horizon coordinate system, the apparent rotation of the heaven, and allows for exploration of the relationship betwixt the horizon and angelic equatorial coordinate systems.

At this particular time in Earth'southward history, at that place happens to exist a star very close to the n angelic pole. Information technology is chosen Polaris, the pole star, and has the distinction of being the star that moves the to the lowest degree amount as the northern sky turns each day. Because it moved so little while the other stars moved much more, it played a special part in the mythology of several Native American tribes, for case (some chosen it the "fastener of the sky").

What's Your Angle?

Astronomers measure how far apart objects appear in the sky by using angles. Past definition, there are 360° in a circumvolve, and so a circle stretching completely around the celestial sphere contains 360°. The half-sphere or dome of the sky so contains 180° from horizon to opposite horizon. Thus, if two stars are 18° autonomously, their separation spans about 1/10 of the dome of the sky. To give yous a sense of how big a degree is, the full Moon is about one-half a degree across. This is about the width of your smallest finger (pinkie) seen at arm'southward length.

Rise and Setting of the Sun

Nosotros described the movement of stars in the night sky, merely what almost during the daytime? The stars continue to circle during the mean solar day, only the luminescence of the Sun makes them hard to meet. (The Moon tin often be seen in the daylight, even so.) On any given day, we can think of the Sun as being located at some position on the hypothetical celestial sphere. When the Sun rises—that is, when the rotation of Earth carries the Sunday above the horizon—sunlight is scattered by the molecules of our atmosphere, filling our sky with light and hiding the stars higher up the horizon.

For thousands of years, astronomers accept been enlightened that the Sun does more than merely ascent and ready. It changes position gradually on the celestial sphere, moving each solar day nearly ane° to the east relative to the stars. Very reasonably, the ancients thought this meant the Lord's day was slowly moving around Globe, taking a catamenia of time nosotros phone call 1 year to make a full circle. Today, of course, we know it is Globe that is going effectually the Sun, but the effect is the same: the Sun's position in our heaven changes 24-hour interval to 24-hour interval. Nosotros have a similar experience when we walk around a campfire at night; we see the flames appear in forepart of each person seated well-nigh the fire in turn.

The path the Sun appears to take around the angelic sphere each twelvemonth is chosen the ecliptic (Figure four). Because of its motion on the ecliptic, the Sun rises virtually four minutes later each twenty-four hours with respect to the stars. Earth must make only a flake more than one consummate rotation (with respect to the stars) to bring the Lord's day upwards again.

Figure 4: Star Circles at Different Latitudes. The turning of the sky looks different depending on your latitude on Globe. (a) At the North Pole, the stars circle the zenith and do not rise and set. (b) At the equator, the angelic poles are on the horizon, and the stars rise straight up and set direct down. (c) At intermediate latitudes, the north celestial pole is at some position betwixt overhead and the horizon. Its angle above the horizon turns out to exist equal to the observer'southward latitude. Stars rising and set at an angle to the horizon.

As the months go by and we look at the Sun from unlike places in our orbit, we run across it projected against different places in our orbit, and thus confronting different stars in the background (Figure 5 and Tabular array i)—or we would, at least, if nosotros could see the stars in the daytime. In practice, nosotros must deduce which stars prevarication behind and beyond the Sun past observing the stars visible in the opposite management at night. Later a year, when Earth has completed ane trip around the Sun, the Sun will appear to have completed 1 circuit of the sky along the ecliptic.

Figure 5: Constellations on the Ecliptic. As Earth revolves around the Sun, we sit down on "platform Earth" and see the Lord's day moving effectually the heaven. The circumvolve in the sky that the Sun appears to make around us in the course of a year is called the ecliptic. This circle (similar all circles in the sky) goes through a fix of constellations. The ancients thought these constellations, which the Sunday (and the Moon and planets) visited, must be special and incorporated them into their organisation of star divination. Note that at any given time of the year, some of the constellations crossed by the ecliptic are visible in the night sky; others are in the solar day heaven and are thus hidden past the brilliance of the Lord's day.

Table ane. Constellations on the Ecliptic
Constellation on the Ecliptic	Dates When the Sun Crosses Information technology
Capricornus	January 21–February sixteen
Aquarius	February 16–March 11
Pisces	March xi–April 18
Aries	Apr 18–May 13
Taurus	May 13–June 22
Gemini	June 22–July 21
Cancer	July 21–August ten
Leo	August 10–September xvi
Virgo	September xvi–October 31
Libra	October 31–November 23
Scorpius	November 23–November 29
Ophiuchus	November 29–December 18
Sagittarius	Dec 18–January 21

The ecliptic does not lie along the angelic equator but is inclined to it at an bending of near 23.five°. In other words, the Sunday'south annual path in the heaven is non linked with Earth's equator. This is because our planet'due south axis of rotation is tilted by most 23.five° from a vertical line sticking out of the plane of the ecliptic (Effigy 6). Being tilted from "direct up" is not at all unusual among celestial bodies; Uranus and Pluto are actually tilted and so much that they orbit the Sun "on their side."

Figure 6: The Angelic Tilt. The celestial equator is tilted past 23.5° to the ecliptic. Every bit a result, North Americans and Europeans see the Lord's day north of the celestial equator and high in our heaven in June, and south of the celestial equator and depression in the sky in Dec.

The inclination of the ecliptic is the reason the Dominicus moves n and south in the sky as the seasons change. In Globe, Moon, and Sky, we talk over the progression of the seasons in more particular.

Stock-still and Wandering Stars

The Lord's day is not the only object that moves amidst the fixed stars. The Moon and each of the planets that are visible to the unaided heart—Mercury, Venus, Mars, Jupiter, Saturn, and Uranus (although but barely)—also change their positions slowly from 24-hour interval to solar day. During a unmarried 24-hour interval, the Moon and planets all rise and ready equally Globe turns, just as the Sun and stars exercise. Simply like the Dominicus, they accept independent motions among the stars, superimposed on the daily rotation of the celestial sphere. Noticing these motions, the Greeks of 2000 years ago distinguished between what they chosen the fixed stars—those that maintain fixed patterns among themselves through many generations—and the wandering stars, or planets. The word "planet," in fact, means "wanderer" in aboriginal Greek.

Today, we exercise not regard the Sun and Moon every bit planets, simply the ancients applied the term to all seven of the moving objects in the sky. Much of ancient astronomy was devoted to observing and predicting the motions of these celestial wanderers. They even dedicated a unit of time, the week, to the seven objects that motility on their own; that's why in that location are 7 days in a week. The Moon, being World'due south nearest celestial neighbour, has the fastest apparent movement; information technology completes a trip around the sky in about ane month (or moonth). To do this, the Moon moves about 12°, or 24 times its own apparent width on the sky, each day.

Example one: Angles in the Sky

A circle consists of 360 degrees (°). When we measure the angle in the sky that something moves, we can employ this formula:

[latex]\displaystyle\text{speed}=\frac{\text{distance}}{\text{time}}[/latex]

This is truthful whether the motility is measured in kilometers per hour or degrees per 60 minutes; nosotros just need to employ consistent units.

As an case, let's say you notice the vivid star Sirius due due south from your observing location in the Northern Hemisphere. You annotation the time, and then later, you lot notation the time that Sirius sets below the horizon. You find that Sirius has traveled an angular altitude of most 75° in 5 h. Well-nigh how many hours volition it take for Sirius to render to its original location?

Check Your Learning

The Moon moves in the heaven relative to the background stars (in addition to moving with the stars as a event of Globe'due south rotation.) Get outside at night and note the position of the Moon relative to nearby stars. Echo the observation a few hours after. How far has the Moon moved? (For reference, the diameter of the Moon is about 0.five°.) Based on your estimate of its move, how long will it take for the Moon to return to the position relative to the stars in which you first observed it?

The individual paths of the Moon and planets in the sky all lie close to the ecliptic, although not exactly on information technology. This is because the paths of the planets almost the Sun, and of the Moon about Earth, are all in nearly the same plane, every bit if they were circles on a huge canvas of paper. The planets, the Sun, and the Moon are thus always plant in the sky within a narrow xviii-degree-wide chugalug, centered on the ecliptic, called the zodiac (Effigy 5). (The root of the term "zodiac" is the same every bit that of the give-and-take "zoo" and means a collection of animals; many of the patterns of stars inside the zodiac belt reminded the ancients of animals, such equally a fish or a goat.)

How the planets appear to move in the sky equally the months pass is a combination of their actual motions plus the motion of Earth nigh the Sun; consequently, their paths are somewhat complex. As we will see, this complexity has fascinated and challenged astronomers for centuries.

Constellations

The properties for the motions of the "wanderers" in the sky is the canopy of stars. If there were no clouds in the heaven and nosotros were on a apartment patently with zero to obstruct our view, we could see virtually 3000 stars with the unaided center. To find their way effectually such a multitude, the ancients constitute groupings of stars that made some familiar geometric pattern or (more rarely) resembled something they knew. Each culture plant its ain patterns in the stars, much like a modern Rorschach exam in which y'all are asked to discern patterns or pictures in a prepare of inkblots. The ancient Chinese, Egyptians, and Greeks, amid others, found their own groupings—or constellations—of stars. These were helpful in navigating amongst the stars and in passing their star lore on to their children.

You may exist familiar with some of the old star patterns we however use today, such as the Big Dipper, Picayune Dipper, and Orion the hunter, with his distinctive belt of iii stars (Figure 7). However, many of the stars we run into are not part of a distinctive star blueprint at all, and a telescope reveals millions of stars likewise faint for the eye to see. Therefore, during the early decades of the twentieth century, astronomers from many countries decided to plant a more than formal system for organizing the sky.

Figure 7: Orion. (a) The winter constellation of Orion, the hunter, is surrounded past neighboring constellations, as illustrated in the seventeenth-century atlas by Hevelius. (b) A photo shows the Orion region in the heaven. Note the three blue stars that make upward the belt of the hunter. The bright red star above the belt denotes his armpit and is called Betelgeuse (pronounced "Beetel-juice"). The bright blue star below the chugalug is his foot and is called Rigel. (credit a: modification of work past Johannes Hevelius; b: modification of work past Matthew Spinelli)

Today, we use the term constellation to hateful 1 of 88 sectors into which nosotros separate the sky, much equally the United States is divided into 50 states. The modernistic boundaries betwixt the constellations are imaginary lines in the sky running north–southward and east–west, so that each point in the sky falls in a specific constellation, although, like us, non all constellations are the same size. All the constellations are listed in The Constellations. Whenever possible, we have named each modernistic constellation after the Latin translations of one of the ancient Greek star patterns that lies within it. Thus, the mod constellation of Orion is a kind of box on the sky, which includes, amidst many other objects, the stars that fabricated up the aboriginal picture show of the hunter. Some people use the term asterism to denote an peculiarly noticeable star pattern inside a constellation (or sometimes spanning parts of several constellations). For instance, the Big Dipper is an asterism within the constellation of Ursa Major, the Big Bear.

Students are sometimes puzzled because the constellations seldom resemble the people or animals for which they were named. In all likelihood, the Greeks themselves did non name groupings of stars considering they looked similar actual people or subjects (whatsoever more than the outline of Washington state resembles George Washington). Rather, they named sections of the sky in honor of the characters in their mythology and and so fit the star configurations to the animals and people as best they could.

This website almost objects in the heaven allows users to construct a detailed sky map showing the location and information near the Sun, Moon, planets, stars, constellations, and even satellites orbiting Earth. Begin past setting your observing location using the choice in the menu in the upper right corner of the screen.

The direct evidence of our senses supports a geocentric perspective, with the celestial sphere pivoting on the celestial poles and rotating nearly a stationary Globe. We see only half of this sphere at 1 fourth dimension, express by the horizon; the point straight overhead is our zenith. The Sun'due south annual path on the celestial sphere is the ecliptic—a line that runs through the center of the zodiac, which is the 18-degree-wide strip of the heaven within which we e'er find the Moon and planets. The celestial sphere is organized into 88 constellations, or sectors.

Glossary

angelic equator: a bang-up circle on the celestial sphere 90° from the angelic poles; where the celestial sphere intersects the plane of World'southward equator

celestial poles: points about which the celestial sphere appears to rotate; intersections of the celestial sphere with World's polar axis

celestial sphere: the apparent sphere of the sky; a sphere of large radius centered on the observer; directions of objects in the heaven tin can exist denoted by their position on the celestial sphere

circumpolar zone: those portions of the celestial sphere near the celestial poles that are either always above or ever beneath the horizon

ecliptic: the apparent almanac path of the Sun on the celestial sphere

geocentric: centered on Earth

horizon (astronomical): a bang-up circle on the angelic sphere 90° from the zenith; more popularly, the circle around the states where the dome of the sky meets Globe

planet: today, any of the larger objects revolving near the Sun or any similar objects that orbit other stars; in ancient times, any object that moved regularly amid the fixed stars

year: the period of revolution of Globe around the Sunday

zenith: the point on the celestial sphere contrary the direction of gravity; point directly above the observer

zodiac: a belt around the heaven most 18° wide centered on the ecliptic

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Source: https://courses.lumenlearning.com/astronomy/chapter/the-sky-above/

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