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To this point, we have discussed the apparent motions that the stars on the celestial sphere make across the sky above our heads on a daily basis. But why is it that during various times of the year, we see completely distinct patterns of stars, often known as different constellations?
The explanation is that the night sky changes as a result of Earth’s orbit around the Sun, and as you’ll see in a moment, this also explains why the route on the celestial sphere known as the ecliptic is shaped the way it is.
Keep in mind that the celestial sphere is really an illusion brought about by the fact that we cannot perceive depth in space.
In actuality, the Earth travels around the Sun in the manner shown in Figure 2.16, whereas the distant stars that are visible in the night sky are positioned far further off (and themselves located at different distances from Earth). On the other hand, given that we can’t know how far away the stars are just by gazing at them, Figure 2.16 depicts them as being situated exactly beyond the orbit of the Earth around the Sun. In addition, because the Earth travels along the same path in its orbit year after year, we always see the Sun appearing to move among the same set of constellations, which are referred to as the constellations of the zodiac, from one year to the next. This is because the Earth follows the same orbital path.
It is interesting to note that from where we stand on Earth, the Sun seems to travel in a rather constant direction toward the east as it traverses the constellations of the zodiac. You should keep in mind that this route is referred to as the ecliptic, and that the ecliptic is marked on the celestial sphere because of this reason (as we first saw in Figure 2.6).
Follow along as you look at Figure 2.16 to see how the orbit of Earth causes us to view various constellations at different times of the year:
Find the point labeled “March 21” on the orbit of Earth around the Sun. This position indicates where Earth is positioned on March 21 of each year.
Observe that on that day, the Sun seems to be on the celestial sphere at the location of the yellow dot in the constellation Pisces if you draw a line from that position through the Sun. This can be seen if you draw a line from that point through the Sun. Since of this, on March 21 we won’t be able to view the constellation Pisces at night because it will be visible in the daylight sky with the Sun.
If, on the other hand, we turn our gaze away from the Sun and in the direction indicated by the arrow labeled “night,” we will find the constellation Virgo at the position on the celestial sphere that is directly opposite the Sun. This indicates that Virgo will be visible to us ascending at twilight, crossing the meridian around midnight, then descending at dawn.
If you choose another date, you’ll discover that the sun is at a different spot along the ecliptic, and as a result, the constellations you see when you look up at the night sky will be different.
A gathering of stars that seems like it forms a pattern or image is called a constellation. Some examples of constellations are Orion the Great Hunter, Leo the Lion, and Taurus the Bull. Patterns that are clearly identifiable and that assist humans in orienting themselves using the night sky are called constellations. There are 88 constellations that are considered “official.”
Certainly not in every case. Each constellation is made up of a group of stars that are spread out throughout space in three distinct dimensions; also, the stars in each constellation are located at varying distances from Earth. Because we are looking at a constellation from a very, very great distance, all of the stars inside it give the impression that they are lying on the same plane. The size, distance from Earth, and temperature of stars may vary substantially from one another. It’s possible that dim stars are smaller in size, farther distant, or have a lower temperature than brighter stars. In the same vein, the stars with the strongest light may not always represent the nearest ones. The dimmest star in the constellation Cygnus, the Swan, is the one that is nearest to us, while the brightest star is the one that is farthest distant.
The ancient civilizations of the Middle East, Greece, and Rome are responsible for the majority of the names that we use for the constellations. They ascribed deities, goddesses, animals, and other characters and items from their tales to groups of stars. It is essential to have an understanding that these civilizations were not the first ones to decorate the night sky with figures that played an important role in their everyday life. Native Americans, Asians, and Africans are just some of the peoples from all around the globe and throughout history who have created images using those same stars. It’s possible that some constellations had ritual or spiritual significance in certain places and times. In other instances, the star patterns served as a useful means of determining the passage of time between sowing crops and gathering their harvest. There are 48 “old” constellations, and these are the brightest groups of stars; those that can be seen readily by the naked eye are considered to be part of the ancient constellations. There are in fact fifty “old” constellations; astronomers have segmented one of the constellations, Argo, into three separate pieces.
“Modern” constellations such as the Peacock, Telescope, and Giraffe were discovered by later astronomers in the 1500s, 1600s, and 1700s. These astronomers were able to observe the night sky in the southern hemisphere using telescopes, which allowed them to identify “modern” constellations such as these. These researchers “linked” the fainter stars that were previously thought to be separate constellations. There are 38 different constellations in the contemporary era.
In 1930, the International Astronomical Union officially classified 88 current and ancient constellations (one of the ancient constellations was broken into three sections), as well as drew a border around each one of them. The edges of the boundary meet, creating 88 individual segments inside the fictitious sphere known as the celestial sphere that encompasses the Earth. Even though it is not included in the constellation image, astronomers regard every star that falls inside the boundaries of a certain constellation to be a component of that particular constellation.
No, there are billions of stars, but only a small portion of those stars make up the forms of our constellations. These are the stars that can be seen with the naked eye without the use of any other instrument. Ancient astronomers pieced together the star images by connecting these stars.
However, any star may be located inside the confines of one of the 88 areas that make up a constellation. Through the use of contemporary telescopes, astronomers were able to observe the night sky and identify stars that were not a part of the first star images. These stars could be seen in the empty areas surrounding the constellations. During a night when the sky is completely black, you may see some of these stars by looking up at the sky. If you gaze up at the night sky through a pair of binoculars, you will notice that there are even more stars. When you look through a telescope, you’ll be able to see much more! All of the stars that you can see are a part of a single unique collection of heavenly bodies known as the stars that make up our galaxy, the Milky Way.
In ages past, names were given to hundreds of the brightest stars, which are defined as those that can be seen with the naked eye. Eltanin of Draco, also known as the Dragon, and Vega in Lyra, also known as the Lyre, are two examples of them. Because they have been noticed by people of many civilizations, several of these stars have more than one name.
These days, stars are given names that are derived from their positions on the celestial sphere. This is a fictitious sphere that surrounds the planet Earth. It is possible to extend Earth’s north and south poles into space to identify the north and south celestial poles, which are the poles that the sphere revolves around. At the point where the sphere and the expanded north pole meet is where you’ll find Polaris. When the equator of Earth is stretched into space, it meets the sphere at the celestial equator, which is the point at which it divides the sphere into the northern and southern hemispheres. It is possible to plot on a map the positions of all stars and other objects in space, such as constellations, in relation to the celestial sphere’s poles and equator. The term “declination” refers to their location in relation to the celestial equator, which is effectively the same thing as their latitude. Their longitude, also known as their right ascension, is their location in relation to east or west, and it is measured in hours, minutes, and seconds. Right ascension on the celestial sphere is measured from the junction of the ecliptic (the plane of Earth’s orbit) and the celestial equator. On Earth, we measure our longitude east or west from Greenwich, England. On the celestial sphere, right ascension is measured from the same intersection.
Because there are a great number of catalogs of stars, each of which uses a unique system for annotating location, this results in each star having a greater number of names. One of the most well-known star catalogs from the 1800s is called the Bonn Survey. It segments the sky into bands of declination that are 1 degree broad and counts the stars using right ascension, moving from west to east. Vega is designated as “BD+38° 3238” in the Bonn Survey, which indicates that it is the 3238th star in the band that spans 38 degrees north to 39 degrees north. Another collection, known as the Smithsonian Astrophysical Observatory (SAO) catalog, combined the information from ten different catalogs to provide a listing of the locations of more than 250,000 stars. This catalog identifies Vega as object SAO 067174. Astronomers are now able to observe even more stars than before thanks to the Hubble Space Telescope. At the moment, the Hubble Space Telescope Guide Star Catalogue has the coordinates of more than 19 million luminous objects, 15 million of which are categorized as stars.
The stars are very far away things. Although their distances differ, they are all quite far apart from one another. The distance between our planet and the next closest star, Proxima Centauri, is more than four light years. As the Earth rotates on its axis, we, as observers confined to Earth, move past a backdrop consisting of very far away stars. The same reason that our Sun seems to “rise” in the east and “set” in the west is the same reason that the stars appear to travel across our night sky from east to west as the Earth rotates. This effect is caused by the rotation of the Earth.
Stars that are located very near to the celestial poles, which are the fictitious places in space at which the north and south axes of Earth point, have a very tiny circle of spin. If you are able to locate Polaris, also known as the “pole star” of the northern hemisphere, you will notice that it moves in the night sky just very, very little. The greater one’s distance from Polaris, the more expansive the circle that the stars trace. “Circumpolar stars” are the names given to the constellations Big and Little Dippers, which may be seen in the northern hemisphere. These stars make a complete circuit around the celestial pole. They do not go down because they remain in the night sky. Because the celestial poles lie on the horizon near the equator, there are no circumpolar stars to be seen there. Every star may be seen to rise in the east and set in the west when seen from the equator.
When seen over the course of a year, the constellations may be seen to move steadily toward the west. This occurs as a result of the orbit of the Earth around the Sun. The direction that viewers are gazing in during the nighttime hours of space when it is summer is different from the direction that they are looking in during the winter.
What exactly is the Zodiac sign?
Each year, Earth completes one revolution around the Sun. When seen from our vantage point on Earth, the route that our Sun seems to follow is round. This trajectory creates a plane that is referred to as the ecliptic plane (or just the ecliptic). The collection of constellations known as the zodiac, which lie in a line parallel to the plane of the ecliptic, is sometimes called a belt. Our Sun gives the impression of “passing” through each of these constellations during the course of a year. There are 12 zodiac constellations according to astrology, but according to astronomy, there are 13 zodiac constellations: Capricornus, Aquarius, Pisces, Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra, Scorpius, Sagittarius, and Ophiuchus. The constellations of the zodiac are arranged clockwise around the sky. Ancient civilizations calculated the passage of time based on the cycle of the zodiac, which occurred once every year.
The orbits of most of the planets—with the exception of Pluto, of course—are also quite near to the ecliptic plane, which is determined by the motion of Earth (within about 8 degrees above or below). There are between 21 and 24 constellations that make up the zodiac when all of the constellations that are included in this expanded definition of the ecliptic plane are taken into account.
Around 2500 years ago, the astrological signs were recognized and associated with the calendar. However, the current order of Earth’s seasons is not the same as it was millions of years ago. This is due, in part, to the fact that the Earth is somewhat unstable, much like a top, which causes its axis to point in a variety of different directions at certain points in time. This is a repeating pattern of change that has occurred about once every 23,000 years. This wobble will cause a particular season (for example, winter in the northern hemisphere) to occur at a slightly different place over the course of time. Because the direction of the Earth’s axis of rotation determines at which point in the Earth’s orbit the seasons will occur, this wobble will also cause the length of time that each season lasts. Therefore, throughout the course of time, there has been a change in how the seasons correspond to the backdrop of the zodiac constellations. Our Sun was in Taurus when the spring equinox occurred five thousand years ago; however, it will be in Pisces when the spring equinox occurs this year. Should you ever find yourself wondering why your horoscope can be wrong by a little bit… This shift might be the cause of the delay, which could be several thousand years.
The answer to this question is due to Earth’s changing position in relation to the sun and stars. As the earth rotates on its axis, it moves around the sun. This means that different constellations are visible at different times of the year. Additionally, as Earth orbits around the sun, it also wobbles on its axis. This wobble causes our north pole to point in a slightly different direction every day. This means that some constellations can be seen from both hemispheres while others can only be seen from one hemisphere or the other. These principles explain why we see different constellations at different times of year, but there is still much more to learn about astronomy! Have you ever wondered why we see