Introduc
Astronomy is one of the oldest natural sciences, dating back to antiquity, with its origins in the religious, mythological, and astrological practices of pre-history.
(Photo Credit: By Mathias Krumbholz (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons Mathias Krumbholz)
Constellations
Origin of the Constellations
Ever since people first wandered the Earth, great significance has been given to the celestial objects seen in the sky. Throughout human history and across many different cultures, names and mythical stories have been attributed to the star patterns in the night sky to more easily remember and recognize them, giving birth to what we know as constellations.Constellations were used by people for many practical reasons. For example, in agriculture, constellations could help determine when the seasons were coming before the birth of calendars. Constellations also helped navigators and explorers to find their way across the planet. These activities fastened the discovery of new constellations. Nowadays, astronomers still use the names of constellations to explain where a celestial object can be found in the sky.(Text adapted from the constellations page - International Astronomical Union, http://www.iau.org/public/themes/constellations/)
Constellations are formed of bright stars which appear close to each other on the sky, but are really far apart in space. The shapes you see all depend on your point of view. Many societies saw patterns among the stars with gods and goddesses or stories from their culture.Most of the constellations with which we are familiar come from ancient Greece. But other civilizations created their own patterns in the sky based on stories and people that were important to them. Many peoples noticed that the planets, the moon, and comets moved through the sky in a different ways than the stars[1].
Because of the rotation of the Earth and its orbit around the Sun, we divide the constellations into two groups. Some constellations never rise nor set, and they are called circumpolar. All the rest are divided into seasonal constellations. Which constellations will be circumpolar and which seasonal depends on your latitude.
Constellation" is the name we give to seeming patterns of stars in the night sky. "Stella" is the Latin word for star and a constellation is a grouping of stars. In general, the stars in these groups are not actually close to each other in space, they just appear to be close when viewed from Earth.If we could travel by spaceship to another part of the galaxy, we would imagine an entirely different set of constellations. In the meantime, for us on Earth, the constellations are a handy way to locate a star in the sky.
On Earth, we see different constellations as we travel to different parts of the globe. The fact that some constellations were visible in the northern hemisphere and not the southern hemisphere, and vice-versa, was used more than 2000 years ago by Greek astronomers to argue that the Earth is round.
Johannes Hevelius' Orion from URANOGRAPHIA (1690)
Long before the invention of the telescope, early civilizations invented star patterns and named them after animals, objects, heroes, gods, and beasts from stories and myths. Many of these myths were probably created to explain changes in the sky due to seasons, etc. The ancient Greeks named many constellations. For example, they told the story of Orion, the hunter, who leaped into the sea to escape a scorpion’s bite, which explained why the constellation Orion disappears from the sky when the constellation Scorpius rises. Different civilizations imagined different patterns, and some stars were included in more than one pattern. Over time, the situation became confusing.
In 1929 the International Astronomical Union defined 88 constellations that are today recognized as the "official" constellations. Many of these constellations are derived from the complex creations of Greek mythology, like Andromeda, Perseus, and Orion. Others came from ancient cultures such as the Egyptians and Chaldeans and still others were not defined until the 17th and 18th century.
In the past, people used the constellations as markers. Some used the constellations to navigate their boats across the sea, to mark seasons of the year, or to locate special stars. Today, astronomers still use constellations as a handy marker to indicate a general area of the sky where far away celestial objects appear. Many of these extremely distant objects can be seen only with powerful telescopes.
In the next sections you will find short descriptions of some of the constellations in which Chandra images are located. But remember that "in" doesn't really mean "in." It means “in the same direction.” The stars that make up the constellations and many of the sources that Chandra observes are at very different distances from Earth.
We can track the seasons using either the stars or the Sun. The stars all have a fixed pattern relative to each other from year to year, like the pattern of major cities on a map of the Earth. Many ancient cultures selected specific groupings of stars or constellations to represent certain animals or mythological figures. When you see the constellations — for example Ursa Major the great bear, or Orion the hunter, or Pisces the fish — it is not obvious what the patterns are supposed to represent. You have to use your imagination! The constellations are not literal drawings on the sky, but were used as a memory aid for navigation or to pay homage to gods and myths. If you learn a few major constellations, they can help you locate the North Star and other directions in the sky.
By Johannes Hevelius [Public domain], via Wikimedia Commons
Please check out the constellation index here.
How do astronomers calculate the distance to a star?[2]
Astronomers developed two methods to determine the distance of a star. The first one, known as parallax, uses triangulation. Astronomers observe the position of a star on one day (position 1 in the drawing below) and again six months later (position 2 in the drawing below). They can see the angle formed with the star and the Sun. Knowing that the diameter of the Earth’s orbit around the Sun is about 300 million kilometres, astronomers use trigonometry to calculate the distance. This technique works for stars within about 400 light years of Earth.
Another method is related to the brightness of a star. Astronomers observe and determine the real colour spectrum of the star. They compare the real brightness of the star to the brightness seen on Earth. Using these two values, astronomers can deduce the distance of the star from Earth.
Cosmic Calendar[3]
The Cosmic Calendar is a method to visualize the vast history of the universe in which its 13.8 billion year lifetime is condensed down into a single year. In this visualization, the Big Bang took place at the beginning of January 1 at midnight, and the current moment is mapped onto the end of December 31 at midnight.At this scale, there are 438 years per second, 1.58 million years per hour, and 37.8 million years per day. This concept was popularized by Carl Sagan in his book The Dragons of Eden and on his television series Cosmos. In the 2014 sequel series, Cosmos: A Spacetime Odyssey, host Neil deGrasse Tyson presents the same concept of a Cosmic Calendar, but using the revised age of the universe of 13.8 billion years as an improvement on Sagan's 1980 figure of 15 billion years. Sagan goes on to extend the comparison in terms of surface area, explaining that if the Cosmic Calendar is scaled to the size of a football field, then "all of human history would occupy an area the size of [his] hand".
The 13.8 billion year history of the universe mapped onto a single year, as popularized by Carl Sagan. At this scale the Big Bang takes place on January 1 at midnight, the current time is December 31 at midnight, and the longest human life is a blink of an eye (about 1/4 of a second).
Readings:
Video Lesson:
Login to https://ed.ted.com/u/lessons and watch the VIDEO about Constellations and complete the discussion below the video.