Introduction to the Day and Night Sky - Content
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- Introduction to the Sky
- Day and Night
- Celestial Sphere
- Earth's Rotation
- Relative Motion
- Constellations, Patterns in the Sky
- Ancient Civilizations and the Night Sky
- The Ecliptic and the Zodiac
1. Introduction to the Sky
The sky has captivated people for thousands of years. The Sun dominates the daytime sky with its radiance, the day beginning and ending with beautiful sunrises and startling sunsets. But with the disappearance of the Sun, the night sky comes alive. The Moon is the brightest and most recognizable object in the sky at night, and is the closest celestial body to the Earth. A full moon drowns out stars with its light, but a sky devoid of the Moon reveals thousands of stars. From a dark site away from city lights, we can see nearly 3000 stars (compared to only a few hundred from the city). In addition to the Moon and the stars, a few of the planets are clearly visible in our sky. Because planets are further away than the Moon, they appear like the distant stars as points of light. Venus is the brightest object in the sky after the Sun and Moon, and is usually visible just before sunrise or after sunset as a bright "star" low in the sky. Mars, Jupiter and Saturn are also easily visible to the unaided eye (without a telescope), and Mercury is visible as well, although it is much more difficult to spot. There are many objects in the night sky that are extremely far away and too faint for us to see without binoculars or a telescope. A telescope reveals an incredible universe we cannot see while looking up at the night sky from our backyard. There are billions of other stars, small rocks (asteroids), dirty snowballs (comets), gases which glow from the light of nearby stars (nebulae), as well as billions of galaxies. It is truly unbelievable what treasures our night sky holds that are simply too far away and faint to be seen with our eyes.
2. Day and Night
The Sun makes the daytime sky very different from the night sky. During the day, the Sun's brilliance floods the sky and drowns out the light from the other objects in the sky. The Sun's light appears a yellowish white, but is composed of the full spectrum of light. Our atmosphere refracts the blue component of sunlight down to the surface, but it reflects the other colours back into space. This is why the daytime sky appears blue. In contrast, the sky at night is void of the Sun's light and as a result it is dark, allowing the light from thousands of stars to twinkle in our sky. Stars remain present during the day, (the brightest ones visible with a telescope), but cannot be seen because the Sun overwhelms them.
3. Celestial Sphere
The stars in our night sky have remained unchanged for many years. Although stars do have their own motion in space, they are so far away we cannot detect their movement unless observed over many years; their positions relative to each other will not change noticeably within our lifetime. The patterns we see in the sky are virtually no different than the patterns our ancestors saw when they looked up at the sky. The first astronomers believed that the stars were fixed on a celestial sphere surrounding the Earth. Although we now know this to be untrue (the universe is three-dimensional and stars are located at various distances from the Earth), it still helps to use this illustration to better visualize and understand the night sky. Using this representation, the stars do not move with respect to each other, but they do move in our night sky due to diurnal motion, the Earth's rotation on its axis. As the Earth rotates within the celestial sphere, stars will rise in the East and will travel across our sky from East to West. Astronomers use a coordinate system for the celestial sphere much like the coordinate system on the Earth. Right ascension is analogous to longitude, as is declination to latitude. Right ascension is broken into 24 hours, with smaller divisions of minutes and seconds. While it is not a measurement of time, right ascension is related to time because the entire sky rotates once in about 24 hours. Declination is measured in degrees like on the Earth, but it is generally listed as positive and negative instead of north and south.
Stars appear to circle around the north celestial pole, which happens to be near the star called Polaris (better known as the North Star). Just as the North Pole of the Earth is stationary while the Earth turns, the north celestial pole also appears to be stationary. This phenomenon is clearly visible in time lapse photography of the night sky over a few hours; stars will leave trails that circle around Polaris. This motion is apparent simply by looking at a recognizable or prominent star early in the evening and returning a few hours later to see how the star's position has shifted in the sky. The further north one is from the equator, the higher the North Star will be in the sky, until at the North Pole, where Polaris will be directly overhead at the zenith. As the apparent position of the north celestial pole rises in the sky with the increasing latitude on the Earth, more of the sky becomes circumpolar. Circumpolar stars are ones which do not set below the horizon through the course of a year.
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4. Earth's Rotation
The nightly changes in the sky are caused by the Earth's rotation, but the Earth is simultaneously in orbit around the Sun. Because of this, every time the Earth completes one rotation on its own axis (with respect to the celestial sphere), it has moved slightly in its path around the Sun. When the Sun returns to the same spot in the sky, we will have rotated through slightly more than one rotation with respect to the celestial sphere, so the stars will appear to have shifted. The same star will rise almost four minutes earlier every night, and during the course of a year the Sun will appear to have moved through the fixed background stars. This progression of the Sun relative to the stars causes some stars to be visible only at certain times of the year. We can imagine that at any given time, half of the stars in the celestial sphere will be visible while the other half will be on the daytime side of the sky. A star that is in the visible half of the sky will in six months be in the sunlit half because the Earth is on the other side of the Sun.
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- Circumpolar Stellar Rotation (SWF format, 62 KB)
The easiest way to detect the Earth's rotation is by observing the path of the sun across the sky through an entire day. Doing this very thing, the ancients determined that by tracking the location of the sun in the sky, they could decipher the time of day. To do this, they invented sundials. A sundial consists of a gnomon standing vertically on a hemispherical base. Aligning the sundial to the Earth's axis of rotation, the Sun's progression through the sky casts shadows which correspond to the time of day. The largest sundial in Canada is located in Lloydminster, Alberta and is over 60 metres across. The largest sundial in the world* was constructed c. 1724 in Jaipur, India and stands over 30 metres high and occupies an area of nearly one acre. Astronomers claim that the Jaipur sundial is accurate to within mere fractions of a second. At the top of the gnomon, sits an observatory.
*as quoted by encyclopedia.com
Some of the largest sundials
Bud Miller All Seasons Park,
City of Lloydminster
Image courtesy of Alan Levy
6. Relative Motion
The stars do not change their positions relative to each other because they are very far away from the Earth. The Moon and planets, however, are relatively close and are in constant motion in their own orbits around the Earth (the Moon) and the Sun (the planets). As a result their locations in the sky will vary on a nightly and yearly basis. The Moon will rise about an hour later each day until it returns to approximately the same rise time 27.3 days later, when the cycle begins again. The Moon rises later each day because its progression through our sky changes its location when compared to the Sun (this will be explained in greater detail in Module 3). The positions of the planets in our sky also vary from day to day and from year to year. A planet may be visible only during the summer this year, whereas several years ago it was only visible during the winter. The independent orbits of the planets combined with our own orbit around the Sun cause their positions in our sky to constantly change.
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- Celestial Sphere Applet (SWF format, 99 KB)
7. Constellations, Patterns in the Sky
When looking up at the stars, there are discernable patterns in the sky called constellations. Different cultures saw different patterns, and stories were told of their origins. The ancient Greeks envisioned the constellations we use today, and we still use the corresponding Latin designations. While looking up at the night sky it can be very difficult to identify any of the constellations; the sky appears to be a puzzling clutter of hundreds of points of light, and most constellations bear little resemblance to their names. In addition to this, the imaginary lines which are drawn to join the stars (called asterisms) often differ from source to source. The most prominent and easily recognizable constellations in Canada are the Big Dipper (an asterism within the constellation Ursa Major) and Orion the Hunter. To help identify other constellations, we can use pointer stars to find the location of other stars and constellations. The best example of this is using the two right stars of the Big Dipper's bowl to point up to Polaris, the North Star. In addition to the few constellations which are easy to identify, there are several others which are nearly impossible to identify. In 1930, astronomers divided the entire sky into 88 constellations, which are officially recognized today. We cannot see all 88 constellations in Canada because we are never exposed to the stars in the southern celestial sphere; the Earth literally blocks our view of the southern sky. The stars visible to the southern hemisphere are different than the stars we see because the southern hemisphere faces a different spot on the celestial sphere. The ancient civilizations who first named the constellations lived in the northern hemisphere and could not see the southern sky. The constellations in this portion of the celestial sphere were not imagined and named until European explorers traveled to the southern regions of the world. While the northern constellations were created from mythological stories, the southern constellations were named during the 17th and 18th centuries, after important inventions of that time period. As a result, the southern constellations are not as interesting and imaginative; Antlia the Air Pump and Caelum the Chisel are two examples. The 88 constellations can be detected with the help of sky atlases which show the positions of numerous stars and the divisions between the constellations. Star maps can be very simple and show only the brightest stars, or can go into more detail and show small portions of the sky with the positions of thousands of stars. There are also many computer programs that can plot the positions of millions of stars. The stars and the patterns they create can be complicated, but with a good atlas and some patience, it is not difficult to identify numerous constellations.
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8. Ancient Civilizations and the Night Sky
The stars and the patterns they create are visually fascinating and allow us to orient ourselves. They help us find our way around the sky and can also be used to designate the location of certain objects (the star Betelgeuse is in the constellation Orion). The constellations have limited significance to us today, but to ancient civilizations, understanding the patterns in the sky was an important aspect of life. The stars (especially the North Star) helped early explorers navigate their ships.
Calendars were created using the motions of the stars and were an essential component in the production of a successful crop. In ancient Egypt the arrival of the star Sirius warned of the annual flooding of the Nile. The constellations were also important culturally, and told vital stories of the past. The Greeks had extensive stories of gods, human heroes and creatures, and they believed that the most significant heroes and creatures were placed into the heavens once they perished. Two of the most well-known Greek heroes we see represented in the sky are Orion the Hunter and Hercules the Warrior.
9. The Ecliptic and the Zodiac
As explained earlier, the Earth's orbit causes the Sun to follow a linear course in the sky. The path that the Sun traces on the celestial sphere is called the ecliptic. Because the objects of the solar system all orbit the Sun in the same horizontal plane, they never deviate far from this imaginary line. The ecliptic was an important component of ancient astronomy and was the foundation for the origin of the zodiac. The zodiac is the collective term for twelve constellations through which the ecliptic passes. The Sun actually passes through thirteen constellations, but it is believed that twelve were chosen because of the twelve months of the year. The zodiac was very significant to ancient astronomers because the Sun, Moon and planets pass through the zodiac, and the desire to learn more about our solar system was essential to understand our place in the universe. Because we now have extensive knowledge of our solar system, the constellations of the zodiac are best known today because of astrology and horoscopes. Although not a science, astrology helped develop astronomy because astrologers took such detailed records of the night sky. Astrologers no longer observe the heavens as they once did; this duty is now very scientific and has been left to astronomers.
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- Celestial Sphere Applet (SWF format, 87 KB)
The Earth's own rotation combined with its orbit around the Sun cause the wonderful changes we see in our sky on a daily and yearly basis. The daytime sky is dominated by the Sun, but in its absence the night sky is home to the Moon, the planets and thousands of stars. The planets are in constant motion with respect to the stars, and their locations in our sky will continually shift along the ecliptic. The stars also appear to move across our sky from east to west during the night, and as the Earth travels around the Sun new stars become visible throughout the year. The patterns created by the stars are from the imaginations of past peoples who viewed the constellations with much higher importance than we do today. The constellations through which the Sun traveled were especially important, which is why the zodiac is so well known. The patterns formed by stars can be challenging to spot, but are still interesting formations and can be used to familiarize ourselves with the sky. Our night sky is always changing; the constellation Orion is only visible in the winter months while Scorpius is only visible in the summer. With all the diversity in our sky, it is not surprising that astronomy can be so challenging, but also so interesting.
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