Michael Seeds Dana Backman. Chapter 2 User s Guide to the Sky: Patterns and Cycles

Transcription

1 Michael Seeds Dana Backman Chapter 2 User s Guide to the Sky: Patterns and Cycles

2 The Southern Cross I saw every night abeam. The sun every morning came up astern; every evening it went down ahead. I wished for no other compass to guide me, for these were true. - CAPTAIN JOSHUA SLOCUM Sailing Alone Around the World

3 The night sky is the rest of the universe as seen from our planet. When you look up at the stars, you look out through a layer of air only about 100 kilometers deep. Beyond that, space is nearly empty with the planets of our solar system several AU away and the far more distant stars scattered many light-years apart.

4 You can begin your understanding of the natural laws that govern the universe by carefully noting what the universe looks like and how it behaves.

5 Keep in mind that you live on a planet, a moving platform. Earth rotates on its axis once a day. So, from our viewpoint, sky objects appear to rotate around us each day. For example, the sun rises in the east and sets in the west, and so do the stars. The sun, the moon, planets, stars, and galaxies all have an apparent daily motion that is not real but is caused by a real motion of Earth.

6 The Stars On a dark night, far from city lights, you can see a few thousand stars. Your observations can be summarized by naming individual stars and groups of stars and by specifying their relative brightness.

7 Constellations All around the world, ancient cultures celebrated heroes, gods, and mythical beasts by naming groups of stars called constellations.

8 Constellations You should not be surprised that the star patterns do not look like the creatures they are named after any more than Columbus, Ohio, looks like Christopher Columbus.

9 Constellations The constellations named within Western culture originated in Mesopotamia, Babylon, Egypt, and Greece beginning as much as 5,000 years ago. Of these ancient constellations, 48 are still in use.

10 Constellations In those former times, a constellation was simply a loose grouping of bright stars. Many of the fainter stars were not included in any constellation. Regions of the southern sky not visible to the ancient astronomers living at northern latitudes also were not identified with constellations.

11 Constellations Constellation boundaries, when they were defined at all, were only approximate. So, a star like Alpheratz could be thought of as part of Pegasus and also part of Andromeda.

12 Constellations In recent centuries, astronomers have added 40 modern constellations to fill gaps.

13 Constellations In 1928, the International Astronomical Union (IAU) established 88 official constellations with clearly defined permanent boundaries that together cover the entire sky. A constellation now represents not a group of stars but a section of the sky a viewing direction. Any star within the region belongs to only that one constellation.

14 Constellations In addition to the 88 official constellations, the sky contains a number of less formally defined groupings known as asterisms. For example, the Big Dipper is an asterism you probably recognize that is part of the constellation Ursa Major (the Great Bear).

15 Constellations Another asterism is the Great Square of Pegasus that includes three stars from Pegasus and Alpheratz, now considered to be part of Andromeda only.

16 Constellations Although constellations and asterisms are named as if they were real groupings, most are made up of stars that are not physically associated with one another. Some stars may be many times farther away than others in the same constellation and moving through space in different directions.

17 Constellations The only thing they have in common is that they lie in approximately the same direction from Earth.

18 The Names of the Stars In addition to naming groups of stars, ancient astronomers named the brighter stars. Modern astronomers still use many of those names.

19 The Names of the Stars The names of the constellations are in Latin or Greek, the languages of science in Medieval and Renaissance Europe.

20 The Names of the Stars Most individual star names derive from ancient Arabic, much altered over centuries. The name of Betelgeuse, the bright red star in Orion, comes from the Arabic phrase yad aljawza, meaning armpit of Jawza (Orion). Aldebaran, the bright red eye of Taurus the bull, comes from the Arabic aldabar an, meaning the follower.

21 The Names of the Stars Another way to identify stars is to assign Greek letters to the bright stars in a constellation in the approximate order of brightness. Thus, the brightest star is usually designated alpha (α), the second brightest beta (β), and so on.

22 The Names of the Stars For many constellations, the letters follow the order of brightness. However, some constellations are exceptions.

23 The Names of the Stars A Greek-letter star name also includes the possessive form of the constellation name. For example, the brightest star in the constellation Canis Major is alpha Canis Majoris. This name identifies the star and the constellation and gives a clue to the relative brightness of the star. Compare this with the ancient individual name for that star, Sirius, which tells you nothing about its location or brightness.

24 The Brightness of Stars Astronomers measure the brightness of stars using the magnitude scale.

25 The Brightness of Stars The ancient astronomers divided the stars into six brightness groups. The brightest were called first-magnitude stars. The scale continued downward to sixth-magnitude stars the faintest visible to the human eye.

26 The Brightness of Stars Thus, the larger the magnitude number, the fainter the star. This makes sense if you think of the bright stars as first-class stars and the faintest stars visible as sixth-class stars.

27 The Brightness of Stars The Greek astronomer Hipparchus ( BC) is believed to have compiled the first star catalog. There is evidence he used the magnitude system in that catalog.

28 The Brightness of Stars About 300 years later (around AD 140), the Egyptian-Greek astronomer Claudius Ptolemy definitely used the magnitude system in his own catalog. Successive generations of astronomers have continued to use the system.

29 The Brightness of Stars Star brightnesses expressed in this system are known as apparent visual magnitudes (m V ). These describe how the stars look to human eyes observing from Earth.

30 The Brightness of Stars Brightness is quite subjective. It depends on both the physiology of human eyes and the psychology of perception. To be scientifically accurate, you should refer to flux. This is a measure of the light energy from a star that hits one square meter in one second.

31 The Brightness of Stars With modern scientific instruments, you can measure the intensity of starlight with high precision and then use a simple mathematical relationship that relates light intensity to apparent visual magnitude. So, instead of saying that the star known by the charming name Chort (Theta Leonis) is about third magnitude, you can say its magnitude is 3.34.

32 The Brightness of Stars Thus, precise modern measurements of the brightness of stars are still connected to observations of apparent visual magnitude that go back to the time of Hipparchus.

33 The Brightness of Stars Limitations of the apparent visual magnitude system have motivated astronomers to supplement it in various ways.

34 The Brightness of Stars One, some stars are so bright that the scale must extend into negative numbers. Sirius, the brightest star in the sky, has a magnitude of 1.47.

35 The Brightness of Stars Two, with a telescope, you can find stars much fainter than the limit for your unaided eyes. Thus, the magnitude system has also been extended to include numbers larger than sixth magnitude to include fainter stars.

36 The Brightness of Stars Three, although some stars emit large amounts of infrared or ultraviolet light, those types of radiation are invisible to human eyes. The subscript V in m V is a reminder that you are counting only light that is visible. Other magnitudes systems have been invented to express the brightness of invisible light arriving at Earth from the stars.

37 The Brightness of Stars Four, an apparent magnitude informs you only how bright the star is as seen from Earth. It doesn t reveal anything about a star s true power output because the star s distance is not included.

38 The Sky and Its Motions The sky above you seems to be a great blue dome in the daytime and a sparkling ceiling at night. Learning to understand the sky requires that you first recall the perspectives of people who observed the sky thousands of years ago.

39 The Celestial Sphere Ancient astronomers believed the sky was a great sphere surrounding Earth, with the stars stuck on the inside like thumbtacks in a ceiling.

40 The Celestial Sphere Modern astronomers know that the stars are scattered through space at different distances. However, it is still convenient to think of the sky as a great sphere enclosing Earth with stars all at one distance.

41 The Celestial Sphere The celestial sphere is an example of a scientific model, a common feature of scientific thought. You can use the celestial sphere as a convenient model of the sky.

42 The Celestial Sphere As you study the sky, you will notice three important points.

43 The Celestial Sphere One, sky objects appear to rotate westward around Earth each day, but that is a consequence of Earth s eastward rotation. This produces day and night.

44 The Celestial Sphere Two, what you can see of the sky depends on where you are on Earth. For example, Australians see many constellations and asterisms invisible from North America, but they never see the Big Dipper.

45 The Celestial Sphere Three, astronomers measure distances across the sky as angles. These are expressed in units of degrees and subdivisions of degrees called arc minutes and arc seconds.

46 Precession In addition to the daily motion of the sky, Earth s rotation adds a second motion to the sky that can be detected only over centuries.

47 Precession More than 2000 years ago, Hipparchus compared a few of his star positions with those made by other astronomers nearly two centuries before him. He realized that the celestial poles and equator were slowly moving relative to the stars.

48 Precession Later astronomers understood that this apparent motion is caused by a special motion of Earth called precession.

49 Precession If you have ever played with a toy top or gyroscope, you may recall that the axis of such a rapidly spinning object sweeps around relatively slowly in a circle. The weight of the top tends to make it tip. This combines with its rapid rotation to make its axis sweep around slowly in precession motion.

50 Precession Earth spins like a giant top, but it does not spin upright relative to its orbit around the sun. You can say either that Earth s axis is tipped 23.5 from vertical or that Earth s equator is tipped 23.5 relative to its orbit.

51 Precession Earth s large mass and rapid rotation keep its axis of rotation pointed toward a spot near Polaris (alpha Ursa Minoris). Its axis direction would not move if Earth were a perfect sphere.

52 Precession However, Earth has a slight bulge around its middle. The gravity of the sun and moon pull on this bulge, tending to twist Earth s axis upright relative to its orbit.

53 Precession The combination of these forces and Earth s rotation causes Earth s axis to precess in a slow circular sweep taking about 26,000 years for one cycle.

54 Precession As the celestial poles and equator are defined by Earth s rotational axis, precession moves these reference marks. You would notice no change at all from night to night or year to year. Precise measurements, though, reveal their slow apparent motion.

55 Precession Over centuries, precession has dramatic effects. Egyptian records show that 4,800 years ago the north celestial pole was near Thuban (alpha Draconis). Now, the pole is approaching Polaris and will be closest to it in about 2100.

56 Precession In about 12,000 years, the pole will have moved to the apparent vicinity of the very bright star Vega (alpha Lyrae). The figure shows the apparent path followed by the north celestial pole over thousands of years.

57 The Cycle of the Sun Rotation is the turning of a body on its axis. Revolution means the motion of a body around a point outside the body. Earth rotates on its axis and that produces day and night. Earth also revolves around the sun and that produces the yearly cycle.

58 The Annual Motion of the Sun Even in the daytime, the sky is actually filled with stars. However, the glare of sunlight fills Earth s atmosphere with scattered light, and you can only see the brilliant blue sky.

59 The Annual Motion of the Sun If the sun were fainter and you could see the stars in the daytime, you would notice that the sun appears to be moving slowly eastward relative to the background of the distant stars. This apparent motion is caused by the real orbital motion of Earth around the sun.

60 The Annual Motion of the Sun In January, you would see the sun in front of the constellation Sagittarius. By March, it is in front of Aquarius.

61 The Annual Motion of the Sun Note that your angle of view in the figure makes the Earth s orbit seem very elliptical when it is really almost a perfect circle.

62 The Annual Motion of the Sun Through the year, the sun moves eastward among the stars following a line called the ecliptic the apparent path of the sun among the stars. If the sky were a great screen, the ecliptic would be the shadow cast by Earth s orbit. In other words, you can call the ecliptic the projection of Earth s orbit on the celestial sphere.

63 The Annual Motion of the Sun Earth circles the sun in days and, consequently, the sun appears to go once around the sky in the same period. You don t notice this motion because you cannot see the stars in the daytime.

64 The Annual Motion of the Sun However, the motion of the sun caused by a real motion of Earth has an important consequence that you do notice the seasons.

65 The Seasons The seasons are caused by the revolution of Earth around the sun combined with a simple fact you have already encountered. Earth s equator is tipped 23.5 relative to its orbit.

66 The Seasons There are two important principles to note about the cycle of seasons.

67 The Seasons One, the seasons are not caused by variation in the distance between Earth and the sun. Earth s orbit is nearly circular, so it is always about the same distance from the sun.

68 The Seasons Two, the seasons are caused by changes in the amount of solar energy that Earth s northern and southern hemispheres receive at different times of the year resulting from the tip of the Earth s equator and axis relative to its orbit.

69 The Seasons The seasons are so important as a cycle of growth and harvest that cultures around the world have attached great significance to the ecliptic. It marks the center line of the zodiac ( circle of animals ). Also, the motion of the sun, moon, and the five visible planets (Mercury, Venus, Mars, Jupiter, and Saturn) are the basis of the ancient superstition of astrology.

70 The Seasons However, the signs of the zodiac are no longer important in astronomy.

71 The Seasons You can look for the planets along the ecliptic appearing like bright stars. Mars looks quite orange in color.

72 The Seasons As Venus and Mercury orbit inside Earth s orbit, they never get far from the sun and are visible in the west after sunset or in the east before sunrise. Venus can be very bright, but Mercury is difficult to see near the horizon.

73 The Seasons By tradition, any planet in the sunset sky is called an evening star. Any planet in the dawn sky is called a morning star.

74 The Seasons Perhaps the most beautiful is Venus, which can become as bright as magnitude As Venus moves around its orbit, it can dominate the western sky each evening for many weeks. Eventually, its orbit appears to carry it back toward the sun as seen from Earth, and it is lost in the haze near the horizon. A few weeks later, you can see Venus reappear in the dawn sky as a brilliant morning star. Months later, it will switch back to being an evening star.

75 The Cycles of the Moon The moon orbits eastwards around Earth once a month.

76 The Cycles of the Moon Starting this evening, look for the moon in the sky. If it is a cloudy night or if the moon is in the wrong part of its orbit, you may not see it. Keep trying on successive evenings. Within a week or two, you will see the moon. Then, watch for the moon on following evenings. You will see it move along its orbit around Earth and cycling through its phases as it has done for billions of years.

77 The Motion of the Moon If you watch the moon night after night, you will notice two things about its motion. First, you will see it moving relative to the background of stars. Second, you will notice that the markings on its face don t change. These two observations will help you understand the motion of the moon and the origin of the moon s phases.

78 The Motion of the Moon The moon moves rapidly among the constellations. If you watch the moon for just an hour, you can see it move eastward against the background of stars by slightly more than its own apparent diameter. Each night when you look at the moon, you will see it is roughly half the width of a zodiac constellation about 13 degrees to the east of its location the night before. This movement is the result of the motion of the moon along its orbit around Earth.

79 The Cycle of Moon Phases The changing shape of the illuminated part of the moon as it orbits Earth is one of the most easily observed phenomena in astronomy.

80 The Cycle of Moon Phases There are three important points to notice about the phases of the moon.

81 The Cycle of Moon Phases First, the moon always keeps the same side facing Earth, and you never see the far side of the moon. The man in the moon (some cultures see the rabbit in the moon instead) is produced by familiar features on the moon s near side.

82 The Cycle of Moon Phases Second, the changing shape of the moon as it passes through its cycle of phases is produced by sunlight illuminating different parts of the side of the moon you can see.

83 The Cycle of Moon Phases You always see the same side of the moon looking down on you. The changing shadows, though, make the man in the moon shift moods as the moon cycles through its phases.

84 The Cycle of Moon Phases Finally, the orbital period of the moon around Earth is not the same as the length of a moon phase cycle.

85 Eclipses Eclipses are due to a seemingly complicated combination of apparent motions of the sun and moon. Yet, they are actually easy to predict once all the cycles are understood.

86 Eclipses Eclipses are also among the most spectacular of nature s sights you might witness.

87 Solar Eclipses From Earth, you can see a phenomenon that is not visible from most planets. It happens that the sun is 400 times larger than our moon and, on the average, 390 times farther away. So, the sun and moon have nearly equal angular apparent diameters. Thus, the moon is just about the right size to cover the bright disk of the sun and cause a solar eclipse. In a solar eclipse, it is the sun that is being hidden (eclipsed) and the moon that is in the way.

88 Solar Eclipses A shadow consists of two parts. The umbra is the region of total shadow. For example, if you were in the umbra of the moon s shadow, you would see no portion of the sun. The umbra of the moon s shadow usually just barely reaches Earth s surface and covers a relatively small circular zone.

89 Solar Eclipses Standing in that umbral zone, you would be in total shadow unable to see any part of the sun s surface. This is called a total eclipse.

90 Solar Eclipses If you moved into the penumbra, you would be in partial shadow, but could also see part of the sun peeking around the edge of the moon. This is called a partial eclipse.

91 Solar Eclipses If you are outside the penumbra, you see no eclipse at all.

92 Solar Eclipses Due to the moon s orbital motion and Earth s rotation, the moon s shadow sweeps rapidly across Earth in a long, narrow path of totality. If you want to see a total solar eclipse, you must be in the path of totality.

93 Solar Eclipses When the umbra of the moon s shadow sweeps over you, you see one of the most dramatic sights in the sky the totally eclipsed sun.

94 Solar Eclipses The eclipse begins as the moon slowly crosses in front of the sun. It takes about an hour for the moon to cover the solar disk.

95 Solar Eclipses As the last sliver of sun disappears, dark falls in a few seconds. Automatic street lights come on, drivers of cars turn on their headlights, and birds go to roost. The sky usually becomes so dark you can even see the brighter stars.

96 Solar Eclipses The darkness lasts only a few minutes. This is because the umbra is never more than 270 km (170 miles) in diameter on Earth s surface and sweeps across the landscape at over 1,600 km/hr (1,000 mph). The period of totality lasts on average only 2 or 3 minutes and never more than 7.5 minutes.

97 Solar Eclipses During totality you can see subtle features of the sun s atmosphere. These include red flame-like projections that are visible only during those moments when the brilliant disk of the sun is completely covered by the moon.

98 Solar Eclipses As soon as part of the sun s disk reappears, the fainter features vanish in the glare. The period of totality is over. The moon moves on in its orbit and, in an hour the sun, is completely visible again.

99 Solar Eclipses Sometimes, when the moon crosses in front of the sun, it is too small to fully cover the sun. Then, you would witness an annular eclipse. This is a solar eclipse in which an annulus ( ring ) of the sun s disk is visible around the disk of the moon.

100 Solar Eclipses The eclipse never becomes total. It never quite gets dark. You can t see the faint features of the solar atmosphere.

101 Solar Eclipses Annular eclipses occur because the moon follows a slightly elliptical orbit around Earth. If the moon is in the farther part of its orbit during totality, its apparent diameter will be less than the apparent diameter of the sun and you see an annular eclipse.

102 Solar Eclipses Also, Earth s orbit is slightly elliptical. So, the Earth-to-sun distance varies slightly. So does the apparent diameter of the solar disk. These contribute to the effect of the moon s varying apparent size.

103 Solar Eclipses If you plan to observe a solar eclipse, remember that the sun is bright enough to burn your eyes and cause permanent damage if you look at it directly. This is true whether there is an eclipse or not.

104 Solar Eclipses Solar eclipses can be misleading tempting you to look at the sun in spite of its brilliance and thus risking your eyesight.

105 Solar Eclipses During the few minutes of totality, the brilliant disk of the sun is hidden, and it is safe to look at the eclipse. However, the partial eclipse phases and annular eclipses can be dangerous.

106 Solar Eclipses The figure demonstrates a safe way to observe the partially eclipsed sun.

107 Solar Eclipses The table will allow you to determine when some upcoming solar eclipses will be visible from your location.

108 Lunar Eclipses Occasionally, you can see the moon darken and turn copper-red in a lunar eclipse.

109 Lunar Eclipses A lunar eclipse occurs at full moon when the moon moves through Earth s shadow. As the moon shines only by reflected sunlight, you see the moon gradually darken as it enters the shadow.

110 Lunar Eclipses If you were on the moon and in the umbra of Earth s shadow, you would see no portion of the sun.

111 Lunar Eclipses If you moved into the penumbra, you would be in partial shadow and would see part of the sun peeking around the edge of Earth so the sunlight would be dimmed but not extinguished.

112 Lunar Eclipses In a lunar eclipse, it is the moon that is being hidden in the Earth s shadow and Earth that is in the way of the sunlight.

113 Lunar Eclipses If the orbit of the moon carries it through the umbra of Earth s shadow, you see a total lunar eclipse.

114 Lunar Eclipses As you watch the moon, it first moves into the penumbra and dims slightly. The deeper it moves into the penumbra, the more it dims.

115 Lunar Eclipses In about an hour, the moon reaches the umbra, and you see the umbral shadow darken part of the moon. It takes about an hour for the moon to enter the umbra completely and become totally eclipsed.

116 Lunar Eclipses The period of total eclipse may last as long as 1 hour 45 minutes. However, the timing of the eclipse depends on where the moon crosses the shadow.

117 Lunar Eclipses When the moon is totally eclipsed, it does not disappear completely. Although it receives no direct sunlight, the moon in the umbra does receive some sunlight that is refracted (bent) through Earth s atmosphere.

118 Lunar Eclipses If you were on the moon during totality, you would not see any part of the sun as it would be entirely hidden behind Earth. However, you would be able to see Earth s atmosphere illuminated from behind by the sun.

119 Lunar Eclipses The red glow from this ring consisting of all the Earth s simultaneous sunsets and sunrises illuminates the moon during totality and makes it glow coppery red.

120 Lunar Eclipses If the moon passes a bit too far north or south of Earth s shadow, it may only partially enter the umbra. Then, you see a partial lunar eclipse.

121 Lunar Eclipses The part of the moon that remains outside the umbra in the penumbra receives some direct sunlight. The glare is usually great enough to prevent your seeing the faint coppery glow of the part of the moon in the umbra.

122 Lunar Eclipses Lunar eclipses always occur at full moon but not at every full moon. The moon s orbit is tipped about 5 degrees to the ecliptic. So, most full moons cross the sky north or south of Earth s shadow and there is no lunar eclipse that month.

123 Lunar Eclipses For the same reason, solar eclipses always occur at new moon but not at every new moon.

124 Lunar Eclipses The orientation of the moon s orbit in space varies slowly. As a result, solar and lunar eclipses repeat in a pattern called the Saros cycle lasting 18 years 11 1/3 days. Ancient peoples who understood the Saros cycle could predict eclipses without understanding what the sun and moon really were.

125 Lunar Eclipses Although there are usually no more than one or two lunar eclipses each year, it is not difficult to see one. You need only be on the dark side of Earth when the moon passes through Earth s shadow. That is, the eclipse must occur between sunset and sunrise at your location to be visible.

126 Lunar Eclipses The table will allow you to determine when some upcoming lunar eclipses will be visible from your location.