1. Neil F. Comins • William J. Kaufmann III Discovering the Universe
Ninth Edition
CHAPTER 1
Discovering the Night Sky
Stars appear to rotate around Polaris, the North Star (top), in this time exposure, taken January 26, Below Polaris is the 4-m telescope dome at Kitt Peak National Observatory near Tucson, Arizona. The imageis composed of 114 thirty-second exposures of the night sky combined to make the equivalent of a nearly 1-h exposure in which Earth’s rotation causes the stars to appear to move across the night sky. The orange glow on the horizon is from the city of Phoenix, 160 km (100 mi) away. (STAN HONDA/AFP/Getty Images)

2. ASTRONOMER’S ALMANAC
Astronomy is an ancient science, whose history goes back farther than 2136 BC, the earliest event on the chart.

3. DISCOVERING ASTRONOMY
Just a few milestones of the last 150 years.

4. WHAT DO YOU THINK?
Is the North Star—Polaris—the brightest star in the night sky?
What do astronomers define as constellations?
What causes the seasons?
When is Earth closest to the Sun?
How many zodiac constellations are there?
Does the Moon have a dark side that we never see from Earth?
Is the Moon ever visible during the daytime?
What causes lunar and solar eclipses?

5. In this chapter you will discover…
how astronomers organize the night sky to help them locate objects in it.
that Earth’s spin on its axis causes day and night.
how the tilt of Earth’s axis of rotation and Earth’s motion around the Sun combine to create the seasons.
that the Moon’s orbit around Earth creates the phases of the Moon and lunar and solar eclipses.
how the year is defined and how the calendar was developed.

6. The Night Sky Without Light Pollution … and With
FIGURE 1-1 The Night Sky With and Without Light Pollution
Sunlight is a curtain that hides virtually everything behind it. As the Sun sets, places with little smog or light pollution treat viewers to beautiful panoramas of stars that can inspire the artist or scientist in many of us. This photograph shows the night sky in Goodwood, Ontario, Canada, during a power outage.
(b) This photograph shows the same sky with normal city lighting. (© Todd Carlson/SkyNews Magazine)
In (b), the brilliant yellow glow in the windows is from candles, to emphasize the subtlety of the night sky. . In centuries and millennia past, everyone, illiterate or learned, had a solid knowledge of the night sky, phases of the Moon, and the motions of the planets. Astronomers understand these much better today, but relatively few average citizens can find the North Star or say what time a last quarter Moon rises.

7. The universe is huge, and the sizes and distances of objects in the universe vary greatly. Therefore we use scientific notation, which involves powers of ten notation to describe numbers much smaller or much greater than 1.
Some common examples of powers of ten:
POWER DECIMAL NAME METRIC PREFIX
one thousand Kilo
109 1,000,000,000 one billion Giga
one millionth micro
The Metric System and Powers of Ten
Other prefixes are Mega for 10+6 , milli for 10-3, nano for 10-9, and pico for Also, you need to be fluent in scientific notation, since numbers like and also occur.

8. THE SCALES OF THE UNIVERSE
The range of objects we study are from the extremely small subatomic particles, to objects which are gigantic, such as a galaxy or the size of the known universe itself.
Each division up the line indicates an increase in size by 100,000.
FIGURE 1-2 The Scales of the Universe
This curve gives the sizes of objects in meters, ranging from subatomic particles at the bottom to the entire observable universe at the top. Every 0.5 cm up along the arc represents a factor of 10 larger. (Top to bottom: R. Williams and the Hubble Deep Field Team [STScI] and NASA; AAT; L. Golub, Naval Observatory, IBM Research, NASA; Richard Bickel/Corbis; Scientific American Books; Jose Luis Pelaez/Getty Images; Rothamsted Research Centre for Bioimaging; Courtesy of Florian Banhart/University of Mainz)
Note that the red and yellow segments form a logarithmic scale. We will use many such scales in the course. The main idea is that each division represents a given factor. Each segment represents a factor of 100,000 in length. In scientific notation, this is 10 to the 5th, normally written as For ease of typing, the style “10^5” may be used. Astronomy involves physics on all scales, from subatomic to the size of the universe.

9. What Have Astronomers Discovered in Our Universe?
FIGURE 1-3 Inventory of the Universe
Pictured here are examples of the major categories of objects that have been found throughout the universe. You will discover more about each type in the chapters that follow. (a: NASA/Hubblesite; b: NASA; c: Peter Stättmayer/European Southern Observatory; d: Big Bear Observatory; e: NASA/Jeff Hester & Paul Scowen; f: Anglo-Australian Observatory; g: NOAO; h: NASA; i: N. F. Comins and F. N. Owen/NRAO)
… By the way, the Sun is the only star on which we can see any surface detail.

10. In order to more easily locate objects in the sky, we divide the sky into regions named after familiar patterns of stars called constellations.
Figure 1-4 The Constellation Orion
The pattern of stars (asterism) called Orion is prominent in the winter sky. From the northern hemisphere, it is easily seen high above the southern horizon from December through March. You can see in this photograph that the various stars have different colors—something to watch for when you observe the night sky.
(b) Technically, constellations are entire regions of the sky. The constellation called Orion and parts of other nearby constellations are depicted in this photograph. All the stars inside the boundary of Orion are members of that constellation. The celestial sphere is covered by 88 constellations of differingsizes and shapes. (© 2004 Jerry Lodriguss/
The modern “property lines” were selected to correspond closely to tradition. Note that there is no “unowned property” between constellations.
Ancient constellations were imaginary pictures outlined by familiar patterns of stars.
Modern astronomers divide the sky into 88 official constellations or regions of space, many of which contain the ancient star patterns.

11. Some Common Guides to Finding Constellations
Figure 1-5 The Big Dipper as a Guide
In the northern hemisphere, the Big Dipper is an easily recognized pattern of seven bright stars. This star chart shows how the Big Dipper can be used to locate the North Star as well as the brightest stars in three other constellations. While the Big Dipper appears right side up in this drawing of the sky shortly before sunrise, at other times of the night it appears upside down.
The course doesn’t emphasize names of constellations, etc (positional astronomy), but you are encouraged to explore on your own.
Using the “Big Dipper” as a guide

12. The “Winter Triangle” Figure 1-6 The Winter Triangle
This star chart shows the southern sky as it appears during the evening in December. Three of the brightest stars in the sky make up the winter triangle. In addition to the constellations involved in the triangle, Gemini (the Twins), Auriga (the Charioteer), and Taurus (the Bull) are also shown.
The “Winter Triangle”

13. The “Summer Triangle” Figure 1-7 The Summer Triangle
This star chart shows the northeastern sky as it appears in the evening in June. In addition to the three constellations involved in the summer triangle, the faint stars in the constellations Sagitta (the Arrow) and Delphinus (the Dolphin) are also shown.
The “Summer Triangle”

14. Astronomers describe the universe as an imaginary sphere surrounding the Earth on which all objects in the sky can be located, called the CELESTIAL SPHERE.
As viewed from Earth, the celestial sphere appears to rotate around two axis points, the north and south celestial poles, which are located directly above the Earth’s poles.
Between these is the celestial equator, which divides the celestial sphere into northern and southern hemispheres.
We define the position of an object on the celestial sphere using two coordinates, right ascension and declination.
Figure 1-8 The Celestial Sphere
The celestial sphere is the apparent “bowl” or hollow sphere of the sky. The celestial equator and poles are projections of Earth’s equator and axis of rotation onto the celestial sphere. The north celestial pole is therefore located directly over Earth’s North Pole, while the south celestial pole is directly above Earth’s South Pole. Analogous to longitude and latitude, the coordinates in space are right ascension (R.A.) and declination (Decl.), respectively. The star in this figure has the indicated R.A. and Decl.
For navigators, Latitude is measured North or South of the Equator. This makes Declination precisely comparable to Latitude. Longitude on Earth is measured East or West from the Prime Meridian, which runs through London (or, more precisely, Greenwich). Since the Earth rotates, the reference line for R.A. cannot be anything fixed on Earth, but needs to be something fixed in the sky. Astronomers have settled on the Vernal Equinox, which is where the Sun is when it crosses the Equator northbound. Since this occurs on March 21 (plus or minus a day), the term “Vernal Equinox” also applies to this date.

15. Cyclic motions of the Sun and stars in our sky are due to motions of Earth.
ROTATION = the spin of Earth on its axis. It takes one day for Earth to complete one rotation.
REVOLUTION = the movement of Earth in orbit around the sun. It takes one year for Earth to complete one revolution.
PRECESSION = the slow conical (top-like) motion of Earth’s axis of rotation. It takes 26,000 years for Earth to complete one cycle of precession.
Rotation, Revolution, and Precession – Cyclic Motions

16. Angular Distance (example from The Big Dipper)
The angular distance between the two “pointer stars” at the front of the Big Dipper is about 5°. For comparison, the angular diameter of the Moon is about 1⁄2°.
The angular distance between the two “pointer stars” at the front of the Big Dipper is about 5°.

17. Estimating Angles with the Human Hand
Various parts of the adult human hand extended to arm’s length can be used to estimate angular distances and sizes in the sky. (These angles vary slightly from person to person because hands and arms are different sizes.)
Various parts of the adult human hand extended to arm’s length can be used to estimate angular distances and angular sizes in the sky.

18. Circumpolar Star Trails
FIGURE 1-9 Circumpolar Star Trails
This long exposure, taken from Australia’s Siding Spring Mountain and aimed at the south celestial pole, shows the rotation of the sky. The stars that pass between the pole and the ground are all circumpolar stars. Note the lack of a bright, short star arc near the celestial pole compared to that of Polaris in the chapter opening photograph. (Anglo-Australian Observatory/David Malin Images)
The stars near the poles of the celestial sphere (shown here) move in trails that circle the pole and never set. They are called circumpolar.

19. The apparent westward motion of the Sun, Moon, and stars across our sky each day is caused by Earth’s rotation.
At middle latitudes, we see the Sun, Moon, and many of the stars first come into view moving upward, rising at some point along the eastern horizon. Then, they appear to arc across the sky. Finally, they disappear somewhere along the western horizon.
We generalize this motion to make statements such as, “The Sun rises in the east and sets in the west.”
The Apparent Westward Motion of the Sun and Stars is Caused by Earth’s Rotation

20. We can see how different stars appear at different times of day by looking at the position of the Sun against the backdrop of stars. The side of Earth facing the Sun is experiencing “day,” while the side of Earth turned away from the Sun is experiencing “night.”
Figure 1-10 Why Different Constellations Are Visible at Different Times of the Year
On the autumnal equinox each year, the Sun is in the constellation Virgo. As seen from Earth, that part of the sky is then daylight and we see stars only on the other half of the sky, centered around the constellation Pisces.
(b) Six months later, the Sun is in Pisces. This side of the sky is then bright, while the side centered on Virgo is in darkness.
As seen from Earth, the Sun moves through the constellations of the Zodiac, passing through all thirteen (yes, 13: stay tuned) over the course of a year.
MARCH
SEPTEMBER

21. Motion of Stars at the Poles
Because Earth rotates around its poles, stars seen from these locations appear to move in huge, horizontal circles. This is the same effect you would get by standing up in a room and spinning around; everything would appear to move in circles around you. At the North Pole, stars move left to right, while at the South Pole, they move right to left.
FIGURE 1-11 Motion of Stars at the Poles
Because Earth rotates around the axis through its poles, stars seen from these locations appear to move in huge, horizontal circles. This is the same effect you would get by standing up in a room and spinning around; everything would appear to move in circles around you. At the North Pole, stars move left to right, while at the South Pole they move right to left.

22. Rising and Setting of Stars at the Equator
FIGURE 1-12 Rising and Setting of Stars at the Equator
Standing on the equator, you are perpendicular to the axis around which Earth rotates. As seen from there, the stars rise straight up on the eastern horizon and set straight down on the western horizon. This is the same effect you get when driving straight over the crest of a hill; the objects on the other side of the hill appear to move straight upward as you descend.
Standing on the equator, you are perpendicular to the axis
around which Earth rotates. As seen from there, the stars
rise straight up on the eastern horizon and set straight down on
the western horizon.

23. Rising and Setting of Stars at Middle Northern Latitudes
FIGURE 1-13 Rising and Setting of Stars at Middle North Latitudes
Unlike the motion of the stars at the poles (see Figure 1-11), the stars at all other latitudes do change angle above the ground throughout the night. This time-lapse photograph shows stars setting. The latitude determines the angle at which the stars rise and set. (David Miller/DMI)
If you track a star (or the Sun or Moon) from an hour before it sets, you will observe a path like these.
Unlike the motion of the stars at the poles, the stars at all other latitudes do change angle above the ground throughout the night. This time-lapse photograph shows stars setting. The latitude determines the angle at which the stars rise and set.

24. Earth also revolves around the Sun, which changes our view of the stars.
Figure 1-14 The Ecliptic
The ecliptic is the apparent annual path of the Sun on the celestial sphere.
(b) The ecliptic is also the plane described by Earth’s path around the Sun. The planes created by the two ecliptics exactly coincide. As in (a), the rotation axis of Earth is shown here tilted 23½° from being perpendicular to the ecliptic.
Think of the Earth’s spin axis pointing in the same direction in space (e.g., towards a very distant star), while Earth orbits the Sun.
From our perspective, the Sun appears to move through the stars along a special path called the ecliptic.
From an outside view, we see Earth revolve around the Sun. We define the plane of Earth’s orbit as the ecliptic plane.

25. Where is the Sun? At noon, the Sun is in the constellation Leo.
In which constellation is it at 6 PM?
Leo
Taurus (3 constellations right of Leo)
Scorpius (3 constellations left of Leo)
Ophiuchus
Scroll down for answer
A. Leo. The Sun spends about one month in each constellation.

26. Seasons are caused because Earth’s axis is tilted, and as Earth revolves around the Sun, different parts of Earth receive more direct sunlight (summer), whereas other parts of Earth receive sunlight that is more spread out (winter).
Figure 1-15 The Tilt of Earth’s Axis
Earth’s axis of rotation is tilted 23½° from being perpendicular to the plane of Earth’s orbit. Earth maintains this orientation (with its North Pole aimed at the north celestial pole near the star Polaris) throughout the year as it orbits the Sun. Consequently, the amount of solar illumination and the number of daylight hours at any location on Earth vary in a regular fashion with the seasons.
The four positions of Earth represent June 21 (left), September 22 (bottom), December 21 (right), and Mar 21 (top. Take a look at the left-position Earth, at the Summer Solstice, which occurs typically on June 21. At high noon over La Paz, Baja California (near the tip, some 900 km south of San Diego), the Sun is passing directly overhead, meaning a vertical flagpole casts no shadow. Furthermore, the Earth turns more than 180 degrees between sunrise and sunset. Hence: daylight is more direct; and there are more hours of daylight. That combination makes summer. Note that for Australia, both effects are reversed, so it’s winter there on June 21. Now, on the same position of Earth, consider a point close to the North Pole. Note that it’s in daylight for the entire spin of the Earth, so it’s daylight 24/7 for weeks or months on end (“Land of the Midnight Sun”). And the Winter Solstice (Dec 21 or 22) has the opposite effects. Understand the meaning and significance of these terms: solstice, equinox, Tropics of Cancer/Capricorn, Arctic/Antarctic Circle.

27. The seasons we experience are linked to the motion of the Sun along the celestial sphere.
The point of the Sun’s path farthest north on the celestial sphere is called the summer solstice (JUN 21), whereas the point of the ecliptic farthest south is called the winter solstice (DEC 21).
The two points on the ecliptic where the Sun crosses the celestial equator are called equinoxes. During the vernal equinox (MAR 21), the Sun is moving north, while during the autumnal equinox (SEPT 21), the Sun is moving south.
Remember that the seasonal names of the equinoxes and the solstices refer to seasons in the NORTHERN hemisphere. The seasons occurring in the SOUTHERN hemisphere are exactly opposite.
Figure 1-16 The Seasons Are Linked to Equinoxes and Solstices
The ecliptic is inclined to the celestial equator by 23½° because of the tilt of Earth’s axis of rotation. The ecliptic and the celestial equator intersect at two points called the equinoxes. The northernmost point on the ecliptic is called the summer solstice; the southernmost point is called the winter solstice.
A geocentric perspective of the previous slide. The Sun takes a YEAR to go around the tilted circle while the Earth spins rapidly, in the same sense as the arrow for the Sun is moving.

28. The Sun’s Daily Path and the Energy It Deposits Here
On the winter solstice―first day of winter,―the Sun rises farthest south of east, it is lowest in the noontime sky, stays up the shortest time, and its light and heat are least intense (most spread out) of any day of the year in the northern hemisphere
On the vernal equinox―first day of spring―the Sun rises precisely in the east and sets precisely in the west. Its light and heat have been growing more intense, as shown by the brighter oval of light than in (a)
FIGURE 1-17 The Sun’s Daily Path and the Energy It Deposits Here
On the winter solstice—the first day of winter—the Sun rises farthest south of east, is lowest in the noontime sky, stays up the shortest time, and its light and heat are least intense (most spread out) of any day of the year in the northern Hemisphere.
(b) On the vernal equinox—the first day of spring—the Sun rises precisely in the east and sets precisely in the west. Its light and heat have been growing more intense, as shown by the brighter oval of light than in (a).
(c) On the summer solstice—the first day of summer—the Sun rises farthest north of east of any day in the year, is highest in the noontime sky, stays up the longest time, and its light and heat are most intense of any day in the northern hemisphere.
(d) On the autumnal equinox, the same astronomical conditions exist as on the vernal equinox.
This is showing the effect of the Sun being lower in the sky. Each cylinder of solar radiation carries the same amount of energy, but it gets spread over more area in winter. So, a given square meter of the surface receives less energy and doesn’t heat up as much.
(c) On the summer solstice―first day of summer― the Sun rises farthest north of east of any day in the year, is highest in the sky at noontime, stays up the longest time, and its light and heat are most intense of any day in the northern hemisphere.
(d) On the autumnal equinox, the same astronomical conditions exist as on the vernal equinox.

29. The Midnight Sun
FIGURE 1-18 The Midnight Sun
This time-lapse photograph was taken on July 19, 1985, at 69° north latitude in northeastern Alaska. At that latitude, the Sun is above the horizon continuously from mid-May until the end of July. (Doug Plummer/Science Photo Library)
This time-lapse photograph was taken on July 19, 1985, at 69°
north latitude in northeastern Alaska. At that latitude,
the Sun is above the horizon continuously from mid-May
until the end of July.

30. Where on Earth is this?
Is this location North or South of the Arctic Circle??
North
South
Precisely on the Arctic Circle
Cannot be determined
Scroll Down for answer
>
> > > > > > A. North. Otherwise you NEVER get the midnight Sun

31. Different parts of the world experience different times of day as Earth rotates.
TIME ZONES can be used to calculate the time of day in any given part of the world.
FIGURE 1-19 Time Zones of the World
For convenience, Earth’s 360° circumference is divided into 24 time zones. Ideally, each time zone would run due north-south. However, political considerations make many zones irregular. Indeed, there are even a few zones only a half-hour wide.

32. FIGURE 1-20 Precession and the Path of the North Celestial Pole
The gravitational pulls of the Moon and the Sun on Earth’s equatorial bulge cause Earth to precess.
(b) The situation is analogous to the motion of a gyroscope. The top of the gyroscope shows the motion of Earth’s North Pole or South Pole, while the point on which the gyroscope spins represents the center of Earth. As the gyroscope spins, Earth’s gravitational pull causes the gyroscope’s axis of rotation to move in a circle—to precess.
Earlier, we said think of the Earth’s rotation axis as pointing in a constant direction in space. Actually, the direction changes very slowly. The reason is exactly analogous to the gyroscope, or to a toy top, which precesses slowly compared to its spin rate. In the case of the Earth, the precession takes about 26,000 years to complete a cycle.
Gravitational forces of the Sun and the Moon pulling on Earth as it rotates cause Earth to undergo a top-like motion called precession. Over a period of 26,000 years, Earth’s rotation axis slowly moves in a circular motion.

33. Which is NOT a reason for the seasons
Angle of sunlight at high noon
Distance of Sun from Earth
Number of hours of daylight
Tilt of the Earth’s Axis
Scroll for answer
> > > >> > > > > > B.

34. This precession causes the position of the North Celestial Pole to slowly change over time. Today, the North Celestial Pole is near the star Polaris, which we call the “North Star.” However, in 3000 BC, Thuban was close to the North Celestial Pole and in 14,000 AD, Vega will be in this location.
FIGURE 1-20 Precession and the Path of the North Celestial Pole
(c) As Earth precesses, the north celestial pole slowly traces out a circle among the northern constellations. At the present time, the north celestial pole is near the moderately bright starPolaris, which serves as the pole star. The total precession period is about 26,000 years.
Look at AD 1 and today on the star chart; imagine where the spin axis, which marks true north, was in the epoch for Bartholomeu Dias, Vasco da Gama, and Christopher Columbus. Note that Thuban was an excellent North Star for the Egyptians in the Old Kingdom, when construction of the pyramids began; but that for the New Kingdom (ca 1400 BCE), a star in the dipper part of Ursa Minor was the mediocre best.

35. Find your Birthday. Find your Sign. Is it what you thought?
The traditional astrologers’ dates correspond to the Earth’s orientation circa 150 AD.
From your birthday, find your Zodiac sign in the table above. For most people, this is different from your traditional sign, and the traditional sign is the one below. The traditional dates for Zodiac signs have been fixed since 150 AD.
Since that time, the Earth’s axis has precessed 1850/26000 of the circuit, or almost one constellation. Furthermore, due to exact boundary lines, a 13th constellation is in the Zodiac.
Be sure to read about the Great Calendar Reform of 1582, and why we use the “Gregorian Calendar.” The calendar is set to keep the Vernal Equinox (the date the Sun crosses the Equator northbound) on March 21 (plus/minus a day), NOT to keep March 21 the date that the Sun reaches some point in some constellation. Two original basic tasks of Astronomy were to determine the calendar and the clock.

37. Figure 1-21 The Phases of the Moon
The diagram shows the Moon at eight locations on its orbit as viewed from far above Earth’s North Pole. Light from the Sun illuminates one-half of the Moon at all times, while the other half is dark. It takes about 29½ days for the Moon to go through all its phases. The inset drawings with photographs show the resulting lunar phases as seen from Earth. (Yerkes Observatory and Lick Observatory)
At any point in time, half of the Moon is illuminated by the Sun (barring lunar eclipses), and half of the Moon is visible from Earth. The Full Moon occurs when these halves are the same. A New Moon occurs when the halves are exactly opposite. The degree of overlap between these halves gives all of the other phases of the Moon. This corresponds to the angle between the Sun and Moon, as seen from Earth, and it also corresponds to the time the Moon will rise and set. For example, the First Quarter Moon rises about noon, is high in the southern sky at sunset, and sets around midnight.
Another familiar cycle is the lunar cycle. When the Moon orbits Earth, the amount of the side facing Earth that is lit changes, creating the Moon’s phases. This phase cycle is called the synodic period and is 29½ days long.

38. Test your understanding
What fraction of the Moon is illuminated
by the Sun at any point in time?
A. All of it
50%
None
Depends on the phase of the Moon
The answer is (scroll down)
B. 50%

39. One common misconception is that the Moon is only visible at night
One common misconception is that the Moon is only visible at night. However, the time of day in which the Moon is in our sky varies depending on its phase. This picture clearly displays the Moon, visible during the day.
FIGURE 1-22 The Moon During the Day
The Moon is visible at some time during daylight hours virtually every day. The time of day or night it is up in our sky depends on its phase. (Richard Cummins/SuperStock)

40. A synodic month is the time it takes for the Moon to orbit Earth with respect to the Sun and is 29½ days long A sidereal month is the time it takes for the Moon to orbit Earth with respect to the stars and is 27.3 days long.
The two times are different because Earth moves in its orbit around the Sun as the Moon moves in its orbit around Earth.
Figure 1-23 The Sidereal and Synodic Months
The sidereal month is the time it takes the Moon to complete one revolution with respect to the background stars, about 27.3 days. However, because Earth is constantly moving in its orbit about the Sun, the Moon must travel through more than 360° to get from one new Moon to the next. The synodic month is the time between consecutive new Moons or consecutive full Moons, about 29½ days.
Key Term “sidereal” means “with respect to a reference frame fixed with the distant stars.
Key Term “synodic” means “with respect to a reference line between Earth and Sun;” this reference line is moving as the Earth orbits.
The terms are illustrated as applied to the Moon, but they are also applied to the planets.

41. Test your Understanding
Which is longer, a sidereal month or
a synodic month?
Sidereal
Synodic
Both are equal
Depends on the month
Scroll down for answer
B. Synodic is longer

42. During a new or full moon phase, when the Moon, Sun, and Earth are aligned, the Moon may enter the shadow of Earth, or the shadow of the Moon may reach Earth, creating eclipses. However, these eclipses do not occur during every full or new moon because the Moon’s orbit is tilted by 5 with respect to the Earth-Sun (ecliptic) plane. “Ecliptic” has to do with “eclipses.”
Figure 1-24 Conditions for Eclipses
The Moon must be very nearly on the ecliptic at new Moon for a solar eclipse to occur. A lunar eclipse occurs only if the Moon is very nearly on the ecliptic at full Moon. When new Moon or full Moon phases occur away from the ecliptic, no eclipse is seen because the Moon and Earth do not pass through each other’s shadows.
Recall that the ecliptic, or ecliptic plane, is the plane of the Earth’s orbit around the Sun. By this definition, Earth and Sun are always in the ecliptic plane. The Moon’s orbital plane is tilted about 5 degrees compared to the ecliptic, with the Moon coming above the ecliptic, passing through it, going below the plane, and cycling back. Eclipses are possible only if the Moon is in (or very near) the ecliptic plane when it is at the proper phase. Maybe that’s why it’s called “ECLIPTIC.”

43. Lunar Eclipse: Moon passes through Earth’s Shadow
PENUMBRAL = the Moon appears dimmed.
PARTIAL = part of the Moon enters Earth’s umbra and is darkened.
TOTAL = all of the Moon enters Earth’s umbra and becomes a reddish color, only lit from light bent by Earth’s atmosphere.
FIGURE 1-25 Three Types of Lunar Eclipses
People on the nighttime side of the Earth see a lunar eclipse when the Moon moves through the Earth’s shadow. The umbra is the darkest part of the shadow. In the penumbra, only part of the Sun is covered by the Earth. The inset shows the various lunar eclipses that occur, depending on the Moon’s path through Earth’s shadow.
The Earth is 3½ times the size of the Moon. Since the Sun is about 100 times the size of the Earth, Earth’s umbra tapers down, but is still 2½ times as big as the Moon at the distance to the Moon. The center of the bullseye corresponds to the ecliptic plane; the penumbra is still fairly close; the Moon might be above the penumbra by the diameter of the penumbra

44. FIGURE 1-25 Three Types of Lunar Eclipses
(b) This sequence of nine photographs was taken over a 3-h period during the total lunar eclipse of January 20, During the total phase, the Moon has a distinctly reddish color. (Fred Espenak, NASA/Goddard Space Flight Center; © Fred Espenak, MrEclipse.com)
The Moon is a dull red when totally eclipsed. The Earth’s atmosphere bends and scatters some light so that the shadow isn’t perfectly dark. But when light goes through the atmosphere, the red part gets dimmed less than other colors – a phenomenon you have noticed with sunsets and sunrises
During a total lunar eclipse, the Moon moves in and out of the umbra of Earth’s shadow.

45. If you are located where the umbra of the Moon’s shadow reaches, you will see a total solar eclipse, during which the entire disk of the Sun is covered by the Moon, revealing the faint solar corona surrounding the Sun.
FIGURE 1-26 A Total Eclipse of the Sun
During a total solar eclipse, the Moon completely covers the Sun’s disk, and the solar corona can be photographed. This halo of hot gases extends for millions of kilometers into space. This gorgeous image was taken in southwestern Mongolia during the August 1, 2008, solar eclipse. (© 2008 Miloslav Druckmüller, Martin Dietzel, Peter Aniol, Vojtech Rušin)
Aside from being a spectacular phenomenon, solar eclipses allow scientific study of aspects that can’t be seen otherwise.

46. Unlike lunar eclipses, solar eclipses occur at specific places on Earth, indicated by the arrow.
FIGURE 1-27 The Geometry of a Total Solar Eclipse
During a total solar eclipse, the tip of the Moon’s umbra traces an eclipse path across Earth’s surface. People inside the eclipse path see a total solar eclipse, whereas people inside the penumbra see only a partial eclipse. The photograph in this figure shows the Moon’s shadow on Earth. It was taken from the Mir space station during the August 11, 1999, total solar eclipse. The Moon’s umbra appears as the very dark spot on the eastern coast of the United States. The umbra is surrounded by the penumbra. (Jean-Pierre Haigneré, Centre National d’Etudes Spatiales, France/GSFS)
The relative sizes and distances of the Sun and Moon are tuned such that the tip of the umbra can barely reach Earth, as shown. In fact the orbits aren’t circles, so both distances vary: Moon’s by 5 percent, Sun’s by 1½ percent.
When the Moon is relatively farther away, the tip of the umbra does NOT reach Earth; the Moon lines up perfectly, but leaves a “ring of fire.”

47. Test your understanding
At what phase of the Moon can
a solar eclipse occur?
Full
New
First or Last Quarter
Any; only the Sun is involved in
a solar eclipse
Scroll down for answer
B. New. The Moon IS involved.

48. Solar Eclipse – Actual Scale
Fig prepared by R. T. Skelton. The Moon’s orbit is inclined by 5 degrees relative to the ecliptic plane. As a result, the region of totality is nowhere on Earth unless the Moon is very close to the ecliptic plane at New Moon.
The various Moons represent the Moon at various levels it can achieve above and below the ecliptic plane.
Note also that if the Moon is moved back by several percent of its distance, the tip of the umbra doesn’t reach Earth, and one has an annular eclipse.
The Sun is almost 400 times the size of the Moon, and the same factor farther away from Earth.

49. Eclipse Paths for Total and Annular Eclipses 2001–2020
FIGURE 1-28 Eclipse Paths for Total and Annular Eclipses 2001–2020
This map shows the eclipse paths for the 14 total solar and 13 annular eclipses that occur between 2001 and In each eclipse, the Moon’s shadow travels along the eclipse path in a generally eastward direction across Earth’s surface. (Courtesy of Fred Espenak, NASA/Goddard Space Flight Center)
Twenty year’s worth of solar eclipses. No place gets eclipses commonly. Only the fortunate few actually see a total solar eclipse – fortunate enough to live there, or fortunate to be rich enough to pay the travel/lodging rates which get raised for the event. Clear weather is also required to appreciate it properly.
This map shows the eclipse paths for the 14 total solar and 13 annular eclipses that occur between 2001 and In each eclipse, the Moon’s shadow travels along the eclipse path in a generally eastward direction across Earth’s surface.

50. Sometimes eclipses occur when the Moon is too far away from Earth to completely cover the Sun in our sky.
When this occurs, the Moon appears in the center and a thin ring, or “annulus,” of light surrounds it. These are called annular eclipses.
FIGURE 1-29 An Annular Eclipse of the Sun
This composite of five exposures taken at sunrise in Costa Rica shows the progress of an annular eclipse of the Sun that occurred on December 24, Note that at mid-eclipse the edge of the Sun is visible around the Moon. (Dennis Di Cicco)
An annular eclipse is also called a “ring of fire.”

51. Test your understanding
About how often does a lunar eclipse occur?
Every month
Every 6 months
Once per year
Once in a blue moon
Scroll down for answer
B. Every 6 months. The Moon has to be crossing through the ecliptic plane in the New or Full phase.

52. Summary of Key Ideas

53. Sizes in Astronomy
Astronomy examines objects that range in size from the parts of an atom (1015 m) to the size of the observable universe (1026 m).
Scientific notation is a convenient shorthand for writing very large and very small numbers.

54. Patterns of Stars
The surface of the celestial sphere is divided into 88 unequal areas called constellations.
The boundaries of the constellations run along lines of constant right ascension or declination.

55. Earthly Cycles
The celestial sphere appears to revolve around Earth once in each day-night cycle. In fact, it is the Earth’s rotation that causes this apparent motion.
The poles and equator of the celestial sphere are determined by extending the axis of rotation and the equatorial plane of Earth out onto the celestial sphere.

56. Earthly Cycles
Earth’s axis of rotation is tilted at an angle of 23½° from a line perpendicular to the plane of Earth’s orbit (the plane of the ecliptic). This tilt causes the seasons.
Equinoxes and solstices are significant points along Earth’s orbit that are determined by the relationship between the Sun’s path on the celestial sphere (the ecliptic) and the celestial equator.

57. Earthly Cycles
Earth’s axis of rotation slowly changes direction relative to the stars over thousands of years, a phenomenon called precession. Precession is caused by the gravitational pull of the Sun and Moon on Earth’s equatorial bulge.
The length of the day is based upon Earth’s rotation rate and the average motion of Earth around the Sun. These effects combine to produce the 24-hour day upon which our clocks are based.

58. Earthly Cycles
The phases of the Moon are caused by the relative positions of Earth, the Moon, and the Sun. The Moon completes one cycle of phases in a synodic month, which averages 29½ days.
The Moon completes one orbit around Earth with respect to the stars in a sidereal month, which averages 27.3 days.

59. Eclipses
The shadow of an object has two parts: the umbra, where direct light from the source is completely blocked; and the penumbra, where the light source is only partially obscured.
A lunar eclipse occurs when the Moon moves through Earth’s shadow. During a lunar eclipse, the Sun, Earth, and the Moon are in alignment with Earth between the Sun and the Moon, and the Moon is in the plane of the ecliptic.

60. Eclipses
A solar eclipse occurs when a strip of Earth passes through the Moon’s shadow. During a solar eclipse, the Sun, Earth, and the Moon are in alignment with the Moon between Earth and the Sun, and the Moon is in the plane of the ecliptic.
Depending on the relative positions of the Sun, Moon, and Earth, lunar eclipses may be penumbral, partial, or total, and solar eclipses may be annular, partial, or total.

61. Key Terms angle angular diameter (angular size) annular eclipse
arc angle
autumnal equinox
celestial equator
celestial sphere
circumpolar star
constellation
declination
degree
diurnal motion
eclipse path
ecliptic
equinox
gravitation
line of nodes
lunar eclipse
lunar phase
north celestial pole
partial eclipse
penumbra
penumbral eclipse
precession
precession of the equinoxes
revolution
right ascension
rotation
scientific notation
sidereal month
sidereal period
solar corona
solar day
solar eclipse
south celestial pole
summer solstice
synodic month
terminator
time zone
total eclipse
umbra
vernal equinox
winter solstice
zenith
zodiac

62. WHAT DID YOU THINK?
Is the North Star—Polaris—the brightest star in the night sky?
No. Polaris is a star of medium brightness compared with other stars visible to the naked eye.

63. WHAT DID YOU THINK? What do astronomers define as constellations?
Astronomers sometimes use the common definition of a constellation as a pattern of stars. Formally, however, a constellation is an entire area of the celestial sphere and all the stars and other objects in it. Viewed from Earth, the entire sky is covered by 88 different-sized constellations. If there is any room for confusion, astronomers refer to the patterns as asterisms.

64. WHAT DID YOU THINK? What causes the seasons?
The tilt of Earth’s rotation axis with respect to the ecliptic causes the seasons. They are not caused by the changing distance from Earth to the Sun that results from the shape of Earth’s orbit.

65. WHAT DID YOU THINK? When is Earth closest to the Sun?
On or around January 3 of each year.

66. WHAT DID YOU THINK? How many zodiac constellations are there?
There are 13 zodiac constellations, the least-known one being Ophiuchus.

67. WHAT DID YOU THINK?
Does the Moon have a dark side that we never see from Earth?
Half of the Moon is always dark. Whenever we see less than a full Moon, we are seeing part of the Moon’s dark side. So, the dark side of the Moon is not the same as the far side of the Moon, which we never see from Earth.

68. WHAT DID YOU THINK? Is the Moon ever visible during the daytime?
The Moon is visible at some time during daylight hours almost every day of the year. Different phases are visible during different times of the day.

69. WHAT DID YOU THINK? What causes lunar and solar eclipses?
When the Moon is crossing the ecliptic in the full or new phase, the shadows of Earth or the Moon, respectively, then fall on the Moon or Earth. These shadows on the respective surfaces are eclipses.