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In this chapter you will discover…In this chapter you will discover… how astronomers organize the night sky to help them how astronomers organize the night sky to help them
locate objects in itlocate objects in it that Earth’s spin on its axis causes day and nightthat Earth’s spin on its axis causes day and night how Earth’s orbit around the Sun combined with the tilt how Earth’s orbit around the Sun combined with the tilt
of Earth’s axis of rotation relative to its orbit create the of Earth’s axis of rotation relative to its orbit create the seasonsseasons
that the Moon’s orbit as seen from Earth creates the that the Moon’s orbit as seen from Earth creates the phases of the Moon and lunar and solar eclipsesphases of the Moon and lunar and solar eclipses
how the year is defined and how the calendar was how the year is defined and how the calendar was developeddeveloped
The Night Sky With and Without Light Pollution
(a) 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.
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:Some common examples of powers of ten:
POWERPOWER DECIMALDECIMAL NAMENAME METRIC PREFIXMETRIC PREFIX
101033 10001000 one thousandone thousand KiloKilo
101099 1,000,000,0001,000,000,000 one billionone billion GigaGiga
1010-6-6 0.0000010.000001 one millionthone millionth micromicro
THE SCALES OF THE SCALES OF THE UNIVERSETHE UNIVERSE
The range of objects we study are from the extremely small subatomic particles to objects that 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 a factor of 10.
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.
Knowing where the constellations are can be usefulKnowing where the constellations are can be useful
In order to more easily locate objects in the sky, we divide In order to more easily locate objects in the sky, we divide the sky into regions named after familiar patterns of stars the sky into regions named after familiar patterns of stars called called constellationsconstellations..
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.
Some Common Guides to Finding ConstellationsSome Common Guides to Finding Constellations
Using the “Big Dipper” as a guide to find other constellationsUsing the “Big Dipper” as a guide to find other constellations
The “Winter Triangle”The “Winter Triangle”
The “Summer Triangle”The “Summer Triangle”
Astronomers describe the universe as an imaginary sphere Astronomers describe the universe as an imaginary sphere surrounding Earth on which all objects in the sky can be located, surrounding Earth on which all objects in the sky can be located,
called the called the CELESTIAL SPHERECELESTIAL SPHERE..
As viewed from Earth, the celestial sphere As viewed from Earth, the celestial sphere appears to rotate around two axis points, appears to rotate around two axis points, the the north and south celestial polesnorth and south celestial poles, which , which are located directly above Earth’s poles.are located directly above Earth’s poles.
Between these is the Between these is the celestial equator,celestial equator, which divides the celestial sphere into which divides the celestial sphere into northern and southern hemispheres.northern and southern hemispheres.
We define the position of an object on the We define the position of an object on the celestial sphere using two coordinates celestial sphere using two coordinates analogous to longitude and latitude, analogous to longitude and latitude, right right ascensionascension and and declinationdeclination. .
Students being shown a celestial sphereStudents being shown a celestial sphere
The Alt-Azimuth Coordinate System
Altitude and azimuth create a local coordinate system for every place on Earth. Altitude is the angle straight up from the horizon. Azimuth is the angle eastward from north along the horizon. To find Mizar at 10 P.M. on March 14, 2013, as seen from Orono, Maine, we calculate its coordinates there as altitude 47° and azimuth 52° and use them to locate it in the night sky.
The EclipticThe Ecliptic
(a) 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.
Earthly Cycles Earth’s spinning (Rotation), which causes day and Earth’s spinning (Rotation), which causes day and
night, as well as causing the apparent daily motion of night, as well as causing the apparent daily motion of the celestial sphere and all the objects on it.the celestial sphere and all the objects on it.
Earth’s orbit around the Sun (Revolution), which Earth’s orbit around the Sun (Revolution), which creates the seasons, the year, and the change in creates the seasons, the year, and the change in times at which constellations are up at night.times at which constellations are up at night.
The slow conical (toplike) motion of Earth’s axis of The slow conical (toplike) motion of Earth’s axis of rotation (Precession). It takes 26,000 years for Earth rotation (Precession). It takes 26,000 years for Earth to complete one cycle of precession. to complete one cycle of precession.
The Moon’s orbit around Earth, which creates the The Moon’s orbit around Earth, which creates the lunar phases, the cycle of tides, and the spectacular lunar phases, the cycle of tides, and the spectacular phenomena we call eclipses.phenomena we call eclipses.
We can see how different stars appear at different times of day by looking at the 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 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 is experiencing “day,” while the side of Earth turned away from the Sun is experiencing “night.”experiencing “night.”
SEPTEMBER MARCH
Circumpolar Star Trails
The stars near the poles of the celestial sphere move in trails that circle the The stars near the poles of the celestial sphere move in trails that circle the pole and never set. They are called pole and never set. They are called circumpolar.circumpolar. (Shown here is the south (Shown here is the south celestial pole.)celestial pole.)
The apparent westward motion of the Sun, Moon, The apparent westward motion of the Sun, Moon, and stars across our sky each day is caused by and stars across our sky each day is caused by Earth’s Earth’s rotationrotation. .
At middle latitudes, we see the Sun, Moon, and many At middle latitudes, we see the Sun, Moon, and many of the stars first come into view moving upward, rising of the stars first come into view moving upward, rising at some point along the eastern horizon. Then, they at some point along the eastern horizon. Then, they appear to arc across the sky. Finally, they disappear appear to arc across the sky. Finally, they disappear somewhere along the western horizon.somewhere along the western horizon.
We generalize this motion to make statements such as, We generalize this motion to make statements such as, “The Sun rises in the east and sets in the west.”“The Sun rises in the east and sets in the west.”
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.
Rising and Setting of Stars at the Equator
Standing on the equator, you are perpendicular to the axisaround which Earth rotates. As seen from there, the starsrise straight up on the eastern horizon and set straight down onthe western horizon.
Rising and Setting of Stars at Middle Northern Latitudes
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.
Seasons are caused because Earth’s axis is tilted, and as Earth revolves around 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), while the Sun, different parts of Earth receive more direct sunlight (summer), while other parts of Earth receive sunlight that is more spread out (winter).other parts of Earth receive sunlight that is more spread out (winter).
The Tilt of Earth’s Axis
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 22).
The two points on the ecliptic where the Sun crosses the celestial equator are called equinoxes. During the vernal equinox (MAR 20), the Sun is moving north, while during the autumnal equinox (SEPT 22), 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.
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 Sun’s Daily Path and the Energy It Deposits
(a) 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. (b) 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). (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.
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.
Different parts of the world experience different times of day as Earth rotates. 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 TIME ZONES can be used to calculate the time of day in any given part of the world. world.
Time Zones of the World
The two times are different because Earth moves in its orbit around the Sun as the Moon moves in its orbit around Earth.
Combining Earth’s rotation and its orbit around the Sun, the length of the sidereal day can be seen (on average) to be 4 minutes shorter than the solar day. The daily motion of Earth is exaggerated here for clarity.
Sidereal and Solar Days
Gravitational forces of the Sun and the Moon pulling on Earth as it rotates Gravitational forces of the Sun and the Moon pulling on Earth as it rotates cause Earth to undergo a toplike motion called precession. Over a period of cause Earth to undergo a toplike motion called precession. Over a period of 26,000 years, Earth’s rotation axis slowly moves in a circular motion.26,000 years, Earth’s rotation axis slowly moves in a circular motion.
Precession
This precession causes This precession causes the position of the north the position of the north celestial pole to slowly celestial pole to slowly change over time. change over time. Today, the north Today, the north celestial pole is near the celestial pole is near the star Polaris, which we star Polaris, which we call the “North Star.” call the “North Star.” However, in 3000 B.C., However, in 3000 B.C., Thuban was close to the Thuban was close to the north celestial pole and north celestial pole and in 14,000 A.D., Vega will in 14,000 A.D., Vega will be in this location. be in this location.
The Path of the North Celestial Pole
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. The boundary between the bright and dark regions is called the terminator.
The Phases of the Moon
One common misconception One common misconception is that the Moon is only is that the Moon is only visible at night. However, visible at night. However, the time of day in which the the time of day in which the Moon is in our sky varies Moon is in our sky varies depending on its phase. This depending on its phase. This picture clearly displays the picture clearly displays the Moon, visible during the day. Moon, visible during the day.
The Moon During the Day
The Sidereal and Synodic MonthsThe 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.
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.
Conditions for Eclipses
PENUMBRAL = the Moon appears dimmed.PARTIAL = part of the Moon enters the umbra of Earth’s shadow and is darkened.TOTAL = all of the Moon enters Earth’s shadow and becomes a reddish color, only lit from light bending around Earth’s atmosphere.
People on the nighttime side of Earth see a lunar eclipse when the Moon moves through Earth’s shadow. The umbra is the darkest part of the shadow. In the penumbra, only part of the Sun is covered by Earth. The inset shows the various lunar eclipses that occur, depending on the Moon’s path through Earth’s shadow.
Three Types of Lunar Eclipses
During a total lunar eclipse, the Moon moves in and out of the umbra of Earth’s shadow.
Total Lunar Eclipse
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.
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.
A Total Eclipse of the Sun
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.
An Annular Eclipse of the Sun
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 2020. In each eclipse, the Moon’s shadow travels along the eclipse path in a generally eastward direction across Earth’s surface.
Summary of Key IdeasSummary of Key Ideas
Sizes in AstronomyAstronomy examines objects that range in Astronomy examines objects that range in
size from the parts of an atom (size from the parts of an atom (10101515 m) m) to the size of the observable universe to the size of the observable universe ((10102626 m). m).
Scientific notation is a convenient Scientific notation is a convenient shorthand for writing very large and very shorthand for writing very large and very small numbers.small numbers.
Patterns of StarsThe surface of the celestial sphere is The surface of the celestial sphere is
divided into 88 unequal areas called divided into 88 unequal areas called constellations.constellations.
The boundaries of the constellations run The boundaries of the constellations run along lines of constant right ascension or along lines of constant right ascension or declination.declination.
Earthly Cycles The celestial sphere appears to revolve around
Earth once in each day-night cycle. In fact, it is 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.
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.
Earth’s axis of rotation slowly changes direction Earth’s axis of rotation slowly changes direction relative to the stars over thousands of years, a relative to the stars over thousands of years, a phenomenon called precession. Precession is phenomenon called precession. Precession is caused by the gravitational pull of the Sun and caused by the gravitational pull of the Sun and Moon on Earth’s equatorial bulge.Moon on Earth’s equatorial bulge.
The length of the day is based on Earth’s The length of the day is based on Earth’s rotation rate and the average motion of Earth rotation rate and the average motion of Earth around the Sun. These effects combine to around the Sun. These effects combine to produce the 24-hour day upon which our clocks produce the 24-hour day upon which our clocks are based.are based.
Earthly Cycles
The phases of the Moon are caused by the The phases of the Moon are caused by the relative positions of Earth, the Moon, and the relative positions of Earth, the Moon, and the Sun. The Moon completes one cycle of phases Sun. The Moon completes one cycle of phases in a synodic or lunar month, which averages in a synodic or lunar month, which averages 29½ days.29½ days.
The Moon completes one orbit around Earth with The Moon completes one orbit around Earth with respect to the stars in a sidereal month, which respect to the stars in a sidereal month, which averages 27.3 days.averages 27.3 days.
Lunar Cycles
EclipsesEclipses The shadow of an object has two parts: the The shadow of an object has two parts: the
umbra, where direct light from the source is umbra, where direct light from the source is completely blocked; and the penumbra, where completely blocked; and the penumbra, where the light source is only partially obscured.the light source is only partially obscured.
A lunar eclipse occurs when the Moon moves A lunar eclipse occurs when the Moon moves through Earth’s shadow. During a lunar eclipse, through Earth’s shadow. During a lunar eclipse, the Sun, Earth, and the Moon are in alignment, the Sun, Earth, and the Moon are in alignment, with Earth between the Sun and the Moon, and with Earth between the Sun and the Moon, and the Moon is in the plane of the ecliptic.the Moon is in the plane of the ecliptic.
EclipsesEclipses A solar eclipse occurs when a strip of Earth A solar eclipse occurs when a strip of Earth
passes through the Moon’s umbra. During a passes through the Moon’s umbra. During a solar eclipse, the Sun, Earth, and the Moon are solar eclipse, the Sun, Earth, and the Moon are in alignment with the Moon between Earth and in alignment with the Moon between Earth and the Sun, and the Moon is in the plane of the the Sun, and the Moon is in the plane of the ecliptic.ecliptic.
Depending on the relative positions of the Sun, Depending on the relative positions of the Sun, Moon, and Earth, lunar eclipses may be Moon, and Earth, lunar eclipses may be penumbral, partial, or total, and solar eclipses penumbral, partial, or total, and solar eclipses may be annular, partial, or total.may be annular, partial, or total.
angleangular diameter (angular size)annular eclipsearc angleasterismautumnal equinoxblack holecelestial equatorcelestial spherecircumpolar starcometconstellationdeclinationdegreediurnal motioneclipse path
eclipticequinoxgravitationline of nodeslunar eclipselunar phasenorth celestial polepartial eclipsepenumbrapenumbral eclipseprecessionprecession of the equinoxesrevolutionright ascensionrotationscientific notation
sidereal monthsidereal periodsolar coronasolar daysolar eclipsesouth celestial polesummer solsticesynodic monthterminatortime zonetotal eclipseumbravernal equinoxwinter solsticezenith zodiac
Key TermsKey Terms