Earth and Sky Flashcards
What defines the locations of its North and South Poles and of its equator, halfway between?
Earth’s rotation
______is the direction toward which Earth rotates, and _____is its opposite.
east, west
We can use what ideas to define a system of coordinates attached to our planet.
the ideas of direction (north south east and west)
A great circle
is any circle on the surface of a sphere whose center is at the center of the sphere.
meridian
a series of great circles that pass through both the North and South Poles. Each of the circles is called a meridian; they are each perpendicular to the equator, crossing it at right angles.
How does one determine their lattitude?
Your latitude (or north-south location) is the number of degrees of arc you are away from the equator along your meridian.
How are lattitude’s measured?
Latitudes are measured either north or south of the equator from 0° to 90°.
N. The latitude of the South Pole
90 degrees
The latitude of the north Pole
90 degrees
Latitude at the equator
(The latitude of the equator is 0°.)
What do astronomers use to denote objects in the sky?
however, astronomers use coordinates called declination and right ascension.
markers in the sky to set up a system of celestial coordinates.
north celestial pole and the south celestial pole. celstial equator
vernal equinox
a point in the sky where the ecliptic (the Sun’s path) crosses the celestial equator.
Right ascension (RA)
is like longitude, except that instead of Greenwich, the arbitrarily chosen point where we start counting is the vernal equinox
How can RA be expressed?
RA can be expressed either in units of angle (degrees) or in units of time.
RA can be expressed either in units of angle (degrees) or in units of time. Why?
This is because the celestial sphere appears to turn around Earth once a day as our planet turns on its axis.
Jean Foucault
provide an unambiguous demonstration of this rotation. In 1851, he suspended a 60-meter pendulum with a mass of about 25 kilograms from the dome of the Pantheon in Paris and started the pendulum swinging evenly. (pendulum prove earth’s rotation)
How did Jean Foucault’s pendulum prove the earth was rotating?
If Earth had not been turning, there would have been no alteration of the pendulum’s plane of oscillation, and so it would have continued tracing the same path. Yet after a few minutes Foucault could see that the pendulum’s plane of motion was turning. Foucault explained that it was not the pendulum that was shifting, but rather Earth that was turning beneath it
Why doesn’t the earth get hotter when it is closer to the sun?
Although Earth’s orbit around the Sun is an ellipse, its distance from the Sun varies by only about 3%. That’s not enough to cause significant variations in the Sun’s heating.
What are seasons caused by?
seasons? As we shall show, the seasons are actually caused by the 23.5° tilt of Earth’s axis.
As Earth travels around the Sun, in which month the Northern Hemisphere “leans into” the Sun and is more directly illuminated.
June
How does the Sun’s favoring one hemisphere translate into making it warmer for us down on the surface of Earth?
There are two effects we need to consider. When we lean into the Sun, sunlight hits us at a more direct angle and is more effective at heating Earth’s surface. The second effect has to do with the length of time the Sun spends above the horizon
Why does direct sunlight make the world more hot
The sunlight isn’t spread out as it would be if it were indirect
the hours of daylight increase in summer and decrease in winter. why?
In the summer months, the northern half of the Earth, where we live, tilts towards the Sun. This means we get more sunlight, making the days longer.
summer solstice
On or about June 21 (the date we who live in the Northern Hemisphere call the summer solstice or sometimes the first day of summer), the Sun shines down most directly upon the Northern Hemisphere of Earth.
Arctic Circle
. As Earth turns on its axis, the North Pole is continuously illuminated by the Sun; all places within 23° of the pole have sunshine for 24 hours. The Sun is as far north on this date as it can get; thus, 90° – 23° (or 67° N) is the southernmost latitude where the Sun can be seen for a full 24-hour period (sometimes called the “land of the midnight Sun”).
winter solstice
first day of winter in the Northern Hemisphere
Halfway between the solstices, where is the sun?
on about March 21 and September 21, the Sun is on the celestial equator. From Earth, it appears above our planet’s equator and favors neither hemisphere. Every place on Earth then receives roughly 12 hours of sunshine and 12 hours of night.
The points where the Sun crosses the celestial equator are called
the vernal (spring) and autumnal (fall) equinoxes.
where are all seasons are much the same?
near the equator
The Sun is north of the celestial equator from about March 21 to September 21, so at the North Pole, the Sun rises when it reaches what?
the vernal equinox
Where does the sun set at the north pole?
sets when it reaches the autumnal equinox.
atmosphere. In reality, however, the atmosphere has the curious effect of allowing us to see a little way “over the horizon.” Why?
because of refraction
refraction
the bending of light passing through air or water
the Sun appears to rise earlier and to set later than it would if no atmosphere were present. Why?
refraction
morning twilight
as beginning when the Sun is 18° below the horizon, and evening twilight extends until the Sun sinks more than 18° below the horizon.
atmospheric effects like refraction require
require small corrections in many of our statements about the seasons.
The measurement of time is based on
the rotation of Earth.
Throughout most of human history, time has been reckoned by
positions of the Sun and stars in the sky.
The most fundamental astronomical unit of time
is the day
solar day
the rotation period of Earth with respect to the Sun,
sidereal day
is defined in terms of the rotation period of Earth with respect to the stars.
Is a solar or sidereal day longer?
solar
A solar day is slightly longer than a sidereal day because:
Earth not only turns but also moves along its path around the Sun in a day. It has to do this because it moves with the respect to the stars and in order to move a little bit more each day it must move 1/364th more than on a normal day (about 4 min)
apparent solar time
as time reckoned by the actual position of the Sun in the sky
local meridian
(the great circle in the sky that passes through our zenith).
before midday (ante meridiem, or a.m.),
before the Sun reaches the local meridian.
p.m. (post meridiem),
after the Sun reaches the local meridian.
Although apparent solar time seems simple, it is not really very convenient to use. Why
use. The exact length of an apparent solar day varies slightly during the year. The eastward progress of the Sun in its annual journey around the sky is not uniform because the speed of Earth varies slightly in its elliptical orbit. Another complication is that Earth’s axis of rotation is not perpendicular to the plane of its revolution. Thus, apparent solar time does not advance at a uniform rate.
After the invention of mechanical clocks that run at a uniform rate, it became necessary to
abandon the apparent solar day as the fundamental unit of time.
mean solar time
is based on the average value of the solar day over the course of the year. A mean solar day contains exactly 24 hours and is what we use in our everyday timekeeping.
Although mean solar time has the advantage of progressing at a uniform rate, it is still inconvenient for practical use because:
it is determined by the position of the Sun. For example, noon occurs when the Sun is highest in the sky on the meridian (but not necessarily at the zenith). But because we live on a round Earth, the exact time of noon is different as you change your longitude by moving east or west.
If mean solar time were strictly observed:
people traveling east or west would have to reset their watches continually as the longitude changed, just to read the local mean time correctly.
Until near the end of the nineteenth century, every city and town in the United States kept its own local mean time. What changed this?
With the development of railroads and the telegraph, however, the need for some kind of standardization became evident
How do time zones work in the US
. In 1883, the United States was divided into four standard time zones (now six, including Hawaii and Alaska), each with one system of time within that zone.
time zones
Within each zone, all places keep the same standard time, with the local mean solar time of a standard line of longitude running more or less through the middle of each zone.
When do travelers reset their watches to account for a time zone change?
Now travelers reset their watches only when the time change has amounted to a full hour.
Daylight saving time
is simply the local standard time of the place plus 1 hour. It has been adopted for spring and summer use in most states in the United States, as well as in many countries, to prolong the sunlight into evening hours, on the apparent theory that it is easier to change the time by government action than it would be for individuals or businesses to adjust their own schedules to produce the same effect.
You pass into a new time zone, on the average, about every [degree of longitude]
15 degree of longitude
By the time you have completed your trip, you have set your watch ahead a full 24 hours and thus gained a day over those who stayed at home.
The solution to this dilemma is the
International Date Line,
International Date Line,
set by international agreement to run approximately along the 180° meridian of longitude. The date line runs down the middle of the Pacific Ocean, although it jogs a bit in a few places to avoid cutting through groups of islands and through Alaska (Figure 4.11). By convention, at the date line, the date of the calendar is changed by one day. Crossing the date line from west to east, thus advancing your time, you compensate by decreasing the date; crossing from east to west, you increase the date by one day. To maintain our planet on a rational system of timekeeping, we simply must accept that the date will differ in different cities at the same time.
There are two traditional functions of any calendar.
. First, it must keep track of time over the course of long spans, allowing people to anticipate the cycle of the seasons and to honor special religious or personal anniversaries. Second, to be useful to a large number of people, a calendar must use natural time intervals that everyone can agree on—those defined by the motions of Earth, the Moon, and sometimes even the planets.
The natural units of our calendar are the
day
The diffuculties have arrised on the calender because the three periods (day month year) are not commensurable; What does that mean?
one does not divide evenly into any of the others.
Mayan calander
Maya did not attempt to correlate their calendar accurately with the length of the year or lunar month. Rather, their calendar was a system for keeping track of the passage of days and for counting time far into the past or future. Among other purposes, it was useful for predicting astronomical events, such as the position of Venus in the sky
Chinese calandar
In addition to the motions of Earth and the Moon, they were able to fit in the approximately 12-year cycle of Jupiter, which was central to their system of astrology. The
Western calandar
derives from a long history of timekeeping beginning with the Sumerians, dating back to at least the second millennium BCE, and continuing with the Egyptians and the Greeks around the eighth century BCE. These calendars led, eventually, to the Julian calendar, introduced by Julius Caesar, which approximated the year at 365.25 days, fairly close to the actual value of 365.2422. The Romans achieved this approximation by declaring years to have 365 days each, with the exception of every fourth year.
flaw with the Julian calendar
represented a great advance, its average year still differed from the true year by about 11 minutes, an amount that accumulates over the centuries to an appreciable error.
Gregorian calendar reform consisted of two steps:
First, 10 days had to be dropped out of the calendar to bring the vernal equinox back to March 21; by proclamation, the day following October 4, 1582, became October 15. The second feature of the new Gregorian calendar was a change in the rule for leap year, making the average length of the year more closely approximate the tropical year.
phases of the moon
appearances), with the Moon starting dark and getting more and more illuminated by sunlight over the course of about two weeks.
Although we know that the Sun moves [what fraction] of its path around the sky each month,
1/12
As we watch the Moon from our vantage point on Earth, how much of its face we see illuminated by sunlight depends on the
angle the Sun makes with the Moon.
The Moon is said to be new
when it is in the same general direction in the sky as the Sun (position A).
After about one week, the Moon is [fraction] of the way around its orbit
one-quarter
During the week after the first quarter phase, do we see less and less or more and more of the Moon’s illuminated hemisphere
more and more
What is the moon when it is opposite the Sun in the sky.
full
when is the full moon highest in the sky and most noticeable?
midnight
Moon’s sidereal period
the period of its revolution about Earth measured with respect to the stars—
solar month,
The time interval in which the phases repeat—
The difference between a sidreal period and a solar month results from
Earth’s motion around the Sun. The Moon must make more than a complete turn around the moving Earth to get back to the same phase with respect to the Sun.
Does the moon rotate on it’s axis faster or slower than it revolves around the earth
same anout of time
The Moon rotates on its axis in exactly the same time that it takes to revolve about Earth. As a consequence,
the Moon always keeps the same face turned toward Earth (Figure 4.15).
synchronous rotation.
rotating on the axis at the same speed of a revolution
The differences in the Moon’s appearance from one night to the next are due to
changing illumination by the Sun, not to its own rotation.
Why does gravity have a stronger pull on some parts of the earth than others
. These forces differ slightly from one another because Earth is not a point, but has a certain size: all parts are not equally distant from the Moon, nor are they all in exactly the same direction from the Moon.
differential forces
As a result, the differences among the forces of the Moon’s attraction on different parts of Earth
what do differntial forces cause:
cause Earth to distort slightly. forces tend to stretch Earth slightly into an oblate spheroid
Moon’s gravitational effect if earth were all water
it would distort until the Moon’s differential forces over different parts of its surface came into balance with Earth’s own gravitational forces pulling it together.
why Earth does not distort enough to balance the Moon’s differential forces with its own gravity.
Because the tidal distortion of the solid Earth amounts—at its greatest—to only about 20 centimeters,
The Formation of Tides
The tide-raising forces, acting over a number of hours, produce motions of the water that result in measurable tidal bulges in the oceans. Water on the side of Earth facing the Moon flows toward it, with the greatest depths roughly at the point below the Moon.opposite the Moon, water also flows to produce a tidal bulge
Does the sun also produce tides on earth?
The Sun also produces tides on Earth, although it is less than half as effective as the Moon at tide raising.
Spring tides
When the Sun and Moon are lined up (at new moon or full moon), the tides produced reinforce each other and so are greater than normal
When the Moon is at first quarter or last quarter (at right angles to the Sun’s direction), how are the tides
direction), the tides produced by the Sun partially cancel the tides of the Moon, making them lower than usual. These are called neap tides.
“simple” theory of tides,
described in the preceding paragraphs, would be sufficient if Earth rotated very slowly and were completely surrounded by very deep oceans.
Which appears bigger, sun or moon
they appear about the same size
an eclipse occurs whenever
any part of either Earth or the Moon enters the shadow of the other.
The shadows of Earth and the Moon consist of two parts:
a cone where the shadow is darkest, called the umbra, and a lighter, more diffuse region of darkness called the penumbra
the most spectacular eclipses occur when an object enters the ______
umbra.
If the path of the Moon in the sky were identical to the path of the Sun (the ecliptic), we might expect to see an eclipse of the sun and moon how often?
each month
How often do we see an elipse
2 a year
Why do we only see an elipse 2 a year?
the Moon’s orbit is tilted relative to the plane of Earth’s orbit about the Sun by about 5°
“annular eclipse,”
Moon looks slightly smaller than the Sun and cannot cover it completely, even if the two are perfectly aligned.
if an eclipse of the Sun occurs when the Moon is somewhat nearer than its average distance, the
Moon can completely hide the Sun, producing a total solar eclipse.
total solar ellipse
eclipse where moon can completely hide the sun
If the moon and sun are properlyaligned what will happen with the moon’s shadow
. If the Sun and Moon are properly aligned, then the Moon’s darkest shadow intersects the ground at a small point on Earth’s surface.
Anyone on Earth within the small area covered by the tip of the Moon’s shadow will, for a few minutes, be unable to see the Sun and will witness a total eclipse.
The corona
the Sun’s outer atmosphere, consisting of sparse gases that extend for millions of miles in all directions from the apparent surface of the Sun.
A lunar eclipse occurs when
when the Moon enters the shadow of Earth.
Unlike a ______ eclipse, which is visible only in certain local areas on Earth, a ______ eclipse is visible to everyone who can see the Moon.
solar, lunar
An eclipse of the Moon is total only if
only if the Moon’s path carries it though Earth’s umbra. If the Moon does not enter the umbra completely, we have a partial eclipse of the Moon.
Which last longer, solar or lunar ecllipses and why?
because Earth is larger than the Moon, its umbra is larger, so that lunar eclipses last longer than solar eclipses,
A lunar eclipse can take place only when
the Sun, Earth, and Moon are in a line.
reversed. The total duration of the eclipse depends on
on how closely the Moon’s path approaches the axis of the shadow.
The sun can be seen at the zenith twice during the year here:
equator
North circumpolar stars are seen
North of the Equator
The sun can be seen at the zenith only once during the year when north of the equator.
tropic of cancer
The sun can be seen at the zenith only once during the year when south of the equator
Tropic of capricorn
The north celestial pole can be seen at the zenith
North Pole
All the stars rise and set
Equator
All northern stars are circumpolar
North Pole
Celestial poles are seen on the horizon
Equator
South celestial poles is seen at the zenith
South Pole
All southern stars are circumpolar
South Pole
The ecliptic (path of the sun) is directly overhead at local noon on the northern hemisphere’s summer solstice
Tropic of Cancer
The sun is at the zenith on the equinoxes
Tropic of Capricorn
The sun fails to rise above the horizon between the northern hemisphere’s spring and autumn equinoxes
South Pole
The Sun passes through the zenith and winter solstice
Tropic of Capricorn
Arctic Circle (66.5 N): Why is this a special lattitude
Sun never sets on
summer solstice.
Tropic of Cancer (23.5 N)
Sun directly
overhead at noon on
summer solstice
