Astronomy Chapter 2

The north celestial pole is ______ away from the celestial equator.

90 degrees

Stars located close to the north celestial pole never dip below the horizon.

:D

Because Earth is much larger than the moon, Earth's shadow at the distance of the Moon is over 2 times the diameter of the Moon.

A total lunar eclipse lasts as long as 1 hour and 40 minutes. A total lunar eclipse is often called a blood-red Moon in literature and poetry.

During the summer, earth's surface at your location is more nearly face-on in the incoming sunlight, so more energy falls on each square meter of ground ash second.

During the winter, the surface of earth where you are is more tilted with respect to the sunlight, so less energy falls on each square meter of the ground each second. This is the main reason why it is hotter in the summer and colder in the winter.

Everywhere on Earth, all the time, half of the sky is...

"...below the horizon."

Kepler's second law of planetary motion:

(also called Kepler's law of equal areas) states that the area swept out by a planet in the same amount of time is always the same, regardless of the location of the planet in its orbit.

A planet moves fastest when...

... it is closest to the sun. (and slowest when it is farthest from the sun)

The more elongated an ellipse becomes, the close it is eccentrically gets to ___.

1

Earth rotates about ____________ at the equator.

1,670 kilometers per hour (km/h)

Earth's average distance from the sun is _______________.

1.50x10^8 km This distance is called an astronomer's unit. (AU) and is used for measuring distances in the Solar System.

The moon takes just under _______ to move through one full set of phases, from full moon to full moon.

30 days

Directly above earth's north pole is its north celestial pole.

Above its South Pole is its south celestial pole. Above the earth's equator is its celestial equator.

A ___________ occurs when the moon is slightly father away from the Earth in its noncircular orbit, so it appears slightly smaller in the sky. It is centered over the disk in the sun but does not block the entire disk. A ring is visible the clocked portion.

Annular solar eclipse.

As seen from Earth's surface, the path each celestial body makes across the sky is called its ___________________.

Apparent daily motion

At the north pole, our horizon makes a 90 degree angle with the north celestial pole at our zenith.

As we move south our horizon tilts and our zenith moves away from the north celestial pole.

What does revolve mean?

Astronomers distinguish between two terms, using rotate to mean that an object spins around an axis through its center, and revolve to mean that one object orbits another. Earth rotates about its axis and revolves around the sun.

At a latitude of 45 degrees south, the south celestial pole has an altitude of 45 degrees.

At the south pole, latitude 90 degrees south) the doth celestial pole is at the zenith, 90 degrees above the horizon.

The celestial sphere is useful because it is easy to draw and visualize.

But always remember that it is imaginary. Each point in the celestial sphere indicates a direction in space.

The constellations that lie along the ecliptic are called the ________________________.

Constellations of the zodiac.

Who came up with the idea of a heliocentric solar system?

Copernicus

As viewed from above Earth's North Pole, Earth rotates ______________.

Counterclockwise, As the rotating Earth carries us from west to east, objects in the sky appear to move in the other direction, from eat to west.

There are two motions associated with Earth and its axis.

Earth spins on its axis, but its axis also wobbles like the axis of a spinning top. The wobble is very slow, taking about 26,000 years to complete one cycle.

___________________ sets the very rhythm of life on Earth-- the passage of day and night.

Earth's rotation on its axis

A star on the celestial equator rises due ______ and sets due ________.

East, west

The shape of an ellipse is determined by its ____________.

Eccentricity.

If we correspondingly note the position of the Sun relative to the stars each day for a year. We find that it traces out a path against the background of the stars called ____________.

Ecliptic.

The view from the south pole is much the same as the north pole but with two major differences:

Fist the south pole is on the opposite side of the earth from the north pole so the visible half of the sky at the south pole is precisely the half that is hidden from the north pole. The second difference is that indent of appearing to move counterclockwise around the sky, stars appear to move clockwise around the south celestial pole.

The highest point is almost never the zenith. You have to be in a special place on a special day for the Sun to be drecly over your head at noon.

For example, at 23.5 egress north latitude on June 21. "Local Midnight" occurs when the Sun again crosses the meridian on the other side of Earth.

Earth's north pole points in the same direction hour after hour and day after day.

For this reason, the objects visible there follow circular paths that always have the same altitude or angle above the horizon.

Today's calendar is the ___________ which is based on the ____________.

Gregorian Calendar, tropical year

In the northern hemisphere, the north star, or Polaris, happens to be located close to the north celestial pole.

If you can find Polaris in the sky and measure its altitude, then you known your latitude. If you are in Phoenix, Arizona, for example, the north celestial pole has an altitude of 33.5 degrees. (longitude is more difficult to calculate because of earth's rotation)

As seen from the Northern Hemisphere, stars north of the celestial equator remain above the horizon for more than 12 hours a day. The farther north the star is, the longer it stays up. Circumplar stars are the extreme example of this phenomenon; they are up 24 hours a day.

In contrast stars south of the celestial equator are above the horizon for less than 12 hours a day. The farther south a star is, the less time it stays up. Stars located close to the south celestial pole never rise above the horizon. in the northern hemisphere. If you are in the southern hemisphere, the same phenomenon holds with the directions reversed.

Since we do have seasons, we can conclude that Earth's axis of rotation is not exactly perpendicular to the plane of the ecliptic.

In fact, it is tilted 23.5 degrees from the perpendicular.

In 1953 Copernicus proposed a heliocentric model that explained retrograde motion much more simply than the geocentric model did.

In the Copernican model, the outer planets move in apparent retrograde motion when Earth overtakes them in their orbits. Retrograde motion is only apparent, not real.

Kepler used Tycho's data to deduce three rules that elegantly and accurately describe the motions of the planets. These three rules are now generally referred to as ____________.

Kepler's Laws

Kepler discovered a mathematical relationship between the period of a planet's orbit an dits average distance from the Sun.

Kepler's third law states that period squared is equal to distance cubed.

_______ are used in our calendar to make up for most of the extra fraction of the day.

Leap years

Wherever you are, you can divide raw sky into an east half and a west half with an imaginary both and south line called the _______.

Meridian

Lengthy explanation of the phases of the moon and its illustrations.

Pages 38-39

If earth's shadow incompletely covered the Moon, some of the disk of the Moon remains bright and part of it is in shadow. This is called a _______________.

Partial lunar eclipse

A ____________ occurs when the Moon partial covers the disk of the sun.

Partial solar eclipse

What does altitude mean?

Refers to the angle formed between the imaginary line from an observer to an object and a second line from the observer to the point on the horizon directly below the object.

Earth's North Pole is tilted toward the sun, the sun is north of the celestial equator.

Six months later, when Earth's North Pole is tilted away from the Sun, the Sun is south of the celestial equator.

A _________ occurs when earth moves through the shadow of the moon.

Solar eclipse

On the equator, days and nights are 12 hours long throughout the year. The Sun passes directly overhead on the fist day of spring and the first day of autumn because these are the days when the Sun is on the celestial equator.

Sunlight is most direct at the equator on these days. On the Summer solstice, the Sun is at its northernmost point along the ecliptic. It is on this day and on the winter solstice that the Sun is farthest from the Zeinith at noon, and therefore sunlight is least direct.

Ancient astronomers know that planets would occasionally exhibit apparent retrograde motion.

That is they would seem to turn around , move westwards for a while, and then return to their normal eastward travel.

The differing length of the night through the year is part of the explanation for seasonal temperature changes, but it is not the while story. There is another important effect:

The Sun appears higher in the sky during the summer than it does during the winter, so sunlight strikes the ground more directly during the summer than during the winter.

As the earth orbits the Sun, the Sun moves along the ecliptic, which is tilted 23.5 degrees with respect to the celestial equator.

The day when the Sun is most directly in the direction of the North Pole's tilt is called the Summer Solstice.

What is a horizon?

The line that you would see if you held your eyes perfect level and turned all the way around. A line to the horizon always makes a right angle with a line to the zenith.

Kepler's First Law of Planetary Motion:

The orbit of a planet is an ellipse with the Sun at one focus. An ellipse is a specific kind of oval whose shape is determined by two foci.

If you live between the latitudes of 23.5 south and 23.5 north for example--

The sun will be directly overhead at noon twice during the year. The band between these two latitudes is called the Tropics. The northern limit of this region is called the Tropic of Cancer. the sonthern limit is called the tropic of capricorn.

What is a day?

The time during which the Sun is up in the sky and the length of time it takes Earth to rotate once (from midnight to midnight).

The autumnal equinox occurs in the fall, around September 23, halfway between summer solstice and winter solstice.

The vernal equinox occurs in the spring around March 21, between winter solstice and summer solstice.

A tropical year is 365.242189 days long.

The year has an extra almost .25 day.

Between the summer and winter solstice, there are days when the Sun lies fiercely above Earth's equator, so that the entire Earth experiences 12 hours of daylight and 12 hours of darkness.

These are called the equinoxes (equinox means "equal night")

The northern hemisphere, a star north of the celestial equator can be seen above the horizon for more than half the time. This is also true for the sun. So the northern hemisphere's night is shorter than 12 hours.

These are the long days of the northern summer. Six months later on December 22, the Sun is most nearly opposite to the direction of Earth's North Pole tilt. SO the Sun appears in the sky south of the celestial equator. Night will be longer than 12 hours; it is winter in the north.

There are three types of solar eclipse:

Total, partial, and annular

At latitudes north of 66.5 degrees north, (the Arctic Circle) and south of 66.5 degrees south (the Antarctic Circle), the sun is circumpolar for part of the year.

When the sun is circumpolar it is above the horizon 24 hours a day, earning the polar regions the nickname of "land of the midnight sun."

The meridian runs from due north through a point directly overhead, called the ________ to a point due south. It then continues around the far side of the celestial sphere through the _______ (the point directly below you), and back to the starting point due north.

Zenith, nadir

The Latitude angle is equal to the altitude of the north celestial pole. Stars within this angle from the north celestial pole never dip below the horizon. These stars are called ________________ stars.

circumpolar, Another group of stars, close to the south celestial pole, never rise above the horizon l and can never be seen from this latitude. And stars between this region can be seen fro only part of each day. Stars in this intermediate region appear to rise above and set below Earth's shifting horizon as Earth turns.

The only place on Earth where you can see the entire sky over the course of 24 hours is the __________,

equator

ou can see any object that lies along the celestial equator _________.

half of the time (12 hours)

The north celestial pole is not a location in space--

instead it is a direction in space-- the direction parallel to Earth's axis of rotation.

As earth ______ around the sun, our view of the night sky changes.

orbits

There is a diagram of the eclipses.

p.41

Half of the length of the long axis is called the ________ of an orbit, which the distance between that planet and the sun.

semimajor axis.

A ________ never lasts longer then 7 1/2 minutes.

solar eclipse

The _______ occurs each year around June 21, the first day of summer in the Northern Hemisphere.

summer solstice, Six months later, the Sun is most directly opposite to the direction of the North Pole's tilt. This day is the winter solstice. This occurs each year around December 22. The shortest day of the year and the fist day of winter in the Northern Hemisphere.

The moon ratites on its axis once per revolution around the earth, always keeping the same face toward Earth. This phenomenon is called ____________ because the rev lotion and the ration are synchronized (or in sync) with each other. The moon is elongated, which causes its near side always to fall toward the Earth.

synchronous rotation

Copernicus heliocentric theory was the beginning of the Copernician Revolution. Through the work of scientists such as Tycho Brahe, Galileo Galilei, Johannes Kepler, and Sir Isaac Newton...

the heliocentric theory of the Solar System has become one of the most well-corroborated theories in all of science.

Durring each 26,000 year wobble of Earth's axis, the locations of the equinoxes make one complete circuit around the celestial equator. Together, these shifts in position are called ____________________.

the precessions of the equinoxes.

A ___________ occurs when the Moon completely blocks the disk of the Sun.

total solar eclipse

The ______________ measures the time from one vernal equinox to the next-- from the start of the northern hemisphere spring to the start of the next northern hemisphere spring.

tropical year

About _______ per _______ the orbital planes line up and eclipses can occur.

twice, year