Chapter 1: Discovering the Night Sky (Textbook Notes)

diurnal motion (def)

(or daily motion) which causes all objects to circle around the celestial poles

Complete blocking of the sun (totality) during a Solar eclipse never lasts more than....

7 1/2 minutes at any location on the eclipse path

Eclipse Path (def)

As earth turns and the moon orbits, the tip traces am eclipse path across our planet

Total Eclipse of the Sun (describe)

During a total solar eclipse, the Moon completely covers the Sun's disk, and the solar corona can be photographed. This halo of hot gasses extends for millions of kilometers into space.

Explain 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, while people inside the penumbra see only a partial eclipse

When is Earth closest to the sun?

Earth is closest to the Sun on or around January 3 of each year.

How do you use the alt azimuth coordinate system?

First, find the North Pole star, Polaris. Facing the direction of Polaris = facing due north. Look straight at the horizon below Polaris. This is (0 degrees, 0 degrees) in the alt-azimuth system.

Annular Eclipse of the Sun (describe)

If what would otherwise be a total solar eclipse occurs when the Moon is relatively far from Earth, the Moon's umbra falls short of Earth and no one sees a total eclipse. From the Earth's surface, the Moon appears too small to cover the Sun completely, and a thin ring or "annulus" of light is seen around the edge of the Moon at mid-eclipse (never look directly!)

When is it okay to look at a solar eclipse?

It is only safe to look at a total solar eclipse without a filter during the brief time when the Moon completely blocks the Sun

How to find the direction North using the Big Dipper?

Locate the two stars farthest from the Big Dipper's handle (at times in the night, the Big Dipper appears to be upside down). These are called pointer stars and they point towards Polaris (aka North Star)

What are the three types of Lunar eclipses?

Penumbral eclipse, partial eclipse, total eclipse

Because Earth spins on its axis, every object on the celestial sphere is continously moving from our perspective -- what happens to their right ascension and declination? altitude and azimuth angles?

RA and Dec = constant ; altitude & azimuth angles = continually changing

What is the brightest star in the night sky?

Sirius

What are the conditions for a eclipse?

The Moon must be very nearly on the ecliptic at new Moon for a solar eclipse to occur. 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

Information to read over >>>

The constellations were chosen in 1922 by the American astronomer Henry Norris Russell and their boundaries were set in 1930 by the Belgian astronomer Eugene Delporte. This work was done under the auspices of the International Astronomical Union

winter solstice (def)

The day that the Sun rises farthest south of east is around December 22 - the farthest point on the ecliptic farthest south of the celestial equator - also the day when the Sun rises to the lowest height at noon and it signals the day of the year int he northern hemisphere with the fewest number of daylight hours

What are the two parts of Earth's shadow? What would happen if one was in each shadow (what would one see)?

Umbra - the part of the shadow where all direct sunlight is blocked by Earth. If you were in Earth's umbra, looking at Earth, you would not see the Sun behind it at all Penumbra - the part of the shadow where Earth blocks only some of the sunlight. If you were in Earth's penumbra looking at Earth, you would see a crescent sun behind it.

Partial Eclipse (describe)

When just part of the lunar surface passes through the umbra, a bite seems to be taken out of the Moon

Partial Eclipse of the Sun (describe)

When only the penumbra sweeps across Earth's surface, as happens in high latitude regions, the Sun is only covered by the Moon (never look directly!)

What causes lunar and solar eclipses?

When the Moon is crossing the ecliptic in the full or new phase, the shadows of the Earth or the Moon, respectively, then fall on the Moon or Earth. These shadows on the respective surfaces are eclipses.

Total Eclipse (describe)

When the Moon travels completely into the umbra (maximum duration lasting 1 hour to 47 minutes -- occur when the Moon is closest to Earth and is traveling directly through the center of the umbra -- safe to watch with naked eye)

Solar Corona (def)

a halo of hot gases during a total solar eclipse

precession (def)

a slow, circular motion of Earth's axis of rotation; the direction in which Earth's axis of rotation points on the celestial sphere changes -- same behavior as a spinning top

When the Moon completely covers the Sun, the result is...

a total solar eclipse

Earth spins on an ____ running between its .... ; It also moves in an ____ around the _____

axis ; north and south poles ; orbit ; sun

Why are different constellations visible at different times of the year?

because of Earth's revolution around the Sun

Why were leap years created?

because the Earth does not orbit the sun in precisely 365 days.

During a solar eclipse, the Sun seems to be... A solar eclipse occurs when...

blotted from the sky the Moon's shadow moves across Earth's surface. As seen from Earth, the Moon moves in front of the Sun-- a new Moon

Astronomers also use angular measure to describe the apparent sizes of ____ ____

celestial objects

Earth's rotation creates the ? cycle and its revolution defines a ?

day-night ; year

celestial equator (def)

divides the sky into northern and southern hemispheres. includes a north celestial pole as well as a south celestial pole

clocks and calendars are based on...

earth's rotation and revolution

far side vs dark side

far side = the side of the Moon facing away from Earth dark side = the side of the Moon facing away from Earth

At the north pole, stars move ? At the south pole, stars move ?

left to right ; right to left

What is the alt azimuth or alt azimuth coordinate system?

local coordinate system that often expedites navigating around the night sky; depends on your location and the time of night

angular measure (def)

method of describing the size of an angle

Based on the distance of the Sun and the Moon, their angular diameter as seen from Earth is...

nearly the same -- 1/2 degrees

Eclipses do not occur during every....

new or full Moon phase

asterism (def)

patterns of bright stars

The length of the Moon's umbra is nearly 5,000 km ____ than the average distance between the Moon and Earth's surfer. This means what?

shorter; the Moon's shadow often fails to reach Earth, making annular solar eclipses slightly more common than total solar eclipses

horizon (def)

the boundary between Earth and the sky.

sidereal period (def)

the length of any cycle of motion measured with respect to the stars

sidereal day (def)

the length of time from where any star is in one place in the sky until it is next in the same place

lunar phases (def)

the position of the moon relative to the sun

The Moon also has an analogous ...

umbra & penumbra

solar day (def) why would it change/vary?

upon which are 24 hour day is based - ideally this is the interval between when the Sun is highest in the sky on one day until the time it is highest in the sky the next day - this changes for multiple reasons: earths orbit around the sun is not perfectly circular + earth's rotation axis is 23 1/2 degrees from being perpendicular to the ecliptic

Penumbral Eclipse (describe)

when the Moon passes through only Earth's penumbral, it's easy to miss since the Moon still looks full, just a little dimmer than usual and sometimes slightly reddish in color

The angle at which the stars rise and set depend on ?

your viewing latitude

The ecliptic and celestial spheres are different circles, how? Why is this so?

they are tilted 23 1/2 degrees with respect to each other on the celestial sphere ; this occurs because Earth's rotation axis is tilted 23 1/2 degrees away from a line perpendicular to the ecliptic

How long does it take for the Moon to complete a full cycle of phases?

29 1/2 days

To talk about smaller angles, we subdivide the degree into .... ? and that is further subdivided into .... ?

arcminute or arcmin (i.e. 60') ' arcsecond or arcsec (i.e. 60'')

summer solstice (def)

around June 21 when the Sun rises farthest north or east and passes highest in the sky - the day of the year in the northern hemisphere with the most daylight

The 88 constellations that completely cover the sky are each formed around an ____ ; Which is why?

asterism ; they have the same names

During the northern hemisphere's winter months, when the northern hemisphere is tilted ___ from the Sun, the Sun rises in the ____. Daylight lasts for fewer than ___ hours, as the Sun skims low over the southern horizon and sets in the ____. Night is ___ in the northern hemisphere when the Sun is at the winter solstice.

away; southeast; 12; southwest; longest

The reference point (0 h) of right ascension is analogous to Earth's ?

prime meridian

The boundaries of the constellations all meet at ____ ____ and run along lines of...

right angles; constant right ascension and constant declination -- (because so far away, this does not change over our lifetime)

Any orbit measured with respect to the stars is called.... This includes ?

sidereal; including orbits of the planets around the Sun, as well as orbits of moon around their planets

celestial sphere (def)

sky map that has a coordinate system using right ascension (longitude-around) and declination (latitude-up&down)

circumpolar (def) (exm)

stars and constellations that never go below the horizon as seen from a given place on Earth -- exm as seen from the North Pole, no stars rise or set, they just seem to revolve around Polaris in horizontal circles -- same for the South Pole even there isn't a South Pole star equivalent to Polaris

altitude is the angle....

straight up from the horizon toward your zenith* - the point in the sky or celestial sphere directly above an observer.

a full circle is divided into? a right angle measures?

360 degrees ; 90 degrees

What is the angle between the two "pointer stars" in the Big Dipper?

5 degrees

The prime meridian is the ___ value of ___ on Earth ; what is this point in the celestial sphere called?

0 ; longitude ; zero of right ascension = defined to be at the vernal equinox

how is declination measured? right ascension?

0 degrees to 90 degrees north or south of the celestial equator ; measured in hours from 0 hr to 24 hours around the celestial equator

How many stars can the unaided human eye detect in the night sky? At any time, how many stars can be detected? Why?

6000 stars; roughly 3000 at one time because only half of the stars are above the horizon

What's the maximum number of eclipses (lunar plus solar) in a year?

7

How many constellations are in the night sky?

88 w/ diff sizes & shapes

What do astronomers define as constellations?

Astronomers sometimes use the common def of a constellation as a pattern of stars. Formally, a constellation is an entire area of the celestial sphere and all the stars and other objects in it. Astronomers refer to the patterns as asterisms.

Why do eclipses occur less often?

Because the Moon's orbit is tilted 5 degrees from the ecliptic

What eventually happens since the Earth points in a slightly different direction over time?

Causes the north celestial pole to drift away from Polaris

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.

Why is Polaris called the North Star?

It is located almost directly over Earth's North Pole. (There is no star equivalent over the South Pole -- and if you face Polaris, to your right will be East, left West, and behind you South)

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.

If you live in the northern hemisphere, where is Polaris located? What counts as the circumpolar stars in the northern hemisphere?

Polaris is always located above your northern horizon at an angle equal to your latitude ; only the stars and constellations that pass bw/ Polaris and the land directly below it are circumpolar

ecliptic (def)

TWO MEANINGS: the annual path of the Sun on the celestial sphere ; also the plane described by Earth's path around the Sun. The plane created by the two ecliptics exactly coincide. (The two ecliptics are the same)

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

How many zodiac constellations are there?

There are 13 zodiac constellations, the least one known being Ophiuchus.

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 oval shape of Earth's orbit.

constellation (def)

an entire region of the sky and everything in that region

Because the Moon's orbit is tilted 5 degrees from the ecliptic, the Moon is usually located ....

above or below the plane of our orbit around the Sun

how do the stars look as seen from the equator?

all the stars appear to rise straight up in the eastern sky and set straight down in the western sky -- Polaris is barely visible on the northern horizon and doesn't set but all the other stars do which means that none of the stars are circumpolar as seen from the equator

equinox meaning ?

comes from the Latin words meaning "equal night" because when the Sun appears at either of these two points, it is directly over Earth's equator, resulting in 12 hours of daytime and 12 hours of nightime everywhere on Earth on that day

During a lunar eclipse, the brilliant full Moon often... A lunar eclipse occurs when...

darkens to a rusty red the Moon passes through Earth's shadow (this can only happen when the Sun, Earth, & Moon are in a straight line)

Earth's rotation causes the stars - as well as the Sun, the Moon and the planets - to appear to rise along the .... move across the sky and set ...

eastern horizon ; western horizon

How much does the Sun illuminate the Moon?

half of the Moon at all times

The Moon's phase that we see depends on...

how much of its sunlit hemisphere is facing Earth

revolution (def) (exm with Earth)

is the motion of any astronomical object around another astronomical object -- Earth takes 1 year or about 365 and 1/4 days to revolve around the sun (revolution) & a year on Earth is measured by the motion of our planet relative to the stars

What does it mean when the Earth's axis of rotation changes slightly with respect to the celestial sphere??

it "points" in a slightly different direction over time

The time of day or night that the Moon is up in the sky depends on...

its phase

What is the Moon's role, in regards to Earth?

keeps it stabilized & maintains the seasons; without the moon, the Earth would not keep a 23 1/2 degree tilt, but rather would change its angle to the ecliptic

To summarize, the Sun is ____ in the northern sky on the winter solstice. This marks the beginning of ____ in the northern hemisphere. As the Sun moves northward, the amount of daylight and heat deposited ____ daily. The vernal equinox marks a _____ in the amount of light and heat from the Sun onto the northern hemisphere and is beginning of ____. When the Sun reaches the ____ ____, it is highest in the northern sky and is above the ____ for the most hours of any day of the year. This is beginning of the ____. Returning southward, the Sun crosses the ____ ____ once again on the autumnal equinox, the beginning of ____.

lowest; winter; increases; midpoint; Spring; summer solstice; horizon; summer; celestial equator; fall

During the northern hemisphere's summer months, when the northern hemisphere is tilted toward the Sun, the Sun rises in the ____ and sets in the ___. The sun provides more than ___ hours of daylight in the northern hemisphere and passes high in the sky. At the summer solstice, the sun is as far ___ as it gets, giving the greatest number of daylight hours to the northern hemisphere.

northeast; northwest; 12; north

arc angle (def)

often called an angle ; the opening between two lines that meet at a point

In navigating around the celestial sphere, astronomers measure the relative position of two objects by...

the angle between them

To locate the stars on the celestial sphere, what must we know?

the angle from one star to another in the sky, a property that stays fixed over our lifetimes because the stars are all so far away (knowing their distances from earth is not neccessary)

angular diameter / angular size (def)

the apparent diameter of a planet or other celestial object measured by the angle that it subtends at the point of observation.

mean (or average) solar day (def)

the average time interval between consecutive noontimes throughout the year is 24 hours, which also determines the time we use on our clocks

terminator (def)

the boundary between the bright and dark regions on the Moon

The phases of the Moon originally inspired....

the concept of the month

zodiac constellations (def)

the constellations through which the Sun moves throughout the year as it travels along the ecliptic (only 12 in pic!)

what is the basic unit of angular measure?

the degree

The temperature and, hence, the seasons are determined by ....

the duration of daylight at any place and the height of the Sun in the sky there (winds + clouds greatly affect the weather as well but right here we ignore these effects)

autumnal equinox (def)

the equinox in autumn, on about September 22 in the northern hemisphere and March 20 in the southern hemisphere.

vernal equinox (def)

the equinox in spring, on about March 20 in the northern hemisphere and September 22 in the southern hemisphere

What factors may vary the length of the sidereal and lunar month?

the gravitational pull of the Sun on Earth changes Earth's orbital speed throughout the year and the gravitational pull of the Sun on the Moon affects the Moon's speed as it orbits Earth

precession of the equinoxes (def)

the intersections of the celestial equator and the ecliptic shift lowly in the sky year to year

What happens when you go farther north in the southern hemisphere? & farther south in the northern hemisphere?

the number of stars and constellations that are circumpolar decrease for both!

tropical year (def)

the time interval from one vernal equinox to the next

lunar month (def)

the time it takes the Moon to complete one 29 1/2 day cycle of phases

sidereal month (def)

the time it takes the Moon to complete one full orbit of 360 degrees around Earth (how long it takes for the Moon to start one place on the celestial sphere and end up in that same place (27.3 days)

gravitation (gravity) (def)

the universal force of attraction between all matter

how many zodiac constellations are there? why/how?

traditionally there were 12 but in 1930, the boundaries of the constellations were redefined by astronomers - as a result, the Sun now moves through 13 constellations throughout the year

waxing vs waning

waxing = the bright side of the Moon is on the right (west) side of the Waxing Moon waning = the bright side of the Moon on the left (east) side of the Waning Moon

As the Moon orbits the Earth, it moves from ... ?

west to eat (right to left)

An eclipse takes place only...

when the Moon crosses the plane of the ecliptic during its new or full phase