astronomy: chapter 0

idek anymore man.

The celestial sphere is best described as:

An imaginary sphere used to map the sky

The apparent daily motion of stars across the sky is caused by:

Earth’s rotation on its axis

Stars that never rise or set are called

Circumpolar stars

The point in the sky directly overhead an observer is called the

zenith

Which coordinate system depends on the observer’s location and time?

horizon coordinate system

Altitude is measured

above the horizon

Azimuth is measured

along the horizon

Right Ascension is most similar to

longitude

Declination is most similar to

latitude

Right Ascension is measured in

hours

What is the celestial equator?

The projection of Earth’s equator onto the celestial sphere

Why does the horizon coordinate system change throughout the night?

Because Earth rotates and the system depends on the observer’s position

What is a constellation in modern astronomy?

An officially defined region of the sky

Why are constellations not true physical groupings of stars?

The stars only appear close together but are far apart in space

What is the ecliptic?

The apparent yearly path of the Sun across the sky

Explain why planets are always found near the ecliptic.

Planets orbit in nearly the same plane as Earth

What is precession, and how does it affect the position of the North Star over time?

A slow wobble of Earth’s axis that changes the celestial poles over time

Describe the motion of circumpolar stars as seen from Earth.

They move in circles around the celestial pole and never set

Explain one advantage of using the equatorial coordinate system instead of the horizon system.

It does not change with time or location

How did early sky charting help civilizations on Earth?

Navigation, calendars, timekeeping, and cultural purposes

planet

one of eight major bodies that orbit the sun, visible to us by reflected sunlight

star

a glowing ball of gas held together by its own gravity and powered by nuclear fusion in its core

galaxy

gravitationally bound collection of a large number of stars. the sun is a star in the milky way galaxy

universe

the totality of all space, time, matter, and energy

astronomy

branch of science dedicated to the study of everything in the universe that lies above earth’s atmosphere

constellations

a human grouping of stars in the night sky into recognizable patterns

celestial sphere

imaginary sphere surrounding earth to which all objects in the sky were once considered to be attached

rotation

spinning motion of a body about an axis

north celestial pole

point on the celestial sphere directly above earth’s north pole

south celestial pole

point on the celestial sphere directly above earth’s south pole

celestial equator

the projection of earth’s equator onto the celestial sphere

celestial coordinates

pair of quantities—right ascension and declination—similar to longitude and latitude on earth, used to pinpoint locations of objects on the celestial sphere

declination

celestial coordinate used to measure latitude above or blow the celestial equator on the celestial sphere

right ascension

celestial coordinates used to measure longitude on the celestial sphere. the zero point is the position of the sun at the vernal equinox

solar day

the period of time between the instant when the sun is directly overhead (i.e., noon) to the next time it is directly overhead

diurnal motion

apparent daily motion of the stars caused by earths rotation

sidereal day

the time needed between successive risings of a given star

revolving

when an object orbits around another (the earth around the sun)

ecliptic

apparent path of the sun, relative to the stars on the celestial sphere, over the course of a year

zodiac

the 12 constellations on the celestial sphere through which the sun appears to pass during the course of a year

summer solstice

point on the ecliptic where the sun is at its northernmost point above the celestial equator, occurring on or near june 21

winter solstice

point on the ecliptic where the sun is at its southernmost point below the celestial equator, occurring on or near december 21

seasons

changes in average temperature and length of day that result from the tilt of earths (or any planets) axis with respect to the plan of its orbit

equinox

earths rotation axis is perpendicular to the line joining earth to the sun

autumnal equinox

date on which the sun crosses the celestial equator moving southward, occurring on or near september 22

vernal equniox

date on which the sun crosses the celestial equator moving northward, occurring on or near march 21

tropical year

the time interval between one vernal equinox and the next

sidereal year

time required for the constellations to complete one cycle around the sky and return to their starting points, as seen from a given point on earth. earths orbital period around the sun is 1 sidereal year

precession

slow change in the direction of the rotation axis of a spinning object, caused by some external gravitational influence

phases

appearance of the sunlit face of the moon at different points along its orbit, as seen from earth

new moon

phase of the moon during which none of the lunar disk is visible

quarter moon

lunar phase in which the moon appears as a half disk

sidereal month

time required for the moon to complete one trip around the celestial sphere

synodic month

time required for the moon to complete a full cycle of phases

eclipse

event during which one body passes in front of another, so that the light from the occulted body is blocked

lunar eclipse

celestial event during which the moon passes through the shadow of earth, temporarily darkening its surface

partial eclipse

celestial event during which only a part of the occulted body is blocked from view

total eclipse

celestial event during which one body is completely blocked from view by another

solar eclipse

celestial event during which the new moon passes directly between earth and the sun, temporarily blocking the suns light

umbra

central region of the shadow cast by an eclipsing body. the central region of sunspot, which is its darkest and coolest part

penumbra

portion of the shadow cast by an eclipsing object in which the eclipse is seen as partial. the outer region of sunspot, surrounding the umbra, which is not as dark and not as cool as the central region

annular eclipse

solar eclipse occurring at a time when the moon is far enough away from earth that it fails to cover the disk of the sun completely, leaving a ring of sunlight visible around its edge

triangulation

method of determining distance based on the principles of geometry. a distant object is sighted from two well-separated locations. the distance between the two locations and the angle between the line joining them and the line to the distant object are all that are necessary to ascertain the objects distance

cosmic distance scale

collection of indirect distance measurement techniques that astronomers use to measure distances in the universe

baseline

distance between two observing locations used for the purposes of triangulation measurements. the larger the baseline, the better the resolution attainable

science

step by step process for investigating the physical world based on natural laws and observed phenomena

theory

framework of ideas and assumptions used to explain some set of observations and make predictions about the real world

Our Galaxy is about 1 million times larger than Earth. (T/F)

false

The stars in a constellation are physically close to one another. (T/F)

false

The solar day is longer than the sidereal day. (T/F)

true

The seasons are caused by the precession of Earth’s axis. (T/F)

false

A lunar eclipse can occur only during the full phase. (T/F)

true

The angular diameter of an object is inversely proportional to its distance from the observer. (T/F)

true

If we know the distance of an object from Earth, we can determine the object’s size by measuring its parallax. (T/F)

false

If Earth rotated twice as fast as it currently does, but its motion around the Sun stayed the same, then

the night would be half as long

A long, thin cloud that stretched from directly overhead to the western horizon would have an angular size of

90 degrees

When a thin crescent of the Moon is visible just before sunrise, the Moon is in its

waning phase

If the Moon’s orbit were a little larger, solar eclipses would be

more likely to be annular

If the Moon orbited Earth twice as fast, but in the same orbit, the frequency of solar eclipses would

stay the same

According to Figure 0.8 (The Zodiac), in January the Sun is in the constellation

aquarius

In Figure 0.19 (Triangulation), using a longer baseline would result in

a more accurate distance to the tree

In Figure 0.20 (Parallax), a smaller Earth would result in

a smaller parralax angle

The vernal equinox is now just entering the constellation Aquarius. In what constellation will it lie in the year a.d. 10,000?

scorpius

Given that Earth orbits at 150,000,000 km from the Sun, through what distance does Earth move in a second? An hour? A day?

29.9 km, 1.08 × 105 km, 2.58 × 106 km

How, and by roughly how much, would the length of the solar day change if Earth’s rotation were suddenly to reverse direction?

it would decrease by 8 minutes

How long does it take for the Moon to move a distance equal to its own diameter of angular 0.5°?

(a) 33 arc minutes, (b) 33 arc seconds, (c) 0.55 arc seconds; 56.4 minutes

Given that the distance to the Moon is 384,000 km, and taking the Moon’s orbit around Earth to be circular, estimate the speed (in kilometers per second) at which the Moon orbits Earth.

1 km/s

Use reasoning similar to that illustrated in Figure 0.7 to verify that the length of the synodic month (the time from one full Moon to the next; Section 0.3) is 29.5 days.

it is!

The baseline in Figure 0.19 is 100 m and the angle at B is 60°. By constructing the triangle on a piece of graph paper, determine the distance from A to the tree.

173 m

Use reasoning similar to that in Discovery 0-1 (but now using a circle centered on the object and containing the baseline) to determine the distance to an object if its parallax, as measured from either end of a 1000-km baseline, is

(a) 57,300 km, (b) 3.44 × 106 km, (c) 2.06 × 108 km

What would the measured angle in Discovery 0-1 have been if Earth’s circumference were 100,000 km instead of 40,000 km?

2.9 degrees