🕮 These are the asteroids to worry about!

Chapter 3 Key Terms

 

angular momentum

the measure of the motion of a rotating object in terms of its speed and how widely the object’s mass is distributed around its axis

aphelion

the point in its orbit where a planet (or other orbiting object) is farthest from the Sun

apogee

the point in its orbit where an Earth satellite is farthest from Earth

asteroid belt

the region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located; the main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun

astronomical unit (AU)

the unit of length defined as the average distance between Earth and the Sun; this distance is about 1.5 × 108 kilometers

density

the ratio of the mass of an object to its volume

eccentricity

in an ellipse, the ratio of the distance between the foci to the major axis

ellipse

a closed curve for which the sum of the distances from any point on the ellipse to two points inside (called the foci) is always the same

escape speed

the speed a body must achieve to break away from the gravity of another body

focus

(plural: foci) one of two fixed points inside an ellipse from which the sum of the distances to any point on the ellipse is constant

gravity

the mutual attraction of material bodies or particles

Kepler’s first law

each planet moves around the Sun in an orbit that is an ellipse, with the Sun at one focus of the ellipse

Kepler’s second law

the straight line joining a planet and the Sun sweeps out equal areas in space in equal intervals of time

Kepler’s third law

the square of a planet’s orbital period is directly proportional to the cube of the semimajor axis of its orbit

major axis

the maximum diameter of an ellipse

mass

a measure of the amount of material within an object

momentum

the measure of the amount of motion of a body; the momentum of a body is the product of its mass and velocity; in the absence of an unbalanced force, momentum is conserved

Newton’s first law

every object will continue to be in a state of rest or move at a constant speed in a straight line unless it is compelled to change by an outside force

Newton’s second law

the change of motion of a body is proportional to and in the direction of the force acting on it

Newton’s third law

for every action there is an equal and opposite reaction (or: the mutual actions of two bodies upon each other are always equal and act in opposite directions)

orbit

the path of an object that is in revolution about another object or point

orbital period (P)

the time it takes an object to travel once around the Sun

orbital speed

the speed at which an object (usually a planet) orbits around the mass of another object; in the case of a planet, the speed at which each planet moves along its ellipse

perigee

the point in its orbit where an Earth satellite is closest to Earth

perihelion

the point in its orbit where a planet (or other orbiting object) is nearest to the Sun

perturbation

a small disturbing effect on the motion or orbit of a body produced by a third body

satellite

an object that revolves around a planet

semimajor axis

half of the major axis of a conic section, such as an ellipse

velocity

the speed and direction a body is moving—for example, 44 kilometers per second toward the north galactic pole

Chapter 2 Key Terms

 

accelerate

to change velocity; to speed up, slow down, or change direction.

apparent magnitude

a measure of how bright a star looks in the sky; the larger the number, the dimmer the star appears to us

astrology

the pseudoscience that deals with the supposed influences on human destiny of the configurations and locations in the sky of the Sun, Moon, and planets

celestial equator

a great circle on the celestial sphere 90° from the celestial poles; where the celestial sphere intersects the plane of Earth’s equator

celestial poles

points about which the celestial sphere appears to rotate; intersections of the celestial sphere with Earth’s polar axis

celestial sphere

the apparent sphere of the sky; a sphere of large radius centered on the observer; directions of objects in the sky can be denoted by their position on the celestial sphere

circumpolar zone

those portions of the celestial sphere near the celestial poles that are either always above or always below the horizon

cosmology

the study of the organization and evolution of the universe

ecliptic

the apparent annual path of the Sun on the celestial sphere

epicycle

the circular orbit of a body in the Ptolemaic system, the center of which revolves about another circle (the deferent)

geocentric

centered on Earth

heliocentric

centered on the Sun

horizon (astronomical)

a great circle on the celestial sphere 90° from the zenith; more popularly, the circle around us where the dome of the sky meets Earth

horoscope

a chart used by astrologers that shows the positions along the zodiac and in the sky of the Sun, Moon, and planets at some given instant and as seen from a particular place on Earth—usually corresponding to the time and place of a person’s birth

parallax

the apparent displacement of a nearby star that results from the motion of Earth around the Sun

planet

today, any of the larger objects revolving about the Sun or any similar objects that orbit other stars; in ancient times, any object that moved regularly among the fixed stars

precession (of Earth)

the slow, conical motion of Earth’s axis of rotation caused principally by the gravitational pull of the Moon and Sun on Earth’s equatorial bulge

retrograde motion

the apparent westward motion of a planet on the celestial sphere or with respect to the stars

year

the period of revolution of Earth around the Sun

zenith

the point on the celestial sphere opposite the direction of gravity; point directly above the observer

zodiac

a belt around the sky about 18° wide centered on the ecliptic

Sun

Due to the sun's extreme temperatures, these elements stay in a gas-like phase called plasma.
In the core, temperatures reaching at least 27 million degrees Fahrenheit, combined with the sun's powerful gravity, fuse together hydrogen molecules to create helium called thermonuclear fusion.
This releases an enormous amount of energy in the form of radiation, electricity, solar wind, and, as we experience on Earth, life giving heat and light.
Such volatility is contained thanks to the Sun's tremendous gravity, it's strong enough to hold the solar system intact and is primarily due to the Sun's size and mass

♞ Gravity is a “built-in” property of mass ⚠️

Gravity is a “built-in” property of mass. Whenever there are masses in the universe, they will interact via the force of gravitational attraction. The more mass there is, the greater the force of attraction. Here on Earth, the largest concentration of mass is, of course, the planet we stand on, and its pull dominates the gravitational interactions we experience. But everything with mass attracts everything else with mass anywhere in the universe.

Newton’s law also implies that gravity never becomes zero. It quickly gets weaker with distance, but it continues to act to some degree no matter how far away you get. The pull of the Sun is stronger at Mercury than at Pluto, but it can be felt far beyond Pluto, where astronomers have good evidence that it continuously makes enormous numbers of smaller icy bodies move around huge orbits. And the Sun’s gravitational pull joins with the pull of billions of others stars to create the gravitational pull of our Milky Way Galaxy. That force, in turn, can make other smaller galaxies orbit around the Milky Way, and so on.

Why is it then, you may ask, that the astronauts aboard the Space Shuttle appear to have no gravitational forces acting on them when we see images on television of the astronauts and objects floating in the spacecraft? After all, the astronauts in the shuttle are only a few hundred kilometers above the surface of Earth, which is not a significant distance compared to the size of Earth, so gravity is certainly not a great deal weaker that much farther away. The astronauts feel “weightless” (meaning that they don’t feel the gravitational force acting on them) for the same reason that passengers in an elevator whose cable has broken or in an airplane whose engines no longer work feel weightless: they are falling (Figure 3.9).2

Figure 3.9 Astronauts in Free Fall. While in space, astronauts are falling freely, so they experience “weightlessness.” Clockwise from top left: Tracy Caldwell Dyson (NASA), Naoko Yamazaki (JAXA), Dorothy Metcalf-Lindenburger (NASA), and Stephanie Wilson (NASA). (credit: NASA)

When falling, they are in free fall and accelerate at the same rate as everything around them, including their spacecraft or a camera with which they are taking photographs of Earth. When doing so, astronauts experience no additional forces and therefore feel “weightless.” Unlike the falling elevator passengers, however, the astronauts are falling around Earth, not to Earth; as a result they will continue to fall and are said to be “in orbit” around Earth (see the next section for more about orbits).

Ellipse

Kepler initially assumed that the orbits of planets were circles, but doing so did not allow him to find orbits that were consistent with Brahe’s observations. Working with the data for Mars, he eventually discovered that the orbit of that planet had the shape of a somewhat flattened circle, or ellipse.

General Astronomy 🔭 Angles