QUARKS 2 COSMOS CH 1

Fall 2024

Universe

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

Astronomy

The study of the universe

Light-year

The distance that light, moving at a constant speed of 300,000 km/s, travels in one year. One light-year is about 10 trillion kilometers.

Theory

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

Theoretical model

An attempt to construct a mathematical explanation of a physical process or phenomenon, within the assumptions and confines of a given theory. In addition to providing an explanation of the observed facts, the model generally makes new predictions that can be tested by further observation or experimentation.

Scientific method

The set of rules used to guide science, based on the idea that scientific “laws” be continually tested, and modified or replaced if found inadequate

4 important defining characteristics of modern scientific theories

1. Must be testable

   • Must admit the possibility that their underlying assumptions and their predictions can be, in principle, exposed to experimental verification

2. Must be continually tested, and their consequences tested too

3. Should be simple

   • A good theory should be no more complex than is absolutely necessary

4. Should in some sense be elegant

   • It’s widely regarded as a strong point in favor of a new theory when that theory’s clearly stated simple principle naturally ties together and explains several different phenomena previously thought to be completely distinct

Constellation

A human grouping of stars in the night sky into a recognizable pattern

Celestial sphere

Imaginary sphere surrounding Earth to which all objects in the sky were once considered to be attached - still used as a fictitious model to help us visualize the positions and motions of stars in the sky

Rotation

Spinning motion of a body about an axis

Celestial pole

Projection of Earth’s North or South pole onto the celestial sphere

Celestial equator

The projection of Earth’s equator onto the celestial sphere

Solar day

The period of time between the instant when the Sun is directly overhead (noon) to the next time it is directly overhead

Diurnal motion

The daily progress of the Sun and the other stars across the sky

Sidereal day

The time needed between successive risings of a given star / a day measured by the stars

Revolution

Orbital motion of one body about another, such as Earth about the Sun

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

Seasons

Changes in average temperature and length of day that result from the tilt of Earth’s (or any planet’s) axis with respect to the plane of its orbit

Autumnal equinox

Date on which the Sun crosses the celestial equator moving southward, occurring on or near September 21

Vernal equinox

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

Precession

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

Sidereal year

The 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; Earth’s orbital period around the Sun is one sidereal year

Phases

Appearance of the sunlit face of the moon at different points along its orbit, as seen from Earth

Sidereal month

Time required for the Moon to complete on 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 lunar 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 Sun’s light

Umbra

Central region of the shadow cast by an eclipsing body; the central region of a sunspot, which is its darkest and coolest spot

Penumbra

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

Annular eclipse

Solar eclipse occuring at a time when the Moon is far enough away from Earth that it fails to cover the disk of the Su completely, leaving a ring of sunlight visible around its edge

Eclipse season

Time of the year when the Moon lies in the same plane as Earth and Sun, so that eclipses are possible

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 object’s distance.

Cosmic distance scale

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

Baseline

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

Parallax

The apparent motion of a relatively close object with respect to a more distant background as the location of the observer changes