Astronomy test

Ecliptic

The apparent path of the Sun across the sky; the plane in which the planets orbit the Sun.

Miles vs. Kilometers vs. AU vs. Light-Years

Miles are generally not used for space distances. Kilometers are used for distances on a planet or to its moon. AU is used for distances within the solar system. Light-years are used for distances outside the solar system.

Astronomical Unit (AU)

The average distance between Earth and the Sun; used to measure distances within the solar system.

Light-Year

A unit of distance used primarily for objects outside our solar system; the distance light travels in one year.

Bayer Designation

A system that names stars using a Greek letter followed by the constellation's Latin name, such as Alpha Leonis.

Alpha

The first Greek letter; commonly used as the designation for the brightest star in a constellation.

Beta

The second Greek letter; used as a star designation after Alpha.

Gamma

The third Greek letter; used as a star designation after Beta.

Delta

The fourth Greek letter; used as a star designation after Gamma.

Alpha Leonis

An example of a Bayer designation: Alpha (Greek letter) + Leonis (Latin form of Leo).

Constellation

One of 88 officially recognized regions of the sky.

Asterism

A recognizable pattern or group of stars that is not an official constellation and can cross constellation boundaries.

Constellations vs. Asterisms

There are 88 official constellations, while there are many asterisms. Asterisms can cross multiple constellations.

Stars in the Same Constellation

Stars in the same constellation are not necessarily physically close to each other or related; they only appear in the same region of the sky from Earth.

Celestial Object Motion

Objects in the sky appear to move primarily from east to west because Earth rotates from west to east.

Sunrise

The Sun appears to rise in the east.

Sunset

The Sun appears to set in the west.

Polaris

Also called the North Star; it lies very close to the direction of Earth's north celestial pole.

Finding Polaris

Look toward the north and find the Big Dipper; use the two outer stars of its bowl to point toward Polaris.

Why Polaris Appears at Different Heights

Polaris's altitude above the horizon depends on your latitude. It appears higher in the sky as you move farther north.

Magnitude Scale

A scale used to describe the apparent brightness of stars and other celestial objects.

Magnitude Scale: Brighter vs. Dimmer

Smaller magnitude numbers indicate brighter objects. Larger magnitude numbers indicate dimmer objects.

Negative Magnitude

Negative magnitudes represent extremely bright objects.

Telescope and Magnitude

The dimmer an object is, the larger the telescope or more light-gathering ability needed to observe it.

Cause of Earth's Seasons

Earth's seasons are caused primarily by the 23.5° tilt of Earth's rotational axis as Earth revolves around the Sun.

Earth's Axis Tilt

Earth's axis is tilted about 23.5°, causing different parts of Earth to receive different amounts and angles of sunlight throughout the year.

Summer

Occurs when a hemisphere is tilted toward the Sun, giving it more direct sunlight and longer days.

Winter

Occurs when a hemisphere is tilted away from the Sun, giving it less direct sunlight and shorter days.

Seasonal Variation by Location

Seasonal changes become more extreme as you move farther from the equator toward the poles.

Rotation

The spinning of an object around its own axis. Earth takes about 24 hours to complete one rotation.

Revolution

The motion of one object orbiting another. Earth takes about 365.25 days to complete one revolution around the Sun.

Precession

The slow wobble of Earth's rotational axis, which changes the direction Earth's axis points over thousands of years.

Lunar Cycle

The repeating cycle of the Moon's phases, lasting about 29.5 days from one new moon to the next.

New Moon

The Moon is between Earth and the Sun; the side facing Earth is mostly unilluminated.

Waxing Crescent

A phase after the new moon when the illuminated portion is increasing; in the Northern Hemisphere, the right side is generally illuminated.

First Quarter

About one week after the new moon; half of the Moon appears illuminated.

Waxing Gibbous

The phase between first quarter and full moon when more than half of the Moon is illuminated and the illuminated portion is increasing.

Full Moon

Earth is approximately between the Sun and Moon; the side facing Earth is fully illuminated.

Waning Gibbous

The phase after the full moon when more than half of the Moon is illuminated but the illuminated portion is decreasing.

Third (Last) Quarter

About three weeks after the new moon; half of the Moon appears illuminated.

Waning Crescent

The phase before the new moon when a small portion of the Moon is illuminated and the illuminated portion is decreasing.

Why We See Moon Phases

The Moon orbits Earth, causing us to see different portions of its sunlit half from Earth.

Why Eclipses Are Less Common Than Moon Phases

The Moon's orbit is tilted about 5° relative to Earth's orbit around the Sun, so the Sun, Earth, and Moon usually do not line up closely enough for an eclipse.

Solar Eclipse

Occurs when the Moon passes between the Sun and Earth and blocks some or all of the Sun's light from reaching Earth.

Solar Eclipse Moon Phase

A solar eclipse can only occur during a new moon.

Lunar Eclipse

Occurs when Earth passes between the Sun and Moon, causing Earth's shadow to fall on the Moon.

Lunar Eclipse Moon Phase

A lunar eclipse can only occur during a full moon.

Total Solar Eclipse

Occurs when the Moon completely covers the Sun's visible disk for observers in the Moon's umbra.

Annular Solar Eclipse

Occurs when the Moon is farther from Earth and appears too small to completely cover the Sun, leaving a bright ring around it.

Perigee

The point in the Moon's orbit when it is closest to Earth.

Apogee

The point in the Moon's orbit when it is farthest from Earth.

Why the Moon Can Appear Red During a Lunar Eclipse

Earth's atmosphere scatters shorter wavelengths and allows more red/orange light to reach the Moon.

Who Can See a Solar Eclipse

Only people in the relatively narrow region where the Moon's shadow falls on Earth's surface can see the eclipse.

Who Can See a Lunar Eclipse

A lunar eclipse can be seen from much of Earth's nighttime side because Earth's shadow falls across the Moon.

Retrograde Motion

The apparent backward motion of a planet against the background stars.

Cause of Retrograde Motion

Retrograde motion is an apparent effect caused by the relative orbital motions of Earth and other planets; it does not mean the planet actually reverses its orbit.

How Ancient People Distinguished Planets from Stars

Planets change position relative to the background stars over time, while stars maintain their patterns relative to one another.

Kepler's First Law

Planets orbit the Sun in ellipses, with the Sun at one focus.

Ellipse

An oval-shaped orbit; planetary orbits are generally elliptical rather than perfect circles.

Kepler's Second Law

A planet sweeps out equal areas in equal amounts of time, meaning it moves faster when closer to the Sun and slower when farther away.

Perihelion

The point in an object's orbit when it is closest to the Sun.

Aphelion

The point in an object's orbit when it is farthest from the Sun.

Kepler's Third Law

The farther a planet is from the Sun, the longer its orbital period; orbital period and average distance are related.

Orbital Period

The time required for an object to complete one revolution around another object.

Electromagnetic Spectrum

The full range of electromagnetic radiation, from gamma rays to radio waves.

Electromagnetic Spectrum: Shortest to Longest Wavelength

Gamma rays → X-rays → ultraviolet → visible light → infrared → radio waves.

Wavelength

The distance between corresponding points on consecutive waves.

Frequency

The number of waves that pass a point per unit of time.

Wavelength vs. Frequency

As wavelength increases, frequency decreases. As wavelength decreases, frequency increases.

Wavelength vs. Energy

Shorter wavelengths have higher energy; longer wavelengths have lower energy.

Gamma Rays

Have the shortest wavelengths and highest energies; useful for studying extremely energetic objects and processes.

X-Rays

Have short wavelengths and high energy; useful for studying very hot and energetic objects such as black holes and neutron stars.

Ultraviolet (UV)

Has shorter wavelengths than visible light; useful for studying hot stars and energetic processes.

Visible Light

The portion of the electromagnetic spectrum detectable by human eyes.

Infrared

Has longer wavelengths than visible light; useful for observing cool objects, dust, and heat-related radiation.

Radio Waves

Have the longest wavelengths and lowest energies in the electromagnetic spectrum; useful for studying radio-emitting objects and cold gas.

Optical Telescope

A telescope designed to collect and focus visible light.

Radio Telescope

A telescope that detects radio waves from astronomical objects; often uses a large dish or array of dishes.

Why Astronomers Use Different Telescopes

Different astronomical objects and processes emit different wavelengths of electromagnetic radiation, so different instruments reveal different information.

Spectroscope

An instrument that spreads light into its component wavelengths, producing a spectrum.

Spectroscopy

The study of the spectrum of light to determine information such as an object's composition, temperature, and motion.

Filter

A device that allows certain wavelengths or colors of light to pass through while blocking or reducing others.

Broadband Color Filter

A filter that passes a relatively broad range of wavelengths associated with a particular color.

Grayscale Image

A black-and-white representation of an astronomical image where brightness represents the amount of detected light.

Chromatic Ordering

Arranging or combining observations according to wavelength/color, generally keeping shorter wavelengths together and longer wavelengths together.