PHYS 103: Elementary Astronomy Review Flashcards

Comprehensive review flashcards covering introductory astronomy, scientific reasoning, cosmic scales, units, celestial coordinates, seasons, and eclipses for PHYS 103.

What is the definition of astronomy?

The study of everything outside of Earth and its atmosphere, encompassing space, celestial objects, and the Universe as a whole.

How does astrophysical cosmology differ from religious cosmology?

Astrophysical cosmology is a scientific field based on testable theories and experimentation to study the Universe's origin, structures, dynamics, future, and physical laws; religious cosmology involves beliefs regarding the creation or end of the world/Universe, often including deities and behavioral guidelines.

What is the key difference between astronomy and astrology?

Astronomy is a science based on observing celestial objects requiring reproducible results, whereas astrology is the belief that positions of celestial objects influence human personalities, relationships, and events on Earth.

Where did astrology originate approximately 4,000 years ago?

Ancient Babylon (located in present-day Iraq).

What are the definitions of a planetary system, a galaxy, and the Universe?

A planetary system is a group of non-stellar objects gravitationally bound to one or more stars; a galaxy is a gravitationally bound system of stars, gas, dust, and dark matter (typically containing millions to trillions of stars); and the Universe is everything that exists.

What requirement must a scientific observation fulfill to enforce honesty and prevent faked or biased results?

It must be reproducible, meaning anyone repeating the observation under the specified conditions will experience the exact same result.

Why does an observer in Talurjuaq, Nunavut ($69.5^\circ\,\text{N}$) observe different solar motion than an observer in Richmond, VA ($37.5^\circ\,\text{N}$)?

Talurjuaq is located north of $66.5^\circ\,\text{N}$, where the Sun circles without setting in summer and never rises in winter; daily rising in the east and setting in the west only occurs within $66.5^\circ$ of the equator.

What two observational proofs did Aristotle provide to demonstrate that Earth is a sphere?

First, different stars become visible as one travels to different latitudes. Second, Earth's shadow cast on the Moon during a lunar eclipse is always round, and a sphere is the only shape that always casts a round shadow in all directions.

How did Eratosthenes indirectly measure Earth's circumference in the 3rd century BCE?

He measured the shadow angle cast by a stick in Alexandria at summer solstice midday when the Sun was directly overhead in Syene (no shadow), and used geometry ($\frac{360^\circ}{\theta} = \frac{\text{circumference}}{d}$) with the distance $d$ to calculate a circumference of $250,000\,\text{stadia}$ ($38,600\,\text{km}$ to $46,600\,\text{km}$).

In scientific reasoning, how are a model, a hypothesis, and a theory distinguished?

A model is a representation used to make predictions; a hypothesis is an informed, testable prediction or proposed explanation; and a theory is an explanation of the natural world supported by repeated, overwhelming experimental and observational evidence.

Why is string theory currently classified as a mathematical framework rather than an accepted scientific theory?

Because it does not currently make testable predictions.

How does Einstein's theory of General Relativity relate to Newton's theory of gravity?

General Relativity is a broader description of gravity at all scales; in non-extreme conditions like on Earth, its mathematical equations simplify directly to Newton's laws of gravity.

On a 12-month Cosmic Calendar scaling the $13.8 \times 10^9\,\text{year}$ history of the Universe to one year, on what calendar dates do the Solar System form, dinosaurs go extinct, and Homo Sapiens appear?

The Solar System forms on September 1 ($4.6 \times 10^9\,\text{years ago}$), dinosaurs go extinct on December 30 ($66 \times 10^6\,\text{years ago}$), and Homo Sapiens appear on December 31 at 23:53 ($300,000\,\text{years ago}$).

If the Sun were scaled down to the size of a softball, how far away would Proxima Centauri be on that same scale?

Approximately $1,800\,\text{miles}$ ($3,000\,\text{km}$) away.

What power-of-ten multipliers correspond to the metric prefixes kilo, mega, Giga, Tera, micro, nano, and pico?

kilo = $10^3$, mega = $10^6$, Giga = $10^9$, Tera = $10^{12}$, micro = $10^{-6}$, nano = $10^{-9}$, and pico = $10^{-12}$.

What is the definition and distance value of an Astronomical Unit (AU)?

An Astronomical Unit is the average distance from Earth to the Sun, equal to $1.5 \times 10^8\,\text{km}$ ($9.3 \times 10^7\,\text{miles}$).

What is a light-year (ly), what is the speed of light $c$, and how wide is the Milky Way Galaxy?

A light-year is the distance light travels in 1 year ($63,240\,\text{AU}$), the speed of light is $c = 3 \times 10^8\,\text{m/s}$, and the Milky Way Galaxy is approximately $100,000\,\text{ly}$ across.

How is a parsec (pc) defined geometrically, and what are its values in light-years and Astronomical Units?

A parsec is the distance at which an object of length $1\,\text{AU}$ subtends an angle of $1\,\text{arcsecond}$ ($1''$); $1\,\text{pc} = 3.26\,\text{ly} = 206,265\,\text{AU}$.

What are the approximate distances to Proxima Centauri, the center of the Milky Way, the Andromeda Galaxy (M31), the Virgo Cluster, and the observable Universe limit?

Proxima Centauri is $\approx 1.3\,\text{pc}$, the Milky Way center is $\approx 8\,\text{kpc}$ from the Sun, Andromeda (M31) is $\approx 756\,\text{kpc}$, the Virgo Cluster is $\approx 16.5\,\text{Mpc}$, and the observable Universe limit is $\approx 28\,\text{Gpc}$ across.

What is the formula for the inverse-square law of light brightness (flux $b$), and how does brightness change if distance $d$ is doubled?

Brightness is $b = \frac{L}{4\pi d^2}$ (where $L$ is luminosity); doubling the distance makes the object $2^2 = 4$ times dimmer ($\frac{1}{4}$ as bright).

What are Right Ascension (RA) and Declination (Dec) on the celestial sphere, and how are their units measured?

Declination ($\delta$) measures angular distance north or south of the celestial equator from $-90^\circ$ to $+90^\circ$ in degrees; Right Ascension ($\alpha$) measures angular distance east/west along the celestial equator from $0\text{h}$ to $24\text{h}$, starting from the Sun's location at the March equinox.

How are zenith, nadir, and local meridian defined for an observer?

Zenith is the point directly overhead, nadir is the point directly below, and the local meridian is the great circle passing through the north celestial pole, zenith, and south celestial pole.

What is the difference between a sidereal day and a synodic (solar) day, and what are their approximate lengths?

A sidereal day is Earth's rotation period relative to distant background stars ($\approx 23\,\text{hr}\,56\,\text{min}\,4\,\text{sec}$), whereas a synodic day is the rotation period relative to the Sun ($\approx 24\,\text{hr}$), requiring an extra $\approx 1^\circ$ of rotation due to Earth's orbit.

How many arcminutes and arcseconds are in $1^\circ$?

$1^\circ = 60'$ (arcminutes) $= 3,600''$ (arcseconds).

What is the Small Angle Formula, and why is the small angle approximation valid for celestial bodies like the Sun and Moon?

The formula is $\theta = \frac{D}{d}$ (where $\theta$ is in radians, $D$ is physical size, and $d$ is distance); it is valid because astronomical angular sizes are tiny (the Sun and Moon both subtend only $\approx 0.5^\circ \approx 0.009\,\text{radians} \ll 1$).

What approximate angles on the sky correspond to a fingertip and a fist held at arm's length?

A fingertip held at arm's length subtends approximately $2^\circ$, and a fist held at arm's length subtends approximately $10^\circ$.

What causes Earth's seasons, and why is orbital distance not the cause?

Seasons are caused by Earth's $23.5^\circ$ axial tilt, which tilts one hemisphere toward the Sun (receiving higher, concentrated light and longer days) while the other is tilted away; Earth's orbital distance is actually smallest during Northern Hemisphere winter.

What is the ecliptic, and how fast does the Sun move along it?

The ecliptic is the apparent path of the Sun against the background stars on the celestial sphere, defining Earth's orbital plane; the Sun moves along it at approximately $1^\circ$ per day.

What is axial precession, and what is its cycle duration?

Axial precession is the slow conical wobbling of Earth's rotational axis over a $26,000\,\text{year}$ cycle, causing the North Star, solstices, equinoxes, and zodiac constellations to shift over time.

What is the difference between an asterism and a constellation?

An asterism is any informal recognizable grouping of stars, whereas a constellation is one of the 88 officially designated celestial regions/groupings.

What is the Messier Catalog, who created it, and how many objects does it contain?

The Messier Catalog is an astronomical catalog published in 1774 by Charles Messier containing 110 deep-sky objects such as nebulae, star clusters, and galaxies (e.g., M31, the Andromeda Galaxy).

For an observer at latitude $37.5^\circ\,\text{N}$ in Richmond, VA, what is the minimum declination required for a star to be circumpolar?

A star is circumpolar if $\text{Latitude} + \text{Dec} > +90^\circ$; for Richmond ($37.5^\circ\,\text{N}$), $\text{Dec} > +90^\circ - 37.5^\circ = +52.5^\circ$.

What is the southern declination visibility limit from Richmond, VA ($37.5^\circ\,\text{N}$), and is the center of the Milky Way ($\text{Dec} = -29^\circ$) visible?

The southern visibility limit is $\text{Dec} = 37.5^\circ - 90^\circ = -52.5^\circ$; since $-29^\circ > -52.5^\circ$, the Milky Way center is visible from Richmond, reaching a peak altitude of $23.5^\circ$ above the southern horizon.

During which Moon phases do solar and lunar eclipses occur, and why will total solar eclipses eventually no longer happen?

Solar eclipses occur during a New Moon and lunar eclipses occur during a Full Moon; total solar eclipses will cease in approx $0.6$ to $1.4 \times 10^9\,\text{years}$ because the Moon's expanding orbit will reduce its angular size below that of the Sun.