ASTR 111: Solar System Motion and Foundations of Astronomy

Flashcards covering key concepts from ASTR 111, including administrative info, cosmic scales, sky coordinate systems, Earth's motion and seasons, lunar phases, solar and lunar eclipses, Earth's precession, and classical history of astronomy.

Who is the instructor for ASTR 111 Section 001 in Spring 2026?

Dr. Peter A. Becker

What is the primary required textbook for ASTR 111?

Astronomy Today, 9th Edition, Volume 1, by Chaisson & McMillan

How is the final semester grade calculated in ASTR 111?

By taking the average of the top $3$ "Exam Unit" grades out of the $5$ total "Exam Units" for the semester.

What distance corresponds to $1\text{ Astronomical Unit (AU)}$ in kilometers and miles?

$150,000,000\text{ km}$ ($93,000,000\text{ miles}$), which is the Earth-Sun distance.

What is the physical distance of one Light Year in kilometers and miles?

$9.5 \times 10^{12}\text{ km}$ or $5.9 \times 10^{12}\text{ miles}$.

What is Diurnal Motion and what causes it?

It is the apparent daily movement where stars sweep out circular paths across the sky, rising in the east and setting in the west, caused by Earth's daily rotation.

What angle is Earth's spin axis tilted relative to its orbital axis, and what does this tilt cause?

A tilt of $23.5^\circ$, which causes Earth's seasons.

How is the ecliptic defined?

The ecliptic is the apparent path of the Sun through the sky on the celestial sphere.

How is Declination (DEC) measured and what is its range of values?

Declination works like celestial latitude and is measured in angular units (degrees $^\circ$, arc-minutes $'$, arc-seconds $''$), ranging from $-90^\circ$ (South Celestial Pole) to $+90^\circ$ (North Celestial Pole).

How is Right Ascension (RA) measured and why?

Right Ascension works like celestial longitude and is measured in time units (hours, minutes, seconds) due to Earth's $24\text{-hour}$ daily rotation.

What is the precise time length of one Sidereal Day versus one Solar Day?

A Sidereal Day is $23\text{ hours and }56\text{ minutes}$ of solar time (the time between stellar alignments), whereas a Solar Day is $24\text{ solar hours}$ (the time between noons).

Why does the Moon always keep the same face pointed toward Earth?

Because it orbits Earth in a tidally locked orbit.

What is the difference between a Sidereal Month and a Synodic Month?

A Sidereal Month ($27.3\text{ days}$) is the time between stellar alignments of the lunar orbit, while a Synodic Month ($29.5\text{ days}$) is the time between Earth-Moon-Sun phase alignments.

By how many degrees is the Moon's orbit tilted relative to Earth's orbital plane (the ecliptic)?

$$5.2^\circ$$

What is the Line of Nodes?

The intersection line between the Moon's orbital plane and Earth's orbital plane (ecliptic), along which the Earth, Moon, and Sun must align for eclipses to occur.

What are the physical sizes of the umbra and penumbra regions during a solar eclipse?

The umbra (region of totality) is about $270\text{ km}$ across, while the penumbra (partial eclipse region) is about $7,000\text{ km}$ across.

What condition causes an Annular Eclipse instead of a Total Solar Eclipse?

An annular eclipse occurs when the Moon is farther away in its slightly eccentric (non-circular) orbit during an alignment, so it does not completely cover the Sun.

What is Earth's precession period and what causes it?

Earth's precession period is about $26,000\text{ years}$, caused by the gravitational pull of the Moon and Sun acting on Earth's rotation.

How did Aristotle determine that the Earth is round?

He observed that Earth's shadow projected onto the Moon during a lunar eclipse is always curved.

How did Eratosthenes calculate the radius of the Earth?

He measured that the Sun was at the zenith in Syene on June 22 at noon, while it was $7.2^\circ$ from the zenith in Alexandria ($5000\text{ stadia}$ north), calculating a radius of $39,789\text{ stadia}$ ($6,366\text{ km}$).

What three main reasons led ancient Greek scholars to favor a geocentric universe model?

1. Earth does not feel like it is moving. 2. No strong wind is felt from fast movement around the Sun. 3. No stellar parallax could be observed.

What is the difference between prograde motion and retrograde motion of planets?

Prograde motion is the normal eastward movement of a planet relative to background stars, while retrograde motion is temporary apparent westward movement relative to background stars.

How did Ptolemy's geocentric model account for planetary retrograde motion?

By assuming planets traveled in small circular paths called epicycles, whose center orbited the Earth along a larger circle called a deferent.