Comprehensive Notes from Transcript

First Astronomy Night & Observing Assignment

• Event tonight: First Astronomy Night of the semester

• Time: 8:30–9:30 PM

• Purpose: Complete the Observing Assignment early in the semester (worth 10% of ASTR 1 grade)

• Presented by: QC Society of Physics Students (Instagram: @qc_sps)

• Encouragement: Attend if possible to get the assignment out of the way

Light travel time to the Moon & distance calculation

• Question: Recall the distance that light travels in a nanosecond (one billionth of a second).

• Given: Light from the Moon takes about 1.3 seconds to reach us.

• Task: Compute the Moon’s distance in feet; discuss with neighbors.

• Key result (approximate): ~1.28 × 10^9 ft

• Calculation details:

  • Speed of light: $c = 299{,}792{,}458 \ \mathrm{m\,s^{-1}} = 9.835\times 10^{8} \ \mathrm{ft\,s^{-1}}$

  • Distance: $d = c \Delta t = (9.835\times 10^{8} \ \mathrm{ft\,s^{-1}}) \times 1.3 \ \mathrm{s} \approx 1.28\times 10^{9} \ \mathrm{ft}$

  • In miles: $\text{miles} = \frac{1.28\times 10^{9}}{5280} \approx 2.42\times 10^{5} \ \text{mi}$

• Realistic cross-check: The average Earth–Moon distance is ~238,855 miles, which corresponds to roughly 1.27 × 10^9 ft, consistent with the above estimate.

• Concept connection: Distance = speed × time; unit conversions between meters, feet, and miles.

One light-year definition

• Definition shown: One light-year equals

  • $5.8 \text{ trillion miles}$

  • $9.4 \text{ trillion kilometers}$

• Implication: A light-year is the distance that light travels in one year.

• Relevance: Used to describe interstellar and intergalactic distances.

Scale models: galaxies & collisions

• Demonstration idea: "Scale Models" using notecards to represent galaxies (one in each hand).

• Question posed: Are collisions between galaxies common?

• Answer on slide: Yes!

• Supporting context:

  • Galaxies are vast; their outskirts interact gravitationally long before cores collide.

  • Typical real-world example: Interacting galaxies can contain tens to hundreds of billions of stars in each galaxy; collisions/mergers are a common part of galaxy evolution.

• Conceptual takeaway: Galaxy interactions influence star formation, galactic morphology, and the growth of galaxies over cosmic time.

The Mice: NGC 4676 (an interacting galaxy pair)

• Example: The Mice Galaxies (NGC 4676) as a case study for gravitational interactions.

• Instrument: Hubble Space Telescope (Advanced Camera for Surveys)

• Credits: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team and ESA; STScI-PRC02-11d

• Significance: Visual evidence of tidal tails and distortion due to mutual gravity in interacting galaxies.

Online resources & video links

• Page 7: YouTube video link: https://www.youtube.com/watch?v=D-OGaBQ494E

• Page 8: YouTube video link: https://www.youtube.com/watch?v=XwcdcNcoE

• Page 15: Advises studying interactive diagrams at

  • http://www.astronomynotes.com/nakedeye/s4.htm

  • Interactive applet: https://astro.unl.edu/classaction/animations/ancientastro/heliacalrisingsim.html

• Page 17: Additional RA/Dec demonstrations: https://astro.unl.edu/classaction/animations/coordsmotion/radecdemo.html

• Page 17: Brightspace listing: Content > Online Resources

The celestial sphere: directions & angles

• Page 9: Sky directions terminology

  • Zenith: point directly overhead

  • Horizon: plane tangent to the Earth at your location

  • Cardinal directions: N, E, S, W

  • Altitude: angle above the horizon

  • Azimuth: compass direction along the horizon

• Page 10: A handy scale for sky angles

  • Example angular scales shown: 1°, 5°, 10°, 15°, 25° (reference from timeanddate.com)

• Page 11: Question 2: Estimate the Moon’s angular diameter

• Page 12: Moon’s angular diameter is approximately $\tfrac{1}{2}^\circ$ (0.5°)

• Page 12: The scale shows Moon diameter as 0.5°, with reference to the same angular scale as page 10

• Page 13: The celestial sphere components

  • Your horizon

  • Celestial equator

  • Earth’s equator

  • Your zenith

  • North Celestial Pole (NCP)

  • Altitude of the pole

  • Latitude of the observer

• Page 14: The celestial sphere in motion across the sky

  • Angles displayed: 22.5°, 45°, 67.5° (and related zenith/meridian/equator lines)

  • Observation note: “Stars rise in the East” (example from Seattle)

• Page 15: Study advice

  • Draw diagrams similar to the provided figures to answer quiz questions

  • Links to external interactive resources for practice

Recording sky positions: RA/Dec system

• Page 16: RA/Dec as sky coordinates

  • RA (Right Ascension) is like longitude and is measured in hours: $\text{RA} \in [0,24)\ \text{hours}$

  • Dec (Declination) is like latitude and is measured in degrees: $\text{Dec} \in [-90^{\circ}, 90^{\circ}]$

  • Celestial equator is the projection of Earth's equator onto the celestial sphere

  • Celestial poles align with Earth's rotation axis; NCP and SCP denote North/South Celestial Poles

• Page 17: RA/Dec demonstrations

  • Online demonstration: https://astro.unl.edu/classaction/animations/coordsmotion/radecdemo.html

  • Additional concept resources linked on Brightspace under Content > Online Resources

The ecliptic, the Sun’s path, and the tilt of the axis

• Page 18: The ecliptic

  • It marks the Sun’s path across the celestial sphere over the year

  • Tilt relative to celestial equator: $\varepsilon = 23.5^{\circ}$

  • This tilt arises because Earth’s rotation axis is tilted relative to its orbital plane around the Sun

  • Key seasonal markers: Summer Solstice, Winter Solstice, Spring Equinox, Fall Equinox

• Page 19: Question 3 (Quiz): The summer solstice occurs around June 21st. In what direction does the Sun rise on that day?

  • Options: a) Due North b) North of East c) Due East d) South of East e) Due South

  • Answer: b) North of East (Sun rises north of due east at the summer solstice in the Northern Hemisphere)

The zodiac, ecliptic, and related constellations

• Page 20: The ecliptic and zodiac signs

  • Constellations along the ecliptic (zodiac): Aquila, Lyra, Cancer, Gemini, Leo, Libra, Scorpius, Virgo, Capricornus, Sagittarius, Taurus, Aries, Pisces, Orion, Canis Major, etc.

  • Labels: North Celestial Pole (NCP), Celestial Equator, Ecliptic, and Vernal/Summer/Winter Solstices and Equinoxes

• Page 21: Precession of the equinoxes

  • The Earth's rotation axis wobbles over a period of approximately $T_{prec} \approx 2.6\times 10^{4} \ \text{years}$

• Page 22: Planets

  • Greek origin: Planets means “wanderers”

  • Planets sometimes show retrograde (backwards) motion relative to stars

  • Resource: https://apod.nasa.gov/apod/ap031216.html

Early cosmology: origins of models & the scientific method

• Page 23: Flammarion engraving (circa 1888) vs. Early Greeks

  • Combined observations with religious/philosophical ideas

  • Developed scientific method: build simple models and test them with new observations

  • Must be repeatable by others

• Page 24: Key figures in ancient cosmology

  • Aristotle (384–322 BC): Earth at the center; believed Earth did not move; Moon phases; Earth is round; Moon closer than Sun

  • Aristarchus of Samos (310–230 BC): Proposed heliocentric model (Sun at center); disfavored due to lack of observed stellar parallax

  • Eratosthenes (276–194 BCE): Measured Earth's diameter via shadows

  • Hipparchus (c. 190–120 BCE): Precession of the equinoxes and the magnitude system (brightness scale)

Lahaina Noon concept (Sun overhead)

• Page 25: Lahaina Noon definition

  • Moment when the Sun is directly overhead at solar noon

• Page 26: Exercise: Which cities experience Lahaina Noon twice per year?

  • Given cities and coordinates:

  • a) New York City, NY (40.7° N, 74.0° W)

  • b) Nairobi, Kenya (1.3° S, 36.8° E)

  • c) Cape Town, South Africa (33.9° S, 18.4° E)

  • d) Jakarta, Indonesia (6.2° S, 106.8° E)

  • e) Amundsen–Scott South Pole Research Station (90° S, 0° E)

  • Answer concept: Lahaina Noon occurs for latitudes between the Tropics of Cancer and Capricorn (approximately ±23.5°). Thus, latitude must be within −23.5° to +23.5°.

  • Likely correct cities (twice per year): b) Nairobi (1.3° S) and d) Jakarta (6.2° S) are within the Tropics; a) NYC (40.7° N) and c) Cape Town (33.9° S) are outside the Tropics; e) South Pole is outside the Tropics.

  • Conclusion: The cities that will experience Lahaina Noon twice per year are Nairobi and Jakarta.

Real-world relevance and connections

• Observing nights and sky mapping connect to practical astronomy workflows used in observational labs and fieldwork.

• Understanding light travel times helps calibrate distance scales in astronomy and relates to communications (e.g., delays in deep-space signals).

• The RA/Dec coordinate system is foundational for locating objects in telescopes and for cataloging celestial bodies.

• The ecliptic and tilt explain seasonal changes, Sun’s path, and the zodiac; these are essential for historical and cultural astronomy as well as modern navigation.

• Precession affects long-term celestial coordinates, astronomical naming, and calibrations of historical records (e.g., star positions across millennia).

• Early cosmology sections illustrate the development of the scientific method and the importance of testable models.

Summary of key formulas and constants (in LaTeX)

• Distance from light travel time: $d = c \Delta t$

• Speed of light (exact): $c = 299{,}792{,}458 \ \mathrm{m\,s^{-1}}$

• Moon distance in feet (from 1.3 s):

  • $d \approx (9.835 \times 10^{8} \ \mathrm{ft\,s^{-1}}) \times 1.3 \ \mathrm{s} \approx 1.28 \times 10^{9} \ \mathrm{ft}$

• Moon distance in miles: $\text{miles} \approx \frac{1.28 \times 10^{9}}{5280} \approx 2.42 \times 10^{5} \ \mathrm{mi}$

• Moon angular diameter: $\theta_{\text{Moon}} \approx 0.5^{\circ}$

• Moon angular diameter in radians: $\theta \approx \frac{\pi}{180} \times 0.5 \approx 0.0087266 \ \mathrm{rad}$

• Ecliptic tilt: $\varepsilon = 23.5^{\circ}$

• Precession period: $T_{prec} \approx 2.6 \times 10^{4} \ \mathrm{yr}$

• RA/Dec ranges:

  • $\text{RA} \in [0,24)\ \mathrm{hours}$

  • $\text{Dec} \in [-90^{\circ}, 90^{\circ}]$

Resources for further study

• General visualization and notes: http://www.astronomynotes.com/nakedeye/s4.htm

• Helical rising simulator: https://astro.unl.edu/classaction/animations/ancientastro/heliacalrisingsim.html

• RA/Dec motion demo: https://astro.unl.edu/classaction/animations/coordsmotion/radecdemo.html

• Brightspace: Content > Online Resources