The Night Sky - Chapter 2 (PHYS 405/406)

Attendance and Participation (Course Logistics)

• The Attendance and Participation portion accounts for 10% of the final grade (5% attendance, 5% participation), earned by engaging in class.

Landmarks and Coordinates; Star Patterns; Ancient Star Figures

• Celestial landmarks like star patterns (e.g., Betelgeuse in Orion) and ancient figures aid in sky navigation and constellation identification.

Modern Constellations

• The IAU established 88 modern constellations, covering the entire sky with defined boundaries. These include Northern (Latinized Greek names, e.g., Orion) and Southern (Latin names, e.g., Telescopium) types.

Constellations Not Always Easy to Identify

• Light pollution in urban areas makes identifying constellations challenging.

The Summer Triangle; The Winter Triangle

• Key asterisms include the Summer Triangle (Deneb, Vega, Altair) and the Winter Triangle (Castor, Pollux, Procyon), useful for locating surrounding constellations.

Finding Your Way Around (Finding North, etc.)

• The Big Dipper's pointer stars (URSA MAJOR) guide observers to Polaris (the North Star), paired with URSA MINOR for navigation.

The Dome of the Sky; The Dome of a Planetarium

• The sky is conceptually a dome enclosing Earth, physically represented by a planetarium dome.

The Celestial Sphere

• A conceptual sphere centered on Earth with stars imagined fixed on its surface; one hemisphere is visible at any time.

Angular Measurements

• Distances on the celestial sphere are measured as angles (angular separations), in degrees ($360^\circ$ for a full circle) or arcminutes/arcseconds for precision.

From Zenith to Horizon; Zenith and Local Sky Coordinates

• Zenith is the point directly overhead ($90^\circ$ altitude). The local sky uses altitude (0°–90°) and azimuth (compass direction) to locate objects along the meridian (north-south line through zenith).

Coordinate Systems; Latitude and Longitude

• Earth-based coordinates use latitude ($-90^\circ$ to $+90^\circ$) and longitude ($-180^\circ$ to $+180^\circ$ or $0^\circ$ to $360^\circ$), with Greenwich at $0^\circ$ longitude.

Celestial Coordinates

• Sky coordinates use Right Ascension (RA, like longitude, $0^\text{h}$ to $24^\text{h}$) and Declination (Dec, like latitude, $-90^\circ$ to $+90^\circ$). The celestial equator is Earth's equator projected onto the sky, and the ecliptic is the Sun's apparent path, tilted $23.5^\circ$ from the celestial equator.

The Daily Motion of the Stars

• Earth's counterclockwise rotation (once per sidereal day, ~$24 \text{ hours}$) causes stars to appear to rise in the east and set in the west. The Moon also revolves counterclockwise over ~29.5 days.

Star Trails; The Motion Explanations

• Time-exposure photos show star trails from Earth's rotation. Stars near celestial poles appear circular and can be circumpolar at high latitudes; at the equator, all stars rise and set.

The Sun's Motion Among the Stars; The Ecliptic

• The Sun's apparent path, the ecliptic, is tilted $23.5^\circ$ relative to the celestial equator due to Earth's axial tilt. This path dictates the Sun's position in the zodiac throughout the year.

Daily Path of the Sun Across the Sky; Seasons

• The Sun's daily path changes seasonally, causing solstices (Summer: June 21, furthest north; Winter: Dec 21, furthest south) and equinoxes (Vernal: Mar 21; Autumnal: Sep 21, on celestial equator). Seasons result from Earth's axial tilt and varying Sun altitude, not distance to the Sun (Earth is closest in January).

Altitude of the Sun at Noon; Length of the Day

• In the Northern Hemisphere, the Sun's noon altitude is highest, and day length is longest, around June; lowest and shortest in December.

Phases of the Moon; The Earth-Moon System

• The Moon orbits Earth every ~29.5 days (synodic month), showing tidal locking (same face always towards Earth). Phases cycle: New Moon, Waxing Crescent, First Quarter, Waxing Gibbous, Full Moon, Waning Gibbous, Third Quarter, Waning Crescent.

Eclipses

• Eclipses occur when the Sun, Earth, and Moon align, casting shadows (umbra and penumbra). Lunar Eclipses happen when the Moon enters Earth's shadow (Full Moon), not every cycle due to orbital inclination and nodal alignments. Types: Penumbral, Partial, Total (Moon appears red due to Earth's atmospheric scattering).

Solar Eclipses

• Solar Eclipses occur when the Moon blocks the Sun (New Moon), casting its umbra (total eclipse) or penumbra (partial eclipse) on Earth. A Total Solar Eclipse (e.g., Aug 21, 2017) reveals the Sun's corona within a narrow path of totality. An Annular Solar Eclipse leaves a bright ring if the umbra doesn't reach Earth.

Retrograde Motion

• Planets typically move prograde (west to east) against background stars, but occasionally appear to move retrograde (east to west). This apparent motion is caused by Earth overtaking outer planets in their orbits.

Notable Takeaways; Quick Reference Points

• Key points: Earth rotates CCW in 24 hours; Moon revolves CCW over ~29.5 days; Moon phases shift slowly. Eclipses depend on orbital inclinations and nodal alignments. The sky serves as a vital tool for navigation, timekeeping, and understanding seasonal changes.