Origins of Modern Astronomy PSCI 131 ORIGINS OF

Origins of Modern Astronomy PSCI 131

ORIGINS OF ASTRONOMY • • Patterns in the Night Sky Our Solar System Earth Motions Earth-Moon System

Patterns in the Night Sky • Constellation – named pattern (or grouping) of stars • 88 known constellations • The brightest stars in each constellation are named in order of their brightness. – Use the Greek alphabet. • Alpha, beta, etc.

Question • Name as many constellations as you can. • Which constellation is the North Star (Polaris) located in?

Patterns in the Night Sky • All of the stars look equally far away so the ancient Greeks positioned the stars inside a “celestial sphere” that surrounds the earth. • In reality, a star’s brightness is dependent on its distance from us and its actual brightness. HR Diagram

Our Solar System

What Does Our Solar System Look Like?

Planet Radius (km) Density (g/cm 3) Atmosphere Composition Rings Surface Mercury 244000 5. 4 Faint atmosphere – Oxygen, Sodium, Magnesium, others No Solid – Rocky Venus 610000 5. 2 Carbon Dioxide (97%), Nitrogen (3%) No Solid – Rocky Earth 640000 5. 5 Nitrogen (78%), Oxygen (21%), Water Vapor (1%) No Solid – Rocky Mars 340000 4. 0 Thin atmosphere – Carbon Dioxide (95%), Nitrogen (3%), Argon (2%) No Solid – Rocky Jupiter 7100000 1. 3 Hydrogen (89%), Helium (10%) Yes Gas Saturn 6000000 0. 63 Hydrogen (96%), Helium (3%) Yes Gas Uranus 2600000 1. 2 Hydrogen (82%), Helium (15%), Methane (2%) Yes Gas Neptune 2500000 1. 5 Hydrogen (80%), Helium (19%), Methane (1%) Yes Gas Pluto 120000 1. 9 Thin Atmosphere - Nitrogen (90%), Methane (10%) No Solid – Rocky/Ice Eris 116000 2. 52 Faint atmosphere – Nitrogen and Methane No Solid – Rocky/Ice

Nebular Theory • Sun and planets cooled from a cloud of rotating interstellar gases – Composition of planets depends on proximity from sun • Mercury, Venus, Earth, Mars – Materials with high melting temperature (metals/rock) – Collisions and accretion formed four protoplanets that eventually became the terrestrial planets • Jupiter, Saturn, Uranus, Neptune – Gases condensed into ices (water vapor, carbon dioxide, ammonia, methane) – Large size, low density became Jovian planets

PSCI 131: Origins of Astronomy –Earth Motions

PSCI 131: Origins of Astronomy –Earth Motions Rotation • Solar day – Time from noon on one day to noon the next day: 24 hours • Time required for the Sun to return to the same position on two successive nights • Sidereal day – Time required for the “fixed” stars to return to the same position on two successive nights: 23 hrs 56 minutes

PSCI 131: Origins of Astronomy –Earth Motions Revolution • Period of revolution: 365. 25 days – Leap year (every 4 th year) – Earth travels around Sun at 107, 000 km per hour • Perihelion: closest to Sun (January) – 147 million km • Aphelion: farthest from Sun (July) – 152 million km

PSCI 131: Origins of Astronomy –Earth Motions Revolution: Solstices & equinoxes Solstice – Sun directly overhead at either Tropic of Cancer or Tropic of Capricorn • Summer solstice - June 21 st • Winter solstice - December 21 st Equinox – Sun’s rays are directly above the equator • Vernal Equinox - March 20 th • Autumnal Equinox - September 22 nd

PSCI 131: Origins of Astronomy –Earth Motions Precession • Wobble of Earth on its axis; 26, 000 -yr cycle

GEOL 131: Glaciers: Causes of Ice Ages Orbital variations: Obliquity • Tilt angle of rotational axis • 41, 000 -year cycle

GEOL 131: Glaciers: Causes of Ice Ages Orbital variations: Eccentricity • Shape of orbit • 100, 000 -year cycle

PSCI 131: Origins of Astronomy –Earth-Moon System The Earth-Moon System

PSCI 131: Origins of Astronomy –Earth-Moon System The Earth-Moon System • Mean Earth-Moon distance: ~238, 000 mi • Period of rotation: 27. 5 days • Period of revolution: 27. 5 days • “Synchronous” orbit – Same side always faces Earth

The Moon • • About ¼ of Earth’s diameter (2160 km) 225 deg F during day, -243 deg F at night 3. 3 x as dense as water Small mass and low gravity means there is no atmosphere.

PSCI 131: Origins of Astronomy –Earth-Moon System Lunar Phases • Half of lunar surface always lit – Exception: lunar eclipse • Current phase depends on how much of lit surface is visible from Earth

PSCI 131: Origins of Astronomy – Earth-Moon System-Lunar Phases

Eclipses

Solar Eclipse • During new moon • Moon casts shadow on earth – Occurs approx. once every 18 months somewhere on the Earth • Umbra – zone of total eclipse • Penumbra – zone of partial eclipse

Lunar Eclipse • During full moon • Moon in earth’s shadow • Umbra • Penumbra

Eclipses • Why don’t we have eclipses every new and full moon? – Moon’s orbit is tilted with respect to the ecliptic

PSCI 131: Origins of Astronomy End of Origins of Astronomy