Astronomy Physics 102 Goderya Scales of Size and Time Astronomy deals with objects on a vast range of size scales and time scales. Most of these size and time scales are way beyond our every-day experience. Chapter(s): Online Learning Outcomes: 1,2,10,11,12 Humans, the Earth, and even the solar system are tiny and unimportant on cosmic scales. Earth Orbiting Around the Sun The Solar System In order to avoid large numbers beyond our imagination, we introduce new units: 1 Astronomical Unit (AU) = Distance Sun Earth = 150 million km Approx. 100 AU The Solar Neighborhood The Milky Way Galaxy Approx. 17 light years New distance scale: 1 light year (ly) = Distance traveled by light in 1 year = 63,000 AU = 10 13 km = 10,000,000,000,000 km (= 1 + 13 zeros) = 10 trillion km Nearest star to the Sun: Proxima Centauri, at a distance of 4.2 light years Diameter of the Milky Way: ~ 75,000 ly 1
Finding objects in the sky Finding objects in the sky Constellations Orion Nebula Source: Jodrell Bank Observatory Source: Jodrell Bank Observatory Constellations Stars are named by a Greek letter ( ) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Orion Betelgeuse = Orionis Betelgeuze Rigel = Orionis The Magnitude Scale First introduced by Hipparchus (160-127 B.C.): Brightest stars: ~1 st magnitude Faintest stars (unaided eye): 6 th magnitude Rigel More quantitative: 1 st mag. stars appear 100 times brighter than 6 th mag. stars 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude => fainter object!) The Magnitude Scale (Example) Magn. Diff. Intensity Ratio 1 2.512 2 2.512*2.512 = (2.512) 2 = 6.31 5 (2.512) 5 = 100 Betelgeuse Magnitude = 0.41 mag The Magnitude Scale The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): For a magnitude difference of 0.41 0.14 = 0.27, we find an intensity ratio of (2.512) 0.27 = 1.28. Rigel Magnitude = 0.14 mag Sirius (brightest star in the sky): m v = -1.42 Full moon: m v = -12.5 Sun: m v = -26.5 2
Apparent Motion of The Celestial Sphere Precession At left, gravity is pulling on a slanted top. => Wobbling around the vertical. The Sun s gravity is doing the same to Earth. The resulting wobbling of Earth s axis of rotation around the vertical w.r.t. the Ecliptic takes about 26,000 years and is called precession. Precession As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. The Sun and Its Motions It will be closest to Polaris ~ A.D. 2100. There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) ~ 12,000 years from now, it will be close to Vega in the constellation Lyra. Earth s rotation is causing the day/night cycle. The Sun and Its Motions Due to Earth s revolution around the sun, the sun appears to move through the zodiacal constellations. The Sun s apparent path on the sky is called the Ecliptic. Equivalent: The Ecliptic is the projection of Earth s orbit onto the celestial sphere. Earth s axis of rotation is inclined vs. the normal to its orbital plane by 23.5, which causes the seasons. 3
are only caused by a varying angle of incidence of the sun s rays. Steep incidence Summer Shallow incidence Winter Light from the sun They are not related to Earth s distance from the sun. In fact, Earth is slightly closer to the sun in (northernhemisphere) winter than in summer. Earth s distance from the sun has only a very minor influence on seasonal temperature variations. Northern summer = southern winter Northern winter = southern summer Earth in January Earth s orbit (eccentricity greatly exaggerated) Sun Earth in July The Phases of the Moon From Earth, we see different portions of the Moon s surface lit by the sun, causing the phases of the Moon. Lunar Eclipses Earth s shadow consists of a zone of partial shadow, the Penumbra, and a zone of full shadow, the Umbra. If the moon passes through Earth s full shadow (Umbra), we see a lunar eclipse. If the entire surface of the moon enters the Umbra, the lunar eclipse is total. 4
A Total Lunar Eclipse A Total Lunar Eclipse A total lunar eclipse can last up to 1 hour and 40 min. During a total eclipse, the moon has a faint, red glow, reflecting sun light scattered in Earth s atmosphere. Solar Eclipses Total Solar Eclipse Chromosphere and Corona The sun appears approx. as large in the sky (same angular diameter ~ 0.5 0 ) as the moon. When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse. Prominences Diamond Ring Effect Earth and Moon s Orbits Are Slightly Elliptical Perihelion = position closest to the sun Sun Earth Perigee = position closest to Earth (Eccentricities greatly exaggerated!) Apogee = position furthest away from Earth Moon Aphelion = position furthest away from the sun 5
Conditions for Eclipses The moon s orbit is inclined against the ecliptic by ~ 5 0. Conditions for Eclipses Eclipses occur in a cyclic pattern. A solar eclipse can only occur if the moon passes a node near new moon. A lunar eclipse can only occur if the moon passes a node near full moon. 6