Astronomy

1. Astronomy Physics 102 Goderya Chapter(s): Online Learning Outcomes: 1,2,10,11,12

2. 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. Humans, the Earth, and even the solar system are tiny and unimportant on cosmic scales.

3. Earth Orbiting Around the Sun In order to avoid large numbers beyond our imagination, we introduce new units: 1 Astronomical Unit (AU) = Distance Sun – Earth = 150 million km

4. The Solar System Approx. 100 AU

5. The Solar Neighborhood New distance scale: 1 light year (ly) = Distance traveled by light in 1 year = 63,000 AU = 1013 km = 10,000,000,000,000 km (= 1 + 13 zeros) = 10 trillion km Approx. 17 light years Nearest star to the Sun: Proxima Centauri, at a distance of 4.2 light years

6. The Milky Way Galaxy Diameter of the Milky Way: ~ 75,000 ly

7. Finding objects in the sky • Constellations Source: Jodrell Bank Observatory

8. Finding objects in the sky • Orion Nebula Source: Jodrell Bank Observatory

9. Constellations Stars are named by a Greek letter (a, b, g) according to their relative brightness within a given constellation + the possessive form of the name of the constellation: Orion Betelgeuse = a OrionisRigel = b Orionis Betelgeuze Rigel

10. The Magnitude Scale • First introduced by Hipparchus (160 - 127 B.C.): • Brightest stars: ~1st magnitude • Faintest stars (unaided eye): 6th magnitude • More quantitative: • 1st mag. stars appear 100 times brighter than 6th mag. stars • 1 mag. difference gives a factor of 2.512 in apparent brightness (larger magnitude => fainter object!)

11. The Magnitude Scale (Example) Betelgeuse Magnitude = 0.41 mag 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

12. The Magnitude Scale The magnitude scale system can be extended towards negative numbers (very bright) and numbers > 6 (faint objects): Sirius (brightest star in the sky): mv = -1.42Full moon: mv = -12.5Sun: mv = -26.5

13. Apparent Motion of The Celestial Sphere

14. 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.

15. Precession As a result of precession, the celestial north pole follows a circular pattern on the sky, once every 26,000 years. 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.

16. The Sun and Its Motions Earth’s rotation is causing the day/night cycle.

17. 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.

18. The Seasons Earth’s axis of rotation is inclined vs. the normal to its orbital plane by 23.5°, which causes the seasons.

19. The Seasons

20. The Seasons The Seasons are only caused by a varying angle of incidence of the sun’s rays. Steep incidence → Summer Light from the sun Shallow incidence → Winter They are not related to Earth’s distance from the sun. In fact, Earth is slightly closer to the sun in (northern-hemisphere) winter than in summer.

21. The Seasons Northern summer = southern winter Northern winter = southern summer

22. The Seasons Earth’s distance from the sun has only a very minor influence on seasonal temperature variations. Earth’s orbit (eccentricity greatly exaggerated) Earth in January Earth in July Sun

23. 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.

24. 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.

25. A Total Lunar Eclipse

26. 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.

27. Solar Eclipses The sun appears approx. as large in the sky (same angular diameter ~ 0.50) as the moon.  When the moon passes in front of the sun, the moon can cover the sun completely, causing a total solar eclipse.

28. Total Solar Eclipse Chromosphere and Corona Prominences

29. Diamond Ring Effect

30. Earth and Moon’s Orbits Are Slightly Elliptical Apogee = position furthest away from Earth Earth Perihelion = position closest to the sun Moon Perigee = position closest to Earth Sun Aphelion = position furthest away from the sun (Eccentricities greatly exaggerated!)

31. Conditions for Eclipses The moon’s orbit is inclined against the ecliptic by ~ 50. A lunar eclipse can only occur if the moon passes a node near full moon. A solar eclipse can only occur if the moon passes a node near new moon.

32. Conditions for Eclipses Eclipses occur in a cyclic pattern.