User’s Guide to the Sky:

1. User’s Guide to the Sky: Astronomy 101 Lecture by: Prof. Barsony Sub: Abby Fuller Find this lecture at http://www.physics.sfsu.edu/~afuller /117L/117L.htm Patterns and Cycles Perspectives on Astronomy Seeds & Backman Chapter 2 (cont’d.)

2. Part III. Cycles • Eclipses • Seasons • Precession

3. Solar Eclipses

4. Which way is the Sun in this picture?

5. http://en.wikipedia.org/wiki/Umbra The umbra (Latin: "shadow") is the darkest part of a shadow. From within the umbra, the source of light is completely blocked by the object causing the shadow. This contrasts with the penumbra , where the light source is only partially blocked and there is only a partial shadow. Penumbras occur only when the source of light is not a point source. As the sun is an extended, visible disc, unlike the stars, which are point sources, shadows cast by sunlit objects, viewed on Earth, have penumbras.

6. Ingredients for a Total Solar Eclipse • By coincidence, the angular size of the Moon, as seen from Earth, is approximately the same as the angular size of the Sun seen from Earth. This allows for the possibility of the Moon’s face completely obscuring the Sun. • A total solar eclipse occurs only when the Moon is directly between the Sun and the Earth. • Since the umbra of the Moon’s shadow barely touches Earth, the region over which totality occurs on Earth is relatively small (270 km or 170 miles across). • Because of the orbital motion of the Moon, combined with the rotational motion of the Earth, the path of totality sweeps across the Earth’s surface at over 1000 mph or 1600 km/hr.

7. Questions • By coincidence, how much farther is the Sun than the Moon? (Ans. 390 x on average) • How much larger is the Sun’s diameter than the Moon’s? (Ans. 400 x) • What phase is the Moon in during a solar eclipse? (Ans. New Moon) • Why isn’t there a total solar eclipse each New Moon? (Ans. The Moon’s orbit is inclined to Earth’s orbit around the Sun by 5o)

8. If you ever get a chance, you should really see a total solar eclipse for yourself. Because our eyes have a much higher dynamic range (seeing both very bright and very dim light at the same time) than any cameras, the photographs of totality shown here in the last two images, cannot do justice to what you actually can see with your own eyes during a total solar eclipse (but only during the few short minutes of totality!)

9. Annular Eclipse http://apod.nasa.gov/apod/ap980824.html This picture of an annular eclipse of the Sun was taken by a video camera. An annular solar eclipse occurs when the Moon's angular size is slightly less than the Sun's angular size. Therefore, when the Moon is directly in front of the Sun, the edges of the Sun are still visible. This solar ring is so bright that the Moon's surface normally appears dark by comparison. The angular sizes of the Sun and Moon change slightly because of the elliptical nature of the Moon's and Earth's orbit. A total solar eclipse would have occurred were the Moon much closer to the Earth.

10. Since looking into the Sun directly will blind you, you should use this method to see the Sun’s disk during most portions of a solar eclipse. Only during the few minutes of totality is it safe to look directly towards the Sun (but with special, filter glasses on).

12. Lunar Eclipses

13. Ingredients for a Lunar Eclipse • Occurs when the Moon moves through the shadow of the Earth. • A total lunar eclipse occurs when the Moon moves through the umbra of the Earth’s shadow. • A partial lunar eclipse occurs when the Moon moves through the penumbra of the Earth’s shadow. • It typically takes a few hours for the Moon to travel through the Earth’s entire shadow during a lunar eclipse.

14. Questions • What phase is the Moon in during a lunar eclipse? (Ans. Full Moon) • Why isn’t there a lunar eclipse each Full Moon? (Ans. The Moon’s orbit is inclined to Earth’s orbit around the Sun by 5o)

15. Orientation of Earth’s and Moon’s Umbral Shadows To Sun (For clarity, the diameters of the Earth and the Moon have been enlarged to twice their size relative to a true scale drawing.) The Moon’s orbital plane, indicated by the dashed line with the arrows on its end, is tilted with respect to the Earth’s orbital plane around the Sun, whose orientation is indicated by the solid, left-pointing arrow. This mismatch between the two orbital planes accounts for the ease with which the Full Moon can miss Earth’s shadow (therefore, no lunar eclipse), or how easily Moon’s umbra can miss Earth’s surface during New Moon (therefore, no solar eclipse).

16. The combination of light scattered by all Earth’s sunrises and sunsets lights the totally eclipsed Moon’s face in a copper glow.

18. Seasons

19. The Seasons • Refer to cyclical climatic changes in Earth’s “temperate” zones: warmth in summer, cold in winter • In summer, Sun is in sky longer, days are longer, in winter days are shorter • In summer, Sun attains higher altitude in the sky than in winter

20. A few terms we’ll need to know for further discussion of the seasons Object in Sky Altitude: The elevation of an object, in angular measure (e.g., degrees) from your horizon Azimuth: The angle around your horizon, measured from North, in degrees. East is at 90o azimuth, S at 180o, W at 270o. Altitude angle Your horizon The Sun reaches its highest altitude in the sky at local noon. The altitude of the Sun at noon varies from its highest point in mid-summer, to its lowest point, in mid-winter. The azimuth of sunrise varies from northeast to southeast, whereas the azimuth of sunset varies from northwest to southwest. Both of these azimuth variations progress with the seasons. The Sun rises due East and sets due West only twice a year: at the equinoxes.

21. Tilt of Earth’s Rotational Axis Relative to an Axis Perpendicular to the Earth-Sun Line is Responsible for the Seasons Direction perpendicular to Earth-Sun lineca Direction of Earth’s rotational axis = NCP Earth’s Rotational Axis is tilted 23.5o away from the direction perpendicular to the Earth-Sun line Northern Summer To Sun Sun at its furthest north as seen from Earth. Southern Winter Celestial Equator To SCP

22. Tilt of Earth’s Rotational Axis Relative to an Axis Perpendicular to the Earth-Sun Line is Responsible for the Seasons To NCP Perpendicular to Earth-Sun Line Northern Winter Celestial Equator Sun at its furthest South, as seen from Earth To Sun Southern Summer 6 months later... To SCP

23. Kinesthetic Astronomy: Key Point for Seasons Although Earth orbits the Sun, the direction of its spin axis*does not change* during the year as it orbits. One person will represent the Sun. Please come up and stand in the middle of the stage. 2. The rest of us will be Earth. Let’s orbit (walk around the Sun) once, with our spin axis (head-to-toe line) perpendicular to the Sun-Earth axis. Now, let’s orbit (walk around the Sun) with our spin axis tilted *at a constant angle* (from the waist up is OK) as we orbit the Sun.

24. 4. OK to go back to your places & sit down. You have experienced that in the course of your orbit around the Sun (i.e., during the year) your head (Northern hemisphere) is sometimes tilted towards the Sun, and sometimes tilted away from the Sun, although your tilt (spin axis) does not change during the course of the year.

25. The amount of energy received from the Sun depends on the angle at which the Sun’s rays strike a surface. Just as with collecting rain in a bucket, you collect the most water if the rain is pouring straight down, and collect no water if the bucket is angled sideways (perpendicular to the incident rain). The same is true for receiving solar energy. Receive more energy per unit area. Receive less energy per unit area.

26. The intersection point between the Celestial Equator and the ecliptic, when the Sun’s apparent position, as seen from Earth, starts going South When the Sun reaches its Southernmost point in the sky. When the Sun reaches its Northernmost point in the sky. The plane traced out by the Earth-Sun line The intersection point between the Celestial Equator and the ecliptic, when the Sun’s apparent position, as seen from Earth, starts going North Seasons defined by 4 Events

27. Precession

28. Precession is the result of torques acting on a spinning body which tend to change the direction of its spin axis Axis of precession Direction perpendicular to Earth-Sun line NCP To Sun For Earth, the main such torque is due to the Sun’s gravitational pull

29. Earth’s Precession Period is 26,000 years It is observable in the changing direction of the Celestial Pole among the constellations.

30. Circles around the Earth Become Orbits around the Sun Perspectives on Astronomy Seeds & Backman Chapter 3 Astronomy 101 Prof. Barsony

31. Theories of the Heavens • Aristotle - Earth centered, stationary Earth, circular orbits, uniform speed • Ptolemy - Slightly off-Earth centered, stationary Earth, epicyclic circular orbits, uniform speed • Copernicus - Sun centered, circular orbits, uniform speed, rotating Earth • Brahe, Kepler - Sun centered, elliptical orbits, variable speed, rotating Earth

32. Aristotle 384-322 BC Aristotle's cosmological work ``On The Heavens” is the most influential treatise of its kind in the history of humanity. It was accepted for more than 18 centuries from its inception (around 350 B.C.) until the works of Copernicus in the early 1500s. In this work Aristotle discussed the general nature of the cosmos and certain properties of individual bodies.

33. Sun Moon Earth solarsystem.nasa.gov/.../ display.cfm?ST_ID=525 Aristotle proposed that Earth was the center of the Universe and above it the Moon, Sun, and 5 planets orbited Earth, held in place by crystal spheres. Higher than all were the fixed stars, which also revolved around Earth, all embedded in the outermost sphere. Beyond the sphere of the stars the universe continued into the spiritual realm where material things cannot be.

34. Question: What were the 5 known planets in Aristotle’s time? Sun Saturn Earth Venus Mars Jupiter Mercury Moon solarsystem.nasa.gov/.../ display.cfm?ST_ID=525

35. The discoveries of Uranus and Neptune required telescopes. The proof that each is a planet was: 1) Motion against the background stars & 2) An extended appearance, unlike stars which appear point-like. Uranus was discovered by Caroline and William Herschel on March 13, 1781. Neptune was discovered September 23, 1846, at a position that was predicted mathematically. Its presence was deduced from the deviations observed in the orbit of Uranus.

36. Aristotle also claimed that while Earth was corruptible and imperfect, the heavenly objects were perfect and not made of the four elements of the world (believed to be fire, water, air, and earth) but rather of an entirely different element called quintessence (=fifth element), which was also perfect.

37. Aristotelian Motion The idea that all bodies, by their very nature, have a natural way of moving is central to Aristotelian cosmology. Movement is, therefore, endowed to bodies, and is not, he states, the result of the influence of one body on another. Some bodies naturally move in straight lines, others naturally are at rest. There is yet another natural movement: the circular motion. By Aristotelian logic: since to each motion there must correspond a substance, there ought to be some bodies that naturally move in circles. Aristotle then proclaims that such things are the heavenly bodies as they are made of a more exalted and perfect substance than all earthly objects.

38. Since the stars and planets are made of this exalted substance and then move in circles, it is also natural, according to Aristotle, for these objects to be spheres. The cosmos is then made of a central earth (which he accepted as spherical) surrounded by the moon, sun and stars all moving in circles around it.

39. In the fully developed Aristotelian system, the spherical Earth is at the center of the universe. The planets are attached to anywhere from 47 to 55 concentric spheres that rotate around the Earth *in the same direction*. Aristotle says that to determine the exact number of spheres, one should consult the astronomers.

40. Problem: Retrograde Motion

41. Throughout the night, the celestial sphere appears to rotate towards the West around the North Celestial Pole, as shown by the blue arrow. In general, Mars traces a path from West to East in relation to the distant stars, except for episodes of “retrograde” motion, when Mars appears to move East to West relative to the background stars. Kentfield, CA latitude: +37.952o longitude: -122.556o http://www.heavens-above.com

42. Kentfield, CA latitude: +37.952o longitude: -122.556o http://www.heavens-above.com

43. Kentfield, CA latitude: +37.952o longitude: -122.556o http://www.heavens-above.com

44. Kentfield, CA latitude: +37.952o longitude: -122.556o http://www.heavens-above.com

45. Kentfield, CA latitude: +37.952o longitude: -122.556o http://www.heavens-above.com

46. Plato gave his students a major problem to work on. Their task was to find a geometric explanation for the apparent motion of the planets, especially the strange retrograde motion. One key observation is that near and during the time of retrograde motion, Mars appears significantly brighter in the sky than at other times. What might this mean? Plato and his students were also constrained by the Pythagorean worldview, in which the Earth is supposed to be the unmoving center of the planetary motions.

47. Aristarchus Plato’s student, Aristarchus, developed a Sun-centered model that could explain the retrograde motion of Mars, with the planets moving on circular paths. zebu.uoregon.edu/2004/hum399/lec04.html

48. Aristarchus’ Sun-centered model was not accepted because of the ``obvious’’ observations against a moving Earth. Ancient astronomers believed that Earth did not move because they saw no parallax. Parallax is the apparent shift of an object against a background caused by a change in the observer’s position. Question: In the example above, what is ``the change in the observer’s position’’?

49. Orders of Magnitude=Powers of 10 On an interstellar scale, parallax created by the different orbital positions of the Earth causes “nearby” stars to appear to move relative to the more distant stars. Because of the great distances to the stars, this effect is so small it is undetectable without extremely precise measurements. Recall the human eye can distinguish a difference of 1 arcminute in angular measure, at best. This is about 1/30 of the diameter of the full moon. The displacement of the nearest star, Proxima Centauri, when seen from opposite sides of the Earth’s orbit around the Sun (e.g., six months apart), is 0.77233 arcseconds. An arcsecond = 1/60 of an arcminute, so the parallax of the nearest star is 0.77233 x 1/60 arcminute= 0.01287 arcminutes or ~77 times smaller than the human eye can detect.

50. Ptolemy 384-322 BC . 85-165 A.D.