An astronomical potpourri. Szydagis / 11

Transcription

1 An astronomical potpourri Szydagis / 11

2 Nearly all gravitational orbits are one of a few possible cross-sections of two of what shape? A. Cylinder B. Football C. Egg On the second question below: D is both B. & C. D. Cone When does the first day of spring season occur? A. March 20 always (Gregorian calendar) B. Depends which hemisphere you are in C. It is not a day it is a time, and changes 2 / 11

3 Parabola is that shape you learned about in algebra class (quadratic polynomial: x 2 is highest power in it) & projectile trajectories An ellipse is a mathematically precise object. It is not just an oval. Parabola is tipping point between ellipse and hyperbola. Means you lose your kinetic energy (coming to rest at infinity) Circular orbit is not stable (too perfect) Hyperbolic orbit means come and go, but still going at infinite distance Prof. Amanda W. Peet, University of Toronto True of any force decreasing with inverse square of distance There are two more not pictured here: radial orbit (straight-line in or out) and a de-stabilizing spiral (general-relativistic effect) 3 / 11

4 Exact dates vary - March June Sept Dec It is a precise moment in time. For example, this year spring will begin 6:29a EDT 3/20 (common) These season labels refer only to the Northern Hemisphere Southern Hemisphere has OPPOSITE seasons. Is closest to sun in summer First days of seasons called: vernal equinox, summer solstice, autumnal equinox & winter solstice. Different dates in places. Contrary to popular belief, the Earth s orbit is NOT a highly exaggerated ellipse It s quite circular Our seasons are not caused by orbital shape / proximity to Sun Caused by tilt of Earth s axis Planet-wide! 4 / 11

5 The Earth wobbles, like a top, as it rotates about its own axis, and revolves around Sun 1 cycle takes ~26,000 years to complete Caused by tidal force of Sun on equatorial bulge Thuban not Polaris was the Pole Star during time of the Great Pyramid Next Vega in 12 millennia. 5 / 11

6 Tidal locking Stigmatella aurantiaca 6 / 11

7 v=ourbkbaogew (President John F. Kennedy speech about the Moon: We Choose to Go ) Note to self, if low on time: skip to the best part, ~8 minutes in 7 / 11

8 Picture: Jay Pasachoff/Science Faction/Corbis Eclipse is a type of syzygy, more general term for straight-line alignment of celestial objects. Transit is smaller object in front of larger; occultation other way Moon is much smaller than sun, but much closer, so that compensates, making total solar eclipses possible Partial eclipses occur when Moon just clips the Sun Annular eclipses occur when sizes not quite matched up Usually very brief. Invisible solar features may appear Solar flares and prominences 8 / 11

9 In parentheses are numbers known in my old book from Our data have improved. Mercury: 0 Venus: 0 Earth: 1 Mars: 2 Jupiter: 67 (16) Saturn: 62 (18) Uranus: 27 (15) Neptune: 14 (8) [Pluto: 5 (1)] solarsystem.nasa.gov We have to add word natural now because of the many artificial ones we ve put in orbit of the Earth! 9 / 11

10 Crazy people thought this would end the world Friday, May 5th, 2000 (not to scale of course) Planets are orbiting at different speeds at their unique radii They are very rarely (thousands of years) in a nice line like in textbook illustration Can at times be on opposite sides of Sun (bad for missions) Can align in our sky and/or from space 10 / 11

11 Quiz #7 Purpose of having 1 per class is to enhance learning, prevent cram Clear who are not doing the reading: bi-modal distribution 11 / 11