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The Discovery of Neptune: Chapter 20

Seen with a good amateur telescope and CCD. Seen from Voyager 2, 1989, first ever visit!. The Discovery of Neptune: Chapter 20. Uranus. Out story begins with Uranus (Chap 16). His 40-foot long, 4ft-wide (mirror) Telescope he built with his reward. William Hershel 1738-1822,

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The Discovery of Neptune: Chapter 20

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  1. Seen with a good amateur telescope and CCD Seen from Voyager 2, 1989, first ever visit! The Discovery of Neptune: Chapter 20

  2. Uranus • Out story begins with Uranus (Chap 16) His 40-foot long, 4ft-wide (mirror) Telescope he built with his reward William Hershel 1738-1822, Astronomer, musician March 13, 1781 Hershel was just looking around (6.5” mirror) when he found, “A curious either nebulous star or perhaps a comet”. He increased the magnification, object grew, thus the image was not a point. In weeks he saw it move and reported it. The trajectory was worked out, it was circular not eccentric! Planet at 19.2 AU, doubling the size of the solar system, first modern! Hershel receives stipend for life from King George III. So how can you tell when you are seeing a planet, not a star? No twinkle, apparent size through telescope, it moves night to night.

  3. Magnification • When you magnify an object you blow up its image (like zoom or a bigger TV) • But objects smaller than telescope resolution still a point (“unresolved”), no new detail, just fatter magnified resolved planet lower surface brightness, you notice more detail “point”--limit of resolution or “blurring circle” single ray (like single pixel), no more detail visible unresolved star Like your digital camera: you can magnify (zoom) to see more detail until you see the pixels (which is the limit of resolution). Planets are close enough to have angular size which is larger than the blurring circle, and thus are resolved.

  4. Twinkling of Stars Air bends (refracts) starlight. Due to atmospheric turbulence, the bending varies with time (temperature, density of air varies), so the amount of light and direction hitting your eye varies. Star closer to horizon Þ more air, more turbulence Þ more twinkling. Slow motion movie Of star seen through atmosphere, Betelgeuse, every 30 milisec, with high magnification Bright spots are called speckles, varied paths through atmosphere (average over time gives moving blob with size of blurring circle) Magnified starlight shows rapid jiggling of ray

  5. Twinkling of Stars (cont.) • Planets don't twinkle as much, since disks of light (many parallel rays of light) and the randomly jiggling rays tend to cancel out starplanet (big, but extremely far away) (small, much closer) • But even planets twinkle when close to the horizon – lots of atmosphere. Above atmosphere Planet Star below atmosphere

  6. Size Matters How big do stars and planets appear? =width/distance Stars: 1.4x106 km / 4x1013 km=3x10-8 radians = 6 x 10-3” = 0.006” That’s small! Remember that resolution goes as lambda/diameter and 1 m -->0.1” (HST ~ 0.04”) But the atmosphere blurs to ~ 1” so no chance! So stars are unresolved, just a point or ray. Saturn AU Planet: 1.2x105km / (10x1.5x108 km)=10-4 radians=20”. No problem for any telescope to resolve! Eye resolution: diameter of pupil = 6 mm, 150 times smaller than 1 m (0.1”) so ~ 15” So Jupiter, yes, Saturn barely. Because angular size bigger than 1” no twinkling (like having 15x15 pixel image). Resolution vs Magnification: You can thinks of resolution as the # of pixels per inch on a TV (more resolution=HDTV!) and magnification as the relative size of those Pixels (more magnification=big screen TV). Sun Alpha Centauri

  7. Increasing Resolution=more information

  8. More magnification, same resolution! Bigger pixels, same information.

  9. Same magnification, more resolution

  10. Remote Question At night you see headlights coming down the road at you. You also see an unusual turtle in the middle of the road. You calmly judge that the headlights appear as pinpoints of light. A quick calculation tells you you should, a) dive out of the way! b) scream at the turtle to run c) poke the turtle with a stick to see if its alive d) grab the turtle and run out of the road answer, d) Assuming the headlights are from a car (10 cm=0.1 m) and you have fair eyesight (~15”), then D=w*=0.1 m x (2x105 radians/ 15 “)= ~1000 meters which gives you 30 sec to a minute, no need to panic, no time to examine the turtle, enough time to grab it and run. Do NOT try this at home!

  11. Alexis Bouvard, 1767-1843, French Astronomer Uranus The Trouble with Uranus Bouvard calculated tables for Jupiter, Saturn, and Uranus. Uranus was misbehaving (1821) Compensated for Jupiter and Saturn but still Uranus was irregular, hypothesized the existence of an 8th planet farther out. "...I leave it to the future the task of discovering whether the difficulty of reconciling [the data] is connected with the ancient observations, or whether it depends on some foreign and unperceived cause which may have been acting upon the planet.” 1832 George Airy (1801-1892) describes the problem of Uranus's orbit as one of the chief problems of astronomy. By this time, Uranus has started to slow down in its orbital motion Some guessed that a comet had hit Uranus ~1820 throwing it off course!! However, even over short time span something was wrong. Two comets!?

  12. Bode’s Law Planet sequence (s+4)/10 actual (AU) • Curious pattern in distances of planets, Johann Titius and Johann Bode, 1772 Why?!! Next Planet 384 38.8

  13. Dark Matter • 1843 young John Adams (Cambridge U) hypothesizes new planet and attempts to calculate its orbit, assumed Bode’s law for distance (38 AU) and along ecliptic. Finds latitude of ~325 deg fits Uranus. • He gives a folded-up note to James Challis and George Airy (Royal Greenwich Observatory) with few details of his calculations and a raw position ~1845 and suggests a search for the planet. Airy and Challis are skeptical. Nobody has ever found a planet with pencil and paper!

  14. French Competition • Independently, French Celestial Mechanist Le Verrier calculates precise position of perturbing planet, publishes results and wrote to Berlin astronomers to look for planet of 8th mag of 3.3” size • Can a modest Planet X compete with Sun to affect Uranus? • Consider: F=GMMU/R2, so FX/FSun=(RSU/RXU)2(MX/Msun) Let’s assume Bode’s Law (so RSU=RXU) and that Planet X has the same mass at Uranus, so MX=MU so FX/FSun= 9x1026kg/2x1030kg=4x10-4 Weak but consider the new circle: since F=mv2/R, FX/Fsun=4x10-4then we equate that to reduction in v/vold of 2x10-2. Normal speed is 7 km/s so change is 0.1 km/s In 10 years that’s 3x107 km or ~100 Uranus widths which would be noticed! • Airy and Challis now believe Adams, begin the search! FX FSun sun Planet X Uranus

  15. Imagining Neptune 318deg 47’ on 1/1/1847 -Le Verrier

  16. Berlin, Sept 23, 1846 Adam’s prediction Neptune Le Verrier’s prediction Saturn This is a 15 deg field-The Berlin astronomers had a new star chart, d’Arrest and Galle Within 1 hour of receiving Le Verrier’s letter: “That star is not on the map!” Challis had seen it in August but without map, didn’t realize it!

  17. Galileo Almost Discovered it! • Galileo’s drawings showed that he had sighted Neptune Dec 28, 1612 and again Jan 27, 1613. Mistook it for a star! (His telescope was too small to resolve it and it had just turned retrograde so was motionless) • Likely true of many other astronomers

  18. So Who Got the Credit? French did not believe the English claim for their own prior prediction (an international incident!), but Airy convinced them he had the evidence so it was shared. French Newspaper 1960’s: English historians tried to verify, but documents stolen by Olin Eggen from Royal Greenwich Observatory! Recovered in 1998…”Neptune papers” showed Airy inflated the certainty of Adam’s calculations which varied up to 20 degrees and discarded by Adam’s in favor of Le Verrier’s own model, and Adam’s had done less to get astronomers to find Neptune. Conclusion was discovery was somewhat more Le Verrier’s than Adam’s.

  19. Neptune from Voyager Neptune: Planet 8 • About the same size as Uranus, gas giant, 1.6 g/cm3 • H, He; methane atmosphere. • Neptune discovered in 1846 (J. Galle), as a result of analysis of Uranus’ orbit (Le Verrier, Adams). • Neptune perturbs Uranus.

  20. Neptune: end of an odyssey • Voyager 2: August 1989, approached to ~ 5000 km! • Saw complete (but clumpy) rings. Atmosphere much more dynamic than Uranus’. Winds up to 1100 km/hour! Many spots, etc. • “Great Dark Spot” – size of Earth; wispy clouds of methane skirt around. • Magnetic field strange: tilted 50°, and offset from planet’s center (like Uranus’ magnetic field). • Triton: large moon with thin methane atmosphere. Fascinating, varied terrain. Dark plumes – active, icy volcanoes! • Fault blocks, collapsed basins, craters. Great end to Voyager’s 12-year odyssey!

  21. Epilogue: Le Verrier Tries again: Planet Vulcan • Le Verrier hypothesizes planet Vulcan at 0.14 AU in 1859, frequently sited, (likely sunspots) never confirmed, Einstein explains the solution in 1915 Exaggerated, Mercury undergoes perihelion (point of closest approach) precession , ~43” per century

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