CELESTIAL BODIESA History of Planetary Discovery
PART I: CLASSICAL PLANETS“O Venus, beauty of the skies,
To whom a thousand temples rise, Gaily false in gentle smiles, Full of love-perplexing wiles;
O goddess, from my heart remove The wasting cares and pains of love.”
- Sappho, Greece, ~600 BCE
asteres planetai ἀστέρες πλανῆται
Ancient Greek: “wandering stars”
Apparent MagnitudeVisible to the typical human eye
Apparent magnitude
Brightness relative to Vega
Number of stars brighter than apparent magnitude
Yes -1.0 250% 1
0 100% 4
1 40% 15
2 16% 48
3 6.30% 171
4 2.50% 513
5 1.00% 1602
6 0.40% 4800
6.5 0.25% 9096
No 7 0.16% 14000
8 0.06% 42000
9 0.03% 121000
10 0.01% 340000
Mars AM: -3.0 to +1.6 Orbital period: 686 days
Saturn AM: -0.24 to +1.47 Orbital period: 29.5 years
Jupiter AM: -2.94 to -1.6 Orbital period: 11.9 years
Mercury AM: -2.6 to +5.7 Orbital period: 88 days
Venus AM: -4.9 to -3.8 Orbital period: 224 days
Venus The Morningstar and Eveningstar • Brightest objet in the sky, after the moon • Overtakes Earth every 584 days, moving
from being visible in the morning to the evening
Mayan astronomy
Mayans Chack ek - the Great Star • Thought to influence life on Earth;
wars and other important events timed based on its position
• Mayans’ Dresden Codex included an almanac showing full cycle in five sets of 584 days each
Yolngu (Northern Australia) Barnumbirr • In the early hours before dawn, Venus draws a rope of light attached
to Earth • Along this “Morning Star Pole”, the people can communicate with their
ancestors
Babylonian Astronomy
Babylonian Venus Tablets Ammisaduqua, 7th century BCE • Babylonians were first recorded civilization to recognize
astronomical phenomena as periodic and apply math • Venus tablets recorded rising and setting of Venus over 21
years • Babylonian astrologers laid foundation for western
astrology
Ancient Greece • Developed mathematical models for
predicting the paths of planets • Named planets after gods; Roman
interpretations give today’s names • 2nd century ‘Almagest’ provided
geocentric model that was astronomical standard for 1,200 years
PART II: MODERN PLANETS“Le Verrier—without leaving his study, without even looking at the sky—had found the unknown planet
[Neptune] solely by mathematical calculation, and, as it were, touched it with the tip of his pen!”
François Arago, France, 1846
Nicolaus Copernicus De revolutionibus orbium coelestium, 1543 • First to propose heliocentric (sun) model
for universe
Galileo Galilei Father of modern astronomy • Made significant advances to telescope • Recorded phases of Venus • Discovered 4 moons of Jupiter • Controversially advocated for
heliocentricsm; jailed by the inquisition
Johannes Kepler Laws of planetary motion, 1621 • Stated that the orbit of a planet is an
ellipse with the Sun at one of the two foci.
Isaac Newton Philosophiæ Naturalis Principia Mathematica, 1687 • Laid foundation for classical mechanics • Outlined law of universal gravitation:
“Any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them”
Sir William Herschel • Observed Uranus on March 13, 1781 • Noted its movement; initially reported it
to be a comet • Within the year, circular orbit had been
identified and it was noted to be a planet
Uranus • Apparent Magnitude: 5.9 to 5.32 • Orbital period: 84.0 years • Aphelion: 20.09 AU • Perihelion: 18.28 AU • Equatorial radius: 4.01 earths • Moons: Numerous • Ice giant, with more ices (water, ammonia,
methane in its atmosphere than S/J) • Coldest planetary atmosphere in the
solar system (minimum temperature of 49 K)
Aberrations in Uranus’ Orbit…
Discovery of Neptune September 23-24, 1846 • Urbain Le Verrier’s work predicted
position of new planet • Johann Galle found new planet within 1
degree of predicted position • Dramatic confirmation of Newtonian
gravitational theory
Neptune • Apparent Magnitude: 8.02 to 7.78 • Orbital period: 165 years • Aphelion: 30.33 • Perihelion: 29.8 • Equatorial radius: 3.9 Earths • Moons: 14 • Densest of gas giants (also known as an
ice giant)
But what about pluto?
There goes the neighborhood…
Eris was discovered in January 2005 and found to be 27% more massive than Pluto.
Estimated number of dwarf planets in our solar system:
>10,000
PART III: EXOPLANETS“[I]f our solar system is not unusual, then there are so many planets in the universe that, for
example, they outnumber the sum of all sounds and words ever uttered by every human who has ever lived. To declare that Earth must be the only
planet with life in the universe would be inexcusably bigheaded of us.”
Neil deGrasse Tyson
Radial Velocity Detecting minor orbit of star caused by orbiting planet
Transit Detecting decrease in light when planet transits in front of star
Direct Imaging Only possible for large planets that are far away from dim stars
Gravitational Lensing Detecting minor changes in light from source star from lensing planet
Timing variations Orbiting planets cause changes in periodic phenomena
2M1207
TrEs-3b • Orbits the star GSC 03089-00929 • Has an orbital period of just 31 hours;
undergoing orbital decay • Has nearly twice the mass of Jupiter • Found with transiting survey; mass
confirmed with radial velocity measurements of host star
TrEs-3b (artist’s interpretation)Jupiter (really)
Kepler Mission Launched March 7, 2009 • Spacecraft equipped with photometer to find transiting planets
• Studies 145,000 stars in a fixed position in the Milky Way
Kepler-20f • Closest radius to Earth of any
exoplanet discovered • Surface temperature is 705K
(a little too hot for liquid water)
(still just an artist’s interpretation)
Estimated number of solar systems in our galaxy:
>100 Billion
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