Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month- October 3, 2005 Dr. Uwe Trittmann Welcome to

Today’s Topics Observing the Planets The Night Sky in October

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Observing the Planets Jupiter Uranus Saturn Neptune

The Terrestrial Planets Small, dense and rocky Mercury Venus Earth Mars

The Solar System: Top View

Side view: Inclination of Orbits Orbits (here: Mars) are very slightly tilted with respect to the sun-earth plane  Planets appear close to the path of the sun in the sky, the ecliptic

Planetary Motions The sky seems to revolve around us because of Earth’s rotation Additionally, planets move with respect to the fixed stars, that’s why they are called planets (greek: wanderers) Due to the planet’s movement in their orbit, and Earth’s orbital motion, this additional motion – the apparent motion of the planet as seen from Earth - looks complicated.

Apparent Planetary Motion Motion as seen from Earth, which itself is revolving around the Sun.

Explanation 1: Ptolemy (~140 AD) Planets move on circles sitting on circles around Earth  geocentric model dominates scientific thought during the Middle Ages Longest lasting (wrong) theory ever: 1000yrs

Epicycles Ptolemy’s explanation of retrograde motion About 40(!) epicycles necessary to explain all observations  complicated theory

Explanation 2: Copernicus (1473–1543) All planets – including Earth – move around the Sun Planets still on circles  needs 48 epicycles to explain different speeds of planets Not more accurate than Ptolemy Major Work : De Revolutionibus Orbium Celestium (published posthumously)

Correct Explanation: Kepler, Newton All planets move around the sun according to Newton’s theory of gravity Kepler’s laws tell us how the orbits look like, and where a planet is in its orbit

Kepler’s First Law The orbits of the planets are ellipses, with the Sun at one focus

Ellipses a = “semimajor axis”; e = “eccentricity”

Kepler’s Second Law An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal times

Kepler’s Third Law The square of a planet’s orbital period is proportional to the cube of its orbital semi-major axis: P 2  a 3 a P Planet Orbital Semi-Major AxisOrbital Period Eccentricity P 2 /a 3 Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto (A.U.)(Earth years)

The heliocentric explanation of retrograde planetary motion

Inner and Outer Planets Inner Planets: closer to sun than Earth –Mercury & Venus –Always close to sun in the sky Outer Planets: further from sun than Earth –Mars, Jupiter, Saturn, Uranus, Neptune, Pluto –Best viewing when opposite of sun in the sky

Inner Planets Inner planet Earth superior conjunction inferior conjunction western elongation eastern elongation

Outer Planets Outer planet Earth conjunction opposition quadrature

Close Outer Planet Outer planet Earth Size of planet varies a lot as Earth moves

Far-Out Planet Outer planet Earth Size of planet varies little as Earth moves

Mercury Color: yellow-golden Brightness: up to –1m Size: 10” When to observe: several times a year for short periods Difficulty: pretty tough, innermost planet, always very close to the sun

Venus Color: white Brightness: up to –4.5m Size: up to 40” When to observe: all year, except for period around superior conjunction; either west of the sun (morning star), or east of the sun (evening star) Difficulty: very easy

Phases of Venus

Heliocentric Geocentric

Mars Color: orange Brightness: up to –2.2 m Size: up to 25” When to observe: about every 2 years Difficulty: very easy around opposition

Mars Opposition 2005 Date of opposition: November 7, 2005 Constellation: Aries Date of closest distance: October 30, 2005 Closest distance to Earth: million km (43 million miles, or AU) In 2003 (historically close): 55.8 million km

Mars Fairly bright, generally not too hard to see Smaller than Earth Density similar to that of the moon Surface temperature 150–250 K Day ~ 24.6 hours Year ~ 2 Earth years

Apparent Mars Diameter

The Terrestrial Planets Comparable tilt of rotation axis

Martian Seasons

Polar Ice Caps Watch them grow and shrink in the telescope

Mars Atlas

Mars observations Look for surface features Try to determine which side of Mars we see Polar caps Seasonal changes phases

Dust Storms

Jupiter Color: yellowish-white Brightness: up to –2.5m Size: 40” When to observe: most of the year, except for some months around conjunction Difficulty: easy, moons visible in binoculars

Jupiter & Moons

Saturn Color: yellowish Brightness: up to –1.5m Size: 20” When to observe: most of the year, except for some months around conjunction Difficulty: easy, rings and moons visible in small telescopes

Saturn & Moons

Uranus Color: greenish Brightness: around 5.7m Size: 4” When to observe: most of the year, except for some months around conjunction Difficulty: challenging, with binoculars

Neptune Color: greenish Brightness: around 7.8m Size: 2.5” When to observe: most of the year, except for some months around conjunction Difficulty: challenging, good binoculars

Pluto Color: white Brightness: 14m Size: star-like, no disk When to observe: most of the year, except for some months around conjunction Difficulty: very tough, outermost planet, always very far away, very faint; big telescope and several nights to identify

The Night Sky in October The sun is past autumn equinox -> longer nights! Autumn constellations are coming up: Cassiopeia, Pegasus, Perseus, Andromeda, Pisces  lots of open star clusters! Mars is getting close to opposition Saturn is visible later at night

Moon Phases Today (New Moon, 0%) 10 / 10 (First Quarter Moon) 10 / 17 (Full Moon) 10 / 24 (Last Quarter Moon) 11/ 1 (New Moon)

Today at Noon Sun at meridian, i.e. exactly south

10 PM Typical observing hour, early October no Moon Mars Uranus at meridian Neptune

South- West High in the sky: The summer triangle

Due North Big Dipper points to the north pole

High up – the Autumn Constellations W of Cassiopeia Big Square of Pegasus Andromeda Galaxy

“PR” Foto Actual look

East High in the sky: Perseus and Auriga with Plejades and the Double Cluster

South Planets –Uranus –Neptune Zodiac: –Capricorn –Aquarius

Mark your Calendars! Next Starry Monday: November 7, 2005, 7 pm (this is a Monday ) Observing at Prairie Oaks Metro Park: –Wednesday, October 12 –Friday, November 18 Web pages: – (Obs.) – (Physics Dept.)

Mark your Calendars II Physics Coffee is every Wednesday, 3:30 pm Open to the public, everyone welcome! Location: across the hall, Science 256 Free coffee, cookies, etc.