Starry Monday at Otterbein Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month- March 6, 2006 Dr. Uwe Trittmann

Today’s Topics Basic Observations in Astronomy The Night Sky in March

On the Web To learn more about astronomy and physics at Otterbein, please visit http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Observatory) http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)

Basic Observations in Astronomy Positions of objects (sun, moon, planets, stars …) Motion of objects – with respect to you, the observer - with respect to other objects in the sky Changes (day/night, seasons, etc.) Appearance of objects (phases of the moon, etc.) Special events (eclipses, transitions, etc.)  All “in the sky”, i.e. on the Celestial Sphere

What’s up in the night sky? The Celestial Sphere An imaginary sphere surrounding the earth, on which we picture the stars attached Axis through earth’s north and south pole goes through celestial north and south pole Earth’s equator Celestial equator Relative positions of stars do not change -> attached to sphere In ancient times was often literally thought of as a physical sphere rotating about the earth

Celestial Coordinates Earth: latitude, longitude Sky: declination (dec) [from equator,+/-90°] right ascension (RA) [from vernal equinox, 0-24h; 6h=90°] Examples: Westerville, OH 40.1°N, 83°W Betelgeuse (α Orionis) dec = 7° 24’ RA = 5h 52m Relative positions of stars do not change -> attached to sphere In ancient times was often literally thought of as a physical sphere rotating about the earth

What’s up for you? Observer Coordinates Horizon – the plane you stand on Zenith – the point right above you Meridian – the line from North to Zenith to south

…depends where you are! Your local sky – your view depends on your location on earth

Look North in Westerville

Look North on Hawai’i

Daily Rising and Setting Due to the rotation of the Earth around its axis Period of rotation: 1 siderial day= 23h56m4.1s 1 solar day (Noon to Noon) =24h Stars rotate around the North Star – Polaris Circumpolar – never rise or set

Solar vs Siderial Day Earth rotates in 23h56m also rotates around sun  needs 4 min. to “catch up” Consequence: stars rise 4 minutes earlier each night after 1/2 year completely different sky at night!

What time is it? Depends on where you are on the Earth! Time zones ensure that the noon is really noon, i.e. sun is at highest point To avoid confusion, use universal time (UT), the time at the meridian in Greenwich UT = EST + 5 hrs Daylight savings adds one hour in spring, so UT = EDT+ 4 hrs

The Time Zones Established to insure that sun is at highest point approximately at noon in the middle of the time zone

Another Complication: Axis Tilt! The Earth’s rotation axis is tilted 23½ degrees with respect to the plane of its orbit around the sun (the ecliptic) It is fixed in space  sometimes we look “down” onto the ecliptic, sometimes “up” to it Rotation axis Path around sun

Position of Ecliptic on the Celestial Sphere Earth axis is tilted w.r.t. ecliptic by 23 ½ degrees Equivalent: ecliptic is tilted by 23 ½ degrees w.r.t. equator!  Sun appears to be sometime above (e.g. summer solstice), sometimes below, and sometimes on the celestial equator Skyglobe demo 7 visible “planets” incl. the sun and moon Planet = “wanderer” Days of the week named after the planets

The Seasons Change of seasons is a result of the tilt of the Earth’s rotation axis with respect to the plane of the ecliptic Sun, moon, planets run along the ecliptic Equinoxes: day and night of equal length Accounts for the N-S motion of the sun on the celestial sphere

The Zodiac throughout the Year Slow drift across background of fixed stars caused by rotation of earth about sun Period = 1 year (365 ¼ days) Example: In Winter sun in Sagittarius, Gemini at night sky; in summer sun in Gemini, Sagittarius at night sky

Constellations of Stars About 5000 stars visible with naked eye About 3500 of them from the northern hemisphere Stars that appear to be close are grouped together into constellations since antiquity Officially 88 constellations (with strict boundaries for classification of objects) Names range from mythological (Perseus, Cassiopeia) to technical (Air Pump, Compass)

Constellations of Stars (cont’d) Also the Chinese and Egyptians; most constellations today (88) have Greek origins Basically the constellations of the zodiac (12 in number) Pattern not really meant to resemble Orion or whatever; likely named in honor, picture fit later Orion as seen at night Orion as imagined by men

Constellations (cont’d) Note naming scheme Orion “from the side” Stars in a constellation are not connected in any real way; they aren’t even close together!

Zodiacal signs vs. Constellations “Constellation” is a modern, well-defined term - Some constellations are big, some are small on the celestial sphere “Zodiacal sign” is the old way of dividing the year and the Sun’s path into 12 equal parts Slow drift across background of fixed stars caused by rotation of earth about sun Period = 1 year (365 ¼ days) 360/12=30, so each zodiacal sign is exactly 30 degrees “long” 0 degrees: Aries, 30 degrees: Taurus, 60 degrees: Gemini, 90 degrees: Cancer, etc.

Example The vernal equinox happens when the sun enters the zodiacal sign of Aries, but is actually located in the constellation of Pisces.

Reason: Precession of the Equinoxes Precession period about 26,000 years “The dawning of the age of Aquarius” A result of the combined grav pull of the sun and moon on the earth Vernal equinox now in Pisces. 2000 years ago in Aries; around AD 2600 will move into Aquarius

Motion of Sun, Moon and other Planets All major bodies in the Solar System move around ecliptic Slow drift (from W to E) against the background of stars Skyglobe demo 7 visible “planets” incl. the sun and moon Planet = “wanderer” Days of the week named after the planets

Reason: All planets move in same plane! all planets orbit in same direction (ccw as seen from above the north pole) all orbits lie nearly in a single plane (Mercury (7deg) and Pluto (17deg) being most notable exceptions) inner planets are small, dense, rocky (Terrestrial); outer planets are large, gaseous, low density (Jovian) density = mass/volume inner planets close together, outer planets further apart

All planets? - Pluto’s strange Orbit Very far out there: 40 A.U. Pluto’s year = 248 Earth years Orbit inclined 17° w.r.t. ecliptic Very eccentric orbit: Perihelion: 30 A.U. (inside the Neptune orbit!) Aphelion: 50 A.U. A bit smaller than our moon Icy, basically like Triton

Is Pluto a planet? Orbit too weird Too small  Today Pluto would probably not be classified as a planet!

Why do all planets move in the same plane? Reason: Formation process of the Solar System Condenses from a rotating cloud of gas and dust Conservation of angular momentum flattens it Dust helps cool the nebula and acts as seeds for the clumping of matter

Formation of Planets Orbiting dust – planitesimals Planitesimals collide Different elements form in different regions due to temperature Asteroids Remaining gas

Motion of the Moon Moon shines not by its own light but by reflected light of Sun  Origin of the phases of the moon Moon revolves around the Earth period of revolution = 1 month Planets also shine by reflected light!

Phases of the Moon

Phases of the Moon (cont’d) Moon rotates around earth in one month Moon rotates around itself in the same time  always shows us the same side!  “dark side of the moon” (not dark at all!)

Motion of the Planets Along the ecliptic as Sun and Moon But: exhibit weird, “retrograde” motion at times

“Strange” motion of the Planets Note: outer planets are slower Planets usually move from W to E relative to the stars, but sometimes strangely turn around in a loop, the so called retrograde motion.

The heliocentric Explanation of retrograde planetary motion See also: SkyGazer

SkyGazer A computer program that simulates the vision of the sky during day and night Things to observe: Set your position on Earth: observe how view of sky changes as you move E,W, N,S Note the distribution of sunlight on Earth! Rotation is around Polaris which is not in zenith

SkyGazer Things to observe (cont’d): Sun, moon, planets, stars rise (E) and set (W) In the southern hemisphere the sun is highest in the north Planets sometimes move backward Moon phases Planets have phases, too!

Eclipses One celestial object hidden by other or in the shadow of another Solar eclipse: sun hidden by the moon Lunar eclipse: moon in earth’s shadow (sun hidden from moon by earth) Also: eclipses of Jupiter’s moons, etc. Most spectacular because moon and sun appear to be the same size from earth

Solar Eclipses Umbra – region of total shadow Penumbra – region of partial shadow Totality lasts only a few minutes! Why isn’t there a solar eclipse every month?

Solar Eclipse

Solar Corona Thin outer atmosphere, usually swamped by other light and hence invisible

Question Why isn’t there an eclipse every month ? Answer: because the Moon’s orbit is inclined w.r.t. the ecliptic

Lunar Eclipses Moon moves into earth’s shadow… …and out of it Moon appears reddish due to light bent by Earth’s atmosphere Aristotle argues earth is round based on shape of its shadow on the moon …and out of it (takes hours!)

Partial Eclipse

Not an Eclipse !

Towards Totality

Almost total…

Totality Totality

The Night Sky in March Long nights, early observing! Winter constellations are up early: Orion, Taurus, Gemini, Auriga, Canis Major & Minor, the spring constellations come up: Cancer, Leo, Big Dipper Saturn dominates the evening, Jupiter early morning.

Moon Phases Today (First quarter Moon) 3/ 14 (Full Moon) 3 / 22 (Last Quarter Moon) 3 / 29 (New Moon)

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

10 PM Typical observing hour, early March Saturn Mars Moon

South-West Plejades Mars in Aries / Taurus

Zenith Big Dipper points to the north pole

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

South-West The Winter Constellations Orion Taurus Canis Major Gemini Canis Minor

The Winter Hexagon Sirius Procyon Pollux Capella Aldebaran Rigel

South Saturn near Praesepe, an open star cluster

East Spring constellations: Leo Hydra

Mark your Calendars! Next Starry Monday: April 3, 2005, 8 (!!!) pm (this is a Monday ) Observing at Prairie Oaks Metro Park: Friday, May 5, 9:00 pm Web pages: http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.) http://www.otterbein.edu/dept/PHYS/ (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.