Transcript Document
Welcome to Basic Observations in Astronomy Classes without Quizzes -Alumni Weekend at OtterbeinJune 8-10, 2007 Dr. Uwe Trittmann Basic Astronomy • Possible topics: – – – – – Basic observations Star Maps Telescopes The Night Sky in June Eclipses 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 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 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 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! 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 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 The Zodiac throughout the Year 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) Orion as seen at night Orion as imagined by men Constellations (cont’d) Orion “from the side” Stars in a constellation are not connected in any real way; they aren’t even close together! Understanding and using Star Maps • The night sky appears to us as the inside of a sphere which rotates • Problem: find a map of this curved surface onto a plane sheet of paper • Let’s explore our turning star map! Fixed and unfixed Stuff • The stars are “fixed” to the rotating sky globe They move from East to West and also from near to the horizon to higher up in the sky • The Solar System bodies (Sun, Moon, Planets, Asteroids, Comets) move with respect to the fixed stars • SSB’s have complicated paths: their own motion is added to the overall motion of the celestial sphere 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 Reason: All planets move in same plane! 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 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 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! Telescopes • Light collectors • Two types: – Reflectors (Mirrors) – Refractors (Lenses) Refraction • Lenses use refraction to focus light to a single spot Reflection • Light that hits a mirror is reflected at the same angle it was incident from • Proper design of a mirror (the shape of a parabola) can focus all rays incident on the mirror to a single place Newtonian Telescope • Long tubes (approx. focal length) • Open at front • Eyepiece on side Schmidt-Cassegrain Telescope (CAT) • Very compact & easy to use • Closed (Corrector plate) • Resonably priced Refractor • Two lenses -> inverted image • Long tube (approx. focal length of objective) • Usually pretty expensive Binoculars • Erect image -> good for terrestrial viewing • Prisms needed to produce erect image • Typical specs: 8x60, means magnifies 8x and objective lens is 60 mm in diameter A good starting point • A pair of binoculars and a star map will keep you busy for a long time – anywhere! – – – – – – – constellations Planets Moon Orion nebula Andromeda Galaxy star clusters … The Night Sky in June • The sun is at its highest -> shortest nights! • Summer constellations are coming up: Hercules, Scorpius, Ophiuchus (Snake Bearer), Snake lots of globular star clusters! • Center of the Milky Way in Sagittarius • Jupiter, Saturn & Venus are easily seen Moon Phases • 6 / 8 (Last Quarter Moon) • 6 / 14 (New Moon) • 6 / 22 (First Quarter Moon) • 6/ 30 (Full Moon) Today at Noon Sun at meridian, i.e. exactly south 10 PM Venus Typical observing hour, early June no Moon Jupiter Pluto (experts only) Saturn SouthWest Virgo and Coma with the VirgoComa galaxy cluster VirgoComa Cluster • Lots of galaxies within a few degrees M87, M88 and M91 Zenith Big Dipper points to the north pole South – – – – Canes Venatici Corona Borealis Bootes Serpens Globular Star Clusters: •M3 •M5 • M 13 Galaxies: • M 51 • M 101 • M 64 (Bl. Eye) M5 SouthEast – Hercules – Ophiuchus – Serpens Globular Star Clusters: • M 13 • M 92 • M 12 • M 10 … Summer is Globular Cluster time! M5 M13: Globular Cluster SouthEast – Ophiuchus – Serpens – Scorpius Globular Star Clusters: •M4 • M 19 • M 62 • M 80 … Summer is Globular Cluster time! M5 Appendix: Eclipses 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 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 (takes hours!) Partial Eclipse Not an Eclipse ! Towards Totality Almost total… Totality Totality