Our Observatory: The Earth 1 Zenith • Point directly overhead, wherever you are 2 Meridian • Meridian – Extends from North to South through zenith. 3 • The Equator runs around the middle of the Earth, through Africa and South America • The “Prime Meridian” runs through Greenwich England The “Obvious” View The celestial sphere: Stars seem to be on the inner surface of a sphere surrounding the Earth They aren’t, but can use two-dimensional spherical coordinates (similar to latitude and longitude) to locate sky objects Celestial Coordinates • Declination: degrees north or south of celestial equator • Right ascension: measured in hours, minutes, and seconds eastward from position of Sun at vernal equinox Vernal what? • Earth is tilted on its axis Ecliptic • The apparent yearly path of the Sun through the stars is called the ecliptic • Equivalent: The Ecliptic is the projection of Earth’s orbit onto the celestial sphere. 8 • Ecliptic is plane of Earth’s path around Sun; at 23.5°to celestial equator • Northernmost point (above celestial equator) is summer solstice ; southernmost is winter solstice ; points where path crosses celestial equator are vernal and autumnal equinoxes Earth’s Orbital Motion – Summer Solstice • The Sun is furthest “north” along the ecliptic---23.5° above the celestial equator. • Longer daylight in Northern hemisphere • Shorter daylight in Southern hemisphere 10 – Summer Solstice • June 21, 2011 • North of Arctic circle, daylight is 24 hours! • South of the Antarctic circle, night is 24 hours! 11 – Equinox • Autumnal (Autumn, Sept. 23, 2010) • Vernal (Spring, March 20, 2011) • On an equinox every place on the Earth sees equal amounts of sunlight. 12 – Winter Solstice • The Sun is furthest “south” along the ecliptic---23.5° below the celestial equator. • Shorter daylight in Northern hemisphere • Longer daylight in Southern hemisphere 13 – Winter Solstice • December 21, 2010 • North of Arctic circle, night is 24 hours! • South of the Antarctic circle, day is 24 hours! 14 Latitude and Longitude Points of reference • Earth is tilted 23.439 o with respect to plane of solar orbit. – Tilt causes seasons on Earth by exposing the northern or southern hemisphere to more or less sunlight. – On longest day of the year, sun is dead overhead at: • Tropic of Cancer (Summer solstice). • Tropic of Capricorn (Winter solstice). 15 – Tropic of Cancer • Northernmost latitude at which the Sun can appear directly overhead at solar noon. • Event occurs at Summer solstice, currently 23° 26 ′ 22 ″ north of the Equator. – The sun now is in Taurus at the June solstice 16 • Tropic of Capricorn – Southernmost latitude at which sun can be directly overhead. – Occurs at Winter Solstice, currently 23° 26 ′ 22 ″ south of the Equator. – In modern times the sun appears in the constellation Sagittarius during this time. Tropic – from the greek “topos” which means “turn” Since it appears that the earth is turning. 17 •• The The eclipticecliptic producesproduces thethe movementmovement ofof sunrisesunrise andand sunsetsunset positionspositions throughoutthroughout thethe yearyear Where is this person standing on the earth? 18 19 Apparent Motion of The Celestial Sphere • Spin the Earth inside the celestial sphere. • The Earth’s rotation cause stars to appear traveling East to West across the night sky. • This apparent motion is called “diurnal motion” 22 Polaris (North Star) The North Celestial Pole points at Polaris Stars close to the celestial poles appear to move in circles. • There is nothing peculiar about Polaris at all (neither particularly bright nor nearby etc.) • North Celestial Pole will be closest to Polaris ~ A.D. 2100. • ~ 12,000 years from now, it will be close to Vega in the constellation Lyra. 24 Precession • At left, gravity is pulling on a slanted top, resulting in a “wobbling” around the vertical. 25 • The Sun’s gravity is doing the same to Earth. • The resulting “wobbling” of Earth’s axis takes about 26,000 years and is called precession . 26 Apparent Motion of the Sun • Now Let’s move the Earth about the sun. • Due to Earth’s orbit around the sun, the sun appears to move through the zodiacal constellations. 27 Constellations • Stars that seem to form a pattern • Define an area on the celestial sphere • 88 official Orion in 3-D • Constellations are not flat, they just look it. • The stars are actually far apart The true three-dimensional relationships among the brightest stars. (The Greek letters indicate brightness—see More Precisely 1-2.) The distances were determined by the Hipparcos satellite in the early 1990s. (See Chapter 17.) 29 Asterisms • pattern of stars seen in Earth's sky which is not an official constellation Zodiac • Constellations that lie on the ecliptic • The sun appears to move through these constellations WINTER SPRING SUMMER FALL The view of the night sky changes as Earth moves in its orbit about the Sun. As drawn here, the night side of Earth faces a different set of constellations at different times of the year. The 12 constellations named here make up the astrological zodiac. 32 •• The The vernalvernal equinoxequinox movesmoves duedue toto precessionprecession soso coordinatescoordinates needneed toto bebe recalculatedrecalculated forfor differentdifferent “epochs”.“epochs”. AtAt presentpresent thatthat pt.pt. isis inin PiscesPisces andand wewe useuse EpochEpoch 20002000 oror 20502050 starstar charts.charts. 34 • The vernal equinox is the point in the sky where the Celestial Meridian, the Celestial Equator and the Ecliptic all meet. It is presently in the southwest of Pisces, moving slowly towards its neighboring constellation, Aquarius. 35.