IS1101 Nothing to Something Mr Goh Hock Leong
Content • Celestial Sphere • declination, right ascension • vernal equinox, autumnal equinox, winter solstice & summer solstice • celestial poles, celestial equator, ecliptic • Seasons
Movement of Earth • Revolution of Earth around the Sun (Heliocentric model) • Earth rotates about its own axis • Axis of rotation tilted at 23.5o
Horizon These star cant be seen as it is above the horizon This star cannot be seen as it is below the horizon
Celestial Sphere • The Earth blocks half of this celestial sphere from view at any instant. • As the Earth rotates different stars come into view. The stars you see depend on • the time of night (i.e. which way the Earth is turned). • whether the Sun is up (the stars are still there but you can't see them). • where you are on the Earth. For example, if you are at the Earth's North Pole the bulk of the Earth will always block the Southern Hemisphere's stars from your view.
Zenith and Meridian • Zenith is the point directly over an observer's head. • Meridianis an imaginary line running north to south through your zenith and dividing the sky in half. • "Meridian" is from an ancient word meaning midday, which is when the Sun crosses your meridian. • When telling time, we use the letters a.m. to indicate morning for ante meridiem, which is Latin for "before midday." • We use p.m. for post meridiem, or "after midday."
Celestial Sphere • Earth at the centre (geocentric model) • Imagine the sky as a great, hollow, sphere surrounding the Earth. The stars are attached to this sphere • Sphere rotates around the stationary Earth roughly every 24 hours (retrogade motion)
Celestial Sphere • The celestial sphere is an imaginary sphere of infinite size surrounding the stationay Earth with the stars embedded in its surface. • Even though we now know that this ancient model of a stationary Earth is incorrect, we still use this model because it is a convenient way to predict the motions of the stars and planets relative to a location on the Earth.
Reference points and lines • Important/Reference points • North Celestial Pole (NCP) • South Celestial pole (SCP) Important/Reference lines • Celestial Equator • Ecliptic
Celestial Pole • Projection of the Earth’s pole out to the sky. • The stars rotate around the NCP & SCP. • These are the points in the sky directly above the geographic north and south poles, respectively.
Celestial Poles • The Earth's axis of rotation intersects the celestial sphere at the celestial poles. • The number of degrees the celestial pole is above the horizon is equal to the latitude of the observer.
Celestial Equator • Same plane as Earth’s equator
Celestial Sphere • Imaginary sphere • Surrounds the Earth • Maps the location of stars • Stars are visualised as to be on the surface of the Celestial Sphere • Infinite size
Globe We use lines of latitude and longitude to pinpoint position on Earth.
Lines of Longitude • Imaginary lines running from pole to pole • ‘Tied’ to Earth • Measures angular distance • Measured in degrees • East or west of Prime Meridian (0 degrees)
Lines of Latitude • Imaginary lines running parallel to equator • ‘Tied’ to Earth • Measures angular distance • Measured in degrees • North or south of Equator (0 degrees)
Coordinate System • Like the longitude and latitude system here on Earth, the celestial sphere has a system to pinpoint locations.
Declination • tells us how far an object is north or south of the celestial equator, measured in degrees • Measures angular distance • Expressed as an angle rather than in metres • Expressed in degrees • + (above celestial equator) • - (below) celestial equator)
RightAscension • Measured in hours, minutes and seconds from a zero point • (* Note: angular measures, not units of time!) • Position of sun at vernal equinox • Increases from west to east
RightAscension • Right ascension tells us how far an object is to the east of the Vernal Equinox (chosen arbitray), that point midway between winter and summer when the ecliptic (the Sun's path) crosses the celestial equator. • Latitude is measured to both the east and west of its starting point, but right ascension is measured only to the east. • Also, right ascension is not measured in degrees, but in hours, minutes, and seconds. • Each hour is equal to 15'.
Food for Thought If the celestial poles, celestial equator, and declination are projections of earthly coordinates, why not simply imagine right ascension as projections of lines of longitude?
Answer View of the night sky changes as the earth orbits and rotates
Ecliptic • A circular path taken by the sun • Inclined at an angle of about 23.5 degrees to the celestial equator • In the course of a year • Against a background of stars
Why do we have seasons? • Different distance from the Sun? • Tilt of Earth? • Area effect? • Atmosphere effect? • Height of the sun above the horizon • Length of the day due to the position of the sun
Equinoxes and Solstices • Earth’s axis is tilted • Our planet orbits the sun • Each hemisphere gets varying amounts of sunlight during the year
Solstice: Days of longest or briefest daylight • Sol : Sun • -stice : Stand Still Equinoxes and Solstices Equinoxes: Days of equal night and day
Summer Solstices • Occurs on June 22 • In the northern hemisphere • Longest day • Sun is the highest in the sky • Marks the beginning of summer • A point in the earth’s orbit • Planet’snorth pole points closest to the sun • Locations spend the greatest portion of their time exposed to the sun
Winter Solstices • Occurs on December 21 • In the Northern Hemisphere • The shortest day • The sun is the lowest in the sky • Marks the beginning of winter • Six months after summer solstice
Equinoxes • Days of equal night and day • When the ecliptic intersects the celestial equator as the sun passes into the northern hemisphere • Vernal Equinox • Occurs March 21 • Marks the beginning of spring • Autumnal Equinox • Occurs September 23 • Marks the beginning of fall
Equinoxes and Solstices Summer Solstice VernalEquinox Autumnal Equinox Winter Solstice
Reference • Astronomy Fifth Edition– A self teaching guide by Dinah L. Moche • Astronomy Today Second Edition by Chaisson/McMillan • The complete Idiot’s Guide to Astronomy Second Edition • http://csep10.phys.utk.edu/astr161/lect/celestial/celestial.html • http://www-astronomy.mps.ohio-state.edu/~pogge/Ast161/Unit4/movearth.html • Iowa State University, Polaris Project north Star, http://www.polaris.iastate.edu/NorthStar/Unit4/unit4_sub1.htm