A Warm Up Exercise The Motion of the Sun Which of the following is NOT true of a circumpolar star? a) It rises and sets from my latitude b) Its direction can be far North c) Its direction can be far South d) At sunset it appears in the sky e) You can see it all night A Warm Up Exercise Which of the following is NOT true of a circumpolar star? a) It rises and sets from my latitude b) Its direction can be far North c) Its direction can be far South d) At sunset it appears in the sky e) You can see it all night A Warm Up Exercise If a star rises in the Southeast, it will set in the a) West b) Northeast c) Southeast d) Northwest e) Southwest A Warm Up Exercise If a star rises in the Southeast, it will set in the a) West b) Northeast c) Southeast d) Northwest e) Southwest The Motion of the Sun The Sun moves Eastward by about 1º per day This apparent motion is the reflection of the Earth s annual orbit around the sun 360º in 365 days During the year we see a sequence of 12 ancient constellations known as the Zodiac. Many date from Babylonian times Familiar from astrological lore. These define an astronomical calendar: See which Zodiacal constellation is on your celestial meridian at midnight. The Sun is in the opposite constellation. The Ecliptic plane defined by the position of the Sun is tilted by 23.5º 1
March 1999: Sun in Pisces Pisces Aquarius June 1999: Sun in Pisces Aquarius What is a Day? The apparent SOLAR DAY is the period from one local noon until the next (noon = when the sun crosses the local meridian) September 1999: Sun in Pisces Aquarius December 1999: Sun in Pisces Aquarius This varies a little bit during year because of the Earth s orbit around the sun The Earth s orbit is not a perfect circle The plane of the Earth s orbit is tilted relative to the equator The mean SOLAR DAY, which averages over these small variations, defines our standard 24 hour day. NO. And don t forget it this will be on a test! Is the 24 hour mean solar day how long it takes the Earth to spin around on its axis once? You have to remember that the Earth is moving on its orbit around the Sun too! In one day, the Earth moves =(360 o )(1day/365days) 1degree along its orbit So from noon to noon the Earth has to rotate not 360 o but 361 o That extra degree takes (24hours)/360=4minutes T=24 h (Solar day) T=23 h 56 m 04 s (Sidereal Day) The Sidereal Day The Mean Solar Day = average time from noon to noon = 24 hours The Sidereal Day = rotation period of the Earth = 23 hours, 56 minutes, 4.091 seconds The sidereal day is also the time it takes a star to cross a meridian from one night to the next Noon T=0 h Not to Scale 2
The Orbit of the Earth About the Sun The Earth goes around the sun in 365.24199. days The Earth s equator is tilted by 23.5 degrees from the ecliptic plane defined by the Earth s orbit s and Equinoxes I Because of the 23.5 degree tilt (the obliquity of the ecliptic ) between the celestial equator and the ecliptic plane, the latitude of the sun relative to the equator changes during the year. North Celestial Pole Celestial Equator =23.5degrees s NCP s occur when the Sun is at its maximum northern or southern declination along the Ecliptic. From Latin sol sistit = sun stands. Occur twice a year in June and December: Summer : Maximum northern declination of the Sun. Winter : Maximum southern declination of the Sun Winter Summer CEq SCP NCP Equinoxes Equinoxes occur when the Sun crosses the Celestial Equator. From Latin equi noctis = equal night Occur twice a year in March and September: Vernal Equinox: Sun crosses the Celestial Equator moving North Autumnal Equinox: Sun crosses the Celestial Equator moving South Winter Autumnal Equinox Vernal Equinox Summer CEq SCP 3
http://www.astronomy.ohiostate.edu/~pogge/ast161/movies/zodiac.mov Summer Vernal & Autumnal Equinoxes Z NCP Winter S W N E CEq s and Equinoxes II Summer (June 21 or 22) the sun reaches its maximum latitude above the equator of 23.5 degrees At the winter solstice It is dark 24 hours per day above the arctic circle (latitude=90 23.5=66.5 deg) It is light 24 hours per day below the antarctic circle (latitude= 90+23.5= 66.5deg) The sun is directly overhead at noon on the Tropic of (latitude=-23.5deg) Winter (December 21 or 22) the sun reaches its maximum latitude below the equator of 23.5 degrees Vernal (Spring) Equinox (March 20 or 21) the sun crosses the celestial equator Autumnul (Fall) Equinox (September 21 or 22) the sun crosses the celestial equator We use the location of the sun at the vernal equinox as the prime meridian for defining celestial longitude. Analemmas Summer Stonehenge: Ancient observatory in England (c. 2950-1600 BC) Analemma photographs show the position of the sun at the same clock time during the course of the year. Winter solstice Sunrise at the Summer 4
The Year The 365.2422.day year we use as our standard is the tropical year which is the time between vernal equinoxes as measured on the Tropic of (+23.5degrees latitude). The sidereal year, the time it takes the Earth to go around the sun, is about 20 minutes longer than the tropical year. This is due to the precession of the equinoxes (which we ll come back to). Summary of Solar Annual Motions Annual Motions reflect the Earth s orbit around the Sun: The Ecliptic: Sun s path relative to the stars. The Obliquity of the Ecliptic: 23.5º Constellations of the Zodiac along the Ecliptic. Equinoxes: Sun crosses the Celestial Equator. s: Sun at maximum declination N & S. Length of the day depends on where this Sun is along the Ecliptic. Precession Because the Earth is not perfectly spherical (the circumference around the poles is 0.3% smaller than around the equator), the Moon s gravity (primarily) makes the Earth precess Precession of the Equinoxes Wobble of the Earth s rotation axis about the Ecliptic Pole. Slow westward drift of the rotation axis Takes ~26,000 years to complete 1 circuit Amounts to ~50"/year, or 1º in 72 years. Discovered by Hipparchus of Nicaea (~150BC), but may have been known to the Babylonians. Caused by tidal torques from the Moon & Sun The Age of Aquarius Precession causes the Equinoxes & s to drift westward over time. Vernal Equinox: Now in Pisces, will enter Aquarius in 2597 AD 1 AD: in Aries Summer : Now leaving & entering Taurus 1 AD: in The North Star Precession also changes which star is the northern pole star over time: 2000 AD: Polaris is 0.75º from the North Celestial Pole. Gets closest to the NCP in 2099. 2700 BC: NCP was near the star Thuban in Draco. Pole star of the Old Kingdom of Egypt. 5
Polestar Celestial Coordinates (Redux) This is simply a means of describing where things are, like longitude and latitude on the Earth Latitude=Declination the angle from the celestial equator along a meridian N. Ecliptic pole 23.5 o Longitude=Right Ascension angle from the Vernal equinox (where the sun lies on the equator in the spring) to the object Right Ascension 6