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PHYS 1025 – Introductory Astronomy Lecture 2, Either Semester - syllabus, information about the course http://astronomy.uconn.edu - observing sessions: Monday – Thursday 9:00 pm, weather permitting http://www.phys.uconn.edu/observatory Richard Crudo Celestial Sphere Our lack of depth perception when we look into space creates the illusion that Earth is surrounded by a celestial sphere. In reality, stars that appear very close together in our sky may actually lie at very different distances from Earth. Remember that we are on the INSIDE of the sphere (on Earth) looking out! Points on the Celestial Sphere • North and south celestial poles • Celestial equator • REMEMBER: These are points /lines on the celestial sphere and NOT on the Earth • From now on: equator = celestial equator The Dome of the Local Sky •Zenith •Nadir •Horizon •Meridian •Transit Horizon coordinate system - coordinates are measured with respect to horizon - change with time and depend on observer • Azimuth: – 0 to 360 degrees around horizon from north towards east – 0° = North, 90 ° = East, 180 ° = South, 270 °= West • Altitude: – 0 to 90 degrees up from horizon – 0 ° = Horizon, 90 ° = Zenith Ecliptic Plane Plane containing the Sun and planets Ecliptic is tilted 23.5° with respect to the Equator Eclipses can only occur when the moon crosses this plane • Ecliptic: – The Sun's apparent annual path among the constellations • Zodiac Constellations – The constellations on the celestial sphere through which the ecliptic passes – Origin of Astrology (Zodiac Sign) Cardinal Points on the Ecliptic • Vernal Equinox • Sun rises due East and sets due West • Length of day = length of night = 12 hours • Summer Solstice • Sun is highest in the sky (this is why it’s so hot during summer) • Autumnal Equinox • Winter Solstice • Sun is lowest in the sky (this is why it’s so cold during winter) Equatorial coordinate system - coordinates fixed on the celestial sphere - time and observer independent • declination (dec) – Analogous to latitude, but on the celestial sphere; it is the angular north-south distance between the celestial equator and a location on the celestial sphere. – Measured in degrees: » 0 ° to 90 ° – north from celestial equator » 0 ° to -90 ° – south from celestial equator • right ascension (RA) – Analogous to longitude, but on the celestial sphere; it is the angular east-west distance between the vernal equinox and a location on the celestial sphere. – Measured in units of time: hours, minutes, seconds » 0 h – 24 h from Vernal Equinox towards east » Ex. Sirius has RA = 6 h 45 m OR 6:45 Don’t confuse RA with time on your watch! Equatorial coordinate system Comparing latitude and longitude to declination and right ascension RA and Dec of the Cardinal Points on the Ecliptic Vernal Equinox – Sun appears on March 21 – RA = 0h Dec = 0˚ Summer Solstice – Sun appears on June 21 – RA = 6h Dec = 23.5˚ Autumnal Equinox – Sun appears on Sept. 21 – RA = 12h Dec = 0˚ Winter Solstice – Sun appears on Dec. 21 – RA = 18h Dec = -23.5˚ RA and Dec of the Cardinal Points on the Ecliptic 23.5° Vernal Equinox Declination – Sun appears on March 21 – RA = 0h Dec = 0˚ Equator 0h 6h 12 h 18 h Summer Solstice – Sun appears on June 21 – RA = 6h Dec = 23.5˚ 24 h Autumnal Equinox – Sun appears on Sept. 21 – RA = 12h Dec = 0˚ Winter Solstice Ecliptic -23.5° – Sun appears on Dec. 21 – RA = 18h Dec = -23.5˚ Example: where is Vega? Its declination tells us that it is 38°44′ north of the celestial equator. We can interpret its right ascension in two ways: As an angle, it means Vega is about 279° east of the vernal equinox As a time, it means Vega crosses the meridian about 18 hours 35 minutes after the spring equinox. Understanding Local Skies 3 classes of stars: circumpolar north - always visible circumpolar south - never visible rising and setting Understanding Local Skies The sky at the North Pole. Understanding Local Skies The sky at the equator Understanding Local Skies The sky at 40˚N latitude. Understanding Local Skies The sky at 30˚S latitude. The altitude of the celestial pole in your sky is equal to your latitude. Everything in the sky rotates around the north celestial pole Sidereal Time Sidereal time 1) Time measured according to the position of stars in the sky rather than the position of the Sun in the sky. 2) How long ago the vernal equinox has transited 3) It’s the Right Ascension of ANY transiting star. Problem 1 • What is the Sidereal Time at noon, December 21? 1. Sidereal Time = RA of transiting Star 2. What star is transiting at noon? Answer: Sun 3. What is significant about Dec. 21? Answer: Winter Solstice: Sun has an RA of 18 hours 4. Therefore, the Sidereal Time at noon on Dec. 21 is 18:00 Equator Problem 2 • Can we see Kapteyn’s star (RA 5 h 9.7 m, Dec -45°) from an observatory at latitude 50° N? 1. Set up a picture: 2. Do the math: Zenith Dec = 50° NCP Equator NCP The horizon is 90° from your zenith Zenith has a dec = your lat = 50° The lowest point in the sky that you can see has a dec of: 50 ° - 90 ° = - 40 ° 3. The star is 5 ° below the Horizon…so we can’t see it Celestial Sphere 50° 40° Lat = 50° Earth Horizon -5° Horizon Equator Dec = -40° Kapteyn’s Star Dec = -45° Problem 4 • A star lies 15 degrees due east of zenith at 10 PM; when will it transit? Zenith 1. Recall that 1 hour is equal to 15 degrees. Why is this? Answer: Earth rotates 360 degrees in 24 hours 360 / 24 = 15 degrees per hour 2. So we know the star will transit 1 hour before or after 10 PM. Since the star is east of the meridian, it hasn’t yet transited. (all stars rise in the east and set in the west as time passes) West East Therefore, the star will transit in one hour: 11 pm Problem 5 • What is the maximum altitude and the azimuth of the sun at noon, September 21 in Storrs, CT.? Zenith Dec = 42° 1. Set up a picture: NCP Celestial Sphere 2. Do the math: Storrs has a lat of 42 ° N NCP The horizon is 90° from your zenith Zenith has a dec = your lat = 42° The lowest point in the sky that you can see has a dec of: 42° Earth 42 ° - 90 ° = - 48 ° Equator Sun Dec = 0° Lat = 42° 48° Equator Horizon Horizon Dec = -48° The sun has a dec of 0 ° on the Autumn Equinox It’s altitude is then: 48 ° Since it is transiting at noon in the south, it’s azimuth must be 180 ° 48 Degrees