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Richard Crudo

Richard Crudo. 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. Celestial Sphere.

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Richard Crudo

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  1. Richard Crudo PHYS 1025 – Introductory AstronomyLecture 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

  2. 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!

  3. 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

  4. The Dome of the Local Sky • Zenith • Nadir • Horizon • Meridian • Transit

  5. 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

  6. 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

  7. 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)

  8. 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)

  9. 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!

  10. Equatorial coordinate system Comparing latitude and longitude to declination and right ascension

  11. 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˚

  12. RA and Dec of the Cardinal Points on the Ecliptic 23.5° • 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˚ Equator Declination 0 h 6 h 12 h 18 h 24 h Ecliptic -23.5°

  13. 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.

  14. Understanding Local Skies 3 classes of stars: circumpolar north - always visible circumpolar south - never visible rising and setting

  15. Understanding Local Skies The sky at the North Pole.

  16. Understanding Local Skies The sky at the equator

  17. Understanding Local Skies The sky at 40˚N latitude.

  18. Understanding Local Skies The sky at 30˚S latitude.

  19. The altitude of the celestial pole in your sky is equal to your latitude. Everything in the sky rotates around the north celestial pole

  20. 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.

  21. Problem 1 • What is the Sidereal Time at noon, December 21? • Sidereal Time = RA of transiting Star • What star is transiting at noon? Answer: Sun • What is significant about Dec. 21? Answer: Winter Solstice: Sun has an RA of 18 hours • Therefore, the Sidereal Time at noon on Dec. 21 is 18:00

  22. NCP Equator Problem 2 50° 40° Horizon -5° • Can we see Kapteyn’s star (RA 5 h 9.7 m, Dec -45°) from an observatory at latitude 50° N? • Set up a picture: • Do the math: 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 Zenith Dec = 50° NCP Equator Celestial Sphere Lat = 50° Equator Horizon Dec = -40° Earth Kapteyn’s Star Dec = -45°

  23. Problem 4 • A star lies 15 degrees due east of zenith at 10 PM; when will it transit? • 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 • 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) Therefore, the star will transit in one hour: 11 pm Zenith West East

  24. NCP Problem 5 48° 42° Horizon Zenith Dec = 42° • What is the maximum altitude and the azimuth of the sun at noon, September 21 in Storrs, CT.? • Set up a picture: • Do the math: Storrs has a lat of 42 ° N 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 ° - 90 ° = - 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 ° NCP Equator Celestial Sphere Sun Dec = 0° Lat = 42° Equator Earth 48 Degrees Horizon Dec = -48°

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