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Lec. 26: The Night Sky - II The Night Sky II Lecture 26 Lec. 26: The Night Sky II 1 Lecture Topics Astronomical coordinates The changing sky Using RA and Dec Finding objects using coordinates What’s up tonight? If time: Planetarium demo See Planetarium S/W link in “interesting astronomy sites” on class web site Lec. 26: The Night Sky II 2 26 - 1 Lec. 26: The Night Sky - II Equatorial Coordinates Astronomers use equatorial coordinates to locate objects on the celestial sphere. Right Ascension Notation: RA or Equivalent to longitude Declination Notation: Dec or Equivalent to latitude Lec. 26: The Night Sky II 3 Defining RA and Dec. RA is measured in hours The range is from 0 to 24 hours increasing on sky towards the east. The “zero point” is towards the constellation Pisces (Vernal Equinox). Dec is measured in degrees. The zero is on the equator North Pole = 90o South Pole = -90o Lec. 26: The Night Sky II 4 26 - 2 Lec. 26: The Night Sky - II Equatorial Coordinates (Cont’d) The equatorial (celestial) coordinate system is fixed on the sky. The coordinates (, ) of the stars and constellations do not change (ignoring precession). Since it is an “earth-centered” system the coordinates (, ) of the sun do change. Lec. 26: The Night Sky II 5 Equinoxes North 23.5o Sun here on first day of fall Autumnal Equinox Equatorial Plane 1st day of summer 12 6 hr 18 Ecliptic Plane 0 1st day of winter Vernal Equinox Sun here on first day of spring Equinoxes at the intersection of the equatorial and ecliptic planes. Lec. 26: The Night Sky II 6 26 - 3 Lec. 26: The Night Sky - II The Changing Sky At the same time each night, a different RA will be on the meridian at different times of the year. For instance at midnight, the RA’s on the meridian are: Sept. 21 Dec. 21 0 hr , 6 hr , Lec. 26: The Night Sky II March 21 12 hr June 21 18 hr 7 Viewed looking down from the north. 6 hr Winter Sun 12 hr Sun 0 hr Fall Midnight Sep. 21 Midnight Dec. 21 18 hr Lec. 26: The Night Sky II 8 26 - 4 Lec. 26: The Night Sky - II Simulation 1: Yearly variation Earth moving around the sun. Why we see different parts of the sky at different times of the year. Here the observer is out at midnight each night Lec. 26: The Night Sky II 9 Simulation 2: Daily variation Rotation of earth as it moves around the sun. How we see different portions of the sky at night. Lec. 26: The Night Sky II 10 26 - 5 Lec. 26: The Night Sky - II Viewed looking down from the north. 6 hr Spring midnight 12 hr on meridian 12 hr 0 hr Sun 4:00 AM 16 hr on meridian Lec. 26: The Night Sky II 18 hr 11 The Changing Sky (continued) Lec. 26: The Night Sky II to sta r sta r mi dn igh t 4 min. to Each night a given object will pass over the meridian 4 minutes earlier. This corresponds to 2 hours earlier each month, or 24 hours in one year. Objects rise and set earlier each day. At a given time, the RA crossing the meridian increases by 4 min. per day. midnight 1 day along orbit Motion of earth along orbit is exaggerated. 12 26 - 6 Lec. 26: The Night Sky - II In-class question You are in Santiago, Chile (in the southern hemisphere). Which direction does the Sun move through the sky? a) b) c) d) e) north to south east to west south to north west to east It doesn’t move Lec. 26: The Night Sky II 13 Using RA to find an object Given two of RA, meridian transit time, and date Find remaining one Method Always work with RA (RAmid) on the meridian at midnight Find closest reference date (via RA or date) Sep 21 (0), Dec 21 (6), Mar 21 (12), Jun 21 (18 hr) Use fact sky changes by ~ 1 hour of RA per 2 weeks. Lec. 26: The Night Sky II 14 26 - 7 Lec. 26: The Night Sky - II On what date does Orion appear on the meridian at midnight? Orion Nebula RA = 5.5 hr Dec = -5.5 deg RA = 6 hr transits at midnight on Dec 21. Orion transits at midnight on Dec 14. Also Orion transits at 9:00 p.m. on Jan 28. Orion transits at 3:00 a.m. on Oct. 31. Lec. 26: The Night Sky II 15 Example 1: What RA is on the meridian for a given date and time? What RA is on the meridian at 3:00 am on Feb. 21? Dec 21 -- 6 hr overhead at midnight Feb 21 -- 2 months later => add 4 hr => 10 hr overhead at midnight 3:00 am => 3 hr later => 13 hr overhead Lec. 26: The Night Sky II 16 26 - 8 Lec. 26: The Night Sky - II Ex. 2: On what day does a given RA cross the meridian at a specific time? A constellation is at RA = 14 hr. When will it cross the meridian at 9:00 pm? When 14 hr crosses at 9:00 pm, 17 hr crosses at midnight. Mar 21: 12 hr crosses at midnight 17-12 = 5 hr => 2.5 months (10 weeks) => June 7 Lec. 26: The Night Sky II 17 RA/Meridian Question What RA is on the meridian at 9:00 pm tonight (October 29)? Where is Vega (RA ~ 18.5 hrs) at this time? What is nearest reference date? a) Mar 21 b) Jun 21 c) Sep 21 d) Dec 21 What is RA at midnight on that date? a) 0 hr b) 6 hr c) 12 hr d) 18 hr There are 23 days between ref. date and today. What is RA at midnight tonight? a) 22 hr b) 0 hr c) 2 hr d) 4 hr 0 + 2 hr = 2 hr What RA is on meridian at 9:00 pm tonight? a) 9 hr b) 23 hr c) 2 hr d) 5 hr 2 - 3 hr = 23 hr Is Vega east or west of this? a) east b) west When did Vega transit? 23 - 18.5 = 4.5 hr a) 3 pm b) 4:30 pm c) 6:30 pm d) 9:00 pm 9:00 - 4:30 = 4:30 pm Lec. 26: The Night Sky II 23 26 - 9 Lec. 26: The Night Sky - II Finding objects (continued) Circumpolar objects can be visible any time of the year For example, Polaris, the pole star. From Ithaca southerly objects are best observed during transit. Really, all objects are best observed when they transit (you look through the least atmosphere) Lec. 26: The Night Sky II 24 What’s “up” tonight? The Moon is near full. Bad time for viewing planets - Not good for star gazing. Mars may be visible at sunset Jupiter will be visible ~ 2.5 hours after sunset Notable constellations Cygnus is west of the meridian at sunset The Big Dipper will be low in the north Orion rises at ~ 10:00 AM Lec. 26: The Night Sky II 25 26 - 10