1. ASTRONOMICAL OBSERVATIONS Astronomical Observations

2. CONSTELLATIONS

3. On a clear night… an unaided eye can see about 3000 points of light. these points of light comprise 88 constellations.

4. Constellations make locating stars easier, and add “story” to the sky. Look for distinguishing features in this figure.

5. Constellations Con = with, stell = star, ation= process. Show patterns/relationships among objects, even though no true connection (in most cases). Ancient man connected the brightest stars into patterns and named them after mythological beings, heroes, creatures, animals and familiar objects. Different cultures – different constellations – different stories, but tended to use same grouping.

6. In older times, constellations were used as navigational guides. Polaris, the “North Star”, start as the guiding point  does not appear to move from one night to the next, or from one season to the next. CONSTELLATION TRACKING

7. PRIMITIVE CALENDARS – PREDICT PLANTING AND HARVESTING SEASONS Winter TriangleSummer Triangle

8. Practical Applications of Constellations Start of seasons (northern hemisphere) Orion signals winter Lyra signals summer Hercules signals spring Pegasus signals fall

9. Star Groupings Constellations = 88 accepted Asterisms A grouping of stars that is not officially a constellation Many are groupings within an official constellation Ex. – Big / Little Dippers Ex. – Winter / Summer Triangle Ex. – Orion’s belt

10. CELESTIAL SPHERE

11. Celestial Sphere An imaginary sphere of the sky, that is centered on the Earth, and all stars located on it. North and South Celestial Pole Points directly above poles. Celestial Equator Imaginary line between poles.

12. Zenith Point directly above an observers head Meridian Imaginary line connecting north to south celestial poles and intersecting the zenith. Line marks the highest point each star reaches above the horizon each night Divides rising and setting stars Sun’s Relation to Time Ante-meridian (a.m.): before crosses Post-meridian (p.m.): after crosses

13. STELLAR MOTION Seasonal Stars: stars rise and set, appear only during certain seasons Circumpolar Stars: Stars move very little, always visible All stars move in circles very close to Polaris (North Star) Apparent motion is caused by Earth’s rotation Circular motion around an axis

14. What stars you see passing through your zenith depends on your location

15. Precession Earth’s ‘wobble’ on its axis  changes directions “North star” (Polaris to Vega) changes approximately every 12,000 years, or ~26,000 year cycle. Caused by torques (twisting motions) on Earth due to gravitational pulls of the Moon and Sun Causes (or due to) Earth’s 23.5° tilt Relate it to the slowing of a spinning top

16. Because of precession, Earth’s physical geographic equator does not line up with the rest of the solar system. …meaning the Sun’s path in our sky changes. Ecliptic (relates to eclipse): The path the sun takes across our sky The ecliptic is also ~23.5° tilted off / from the celestial equator.

17. The ecliptic is where your `sign' comes from. The Zodiac is made up of the 12 constellations which the Sun passes through on its year long trip along the Ecliptic. When you are born, the constellation that the Sun is presently projected against becomes your 'sign‘ – “the Sun rises in the house of ___”.

18. ANGULAR MEASURE Locating stars in the celestial sphere Size and scale – specified by measuring lengths and angles Angles: Circle = 360°, therefore half circle = 180° (horizon to horizon) 1° increment  subdivided fractions = arc minutes 60’ (arc mins) = 1° Degrees, arc mins, arc seconds = ways to measure size and position of objects in universe Angular size depends on actual size and distance from Earth

19. CELESTIAL COORDINATES Method of locating stars Specify constellation Rank stars in order of brightness Limited utility (more stars than Greek Letters) Extend latitude (declination) and longitude (right ascension) onto celestial sphere

20. Like latitude Measures North or South of the celestial equator Measured in degrees Equator - 0° North Pole +90° South Pole -90° DECLINATION

21. RIGHT ASCENSION (RA) Like geographic longitude Measured in hours, minutes, and seconds which increase in the eastward direction Zero RA is arbitrary Taken to be the point in the sky at the instant of vernal equinox 24hrs = 360°, 1 hr = 15° (1 h ),