Positions of Astronomical Objects

1. Positions of Astronomical Objects • The relative positions of the stars are (very nearly) fixed with respect to (w.r.t.) each other. • The Sun, Moon and Planets positions vary w.r.t. the fixed stars. • We measure positions on the sky using projections of the Earth’s features onto the sky. • Groups of stars that APPEAR nearby on the sky are called CONSTELLATIONS. (but they may be at very different distances -- sky is 2D, universe is 3D) • Sun (and planets) appear to move through a group of constellations we call the ZODIAC.

2. The Zodiac

3. Planets vs. Stars • Planets are WANDERERS • Easily seen: Venus, Jupiter, Mars, Saturn (Mercury) • Later discovered: Uranus, Neptune (Pluto) • They stay NEAR the ecliptic plane (Sun’s path) but are NOT in step w/ stars: faster, slower, sometimes even backwards (retrograde loops) • Planets (usually) don’t TWINKLE like stars do. • Stars are so far away that they appear like points: atmospheric fluctuations (“seeing”) makes their location jump around -- our eye’s fovea “loses” them • Planets are actually smaller, but MUCH closer, so they appear like little disks: their image is never completely lost in your eye.

4. Locating Things in the Sky • Constellations are convenient groups of stars in particular parts of the sky: provide rough locations on the CELESTIAL SPHERE . • BUT they are NOT PHYSICALLY ASSOCIATED GROUPS OF STARS --- some in the same constellation are much farther away than others. • Brightest stars in each constellation names from the Greek alphabet: alpha () Orionis (Betelgeuse), beta () Orionis (Rigel), etc. • Other very bright stars have their own names: e.g., Sirius ( Canis Majoris); Vega ( Lyrae); Altair ( Aquilae), etc. • There are now 88 constellations officially recognized by the International Astronomical Union

5. Orion Constellation in 3-D

6. Latitude & Longitude on Earth

7. Celestial Coordinates: Latitutde & Longitude taken to the Sky • BUT ALL STARS (AND GALAXIES) ARE LOCATED IN CELESTIAL COORDINATES. • Equivalent to LATITUDE is DECLINATION (); degrees (), minutes (') and seconds(") (of arc) • from +90 deg (NCP) to -90 deg (SCP). • 1 circle = 360 deg() • 1 deg = 60 arcmin • 1 arcmin = 60 arcsec, so • 1 arcsec = 1/3600 th of a degree or 1/1,296,000 th of a circle. • Sirius has a declination of: -16, 41', 58"

8. Angular Measure

9. Crude Angular Measures You Can Use Moon’s angular diameter: ~0.5o = 30’ = 1800 arcsec

10. Equivalent to LONGITUDE is RIGHT ASCENSION; (R.A. or ): Zero = spring equinox (where Sun on Ecliptic crosses Celestial Equator) • measured in units of time: hours, minutes and seconds, from 0 hours to 23 h, 59 m, 59.999 s. • One hour of RA = 15 deg of angle (360 degrees/24 hr/day) • Sirius: right ascension of: 6 h, 45 m, 09 s. • Vega has dec=+38O44’ and RA=18h35.2m

11. Circumpolar & Equatorial Stars • At the POLES, ALL stars are polar: N (or S) CP is at the ZENITH (directly overhead) • Other visible stars circle the CPs. • The same stars are seen all the time that the sun is below the horizon (i.e. half the year) and the other half are NEVER seen.

12. At the equator, ALL stars are equatorial. • They rise and set each night. • Polaris is always at the northern horizon. • Half of the stars with DEC = 0 will pass through the zenith during the course of a night. • Stars with higher RA will rise and set later in a particular night. • Stars just rising at sunset (so visible all night long) now will be just setting at sunset in six months (thus below the horizon all night long).

13. At positive latitiude L (example is 40 N), • stars within L deg of the NCP will be circumpolar and these stars have Declinations,  > (90-L) deg. • At latitude L, stars with Declinations, , satisfying: • (90-L) > > (L-90) are equatorial, • Stars with  < (L-90) are "south-polar”: never seen.

14. Sky Looks Different from Different Latitudes Exploring the Celestial Sphere

15. Seasonal/Annual Variations • Seasons: Earth’s REVOLUTION around the sun • Sun rises due E and sets due W on the VERNAL (~Mar 21) and AUTUMNAL EQUINOXES (~Sept 22) • Sun rises most N of E and sets most N of W on the Summer Solstice (~June 21) • Sun rises most S of E and sets most S of W: Winter Solstice (~Dec 21) • Earth is closest to Sun in January -- it is the tilt of Earth’s axis from perpendicular to orbital plane, not distance from Sun, that determines seasons. • The SAME CIRCUMPOLAR stars are seen year-round, BUT different groups of EQUATORIAL zone stars rise and set at different times of year.

16. Tilt of Earth’s Axis  Seasonal Variations • Reason for Seasons

17. Annual Solar Path Changes • Local noon is when Sun crosses meridian. • Height varies throughout the year (analemma), but never directly above (at zenith) unless latitude between -23.5 and +23.5 degrees (Tropics of Capricorn & Cancer) • Sun rises and sets due E and W only on equinoxes

18. The Night Sky: More Motions Many ancient peoples understood most of what we’ve discussed so far though they thought the sky, not earth moved. Egyptians Polynesians Chinese Indians Amerindians Mayans Greeks

19. Slow Sky Change: PRECESSION • PRECESSION: caused by gravitational torques by Sun, Moon (+ planets) on the Earth's non-spherical shape. • Angle between the Earth's axis and the perpendicular to the ecliptic stays around 23.5, but direction changes. • The total period of precession is about 26,000 years. • POLARIS is the POLE STAR NOW (very close to NCP), but 5000 years ago it was THUBAN, and 12,000 years in the future it will be VEGA. • There is no Southern Pole star right now. • Precession implies a star's RA and DEC change about 20 arcmin per year. • There is also ``nodding'' or NUTATION: the Moon changes the Earth's tilt angle by less than 20" over an 18.6 year period.

20. Precession of Non-Spherical Bodies Friction slows the top’s spin energy and angular momentum and eventually it falls over. Earth’s spin slowed by tides.