THE NIGHT SKY

Every culture had description of the sky (Babylonians, Greeks, Chinese, etc.)

- observed patterns of bright stars

- imagined figures of their mythology in these patterns

Constellations -“stars together”

Entire sky is now divided into 88 constellations - have definite boundaries - every location in sky is in a constellation

http://stars.astro.illinois.edu/sow/const.html

http://stars.astro.illinois.edu/sow/const.html

Constellations - used as calendars for planting/harvesting - many constellations only visible during certain seasons

Orion (Winter); Leo (Spring) Cygnus (Summer); Pegasus (Autumn)

- used as navigational guides (Pole Star, Polaris, signals North)

-

The “Battle”:

How things appear VS.

How things really are Stars within a constellation

- most are not physically related with each other(!)

stars may actually be very far apart in space

Apparent Motion of the Sky

- stars are very far away from earth

appear to have fixed positions

- motion during night: - constellations move across sky - rise in east, move to west and set - northern constellations appear to spin around north star (Polaris)

Motion that repeats every 24 hours: “Diurnal Motion”

caused by motion of earth around its axis: “Rotation”

Rotation: Spinning motion around an internal axis

http://messenger.jhuapl.edu/the_mission/flyby_movie.html

Additional motion of the sky -from night to night, stars rise - about 4 minutes earlier each night

- a given star will not rise at exactly the same time until 1 year later

Motion that repeats yearly: “Annual Motion”

caused by motion of earth around the sun: “Revolution”

Revolution: Earth spins about an external axis

The Celestial Sphere

represents how the sky appears to eye: - roughly 6000 stars visible - all appear to be at same distance

- all appear to remain in same patterns

Belief of ancient societies: - stars fixed on inside surface of

huge sphere Celestial Sphere

- still useful today as map of the sky!

Project Earth’s equator and poles onto sky:

- North Celestial Pole (NCP)

- Celestial Equator (CE)

- South Celestial Pole (SCP)

Angles - “exact measure of the opening between two connecting straight lines” - measured in degrees or radians

Complete Circle

circle = 360o = 2 rad

Examples: right angle straight line 90o or /2 180o or

Astronomy - use degrees to measure angles

For more precise measurements:

1o = 60 arcminutes = 60’

1’ = 60 arcseconds = 60”

Positions of objects on Earth: latitude (N-S) longitude (E-W)

C Bldg, GPC Clarkston Campus: Lat: 33o 47’ 33” N Lon: 84o 14’ 03” W

Astronomy Dept, Boston University Lat: 42o 21’ 07” N Lon: 71o 06’ 24” W

Positions on Celestial Sphere:

Declination, : - same as latitude on Earth, N-S - measured in degrees, arcmin, arcsec - north of CE = positive, south = negative

Right Ascension, : - similar to longitude on Earth, E-W - measured in time (hours, minutes, sec) - measured starting from location of

vernal equinox - divided into 24 sections (1 hour ea.)

Examples: Equatorial

Betelgeuse: = 5h 55m 10s; =+7o 24’ 25”

Rigel: = 5h 14m 32s; =-8o 12’ 06”

Vega: =_________; =____________’ 01”

____________: = 6h 23m; = -52o 41’

____________: = 14h 15m; = +19o 11’

Examples: North

Dubhe: =_________; =____________

____________: = 0h 40m; = +56o 32’

Examples: South

Mimosa: =_________; =____________

____________: = 1h 37m; = -57o 14’

Free Star Charts: http://www.midnightkite.com/starcharts.html

Time Keeping

System is based on position of Sun

Zenith: Point on Celestial Sphere directly overhead

Meridian: Circle around Celestial Sphere passing through NCP, SCP, Zenith

Local or “High” Noon – sun crosses meridian (above horizon)

a.m. - “Ante-meridian” – sun has not crossed meridian

p.m. - “Post meridian” sun has already crossed meridian

Should 12:00 noon be “am” or “pm”?

Apparent Solar Time: - measured by a sundial

Apparent Solar Day: - time between two successive noons - 24 hours But, each meridian has its own solar time! - 1883, U.S. Congress - 4 “time zones” across the U.S., each ~15o wide - zone uses solar time for center of zone

Sidereal Time: - based on motion of stars

Sidereal Day: - time between two meridian transits of vernal equinox - 1 sidereal day = 23hrs 56min 4.09sec (less than 24 hrs!!) - true rotational period of Earth

Apparent Motion of the Sun

Position of Sun in sky: - changes RA and dec. every day - moves around Celestial Sphere once a

year - follows “path” call ecliptic

Reason:

- Earth moves around sun

Sun passes through different constellations

- origins of signs of Zodiac - “Birth Sign” is where the sun is on your birthday - But, ecliptic has been slowly changing

- Due to complicated motion of Earth

Traditional Signs of Zodiac

Aries Mar 21 – Apr 20 Libra Sep 24 – Oct 23 Taurus Apr 21 – May 21 Scorpio Oct 24 – Nov 22 Gemini May 22 – Jun 21 Sagittarius Nov 23 – Dec 22 Cancer Jun 22 – Jul 22 Capricorn Dec 23 – Jan 20 Leo Jul 23 – Aug 21 Aquarius Jan 21 – Feb 19 Virgo Aug 22 – Sep 23 Pisces Feb 20 – Mar 20

Actual (Current) Signs of Zodiac

Aries Apr 19 - May 13 Scorpius Nov 23 - Nov 29 Taurus May 14 - Jun 19 Ophiuchus Nov 30 - Dec 17 Gemini Jun 20 - Jul 20 Sagitarius Dec 18 - Jan 18 Cancer Jul 21- Aug 9 Capricornus Jan 19 - Feb 15 Leo Aug 10 - Sep 15 Aquarius Feb 16 - Mar 11 Virgo Sep 16 - Oct 30 Pisces Mar 12 - Apr 18 Libra Oct 31 - Nov 22

Observations of sun’s location - height of sun above horizon at noon - changes throughout the year

- ecliptic is a circle tilted with respect to celestial equator

Reason: Earth’s axis is tilted by 23.5o compared to its orbit

Tilt of Earth’s axis - cause of our seasons - at solstices, most of sun’s energy falls on N

or S hemisphere

Key Dates for Seasons:

Vernal Equinox (~March 21) - Sun crosses CE from south to north - 12 hrs daylight / 12 hrs nighttime

Summer Solstice (~June 21) - Sun reaches highest declination “solar standstill” - longest daylight / shortest nighttime (Northern Hemisphere)

Autumnal Equinox (~September 21) - Sun crosses CE from north to south - 12 hrs daylight / 12 hrs nighttime

Winter Solstice (~December 21) - Sun reaches lowest declination - Shortest daylight / longest nighttime (Northern Hemisphere)

Seasons & length of daylight - reversed in Southern Hemisphere

2015 Spring begins March 20, 6:45pm EDT

Summer begins June 21, 12:38pm EDT

Fall begins Sept. 23, 4:20am EDT

Winter begins Dec. 21, 11:48pm EST

2016 Spring begins March 20, 12:30am EDT

Summer begins June 20, 6:34pm EDT

Fall begins Sept. 22, 10:21am EDT

Winter begins Dec. 21, 5:44am EST