PHSC 1053: Astronomy Time and Coordinates
Astronomical Clocks Earth s Rotation on its Axis Time between two successive meridian transits of the sun 1 solar day (our adopted clock time) 24 hours (86,400 seconds)
Local Time Meridian (North-South Line Through Zenith) Meridian Transit (Object Crossing The Meridian) Local Noon = Solar Meridian Transit NCP Zenith N S W
Universal Time The Time At Zero Degrees Longitude = UT (GMT) Universal Time (UT) or Greenwich Mean Time (GMT) Tropic of Cancer Equator Prime Meridian Greenwich Observatory, England Tropic of Capricorn Greenwich Observatory http://www.rog.nmm.ac.uk/mill/meridian.htm
World Time Zones
U.S.A. Time Zones
Earth s Orbit Counter Clockwise (When viewed from above?) Above = from Polaris to North Pole, N. Hemisphere Rotation (Earth s spin about its axis) Revolution (Earth s orbit around the Sun) Sun
Astronomical Clocks Earth s Rotation on its Axis Time between two successive meridian transits of the sun 1 solar day (our adopted clock time) 24 hours (86,400 seconds) Earth s Orbit Around the Sun Sun s Path on the Sky Returns to the Same Constellation 1 solar year 365.2422 days
Astronomical Clocks Earth s Rotation on its Axis Time between two successive meridian transits of the sun 1 solar day (our adopted clock time) 24 hours (86,400 seconds) Earth s Orbit Around the Sun Sun s Path on the Sky Returns to the Same Constellation 1 solar year 365.2422 days Moon s Orbit Around the Earth 1 month (1 moon th) 29.5 days
Days of the Week The 7 heavenly bodies visible with the unaided eye are each honored with their own day.
Solar Day Day Length Observe Successive Meridian Transits of the Sun 24 hours (86,400 seconds) Clock Time Sidereal Day (sidereal = with respect to the stars ) Observe Successive Meridian Transits of a Star 23 hours 56 minutes (86,160 seconds) Sky Time A 4 MINUTE DIFFERENCE!! What Gives?
4 minutes? Solar versus Sidereal Aligned With Star Earth Motion Aligned With Sun
Earth Motion
Earth Motion
Earth Motion
Earth Motion
Earth Motion
Earth Motion
Earth Motion
Earth Motion
Earth Motion
Sidereal Day 1 Sidereal Day later Aligned With Star Since there are ~365 days in the year and 360 degrees in a circle, the earth moves along its orbit about ~1 degree over the course of a day.
Solar Day 1 Sidereal Day later plus 4 more minutes The Sky appears to move westward by 4 minutes each day when compared to solar or adopted clock time. Aligned With Sun
4 Miniscule Minutes Actual Length of Year (365.2422 days) 365.25-365.2422 = 0.0078 days (11 minutes/year)
A Tale of Time Actual Length of Year (365.2422 days) 365.25-365.2422 = 0.0078 days (11 minutes/year) After 1500+ years, this miniscule error accumulates 11 minutes/year X 1500 = 11 days WHO CARES?
Julian Calendar Adopted ~ 45 B.C. Ten months (Mar,, Sept, Oct, Nov, Dec???) Add July (Julius Caesar) Add August (Augustus Caesar) Three Years of 365 days One Year with 366 days (Leap Year) This simulates a Calendar with 365.25 days per year if averaged over the four years.
Easter Astronomically Defined The First Sunday After The First Full Moon After The Vernal Equinox (date when sun is in a particular location on the sky) As Early as March 21 st As Late as April 20 th BUT that 11 days made Easter LATER and LATER
Gregorian Reformation By 1582 A.D. Pope Gregory had had enough. He Made A Proclamation October 4 th would be October 15 th Century Years Divisible by 400 are NOT Leap Years Average Year 265.2425 365.2424-365.2422 = 0.00027 days (23 seconds) Time needed to accumulate 1 day of error (3850 years)
What Time Is IT?! OR What Day is IT Really?! Only the Stars Know
Bowl Diagram Orienting yourself to a local horizon. Zenith NCP Celestial Equator N S W
Altitude - Azimuth Horizon based coordinate system Horizon dependent (different for each observer) E N S W
Altitude - Azimuth Horizon based coordinate system Horizon dependent (different for each observer) Azimuth Angle (degrees) measured CW from N to E N = 0 o, E = 90 o, S = 180 o, W = 270 o W N 0 270 180 S 90 E N E W S
Altitude - Azimuth Horizon based coordinate system Horizon dependent (different for each observer) Azimuth Angle (degrees) measured CW from N to E N = 0 o, E = 90 o, S = 180 o, W = 270 o Altitude Angle(degrees) horizon to star Horizon = 0 o Zenith = 90 o E N S W
Celestial Orientation
Right Ascension - Declination Geocentric Coordinates Observer Independent North Celestial Pole (NCP) Celestial Equator (Earth s Equator) Toward the Vernal Equinox (Position of the Sun on March 21-22)
Right Ascension - Declination Right Ascension (hours) Measured from the Vernal Equinox along the C.E. Vernal Eq. = 0 hours Celestial Equator (Earth s Equator) RA Toward the Vernal Equinox = 0 hours
Right Ascension - Declination Declination (degrees) Measured from the Celestial Equator to the Pole CE = 0 o NCP = 90 o NCP DEC Celestial Equator (Earth s Equator) Toward the Vernal Equinox = 0 hours
Sky Atlas Magnitude Right Ascension Constellation Lyra, Bright Star Vega
Planetary Observations Inner Planets Mercury Venus Outer Planets Mars Jupiter Saturn Uranus Neptune Pluto Inner Planet Sun Earth Orbit Outer Planet
Conjunctions Inner Planets Conjunction (Aligned with the Sun) Inferior Sun Earth
Conjunction Outer Planets Conjunction (Aligned with the Sun) Always Superior, Never Inferior Sun Earth
Conjunctions Inner Planets Conjunction (Aligned with the Sun) Inferior Superior Sun Earth
Planetary Elongation
Elongation Inner Planets Conjunction (Aligned with the Sun) Inferior Superior Greatest Elongation (Farthest From Sun) Eastern Western Sun Earth
Elongation Inner Planets Conjunction (Aligned with the Sun) Inferior Superior Greatest Elongation (Farthest From Sun) Eastern Western Best Time to View Inner Planet = Greatest Elongation Earth
Conjunction Outer Planets Conjunction Aligned with the Sun Always Superior Sun Earth
Opposition Outer Planets Conjunction Aligned with the Sun Always Superior Opposition Opposite the Sun Earth Closest Approach Sun Best Time to View Outer Planet = Opposition
Planetary Alignments
Planetary Observational Terms Summary