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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
N S
W
NCP Zenith
Universal Time • The Time At Zero Degrees Longitude = UT (GMT)
– Universal Time (UT) or Greenwich Mean Time (GMT)
• Greenwich Observatory http://www.rog.nmm.ac.uk/mill/meridian.htm
Tropic of Cancer
Tropic of Capricorn
Equator
Prime Meridian Greenwich Observatory, England
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.”
Day Length • Solar Day
– 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?
Sidereal Day • 1 Sidereal Day later
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.
Aligned With Star
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 21st • As Late as April 20th
• 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 4th would be October 15th – 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)
• Horizon based coordinate system – Horizon dependent (different for each observer)
Altitude - Azimuth
S W
N
E
• Horizon based coordinate system – Horizon dependent (different for each observer)
• Azimuth Angle (degrees) – measured CW from N to E – N = 0o, E = 90o, S = 180o, W = 270o
Altitude - Azimuth
S W
N
E
N
0
180
90 E
S
270 W
• Horizon based coordinate system – Horizon dependent (different for each observer)
• Azimuth Angle (degrees) – measured CW from N to E – N = 0o, E = 90o, S = 180o, W = 270o
• Altitude Angle(degrees) – horizon to star – Horizon = 0o – Zenith = 90o
Altitude - Azimuth
S W
N
E
Right Ascension - Declination • Geocentric Coordinates
– Observer Independent
Toward the Vernal Equinox (Position of the Sun on March 21-22)
Celestial Equator (Earth’s Equator)
North Celestial Pole (NCP)
Right Ascension - Declination
Toward the Vernal Equinox = 0 hours
Celestial Equator (Earth’s Equator)
• Right Ascension (hours) – Measured from the Vernal Equinox along the C.E. – Vernal Eq. = 0 hours
RA
Right Ascension - Declination
Toward the Vernal Equinox = 0 hours
Celestial Equator (Earth’s Equator)
• Declination (degrees) – Measured from the Celestial Equator to the Pole – CE = 0o – NCP = 90o
NCP
DEC
Planetary Observations • Inner Planets
– Mercury – Venus
• Outer Planets
– Mars – Jupiter – Saturn – Uranus – Neptune – Pluto
Sun
Earth Orbit
Inner Planet
Outer Planet
Conjunction • Outer Planets
– Conjunction (Aligned with the Sun) • Always Superior, Never Inferior
Sun
Earth
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
Opposition • Outer Planets
– Conjunction • Aligned with the Sun • Always Superior
– Opposition • Opposite the Sun • Earth Closest Approach
Best Time to View Outer Planet = Opposition
Sun
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