PHSC 1053: Astronomy Time and Coordinates

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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

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.”

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?

Earth Motion • 4 minutes?

– Solar versus Sidereal

Aligned With Star

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

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)

What Time Is IT?!

• OR What Day is IT Really?!

• Only the Stars Know

“Bowl” Diagram Orienting yourself to a local horizon.

N S

W

NCP

Zenith

Celestial Equator

• 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

Celestial Orientation

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

Sky Atlas

Constellation Lyra, Bright Star Vega

Magnitude

Right Ascension

Planetary Observations • Inner Planets

– Mercury – Venus

• Outer Planets

– Mars – Jupiter – Saturn – Uranus – Neptune – Pluto

Sun

Earth Orbit

Inner Planet

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

Best Time to View Outer Planet = Opposition

Sun

Planetary Alignments

Planetary Observational Terms Summary