Cosmology Course Notes

Chapter 4.

Coordinates and Time

4.1. Astronomical Coordinate Systems

• Horizon system
  • Azimuth (0o-360o) along horizon
  • Altitude (0o-90o) up from horizon toward zenith
  • Intuitive system
• Equatorial system
  • Right ascension (0h-24h) along celestial equator
  • Declination (0o-90o) away from celestial equator towards celestial poles
  • Rotates with celestial sphere

4.2. Time Systems And The Calendar

• Sense of time - established in humans by three observations
  • Daily rising and setting of Sun
  • Monthly phases of Moon
  • Annual seasons of Sun
• Sidereal time - time determine by passages of a star over celestial meridian
  • Sidereal day - time interval between successive passages of vernal equinox over celestial meridian
    • Sidereal day divided into 24h, sidereal hour divided into 60m, sidereal minute divided into 60s
    • Sidereal day = 23h 56m 04s of solar day
  • Sidereal year = 365.256d (sidereal days)
• Solar time - time determined by passages of Sun over celestial meridian
  • Solar day - time interval between successive passages of Sun over celestial meridian
    • Solar day divided into 24h, solar hour divided into 60m, solar minute divided into 60s
    • Solar day = 24h 03m 57s of sidereal day
  • Tropical year = 365.242d (solar days)
• Standard time - 1884, divides Earth into 24 time zones whose local solar time differs by 1h
  • Each time zone is 15o of longitude wide and keeps time for central meridian of longitude for zone
  • Standard meridians in US are
    • 60o for Atlantic standard time
    • 75o for Eastern standard time
    • 90o for Central standard time
    • 105o for Mountain standard time
    • 120o for Pacific standard time
  • International date line is 180o meridian, day differs by 1 across

4.3. Calendars

• Lunar calendars
  • Many ancient peoples used synodic month of 29.5 solar days for basis of calendar
  • Year of 12 synodic months (12 x 30d = 360d), so inserted 13th month periodically
• Julian calendar
  • Egyptians first to use calendar of 365.25d
  • Julian calendar consisted of 3 years of 365d and 1 year of 366d
  • Leap years were those divisible by 4, example 1988
  • Over corrects since fractional day in tropical year is 0.242 and not 0.25
• Gregorian calendar
  • Consists of 3 years of 365d and 1 year of 366d
  • Leap years again those divisible by 4 except those century years divisible by 400, example 1600 and 2000
  • Accurate to about 1d in 3300 years
  • Adopted by Catholic countries in 1582 A.D. and Protestant countries in 1700 A.D.