LESSON%2018:%20Time

1
LESSON 18Time

• Learning Objectives
• Comprehend the longitude/time relationship, time
  conversion, zone time determination, and motions
  of the sun as the basis of time.

2
Apparent Solar Time andMean Solar Time

• Apparent solar time - reckoned with respect to
  the passage or the observable sun (the apparent
  sun).
• Mean solar time - uses a hypothetical mean sun,
  whose hour circle moves at a constant rate along
  the celestial equator.
• The mean sun moves at the average rate of motion
  of the apparent sun.
• In 24 hrs, it moves through 360 deg of arc over
  the circumference of the earth, or through 15 deg
  of arc every hour, or through 15 of arc every
  minute.

3
The Equation of Time

• The difference between the mean and apparent
  times is called the Equation of Time. It changes
  slightly from day to day.

• Found in the bottom right hand corner of each
  daily page of the Nautical Almanac.

4
Local Mean Time andArc to Time Conversions

• Local Mean Time - time referenced to the
  observers meridian also called LMT.
• This time is in contrast to GMT (Greenwich Mean
  Time), time referenced to the Greenwich meridian
  and to ZT (zone time), time referenced to the
  central meridian of the various time zones.
• Based on the relationship of the mean sun with
  the observers meridian. LMT differs from ZT by
  the amount of time required for the mean sun to
  traverse the difference in longitude between the
  observers meridian and the standard (central)
  meridian of the time zone.

5
Local Mean Time andArc to Time Conversions

• Since the mean sun moves through 15 deg of arc
  every hour, or through 15 of arc every minute,
  these differences in longitude can be directly
  converted into differences in time.
• The Conversion of Arc to Time table in the
  Nautical Almanac aids in this calculation.
• Just remember that any location east of another
  has a later time, and any location to the west
  has an earlier time.

6
Arc-to-Time Conversion
7
LMT of Apparent Noon
8
Zone Time of Noon Local Apparent Noon (LAN)

• Use arc-to-time conversion method