LESSON 18:LESSON 18:TimeTime

• Learning ObjectivesLearning Objectives– Comprehend the longitude/time

relationship, time conversion, zone time determination, and motions of the sun as the basis of time.

Apparent Solar Time andApparent Solar Time andMean Solar TimeMean Solar Time

• Apparent solar time - reckoned with respect to Apparent solar time - reckoned with respect to the passage or the observable sun (the apparent the passage or the observable sun (the apparent sun).sun).

• Mean solar time - uses a hypothetical mean sun, Mean solar time - uses a hypothetical mean sun, whose hour circle moves at a constant rate along whose hour circle moves at a constant rate along the celestial equator. the celestial equator.

• The mean sun moves at the average rate of The mean sun moves at the average rate of motion of the apparent sun. motion of the apparent sun. – In 24 hrs, it moves through 360 deg of arc over the In 24 hrs, it moves through 360 deg of arc over the

circumference of the earth, or through 15 deg of arc circumference of the earth, or through 15 deg of arc every hour, or through 15’ of arc every minute.every hour, or through 15’ of arc every minute.

The Equation of TimeThe Equation of Time

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

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

Local Mean Time andLocal Mean Time andArc to Time ConversionsArc to Time Conversions

• Local Mean Time - time referenced to the Local Mean Time - time referenced to the observer’s meridian; also called LMT.observer’s meridian; also called LMT.– This time is in contrast to GMT (Greenwich Mean This time is in contrast to GMT (Greenwich Mean

Time), time referenced to the Greenwich meridian; Time), time referenced to the Greenwich meridian; and to ZT (zone time), time referenced to the central and to ZT (zone time), time referenced to the central meridian of the various time zones.meridian of the various time zones.

• Based on the relationship of the mean sun with Based on the relationship of the mean sun with the observer’s meridian. the observer’s meridian. LMT differs from ZT by LMT differs from ZT by the amount of time required for the mean sun to the amount of time required for the mean sun to traverse the difference in longitude between the traverse the difference in longitude between the observer’s meridian and the standard (central) observer’s meridian and the standard (central) meridian of the time zone.meridian of the time zone.

Local Mean Time andLocal Mean Time andArc to Time ConversionsArc to Time Conversions

• Since the mean sun moves through 15 deg of Since the mean sun moves through 15 deg of arc every hour, or through 15’ of arc every arc every hour, or through 15’ of arc every minute, these differences in longitude can be minute, these differences in longitude can be directly converted into differences in time.directly converted into differences in time.

• The “Conversion of Arc to Time” table in the The “Conversion of Arc to Time” table in the Nautical Almanac aids in this calculation.Nautical Almanac aids in this calculation.– Just remember that any location east of another has Just remember that any location east of another has

a later time, and any location to the west has an a later time, and any location to the west has an earlier time.earlier time.

Arc-to-Time ConversionArc-to-Time Conversion

LMT of Apparent NoonLMT of Apparent Noon

• Use arc-to-time conversion methodUse arc-to-time conversion method

Zone Time of Noon = Zone Time of Noon = Local Apparent Noon (LAN)Local Apparent Noon (LAN)