LESSON 18: Time 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 and Mean Solar Time Apparent solar time - reckoned with respect to the passage or the observable sun (the apparent sun).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.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.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.

The 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 and Arc to Time Conversions Local Mean Time - time referenced to the observer’s meridian; also called LMT.Local Mean Time - time referenced to the observer’s 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 observer’s meridian. LMT differs from ZT by the amount of time required for the mean sun to traverse the difference in longitude between the observer’s meridian and the standard (central) meridian of the time zone.Based on the relationship of the mean sun with the observer’s meridian. LMT differs from ZT by the amount of time required for the mean sun to traverse the difference in longitude between the observer’s meridian and the standard (central) meridian of the time zone.

Local Mean Time and Arc 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.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.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.

Arc-to-Time Conversion

LMT of Apparent Noon

Use arc-to-time conversion methodUse arc-to-time conversion method Zone Time of Noon = Local Apparent Noon (LAN)