Lunar distance (navigation)

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In celestial navigation, lunar distance is the angle of the Moon's centre from the Sun or a bright star as measured using a sextant. Given a lunar distance and a nautical almanac, it is possible to make an accurate calculation of the current time and compare with time at a given place and thus longitude.

1 Method
- 1.1 Theory
- 1.2 Practice
2 Errors
3 See also
4 References

[edit] Method

The method relies on the relatively quick movement of the moon across the background sky. Although the moon appears to circle the earth once a day due to the rotation of the earth, it actually circles the earth in 27.3 days. So from a stationary observer (i.e. an observer keeping stationary with respect to the stars) the moon completes a circuit across the background stars every 27.3 days; In other words, the moon moves approximately by its own diameter across the background stars every hour.

[edit] Theory

If there are two people, one at Greenwich and one 15 degrees West, the time by the sun will be one hour later at 15 degrees West. So, if the person observes the position of the moon at Greenwich at noon and another person observes the moon 15 degrees west of Greenwich at their locally determined noon, then due to the one hour difference, although the sun is at its zenith, the moon would have moved approximately its own diameter across the sky.

[edit] Practice

In practice an observation is made to determine the moon's actual position against the background stars or the sun. It is possible to determine the actual position of the observer by comparing the moon's position to that in tables as predicted for a reference position. The number of hours ahead or behind a reference position such as Greenwich may then be calculated and hence longitude.

[edit] Errors

Modern sextants measure to 0.2 minutes of arc, equivalent to a movement of the moon relative to the stars of 21.84 seconds in time (approximately 5 miles in longitude). In practice however, actual errors will be much larger up to several minutes of arc [1] equating to as much as 100 miles in longitude.

[edit] See also

[edit] References

New and complete epitome of practical navigation containing all necessary instruction for keeping a ship's reckoning at sea ... to which is added a new and correct set of tables - by J. W. Norie 1828