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PLANETS AND THEIR SATELLITES

according to the system set forth in our chapter on Time and Longitude.

The principal drawback of this method is that it is not very accurate. Observations of the time of such an eclipse are doubtful to a large fraction of a minute. This corresponds to 15 minutes of longitude, or 15 nautical miles at the equator. In the polar regions the effect of the error is much smaller, owing to the convergence of the meridians. The method is, therefore, most valuable to polar explorers.

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