In celestial navigation, navigators determine the aircraft’s position by observing the celestial bodies. The apparent position of these bodies changes with time. Therefore, determining the aircraft’s position relies on timing the observation exactly. Time is measured by the rotation of the earth and the resulting apparent motions of the celestial bodies. This section considers several different systems of measurement, each with a special use. Before learning the various kinds of time, it is important to understand transit. Notice in Figure 1-12 that the poles divide the observer’s meridian into halves. The observer’s position is in the upper branch. The lower branch is the opposite half. Every day, because of the earth’s rotation, every celestial body transits the upper and lower branches of the observer’s meridian. The first kind of time presented here is solar time.

**Apparent Solar Time**

The sun as it is seen in the sky is called the true sun or the apparent sun. Apparent solar time is based upon the movement of the sun as it crosses the sky. A sundial accurately indicates apparent solar time. Apparent solar time is not useful, because the apparent length of day varies throughout the year. A timepiece would have to operate at different speeds to indicate correct apparent time. However, apparent time accurately indicates upper and lower transit. Upper transit occurs at noon; apparent time and lower transit at midnight apparent time. Difficulties in using apparent time led to the introduction of mean time.

**Mean Solar Time**

A mean day is an artificial unit of constant length, based on the average of all apparent solar days over a period of years. Time for a mean day is measured with reference to a fictitious body, the mean sun, so designed that its hour circle moves westward at a constant rate along the celestial equator. Time computed using the mean sun is called mean solar time. The coordinates of celestial bodies in the Air Almanac are tabulated in mean solar time, making it the time of primary interest to navigators. The difference in length between the apparent day (based upon the true sun) and the mean day (based upon the mean sun) is never as much as a minute. The differences are cumulative, however, so that the imaginary mean sun precedes or follows the apparent sun by approximately 15 minutes at certain times during the year.

**Greenwich Mean Time (GMT)**

Greenwich Mean Time (GMT) is used for most celestial computations. GMT is mean solar time measured from the lower branch of the Greenwich meridian westward through 360° to the upper branch of the hour circle passing through the mean sun. [Figure 1-13] The mean sun transits the Greenwich meridian’s lower branch at GMT 2400 (0000) each day and the upper branch at GMT 1200. The meridian at Greenwich is the logical selection for this reference, as it is the origin for the measurement of Greenwich hour angle (GHA) and the reckoning of longitude. Consequently, celestial coordinates and other information are tabulated in almanacs with reference to GMT. GMT is also called Zulu or Z time.

**Local Mean Time (LMT)**

Just as GMT is mean solar time measured with reference to the Greenwich meridian, local mean time (LMT) is mean solar time measured with reference to the observer’s meridian. LMT is measured from the lower branch of the observers meridian, westward through 360°, to the upper branch of the hour circle passing through the mean sun. [Figure 1-13] The mean sun transits the lower branch of the observer’s meridian at LMT 0000 (2400) and the upper branch at LMT 1200. For an observer at the Greenwich meridian, GMT is LMT. Navigators use LMT to compute local sunrise, sunset, twilight, moonrise, and moonset at various latitudes along a given meridian.

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