Flight Navigator : Time

Time
In celestial navigation, navigators determine the actual aircraft’s
position by observing the celestial bodies. The actual apparent
position of these bodies changes with time. Consequently,
determining the particular aircraft’s position relies on timing the particular
observation exactly. Time can be measured by the actual rotation of
the actual earth and in addition the particular resulting apparent motions of the particular celestial
bodies. This chapter considers a few different systems of
measurement, every with a special use. Prior to learning the
different kinds of time, it is very important to be able to recognize transit.
Notice with regard to picture below that the particular poles divide the actual observer’s meridian into halves.

 The particular observer’s position typically is throughout the upper
branch. The lower branch typically is the actual opposite half. Every day,
due to the actual earth’s rotation, each celestial human body transits
the actual upper not to mention lower branches of the actual observer’s meridian. The particular
at first kind of time presented here can be solar time.

Apparent Solar Time
The particular sun because it is seen throughout the sky can be called the particular true sun or simply
the apparent sun. Apparent solar time can be based upon the actual
movement of the sun as it crosses the particular sky. A sundial
accurately indicates apparent solar time. Apparent solar
time is not valuable, considering the particular apparent length of day varies
throughout the particular year. A timepiece would have in order to operate at
different speeds to be able to indicate correct apparent time. Though,
apparent time accurately indicates upper and lower transit.
Upper transit occurs at noon; apparent time plus lower transit
at midnight apparent time. Difficulties inside utilizing apparent time
led in order to the particular introduction of mean time.


Mean Solar Time
A mean day is an artificial unit of continual length, based
on the average of almost all apparent solar days over a period of
years. Time for a mean day is actually measured with reference with
a fictitious body, the particular mean sun, so tailored that its hour
circle moves westward at a continual rate along the celestial
equator. Time computed using the mean sun is actually called mean
solar time. The coordinates of celestial bodies within the Air
Almanac are really tabulated in mean solar time, creating it the time
of primary interest with navigators. The particular difference within length
between the particular apparent day (based upon the actual true sun) not to mention the particular
mean day (based upon the mean sun) is actually never because a lot because a
minute. The actual differences are generally cumulative, though, thus which the actual
imaginary mean sun precedes or alternatively follows the actual apparent sun by
approximately 15 minutes at certain occasions in the course of the actual year.
Greenwich Mean Time (GMT)
Greenwich Mean Time (GMT) typically is used for most celestial
computations. GMT typically is mean solar time measured within the
lower branch of the Greenwich meridian westward from
360° in order to the upper branch of the particular hour circle passing by
the particular mean sun.

 The particular mean sun transits the actual
Greenwich meridian’s lower branch at GMT 2400 (0000)
each day as well as the actual upper branch at GMT 1200. The actual meridian at
Greenwich is actually the particular logical selection for this reference, as it happens to be the actual
origin for the actual measurement of Greenwich hour angle (GHA)
and in addition the reckoning of longitude. Consequently, celestial
coordinates as well as additional information and facts tend to be tabulated within almanacs
with reference to be able to GMT. GMT typically is in addition generally known as Zulu or alternatively Z time.

Local Mean Time (LMT)
Really because GMT is mean solar time measured with reference
to the particular Greenwich meridian, neighborhood mean time (LMT) can be
mean solar time measured with reference in order to the actual observer’s
meridian. LMT is actually measured from the lower branch of the
observers meridian, westward by 360°, in order to the actual upper
branch of the particular hour circle passing through the particular mean sun.

 The particular mean sun transits the particular lower branch of
the particular observer’s meridian at LMT 0000 (2400) plus the upper
branch at LMT 1200. For an observer at the Greenwich
meridian, GMT is actually LMT. Navigators utilize LMT to be able to compute
local sunrise, sunset, twilight, moonrise, as well as moonset at
many latitudes along a given meridian.