Navigation ( Part 4 )
Celestial or Rational Horizon
The observer’s rational horizon is a great circle on the celestial sphere every point on which is 90 degree away from his zenith. The plane of the observers rational horizon passes through the center of the earth.
True Altitude
True Altitude of a body is the arc of the vertical circle through that body contained between the rational horizon and the center of the body .
Zenith Distance
Zenith Distance of a body is the arc of the vertical circle through the body contained between the observers zenith and the center of the body
Since every point on the rational horizon is 90 degree from the observers zenith ,then the zenith distance = 90 degree – altitude.
Azimuth
Azimuth of a celestial body is the arc of the observers rational horizon or the angle at his zenith contained between the observer’s celestial meridian and the vertical circle through that body
Prime Vertical
It is the vertical circle through the east and west points of the observers rational horizon.
Amplitude
Amplitude of a celestial body is the arc of the observers rational horizon or the angle at his zenith contained between the observers prime vertical and the vertical circle through the body , when the body is on the observer’s rational horizon
The theoretical rising and setting amplitude is therefore measured N or S from the observers East point when the body is rising and from his West point when setting
Vertical Circle
These are the great circle on the celestial sphere passing through the observer’s zenith and nadir.
Summer Solstice
On 21st June , the sun attains its maximum declination north and the sun’s rays fall vertically over the tropic of cancer. The sun is then said to be at the summer solstice.
Autumnal Equinox
On 23rd Sep the tilt of the earth’s axis is in a direction at right angles to the direction from earth to sun . The sun’s rays fall vertically over the equator and the sun’s declination is o degree .
The circle of illumination passes through the two poles . All places on the earth have equal day and night . The sun is now said to be at the Autumnal equinox.
Winter Solstice
On 22nd December the south end of the earth’s axis , the south pole is titled towards the sun by the maximum amount of 23 ½ degree . on this date the sun is said to be at the winter solstice attains its maximum declination south. Sun’s rays then fall vertically over the tropic of capricorn.
Vernal Equinox
On the 21st march the earth axis is tilted in a direction at right angles to the direction from earth to sun. The sun’s rays again fall vertically over the equator and the declination of sun is zero . The sun is said to be vernal equinox .
Aphelion
A planet is said to be aphelion when its orbit , it is farthest from the sun.
Perihelion
A planet is said to be perihelion when in its orbit , it is nearest to the sun.
Perigee and Apogee
The moon in its orbit around the earth is nearest to the earth , they are said to be perigee and when farthest from the earth they are said to be apogee .
Conjunction
A planet or the moon is said to be in conjunction with the sun , when as viewed from the earth it is in the same direction as the sun . ie ) their celestial longitudes are same.
Opposition
A planet or the moon is said to be in opposition with the sun when as
viewed from the earth , it is opposite in direction to the sun. Ie) their
celestial longitudes are 180 degree apart.
Quadrature
A planet or the moon is said to be in quadrature when its elongation is exactly 90 degree east or west.
Sidereal Period of Moon
Its the time taken by the moon to complete one revolution of 360 degree around the earth. The sidereal period is of constant duration equal to 27 days 07 hours 43 minutes and 12 seconds ie ) approximately 27.33 days .
Synodic Period of Moon
Its the period of time between two consecutive new moons or two consecutive full moons. The sunodic period has an average length of about 29 days 12 hours 44 minutes This period may also be called Lunar month or lunation.
Nodes
The points at which the moons orbit intersects the ecliptic are called the moon’s nodes. That node at which the moon crosses the ecliptic from south to north is called the ascending node and the node at which it crosses the ecliptic from north to south is called the descending node.
Solar Eclipse
When the moon is in conjunction with the sun and the center of the three bodies are nearly in a line . The moon appears directly over the sun as viewed from the earth. Blocking of sun’s disc wholly or partly such an occurrence is called solar eclipse.
Lunar Eclipse
A lunar eclipse takes place when the moon passes through the earth’s shadow. This can happen only when the moon is in opposition with the sun.
Day
Is the interval in time between two successive meridian passage of a heavenly body over the same meridian.
Sidereal day
Is the interval in time between two successive meridian passage of the first point of aries over the same meridian.
Apparent solar day
Is the interval in time between two successive transit of the true sun across the same meridian .
Mean sun
Is an imaginary body assumed to move along the equinoctial at a uniform rate , equal to the average rate of motion of the true sun on the ecliptic.
Mean solar day
Is the interval in time between two successive meridian passage of the mean sun across the same meridian.
Greenwich Sidereal Time (GST)
Is the westerly hour angle of first point of aries measured from the Greenwich merdian
Local Sidereal Time (LST)
Is the westerly hour angle of the first point of aries measured from observer’s meridian.
Greenwich Apparent time (GAT)
Is the westerly hour angle of the true sun measured from the inferior meridian of Greenwich.
Local Apparent time (LAT)
Is the westerly hour angle of the true sun measured from the observer’s inferior meridian.
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