TIME & ARC

Convert Time to Arc

Multiply the hours by 15 to obtain degrees of arc. Divide the minutes of time by four to obtain degrees. Multiply the remainder of step 2 by 15 to obtain minutes of arc. Divide the seconds of time by four to obtain minutes of arc. Multiply the remainder by 15 to obtain seconds of arc. Add the resulting degrees, minutes, and seconds.

Convert Arc to Time

Divide the degrees by 15 to obtain hours. Multiply the remainder from step 1 by four to obtain minutes of time. Divide the minutes of arc by 15 to obtain minutes of time. Multiply the remainder from step 3 by four to obtain seconds of time. Divide the seconds of arc by 15 to obtain seconds of time. Add the resulting hours, minutes, and seconds.

CELESTIAL DEFINITIONS 2

Declination Declination
(abbreviated Dec) is one of the two angles that locate a point on the celestial sphere in the equatorial coordinate system, the other being hour angle. Declination's angle is measured north or south of the celestial equator, along the hour circle passing through the point in question.

Right Ascention
Right ascension (abbreviated RA) is the angular distance measured eastward along the celestial equator from the vernal equinox to the hour circle of the point in question. When combined with declination, these astronomical coordinates specify the direction of a point on the celestial sphere in the equatorial coordinate system.

Celestial Equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space.

Azimuth
An azimuth is an angular measurement in a spherical coordinate system. The vector from an observer (origin) to a point of interest is projected perpendicularly onto a reference plane; the angle between the projected vector and a reference vector on the reference plane is called the azimuth.

Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e. the direction in which gravity pulls, is toward the nadir.

Nadir
The nadir is the direction pointing directly below a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface there.

Aries -
first Point ( ) The point of intersection of the ecliptic and the celestial equator (equinoctial) at which the sun is moving from a south to north declination. Also known as the spring equinox or the vernal equinox.

GHA
(Greenwich Hour Angle) GHA indicates the position past the plane of the Greenwich meridian measured in degrees. Equivalent to longitude on earth. It can be followed by East or West.

LHA
(Local Hour Angle) the angle between the meridian of the celestial object and the meridian of the observer.

SHA
(Sideral Hour Angle) The sidereal hour angle of a body on the celestial sphere is its angular distance west of the vernal equinox generally measured in degrees. The SHA of a star changes slowly, and the SHA of a planet doesn't change very quickly, so SHA is a convenient way to list their positions in an almanac.

CELESTIAL DEFINITIONS

EQUATOR
a great circle the plane of which is perpendicular to the earth’s axis.latitudes are measured from equator.

PRIME MERIDIAN
prime meridian is the meridian passing through Greenwich from where longitudes are measured.

LATITUDE
angular distance from equator of any place named north or south expresses in degrees and minutes.

D'LAT
difference in latitudes expressed in degrees and seconds.

D'LONG
difference in longitudes expressed in degrees and minutes.
Knots:distance travelled in nautical mile in one hour.

DEPARTURE
departure is the E-W distance between any two places.

PARALLEL SAILING
when ship sails on any parallel of latitude.

TRUE COURSE
true course is the angle between ship’s head and the meridian.

RHUMB LINE
rhumb line is a straight line on earth’s surface which cuts all meridian at the same angle.this angle is reffered to as true course.

PLANE SAILING
The ship sails on rhumb line.

TRAVERSE TABLE
traverse table given in norie’s nautical table or burton’s table  which can be used in solving parallel and plane sailing problem without using scientific calculator.

VARIATION
variation is the angle between true north and magnetic north.

Deviation
deviation is the angle between magnetic north and compass north.
Compass error:compass error is the angle between true north and compass north or algebraic sum of variation and deviation.

Meridional parts
length of meridian from equator on a maerctor chart between two parallels of latitude measured in units of longitudinal scale.

DMP
DMP is the length of meridian on a Mercator chart between two parallels of latitude measured in units of longitudinal scale.

Mercator chart
chart representing true shape of small areas of earth’s surface and on which rhumbline can be drawn as straight line.
Dead reckoning:a position worked up from last fix for true courses steered by the ship and her speed through water without considering other factors(leeway,current,tideal stream etc.)

Estimated position
position derived from DR position adjusted for estimated effects of  current ,tidal stream and leeway.

Leeway
the effect of wind causing bodily drift of the vessel to the leeward,perpendicular to the course streed.

Current
current is non-tidal horizontal movement of the sea mainly to meteorological ,oceanographical,or,topographical causes.

Tidal stream
a tidal stream is tidal horizontal movement of the sea due to tide raising forces of the sun and moon.
Set:refer’s to the currents direction and is the direction in which the vessel is being drifted by it,generally obtained fron DR position to fix.

Drift
drift is the distance by which the vessel is set in the direction of current.

GMT
it is the angle between the inferior Greenwich meridian and the meridian passing through the mean sun measured clockwise from the inferior Greenwich meridian.

LMT
it is the angle between the inferior observer’s meridian passing through the mean sun measured clockwise from the inferior observer’s meridian.

LIT
it is the longitude of the observer converted in time by dividing the longitude by 15▪.

ZT
zone time uses the standard(central)meridians of the various time zones as reference meridians.
ZDthe zone description of a time zone is the adjustment to be applied to that time zone to determine GMT.

IDL
international date line is modified 180▪ meridian.

Equinoctial
the great circle on the celestial sphee in the same plane as the equator.

Ecliptic
the great circle on the celestial sphere in the same plane as the orbiting plane of the earth.
Obliquity of ecliptic:the angle that the ecliptic makes with the plane of equinoctial 23▪26’.7.
First point of aries:a point where the sun apparently crosses the equinoctial from south to north.
First point of libra:a point where the sun apparently crosses the equinoctial from north to south.

SHA*:
sidereal hour angle of a star is the angle at the celestial poles or arc of equinoctial contained between celestial meridian passing through first point of areies and the celestial meridian passing through the star measured westwards.

RA*:
it is the angle at the celestial poles or arc of equinoctial contained between celestial meridian passing through first point of aries and the celestial meridian passing through the star measured eastwards.

LHAɣ:
local hour angle of first point is the aries is the angle at the celestial poles or arc of equinoctial contained between celestial meridian passing observer and the celestial meridian passing through through the first point of aries measured westwards.

LHA*:
local hour angle of a star is the angle at the celestial pole or arc of equinoctial contained between celestial meridian passing through the observer and the meridian passing through first point of aries measured westwards.
GP:geographical position of a body is a point on surface of the earth vertically below the body.
Celestial sphere:a sphere of infinite radius whose centre is the earth’s centre.
Equinoctial:a great circle on every point on which is 90▪ from the celestial poles.
Greenwich hour angle:an hour angle contained between celestial meridian of Greenwich and celestial meridian of the body measured westward.

Local hour angle:
an hour angle contained between  celestial meridian of the observer and celestial meridian of the body measured westward.
Sidereal hour angle:an hour angle contained between celestial meridian of ɣ  and celestial meridian of the body measured westwards.

Right ascension:
an hour angle contained between celestial meridian of ɣ and celestial meridian of the body measured eastward.
Easterly hour angle:ah hour angle contained between celestial meridian of the observer and celestial meridian of the body measured eastward.

Declination:
it is arc of celestial meridian passing through the body contained between equinoctial and the body.

Visible horizon:
visible horizon is a small circle on earth surface that binds the observer’s view.
Rational horizon:rational horizon is a great circle on the celestial sphere every point which is 90▪ from observer’s zenith.

Sensible horizon:
sensible horizon is a small circle on the celestial sphere whose plane is parallel to the plane of rational horizon and passes through the eye of the observer.

Sextant altitude:
sextant altitude is the altitude of a body above the visible horizon as read off from the sextant.

Observed altitude:
altitude of the  body above the visible horizon is called observed altitude.

Apparent altitude:
it is the altitude of the body above the sensible horizon.

True altitude:
true altitude of the body is arc of vertical circle passing through the body contained between the observer’s rational horizon and the body or it is the angle at the centre of the earth between the observer’s rational and the centre of the body.

Amplitude:
the amplitude of the body is the arc of the horizon contained between the east point and the body when rising,or between the west point and the body when setting.it is defined as the angle at the zenith between prime vertical and the vertical circle of the body when on rational horizon.

Visible horizon:
a small circle on the earth’s surface binding observer’s view.
Rational horizon:a great circle on celestial sphere whose pole is observer’s zenith.

Theoretical sunrise:
theoretical sunrise occurs when the true sun’s centre is on the observer’s rational horizon.

Visible sunrise:
it occurs at the instant when the upper limb of the sun is first visible above the sea horizon.