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Celestial Definitions

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Definitions

Definition (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 south of the clelstial equator, along the hour crcle 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 pont in question when combined with declinatio, these astronomical coordinates specify the direction of the a point on thecelestial 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 in to 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 reference plane; the angle between the projected vector and a reference vector on the reference plane is called the azimuth.
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Zenith

The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in hte vertical direction opposite to hte apparent qravitational force at that location. The opposite direction, I.E the direction in which gravity pulls, is toward the nadir.
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Nadir

The nadir is the direction pointing directly below a partivular location; 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.
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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 Angel)


the angle between the maridian of the celestial object and the meridian of hte 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.
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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 hours, 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.

  • Nautical Time
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    Hour Angles

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    Diagram showing the Earth with angles identified: local hour angle, Greenwich hour angle, local mean sidereal time, Greenwich mean sidereal time, any right ascension.
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    Twilight

    Nautical twilight is defined to begin in the morning, and to end in the evening , when the center of the sun is geometrically 12 degrees below the horizon. in general, nautical twilight ends when navigation via the horizon at sea is no longer possible.
    Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below to horizon . From the end of astronomical twilight is the evening to the begining of astronaut twilight is the morning. the sky (away) from urban light pollution) is dark enough for all astronaomical observations.
    Morning civil twilight begins when the geometric center of the sun is 6 degree below the horizon (civil dawn) and ends at sunrise , Evening civil twilight begins at sunset and ends when the geometric center of the sun reaches 6 degree below the horizon (civil dusk.) Civil twilight can also be descrived as the limit as which twilight illumination is sufficient, under clear weather conditions. for terrestrial objects to be clearly distinguished, at the beginning of the morning civil twilight, or end of evening civil twilight, the horizon is clearly defined and the brightest stars are visivle under clear atmospheric conditions.
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    Intercept


    The intercept method is based on the following principle. The actual distance from the observer to the geographical position (GP) of a celestial body (that is, the point where it is directly overhead ) is "measured " using a sextant. The observer has already estimated his position by dead reckoning and calculated the distance from the estimated position to eh body's GP the different between the "measured" and caculated distances is called the intercept.
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    Great Circle


    Great-circle navigation is the practice of navigating a vessel (a ship or aircraft) along a great circle. A great circle track is the shortest distance between two points on the surface of a sphere; the Earth isn't exactly spherical, but the formulas for a sphere are simpler and are often accurate enough for navigation.

    Course and distance

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    Light Characteristics

    A light characteristic is a graphic and text description of a navigational light sequence or colur displayed on a nautical chart or in a Light List with the chart symbol for a lighthouse, lightvessel, buoy or sea mark with a light on it. The graphic indicates how the real light beacon may be identified when looking at its actual light output type or sequence.
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    Tide

    Tides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the Moon and the Sun and the rotaion of the Earth.
    Abbreviations used in the Admirality Tide Tables:
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    Spring and Neap Explanation
    The semi-diurnal range (the difference in height between high and low waters over about half a day) varies in a two-week cycle. Approximately twice a month, around new moon and full moon when hte Sun, Moon, and Earth form a line, the tidal force due to the sun reinforces that due to the Moon, The tide's range is then at its maximum; this is called the spring tide, or just springs, It is not named after the season, but, like that word, derives from the meaning "jump , burst forth, rise". as in a natural spring.

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    Speed
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    Mescellaneous
    Trigonometry
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