Untitled

Celestial Navigation

Course Overview

  • Field of study: Bachelor of Science in Marine Transportation

  • Focus on essential navigational tools and their application:

    • Errors of the compass and azimuths

Course Objectives

  • Students will learn to:

    • Determine the observed altitude of the sun when the true altitude is zero.

    • Explain the effect of latitude on the accuracy of amplitude observations.

    • Calculate the Local Apparent Time (LAT) and Local Mean Time (LMT) of the theoretical and visible rising and setting of the sun.

    • Extract information from the Nautical Almanac regarding the rising and setting of the sun.

    • Obtain the error of the magnetic compass and gyrocompass by comparing the compass bearing of a body with the true azimuth at the time of observation.

    • Obtain the azimuth of a celestial body using tables, formulas, or calculations with GMT, Nautical Almanac data, and the observer's Dead Reckoning (DR) position.

    • Solve amplitude problems to obtain the true bearing of a heavenly body at rising or setting.

    • Use isogonal lines or chart information to obtain magnetic variations for the observer's position.

    • Apply variations to the magnetic compass errors to find deviations for the ship's head direction.

Azimuth and Altitude

  • Azimuth:

    • Defined as the angle from the North (0 degrees) around the circle of the horizon where an object is located, ranging from 0° to 360°.

    • Increases clockwise.

  • Altitude:

    • Describes the angle of an object above the horizon, ranging from 0° (exactly on the horizon) to 90° (directly overhead).

Understanding the Sun's Movement

  • The sun does not rise or set; it remains at the center of the solar system.

  • The apparent movement of the sun is due to the Earth rotating on its axis (24-hour cycle, turning eastward).

Procedures for Sunset and Sunrise Calculations

  1. Extract the LMT from the daily page of the Nautical Almanac for the observer's latitude, accessing the next smaller number.

  2. Extract the LMT for the next larger number for the same latitude.

  3. Calculate the difference between these LMTs and use Table I in the Nautical Almanac for latitude correction.

  4. Apply the latitude correction to the smaller LMT to find the LMT for the observer's latitude.

  5. Adjust the LMT with the longitude in time to obtain Universal Time (UT).

Gyroscopic Principles

Gyro Compass

  • Definition:

    • A gyroscope features a spinning wheel or rotor within gimbals allowing movement along three perpendicular axes (horizontal, vertical, and spin axis).

    • Rapid spinning develops gyroscopic inertia, leading to stability in the plane of rotation when not considering friction.

Horizontal Earth Rate

  • At the equator, with spin axis pointing east-west, the gyroscope's plane remains consistent due to gyroscopic inertia while the Earth rotates.

  • Observers see the gyroscope rotate instead of the Earth.

  • Maximum effect noted at the equator, zero at the poles.

Vertical Earth Rate

  • Positioned at a geographic pole with a horizontal spin axis, the gyro appears to rotate along its vertical axis.

  • Between the poles and the equator, the gyro displays a combination of horizontal and vertical earth rates.

Fundamental Characteristics of Gyroscopes

  • Gyroscopic Inertia: The tendency of a spinning body to maintain its plane of rotation.

  • Precession: When a couple is applied, causing movement at right angles to both the applied couple and the wheel's spinning axis.

Magnetic Compass

Definition

  • The oldest navigation instrument, crucial for maritime navigation, permits ships to steer selected courses by taking bearings of visible objects for positional determination on charts.

Working Principle

  • Constructed of a magnetized needle able to rotate in a horizontal plane.

  • Earth's magnetic field causes the needle to align in a north-south position, drawing its power from the Earth's magnetic field.

Limitations of the Magnetic Compass

  1. Sensitive to magnetic disturbances.

  2. Useless at magnetic poles; sluggish and unreliable in polar areas.

  3. Deviation varies according to the ship's magnetic properties, affected by structural or magnetic cargo changes.

  4. Deviation changes with ship's heading, as both ship and earth behave as magnets.

  5. Does not point to true north.

Finding Magnetic Variation

  • Methods:

    • By Isogonic Lines:

    • Lines on Earth's surface along which declination remains constant.

    • By Compass Rose:

    • A circular diagram graduated in degrees (0° to 360°) and showing magnetic variation for specific areas.